Europa is a frozen moon, so at first glace, it doesn&#8217;t seem like a great candidate for life. However, the thick icy crust hides a sub-glacial ocean. <a href="https://solarsystem.nasa.gov/europa/overview.cfm" target="_blank">We know about this oceanic wonderland </a>because, among other things, we detected its magnetic field (however, of course, we won&#8217;t be 100% sure until we get there).
Ultimately, the scientific consensus is that <a href="http://www.planetaryexploration.net/jupiter/io/tidal_heating.html">Europa has a liquid ocean due to gravitational interactions</a> between itself and the other bodies in its neighborhood. In short, Jupiter (and a few of its large moons, such as Io, Callisto, and Ganymede) pull and tug on Europa, causing the moon to stretch and contract. Without these gravitational interactions, and the heating that they produce, Europa would be a solid ice moon.
But what if our moon was replaced with Europa? Would we have a majestic water world just next door?
<figure id="attachment_26973" style="width: 1264px" class="wp-caption alignright"><a href="https://wp-assets.futurism.com/2015/03/EuropasOcean_KPHand003.0.jpg" rel="attachment wp-att-26973"><img class="size-full wp-image-26973" src="https://wp-assets.futurism.com/2015/03/EuropasOcean_KPHand003.0.jpg" alt="An illustration showing the calculated volume of water on Europa, compared to Earth. (Kevin Hand (JPL/Caltech), Jack Cook (Woods Hole Oceanographic Institution), Howard Perlman (USGS))" width="1264" height="913" /></a><figcaption class="wp-caption-text">An illustration showing the calculated volume of water on Europa, compared to Earth. (Kevin Hand (JPL/Caltech), Jack Cook (Woods Hole Oceanographic Institution), Howard Perlman (USGS))</figcaption></figure>
<h5>A Watery Moon</h5>
This should go without saying, but thought experiments require estimates and guesstimates. It is important to note that some of these processes can happen on short time scales (days to months), like the formation of a thin atmosphere, or long time scales (tens of thousands to tens of millions of years), like moon-wide glacial melting. The amount of time depends on a number of factors, and the following is a hypothesis based on our best current data.
<figure id="attachment_34369" style="width: 292px" class="wp-caption alignright"><a href="https://wp-assets.futurism.com/2014/12/Europas-Atmosphere.jpg"><img class="wp-image-34369" src="https://wp-assets.futurism.com/2014/12/Europas-Atmosphere.jpg" alt="Artist rendering of Europa's atmosphere (Credit: Kees Veenenbos)" width="292" height="219" /></a><figcaption class="wp-caption-text">Artist rendering of Europa&#8217;s atmosphere (Credit: Kees Veenenbos)</figcaption></figure>
Now, all that said, Europa is currently located beyond the habitable zone in our solar system. As a refresher, the &#8220;habitable zone&#8221; is the region where liquid water can exist (this zone is defined according to the temperature of the planet&#8217;s star). So, if Europa were moved and placed next to Earth, the moon would suddenly find itself within the habitable zone, and it would start to melt.
Due to the lacking atmosphere around the moon (it has one, but it is tenuous and very thin), the <a href="http://www.av8n.com/physics/melt-sublimate.htm">ice transitions straight from a solid to a gas</a>. Soon, Europa would begin to have a thicker and more substantial atmosphere, which would slowly grow thicker and thicker. Ultimately, without any protection of a magnetic field, strong ionizing radiation (what&#8217;s causing the ice to melt) would break apart the water in the atmosphere,  leaving behind hydrogen and oxygen gasses.
And the ionizing photons have another impact on Europa—they create a temporary protective magnetic field!
This is <a href="https://futurism.com/understanding-our-hellish-twin-nasa-investigates-holes-on-venus/">what we currently see around Venus</a> and Mars (to a lesser degree). The solar wind interacts with Venus&#8217; atmosphere, ionizing it, stripping it away, but also creating a temporary magnetic field in the process. This temporary magnetic field deflects some of the solar wind, thus slowing down the stripping process.
For a time, a fine balance would be found between the amount of ionizing radiation and the strength of the magnetic field. So things would look promising&#8230;for a time.
<h5>Glorious water world? Well, not quite.</h5>
At first glance, all of this sounds like a fantastic thing, it appears as if Europa will completely melt, leaving behind an oxygen rich water world. But this wouldn&#8217;t last. Without  a geomagnetic field like that of the Earth or Jupiter, it is doomed. It&#8217;s atmosphere would slowly be stripped away.
So what would we have? Well, something like this&#8230;
<figure id="attachment_12575" style="width: 640px" class="wp-caption aligncenter"><a href="https://wp-assets.futurism.com/2013/12/ku-xlarge1.gif"><img class="wp-image-12575 size-full" src="https://wp-assets.futurism.com/2013/12/ku-xlarge1.gif" alt="Image Credit: NASA/LRO" width="640" height="361" /></a><figcaption class="wp-caption-text">Image Credit: NASA/LRO</figcaption></figure>
Look familiar? Europa is now a small and barren rock orbiting around the Earth; it has become a smaller moon than our current one, as all of its water has been lost to space. What started off as a giant cosmic ice block has melted and gradually been stripped away by the solar wind (alas).
<h5>What have we learned?</h5>
What makes Europa such a nice target to explore is precisely its location beyond the habitable zone. When the planets were forming in our solar system, there was the frost line at about 5 AU, this is the point at which objects are far enough from the sun that ice crystals could begin forming. An icy moon the size of Europa could not form around the Earth, and it couldn&#8217;t continue being an icy moon if it was moved here. It&#8217;s too warm and the moon is too small.
This same principle applies for a moon like Titan. It is able to exist with its methane lakes because it&#8217;s in a cold region of the solar system. If it was closer, it would be too warm for the hydrocarbons to form naturally. Moreover, if it were closer to the sun, it would have had most of its atmosphere stripped away.
So to sum, it&#8217;s very unlikely that a habitable moon can form around an Earth-sized planet located in the habitable zone. The moon would have to be substantially larger, making it more like two planets as opposed to a planet/moon.
Additional commentary provided by Jolene Creighton.

Off World

If Europa were our moon, would we have a water world teeming with alien life just next door?

What if Europa Was Our Moon?

<a href="https://wp-assets.futurism.com/2013/02/563635_417600054993076_86836014_n.jpg"><img class="alignright wp-image-899" src="https://wp-assets.futurism.com/2013/02/563635_417600054993076_86836014_n.jpg" alt="563635_417600054993076_86836014_n" width="710" height="703" /></a> We&#8217;ve all heard the story&#8230; Giant molecular cloud meets gravity, gravity makes star, star meets gravity and makes fusion. Fusion is the sum of hydrogen meets hydrogen (at its most basic form) and makes helium. Eventually hydrogen runs out of more hydrogen and BAM, helium meets helium to make carbon and oxygen&#8230; And so on and so forth until&#8230; Iron doesn&#8217;t want to play nice (it&#8217;s the antisocial element), so when gravity meets iron&#8230; <a title="Classifying Supernovae: The ‘Earth-shattering’ Kabooms" href="https://futurism.com/classifying-supernovae-the-earth-shattering-kabooms/">Supernova</a>e! It is at this point that we would expect to have some sort of supernova remnant left over, a neutron star or a black hole. At least, that is the normal procession of events that we&#8217;ve been hearing for a long time. What if, however, a star didn&#8217;t quite follow the aforementioned pattern? What if the star was so unstable that it blew itself apart prematurely? Can a star explode without leaving a trace? Could this actually happen??
Before I delve any further into this particular question, I am going to let you all come up with a conclusion to the answer! I am going to give you some of the cold hard facts about stars, and then let you make your decision as to whether a star could blow itself to oblivion before its time, leaving no real trace of its existence. Some of you make already know the answer, or most of the information that is to follow, but I must give enough facts for everyone to be able to make an informed decision (and hopefully not make this too long). To begin, it would be best to briefly describe what makes a star &#8220;burn.&#8221; Strictly speaking, stars don&#8217;t actually burn; burning implies combustion, like that of a fire. Although some oxygen does exist within our sun, it doesn&#8217;t burn so much as explodes. Our star is in its main sequence, this means that, right now, it is fusing hydrogen atoms into helium atoms. Many of us have seen film <a href="http://m.youtube.com/watch?v=icCM3MvlXks" target="_blank">clips of hydrogen bombs going off</a> .
Now, imagine an uncountable amount of those bombs going off every second! So why doesn&#8217;t our sun blow itself apart right now? A star is held together by a fine balance between gravitational contraction (trying to collapse the sun) and radiation pressure (trying to blow the sun to oblivion), the gravity is supplied by the mass of our sun and the radiation pressure is LOTS of hydrogen fusion explosions. In short, our star is in hydrostatic equilibrium&#8211;inward pressure is equal outward pressure.
Next, in another five or so billion years time our star will run out of hydrogen to burn at its core. It is at this point that helium burning begins; however, there is a step to consider before this. Because our star is relatively small on the cosmic scale, it doesn&#8217;t have a clean transition to the helium burning stage of its life, so a cataclysm must occur! This is known as <a href="http://www2.astro.psu.edu/users/rbc/a534/lec23.pdf" target="_blank">the Helium Flash</a>, a thermonuclear runaway event where the entire helium core bursts into life in an instant, a massive amount of energy is released. It is, however, not enough to destroy the star. True, it will make it expand considerably as the outer parts take the brunt of the explosion, but gravity will have its way and pull the material back in again until a new equilibrium is reached. This new equilibrium will be larger than on the hydrogen burning stage, but smaller than what it achieved shortly after the helium flash. Larger stars (a little as twice the size as our sun) don&#8217;t have a helium flash, but the reason why is a topic for another day. At this point, it is important to note that <a href="http://m.youtube.com/watch?v=icCM3MvlXks" target="_blank">variable or pulsating stars </a>are the exception to this equilibrium.
<figure id="attachment_15772" style="width: 300px" class="wp-caption alignleft"><a href="https://wp-assets.futurism.com/2014/01/250140_357238014362614_66440795_n.jpg"><img class="size-medium wp-image-15772" src="https://wp-assets.futurism.com/2014/01/250140_357238014362614_66440795_n-300x251.jpg" alt="Image Credit: Marco Lorenzi" width="300" height="251" /></a><figcaption class="wp-caption-text">Image Credit: Marco Lorenzi</figcaption></figure>
Many larger stars at some point during their lives will, for a while, become unstable. When they just can&#8217;t reach an equilibrium, they pulsate, getting brighter and dimmer. Cepheid Variables are just one type of these pulsating stars. They are very good for using as distance calculators, as they do not pulsate themselves out of existence 🙂 These instabilities are quite temporary, so a Cepheid Variable may potentially not be one in ten years time. At the end of a stars life it will end up two different ways: 1) If it&#8217;s a smaller star, it&#8217;ll end up as a white dwarf of some description. 2) If it&#8217;s a larger star, it&#8217;ll go up in a supernova. We&#8217;re just going to focus on the second option here, the supernova and what causes it. I am just briefly going to skim over this subject, it&#8217;s an article on its own! So, as mentioned in the offset, iron doesn&#8217;t play well with others, and gravity somewhat wins.
Through all of the previous stages of a stars life, gravity keeps trying to crush the core. However, it continues to fuse and produce energy; hydrogen fuses with hydrogen to produce helium; helium fuses with helium to produce carbon and oxygen&#8230;this keeps it in hydrostatic equilibrium. Iron won&#8217;t fuse with iron to produce energy so when gravity meets iron, iron literally gets crushed,&#8211;crushed more and more. If the iron core is going to be heavier than 1.4 solar masses, electron degeneracy can&#8217;t hold it up. As a result, the electrons and protons fuse together to make neutrons. If the core is less than 3-5 solar masses then a neutron star will be the supernova remnant, if more, a black hole will emerge. Briefly, with a 2 solar mass neutron star core, when the star runs out of all fuel the iron core shrinks dramatically to about the size of a city (crushed by gravity until neutron degeneracy stops it from contracting into a black hole). And without any more energy coming from the core, the outer parts of the star begin falling in towards the core VERY FAST. When these outer layers come crashing in on the neutron star at the center, they effectively bounce off of it and sail outwards into the universe. This is a Type II Supernova, the ejection of material into the universe because of a VERY hard bounce.
So&#8230; What&#8217;s the Answer?
Is it possible for a star to explode without there being any remnant left over? Am I bluffing you because I haven&#8217;t mentioned how it could potentially happen? Would I write all of this if there wasn&#8217;t something more? Or am I double bluffing you and trying to make you believe that there should be more when there isn&#8217;t? Or even worse is it a triple bluff?? In the comments box give your answer; &#8220;Yes&#8221; if you think a star can blow itself to oblivion, &#8220;No&#8221; if you think there should always be a remnant. If you want you can even give a reason for your answer Now don&#8217;t cheat! Make sure you make your decision and comment before looking at the link below with the answer. If you get the answer right, why not go and share this post to show off to your friends. We will post the answer with the explanation in 24 hours.

FromQuarkstoQuasars

We’ve all heard the story… Giant molecular cloud meets gravity, gravity makes star, star meets gravity and makes fusion. Fusion is the sum of hydrogen meets hydrogen (at its most basic form) and makes helium. Eventually hydrogen runs out of more hydrogen and BAM, helium meets helium to make carbon and oxygen… And so on […]

Can a Star Explode without Leaving any Remnant?

<figure id="attachment_27005" style="width: 825px" class="wp-caption aligncenter"><a href="https://wp-assets.futurism.com/2014/07/light-pollution-dark-city.jpg"><img class="size-full wp-image-27005" src="https://wp-assets.futurism.com/2014/07/light-pollution-dark-city.jpg" alt="Image via Thierry Cohen" width="825" height="550" /></a><figcaption class="wp-caption-text">Image via <a href="http://thierrycohen.com/" target="_blank">Thierry Cohen</a></figcaption></figure>
Light pollution and astronomy really don&#8217;t go hand in hand at all. Since a majority of the world&#8217;s population lives in built up areas, light pollution is becoming more and more of an issue, especially as the built up areas expand farther and farther out into the countryside. Light pollution may not be noticed quite so much by the younger generation, but those may remember what a truly bright, starry sky used to look like. And it is likely that, over the years, you&#8217;ve noticed a real change in how far development has pushed out from the central business district of your city/town are started to invade places that were once (mostly) uninhabitable.
Take Melbourne as an example, housing estates are popping up everywhere. Generally speaking, housing estates aren&#8217;t placed in already built up areas because there is no room, so city planners go out of the city and dump 20,000 homes on the outskirts. A decade or two later, the area has become saturated with people. So they move to the outskirts of this newly built up area to create more housing estates. And so the cycle continues.
The light pollution of a city as large as Melbourne can still be seen from 150km (93 miles) away, and it is clearly visible in a long exposure photo.
So with light pollution stretching out as far as it does, how do we deal with it? 
I&#8217;m going to break this into two parts, that of visual observation and in a photographic sense.
With light pollution there are 4 main things we can do (some ideas are, obviously, much better than others):
<ol>
<li>Just give up, throw in the towel and say that the skies are too polluted and that you want another hobby.</li>
<li>Somehow manage to disrupt all the power in your city for a night so that the only lights working are battery operated and cars. Although there will still be some light produced from these sources, your skies will still be quite clear (editors note: we will not be held accountable if any of you run out and start attacking power plants).</li>
<li>Head out to a location where the skies are darker. This doesn&#8217;t have to be too far either. Although, in the preceding paragraphs, I&#8217;ve mentioned that light pollution stretches out for about 150km in every direction, the sky starts to get noticeably clearer the moment you leave a city or town. In fact, some of the best skies that can be found are within about 120 km of Melbourne. In short, drive an hour or so away from your city in a direction that takes you away from other major light sources, and chances are, you&#8217;ll get some pretty good viewing.</li>
<li>Another good way to somewhat beat light pollution is with the use of filters. One of the more common type of filter for general observing is called a &#8220;Light Pollution Filter.&#8221; The actual name for this type of filter is a<a href="http://www.astronomik.com/en/visual-filters/uhc-filter.html" target="_blank"> UHC filter or Ultra High Contrast</a>. It&#8217;s a broadband filter with narrowband subtraction. In simple terms, it lets through almost all of the light you want while blocking out the light pollution (Sodium and Mercury lines). Narrowband filters such as <a href="http://www.astronomik.com/en/visual-filters/oiii-filter-visuell.html" target="_blank">O-III and Si-II</a> and H-a are also good for specific deep sky viewing.</li>
</ol>
Now onto photography: Moving away from the city lights is, by far, the best option. It allows you to photograph the entire sky and not just the individual objects that you happen to be able to see.
But this is not possible for many people, which is why, in astrophotography, filters are a must (especially with the narrowband filters). If you were to ask around, about 50% of people would say that UHC filters are good, while the other 50% say that they&#8217;re not that useful. However, you can never fully anticipate what conditions you will encounter, so when photographing in light polluted areas, they&#8217;re better to have than not.
Narrowband filters are amazing in astrophotography (and amazingly necessary). They allow you to take spectacular images of deep sky objects without having to worry about either light pollution from cities or the moon&#8217;s glow. The reason for this is that they only allow very specific wavelengths of light though. In the case of the O-III filter, it only lets the light from ionised oxygen in while blocking pretty much all other light. Because of the way that these work, some of the best images taken by amateur astrophotographers have come from heavily light polluted locations. Sadly however, this is beyond the reach of many aspiring photographers, as these types of setups don&#8217;t come cheap.
To Sum: Observing can be done in light polluted areas, it can be done in cities, and even without any filers at all. However (obviously), when observing, light pollution does limit what you can observe, so it may just be a matter of being selective about what you view on any given night.
If there is a full moon, don&#8217;t expect to be able to see much of any deep sky objects, a full moon can do far more damage than a city&#8217;s light pollution. Instead, just look at the moon or the planets, bright double stars (there are quite a few bright ones up there in the sky). If you have a chance to go out for a night away from a city without the moon lighting up the sky, take advantage of it, remember you don&#8217;t have to travel too far to get some amazing views of the sky.
Also remember that filters are an aid, they aren&#8217;t the be all end all. Using a UHC filter near a city will make the object look at a little better but just keep in mind&#8230; NOTHING beats a darker sky 🙂
&nbsp;

Light pollution and astronomy really don’t go hand in hand at all. Since a majority of the world’s population lives in built up areas, light pollution is becoming more and more of an issue, especially as the built up areas expand farther and farther out into the countryside. Light pollution may not be noticed quite […]

Light Pollution: Tips For Seeing The Stars

<figure id="attachment_17737" style="width: 300px" class="wp-caption alignleft"><a href="https://wp-assets.futurism.com/2014/02/tumblr_m8oz9t6ayU1rpdglno1_400.gif"><img class="size-medium wp-image-17737" src="https://wp-assets.futurism.com/2014/02/tumblr_m8oz9t6ayU1rpdglno1_400-300x300.gif" alt="Image Credit: How Stuff Works" width="300" height="300" /></a><figcaption class="wp-caption-text">Image Credit: How Stuff Works</figcaption></figure>
Throughout the universe, there are several underlying principles that we consider to be constants; for example, the <a href="http://www.space.com/15830-light-speed.html">speed of light in a vacuum</a>. So many different aspects of our understanding of the universe comes from this one constant. If the speed of light wasn&#8217;t constant, GPS systems wouldn&#8217;t work, and we wouldn&#8217;t be able to determine the age of distant galaxies in the universe. Most of Einstein&#8217;s equations would fall apart without a constant speed of light; <a href="https://futurism.com/tag/time-dilation/">time dilation</a> and , the mass-energy relationship would fluctuate and <a href="https://futurism.com/special-relativity-simplified/" target="_blank">quantum fluctuations</a> on the Plank scale could potentially have large scale effects.
The speed of light in a vacuum is considered to be constant, never changing over the last 13 billion years. However, there are a number of other constants, and the one we&#8217;re going to be discussing here is the gravitational constant.
<h2>The Gravitational Constant:</h2>
The Gravitational Constant has a value of 6.67384&#215;10^-11 m^3 kg^-1 s^-2. Now, this possibly looks a bit messy but it basically means that gravity has a set strength. A strength that works in 3-dimensional space, is an accelerating force and is determined by mass. It is because of the set value of the gravitational constant that we&#8217;re able to accurate determine the mass of two orbiting objects. We can calculate the amount of force imparted on us by the Earth to keep us on the Earth &#8211; 9.81 m/s^2.
<figure id="attachment_25496" style="width: 300px" class="wp-caption alignright"><a href="https://wp-assets.futurism.com/2014/06/image.jpg"><img class="size-medium wp-image-25496" src="https://wp-assets.futurism.com/2014/06/image-300x224.jpg" alt="A hand drawing of the device that Henry Cavendish used to measure the Gravitational Constant. Image: Henry Cavendish" width="300" height="224" /></a><figcaption class="wp-caption-text">A hand drawing of the device that Henry Cavendish used to measure the Gravitational Constant. Image: Henry Cavendish</figcaption></figure>
So far as we can tell, the gravitational constant has remained constant throughout the entire history of the universe. This has, however, been VERY difficult to prove! Measurements of the gravitational constant over the past 200 years have been erratic. Even as the techniques that we use now are far more advanced and sensitive than were used two centuries ago, the true value of the gravitational constant remains elusive.
In 2013, a group of researchers working out of France <a href="http://phys.org/news/2013-09-gravitational-constant-higher.html" target="_blank">took the measurement</a> of the gravitational constant, using the same machine that they&#8217;d used some 2 years earlier. Improvements were made on the machine to improve the sensitivity and give a more accurate result. The machine, which uses two independent methods to calculate the constant, averages the results of the two. This, in theory, should help reduce systematic errors. What did they find? A different result!
<h2>Why is the Gravitational Constant so hard to pinpoint?</h2>
At first it may seem strange that the gravitational constant is so hard to determine. There are four fundamental forces in the universe: 
• <a href="https://futurism.com/the-strong-nuclear-force/">Strong Force</a> 
• Weak Force 
• Electromagnetism 
• <a href="https://futurism.com/the-universe-gravity-multiple-dimensions-micro-black-holes/">Gravity</a>
Gravity is by far the weakest of the four forces, which, may also sound a little strange considering what we see in the universe. When looking out into the cosmos, gravity appears to be the reigning king of all. Gravity is so strong that it causes stars to fuse hydrogen into helium, collapses stellar cores into neutron stars and black holes, creates quasars and dictates the flow of matter within the entire universe.
<figure id="attachment_25499" style="width: 300px" class="wp-caption alignleft"><a href="https://wp-assets.futurism.com/2014/06/image1.jpg"><img class="size-medium wp-image-25499" src="https://wp-assets.futurism.com/2014/06/image1-300x223.jpg" alt="A table showing the relatively strengths of the four fundamental forces. Image Credit: HyperPhysics" width="300" height="223" /></a><figcaption class="wp-caption-text">A table showing the relatively strengths of the four fundamental forces. Image Credit: <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/forces/funfor.html">HyperPhysics</a></figcaption></figure>
On a large scale, gravity wins. But, as was previously mentioned, gravity is the weakest of the four forces. The reason for this discrepancy is, as a force, gravity travels further and has a slower fall off. The strongest of the four forces, the Strong Force, becomes almost non-existent at distances outside of a nucleus. What makes gravity stronger in macro circumstances is that it is accumulative. The more matter there is, the more gravity.
But still, gravity is weaker. Therefore, when trying to measure it, the other forces can cause systematic errors. It is akin to trying to measure the weight of a feather, outdoors, in a slight breeze, with an old pair of scales. The first thought would be to try to remove the other sources of error. We do this by doing several different experiments and then averaging the results. We are not yet aware of a single perfect test to measure the gravitational constant.
Over the last century, nearly every time the gravitational constant has been measured, we&#8217;ve observed a different value. At first glance, you may think that means we&#8217;re getting closer to its true value; however, it is hard to tell. It is currently uncertain as to whether the constant has actually been changing marginally over time or just compounding systematic errors. Another theory is that there many be a correlation between dark energy and gravity. Yet another theory states that the constant is always fluctuating around an average value and that if we keep testing it over an even longer period of time, we will find the true average value.
It is nice to still have mysteries in the universe, things that we done quite understand. Incredibly small changes in the gravitational constant can affect the rate at which stars form, their size and how long they remain on their main sequence. Maybe we will never know for sure, remaining one of the universes true mysteries.

Throughout the universe, there are several underlying principles that we consider to be constants; for example, the speed of light in a vacuum. So many different aspects of our understanding of the universe comes from this one constant. If the speed of light wasn’t constant, GPS systems wouldn’t work, and we wouldn’t be able to […]

Is The Gravitational Constant Really Constant?

<figure id="attachment_17637" style="width: 269px" class="wp-caption alignright"><a href="https://wp-assets.futurism.com/2014/02/000hubble.jpg"><img class="size-medium wp-image-17637" src="https://wp-assets.futurism.com/2014/02/000hubble-269x300.jpg" alt="Hubble Frontier Field Abell 2744 via HubbleSite" width="269" height="300" /></a><figcaption class="wp-caption-text">Hubble Frontier Field Abell 2744 via HubbleSite</figcaption></figure>
One of the most important things in astronomy &#8211; being able to determine the age of galaxies in the distant universe. Galaxies are good, they&#8217;re large and produce a bright light source for us to detect billions of years later. When finding the distance to objects closer to us, we use techniques such as parallax or a luminosity relationship. <a href="http://astronomy.swin.edu.au/cosmos/T/Trigonometric+Parallax">Parallax</a> works exceedingly well for stars that are close to us; however, it does not work for other galaxies. For a <a href="http://www.atnf.csiro.au/outreach/education/senior/astrophysics/variable_cepheids.html">luminosity relationship</a>, we either use theoretical models of stellar evolution, variable stars or some supernovae (Type Ia for example). For individual galaxies in the early universe, we need a different method.
<h1 style="padding-left: 30px;">Determining Distance Through Spectrography:</h1>
There are several methods that can be used to determine distance over cosmic scales. Firstly, there is <a href="http://www.atnf.csiro.au/outreach/education/senior/astrophysics/spectrographs.html">spectrography</a>. This is where a spectrum is taken of the subject, whether it be a planet, star, galaxy or quasar. This spectrum is effectively its fingerprint, the imprint of not only what it is made of but what&#8217;s between it and us. For determining the distance of galaxies, we can use what is called the <a href="http://www.ucolick.org/~xavier/Science/LL/">Lyman Limit</a>. The Lyman Limit emission is a specific fingerprint that leaves a very distinctive marker. It originates in the Ultraviolet part of the electromagnetic spectrum, and is caused by the most energetic emission of a hydrogen atom. This emission happens at ~91.2nm, as hydrogen is by far the most abundant element, the emission line is very strong and has a distinctive signature. If we do a spectrographic analysis of a distant galaxy, detecting this line at ~912 nm, in the near infrared, we know that it&#8217;s at a <a href="https://futurism.com/gravitational-redshift-the-universe-in-motion/">redshift</a> of ~9, less than a billion years after the Big Bang. As those photons make their 13 billion year journey, they are going to pass through clouds of hydrogen. While doing this, they are going to pick up the Lyman absorption lines (there are several). So, not only do we get a picture of when the galaxy existed in the first place, but the placement of hydrogen gas in between.
<h1 style="padding-left: 30px;">Determining Distance Through Filters:</h1>
Doing spectrographic analysis is the only way of getting a truly accurate idea on the ages of different galaxies and quasars so early in the universe. The problem with this method, although accurate, is it is very time consuming. We need to get a half decent signal (enough photons) to get an accurate spectrum, where as in an image, it doesn&#8217;t need to be as good. We also have to get the spectrum of every galaxy, where do we start? This is where taking images through multiple filters can make all the difference. The Lyman Limit creates a very sharp cut-off in emissions, this is why it is so distinctive and easy to spot. For us astronomers, it has another benefit.
<figure id="attachment_24080" style="width: 776px" class="wp-caption aligncenter"><a href="https://wp-assets.futurism.com/2014/05/image1.jpg"><img class="size-full wp-image-24080" src="https://wp-assets.futurism.com/2014/05/image1.jpg" alt="The spectrum of two quasars taken as a part of the SSDS catalog. Image Credit: Christensen. L et al " width="776" height="588" /></a><figcaption class="wp-caption-text">The spectrum of two quasars taken as a part of the SSDS catalog. (<a href="http://www.aanda.org/articles/aa/pdf/2009/39/aa12015-09.pdf">Image Credit: Christensen. L et al</a>)</figcaption></figure>
Galaxies slowly get redshifted as we look further and further back into the past, the sharp Lyman Limit cut-off gradually gets pushed out of the UV part of the spectrum and into the blue, green, red and infrared. How does this help astronomers? It allows us to categorise galaxies by what filters they appear in. To explain, let&#8217;s make 4 hypothetical filters with the transmissions as follows:
<figure id="attachment_24082" style="width: 300px" class="wp-caption alignright"><a href="https://wp-assets.futurism.com/2014/05/image2.jpg"><img class="size-medium wp-image-24082" src="https://wp-assets.futurism.com/2014/05/image2-300x221.jpg" alt="An illustration of how different filters view a distant galaxy. Image Credit: Space Telescope Science Institute" width="300" height="221" /></a><figcaption class="wp-caption-text">An illustration of how different filters view a distant galaxy. Image Credit: <a href="http://www.stsci.edu/~dcoe/BPZ/intro.html">Space Telescope Science Institute</a></figcaption></figure>
<ul>
<li>100-380 nm (Ultraviolet)</li>
<li>380-550 nm (Blue to mid green)</li>
<li>550-720 nm (Mid green through to red)</li>
<li>720-1000 nm (Infrared)</li>
</ul>
Galaxies viewed in the nearby universe will emit pretty strongly in all four of these filters. At a redshift of 4, the Lyman Limit is pushed from 91.2nm to 273.6nm. Although it will still appear in all four filters, it&#8217;ll be a weaker signal in filter 1. It&#8217;s still not enough to get an idea of redshift however. At a redshift of 6, the break is now at 456nm. As the cut-off has now pushed out of the first filters passbands, it&#8217;ll be virtually undetectable in that image. This allows us to break galaxies up into filters that they appear in. Galaxies that do not appear in the first filter, these exist from redshift 4.2 and onwards. If not in the second filter, redshift 6 and onwards. If not in the third filter, 7.9 and onwards. If not in any of these four filters, it exists further than redshift 11.
<h1 style="padding-left: 30px;">Putting it All Together:</h1>
To truly understand the universe, we have to know where and when galaxies exist. Doing spectrographic analysis is the only way to get an accurate measure on its distance. If all we&#8217;re interested in is the very distant universe, let&#8217;s say, redshift 8 and beyond, we don&#8217;t want to have to get a spectrum of thousands of galaxies just to find a few dozen candidates. To narrow the search field, we use filters. Using the above example, we know that every galaxy that is visible in the first two filters, exists between the present and redshift 7.9. The passbands of actual filter sets (like Sloan or Johnson-Cousins) are far more useful than the above example. More filters covering the same 100-1000 nm range for more precise measurements.
<h1>Determining Distance Compartmentalized:</h1>
<figure id="attachment_24247" style="width: 864px" class="wp-caption aligncenter"><a href="https://wp-assets.futurism.com/2014/05/Scaling-the-Universe.jpg"><img class="size-full wp-image-24247" src="https://wp-assets.futurism.com/2014/05/Scaling-the-Universe.jpg" alt="Image Credit: Tim Jones/Damond Benningfield" width="864" height="672" /></a><figcaption class="wp-caption-text">Image Credit: Tim Jones/Damond Benningfield</figcaption></figure>

One of the most important things in astronomy – being able to determine the age of galaxies in the distant universe. Galaxies are good, they’re large and produce a bright light source for us to detect billions of years later. When finding the distance to objects closer to us, we use techniques such as parallax […]

Determining Distance Through The Lyman Limit

<figure id="attachment_4312" style="width: 600px" class="wp-caption aligncenter"><a href="https://wp-assets.futurism.com/2013/08/image.jpg"><img class="wp-image-4312 size-full" src="https://wp-assets.futurism.com/2013/08/image.jpg" alt="image" width="600" height="308" /></a><figcaption class="wp-caption-text"><a href=" http://www.pas.rochester.edu/~afrank/A105/LectureXII/LectureXII.html" target="_blank">Image Source</a></figcaption></figure>
<a href="https://wp-assets.futurism.com/2013/08/image.jpg"> 
</a>Supernova, something that we&#8217;ve all heard about at one time or another, are<a href="http://www.mpa-garching.mpg.de/~thj//popular/Collapsing_stars.html" target="_blank"> simply amazing celestial events</a>. The type that we&#8217;re going to be looking at is today is Type II supernova. These occur when a neutron star or a black hole is left behind after the death of the star ie., after the supernova occurs. For a bit of background information, I&#8217;ve created a chart that we can use as a rough guide so as to be able to work out what will be left behind during this final stage of a star’s life (scroll to the bottom to see the chart).
So let’s say we have a 15 solar mass star, based on the stellar evolution chart below, we can see that there will be a supernova and a neutron star leftover. However, even with all of our advanced supercomputers and everything that we currently know about physics and the universe, running a computer model of a supernova still proves to be a very difficult task. What we think happens during a supernova is that, when our 15 solar mass star runs out of fuel, the iron core will shrink from about the size of the Earth to the size of a town on the order of 150 milliseconds&#8230; That&#8217;s unbelievably fast! The only force stopping it from getting any smaller is neutron degeneracy, meaning that the neutrons cannot be pushed any closer together.
As you can imagine, if the core of the star (which may already be as much as twice the mass of our sun) is compressed into a 10 km sphere, gravity is going to be quite intense. The moment that the iron core shrinks the rest of the star falls in towards the core as well, sometimes reaching speeds up to 25% of the speed of light! All of this material then hits the stellar core and bounces&#8230; and it bounces hard. Tremendous amounts of energy are released, about half of it goes into kinetic energy, blowing the star apart, and the other half is turned directly into heat.
<figure id="attachment_23946" style="width: 601px" class="wp-caption aligncenter"><a href="https://wp-assets.futurism.com/2013/07/supnovform.jpg"><img class="wp-image-23946 size-full" src="https://wp-assets.futurism.com/2013/07/supnovform.jpg" alt="Image via HyperPhysics" width="601" height="372" /></a><figcaption class="wp-caption-text">Image via <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/astro/snovcn.html" target="_blank">HyperPhysics</a></figcaption></figure>
However, current computer modeling suggests that a core bounce does not quite provide enough energy to power a supernova, there must be more to it. It is well known and documented that supernovae are basically just massive neutrino factories. A supernova is so bright that it will typically be brighter than an entire galaxy&#8211; as you can imagine, this is very bright. The neutrino output, however, is about 100 times brighter still!
What some more recent computer models have shown is that, during the core collapse, not all of the neutrinos are able to escape. These very small and not overly energy particles don&#8217;t much like interacting with normal matter. They do interact with normal matter, but quite infrequently. It is estimated that tens of thousands of these tiny particles pass through the human body every second of every day over the course of our entire lives. Most pass straight through the Earth as if it wasn&#8217;t even there.
As a large star is undergoing a <a href="http://thesis.library.caltech.edu/7118/2/thesis_oconnor_wlinks.pdf" target="_blank">core collapse</a>, it become very dense at the area just above the newly fledged neutron star, so dense that a higher concentration of the neutrinos actually interact with the in-falling gas and boil it. This is called “neutrino heating,” the neutrinos passing along their energy as well as the energy from the core bounce, that we have enough energy for a supernova.
When running computer models of supernova, they make far more sense when both core bounce and neutrinos are used in the simulation. As our understanding of what is actually going on in these weird and wonderful parts of the universe increases, and our processing power increases, we&#8217;re getting closer to being able to successfully emulate what we&#8217;re seeing out in the universe. We may not quite be there yet but we&#8217;re working on it.
&#8212;&#8212;&#8212;&#8212;&#8212;&#8212;&#8212;&#8212;&#8212;&#8212;&#8212;&#8212;&#8212;&#8212;&#8212;&#8212;&#8212;&#8212;&#8212;&#8212;&#8212; 
M represents the stars’ mass initial in solar masses (1 solar mass is the mass of our sun). Essentially, M represents any stars that fall in between the number on the left and the number on the right. The sentence that follows describes what we would expect to see at the end.
&#8211;0.08 &lt; M &gt; 0.5 Will end in a planetary nebula and a helium white dwarf. 
&#8211;0.5 &gt; M &lt; 8 Will end in a planetary nebula, but will also leave behind a carbon/oxygen dwarf remnant as the core is hot enough for helium burning to occur. 
&#8211;8 &lt; M &gt; 20 Will end with a supernova and a neutron star. A very good and beautiful example of this is with the Crab Nebula which is the remnant of the 1054 AD supernova, this was well documented by the Chinese and several other nations at the time as a star visible during the day. 
&#8211;20 &lt; M &gt; 160 Will end with a supernova and a black hole. 
&#8211;160 &lt; M &gt; 240 Will end as a pair instability supernova, where there will be nothing left over!
For more information on how nothing is sometimes leftover, read the following article:
<blockquote data-secret="lwLyVbAVrA" class="wp-embedded-content"><a href="https://futurism.com/can-a-star-explode-without-leaving-any-remnant-the-explanation/">Can a Star Explode without Leaving any Remnant? &#8211; The Explanation.</a></blockquote>
<iframe class="wp-embedded-content" sandbox="allow-scripts" security="restricted" style="position: absolute; clip: rect(1px, 1px, 1px, 1px);" src="https://futurism.com/can-a-star-explode-without-leaving-any-remnant-the-explanation/embed/#?secret=lwLyVbAVrA" data-secret="lwLyVbAVrA" width="500" height="282" title="&#8220;Can a Star Explode without Leaving any Remnant? &#8211; The Explanation.&#8221; &#8212; Futurism" frameborder="0" marginwidth="0" marginheight="0" scrolling="no"></iframe>
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Supernova, something that we’ve all heard about at one time or another, are simply amazing celestial events. The type that we’re going to be looking at is today is Type II supernova. These occur when a neutron star or a black hole is left behind after the death of the star ie., after the supernova […]

What is a Type II Supernova?

<figure id="attachment_19139" style="width: 236px" class="wp-caption alignright"><a href="https://wp-assets.futurism.com/2014/02/NGC-4402.jpg"><img class=" wp-image-19139" src="https://wp-assets.futurism.com/2014/02/NGC-4402-300x300.jpg" alt="Image Credit: NASA/ESA/Hubble" width="236" height="228" /></a><figcaption class="wp-caption-text">Image Credit: NASA/ESA/Hubble</figcaption></figure>
Recently we published an article in regards to the <a href="https://futurism.com/trends-in-star-formation-when-will-the-universe-run-out-of-fuel-for-stars/">trend of slowing star formation</a> as the universe progresses towards its maturity. In 2012, a research study involving the Subaru Telescope (NAOJ), Very Large Telescope (VLT) and United Kingdom Infrared Telescope (UKIRT) discovered that the universe hit its star formation peak 11 billion years ago, and that it has been declining every since. The study suggests that 95% of all stars that will ever exist, have already been &#8220;born&#8221;. <a href="http://www.naoj.org/Pressrelease/2012/11/05/index.html">Go here for the original press release</a>.
There are many different theories as to why this is happening in the first place. Most people seem to be against the idea that only 5% of stars are still unborn. Let&#8217;s look at why it is not at all an unreasonable conclusion to come to.
<h1 style="padding-left: 30px;">Is it Unreasonable?</h1>
<figure id="attachment_11123" style="width: 252px" class="wp-caption alignleft"><a href="https://wp-assets.futurism.com/2013/11/opo0635a.jpg"><img class=" wp-image-11123" src="https://wp-assets.futurism.com/2013/11/opo0635a-300x240.jpg" alt=" Image Credit: NASA, ESA and the Hubble Heritage Team STScI/AURA" width="252" height="175" /></a><figcaption class="wp-caption-text">Image Credit: NASA, ESA and the Hubble Heritage Team STScI/AURA</figcaption></figure>
So far, the universe has been around for nearly 14 billion years, and if it keeps going the way that it has, it will just continue to expand forever. The eventual fate of the universe is most likely to be, <a href="https://futurism.com/four-ways-the-universe-could-end/">The Big Chill</a>. Basically this means that anything that is reasonably close together will remain so &#8211; our local galaxy group for example. Anything that is currently accelerating away from us due to the expansion of the universe, will likely continue to do so. The Big Chill is a very boring way for the universe to &#8220;end.&#8221; Due to the ever expanding universe, most of the galaxies will eventually recess far away from us, leaving us with nothing outside of our SUPER galaxy.
Whatever gravity&#8217;s little fingers can keep a hold of &#8211; our <a href="http://arxiv.org/abs/astro-ph/0204249">local group of some 50 dwarf galaxies</a>&#8211; will eventually collide, forming a super sized elliptical galaxy. Imagine, the entire Local Group as one galaxy! For an interesting read on the future of the universe, check out <a href="http://www.futuretimeline.net/beyond.htm#.U0-A_CcaySM">Future Timeline</a>, start with &#8220;The Far Future&#8221; and then move to &#8220;Beyond.&#8221;
Clearly, star formation is still going to go on for a long time. The idea that 95% of all stars that are ever going to exist having already been born &#8211; it can seem like a bit of a stretch. Only 5% more stars in a trillion or more years, almost sounds like a joke. So, let&#8217;s have a look at some of the facts.
<h1 style="padding-left: 30px;">What Causes Star-Formation?</h1>
There are several different mechanisms that cause star formation. Most interesting at this point is the &#8220;intense bursts.&#8221; Star formation will occur for a very very long time of its own accord, continuing at a slowly and steady rate. It&#8217;s the bursts of formation that form the mass amounts of stars, especially when an entire galaxy is undergoing a burst. These are known as <a href="https://astronomy.swin.edu.au/cosmos/S/Starburst+Galaxy">Star Burst Galaxies</a>, the name is pretty self-explanatory.
Star formation is started by shock waves, which are generally caused by a nearby supernova. The shock waves of a supernova will hit a molecular cloud, causing some parts to become more dense than others, triggering star formation. At times, one molecular cloud can impact on another, and the collision of two large gas clouds sends shock waves through both of them triggering star formation. The arms of spiral galaxies are caused by a pressure density wave, and these waves are then followed by star formation. This explains why the spiral arms are always newly born, hot, bright stars. This is best seen in <a href="http://astronomy.swin.edu.au/cosmos/g/grand+design+spiral">Grand Design Spiral Galaxies</a>.
<figure id="attachment_22427" style="width: 300px" class="wp-caption alignright"><a href="https://wp-assets.futurism.com/2014/04/image3.jpg"><img class="size-medium wp-image-22427" src="https://wp-assets.futurism.com/2014/04/image3-300x225.jpg" alt="The grand spiral galaxy, M74, as seen almost completely face on. Image Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration" width="300" height="225" /></a><figcaption class="wp-caption-text"><a href="http://www.nasa.gov/multimedia/imagegallery/image_feature_2132.html">Image Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration</a></figcaption></figure>
The above explains normal star formation rates within a galaxy, but what causes star burst galaxies? Galactic collisions. When galaxies merge together, star formation rates go through the roof. Although individual stars have a next to zero chance of colliding with one another, this isn&#8217;t the case for molecular clouds. Compared to stars, the clouds are enormous and diffuse. It really doesn&#8217;t take much for them to be disrupted and for a flurry of star formation to occur. Even just the gravitational interaction of another galaxy after this initial flurry of large stars have formed, they go supernova, creating even more shock waves.
<h1 style="padding-left: 30px;">Tying it All Together:</h1>
Star burst galaxies happen in our time, just not as often as in the early universe. Galaxies are still colliding, and passing close to one another, the frequency of it happening is dramatically lower however. This makes the figure of 95% of all stars existing already do. There is strong evidence that shows that in the early universe many smaller galaxies existed and that they interacted far more.
<figure id="attachment_4947" style="width: 265px" class="wp-caption alignleft"><a href="https://wp-assets.futurism.com/2013/09/The-Antennae.jpg"><img class=" wp-image-4947" src="https://wp-assets.futurism.com/2013/09/The-Antennae-300x250.jpg" alt="Yet another look at the Antennae galaxies (Credit: NASA, ESA/Hubble Heritage Team)" width="265" height="211" /></a><figcaption class="wp-caption-text">Yet another look at the Antennae galaxies (Credit: NASA, ESA/Hubble Heritage Team)</figcaption></figure>
This is the basis of <a href="http://astronomy.swin.edu.au/cosmos/h/hierarchical+clustering">hierarchical clustering</a>, many smaller galaxies merging to become fewer larger ones. All of this galaxy interaction, the merging of galaxies, the entire universe in the early epoch was undergoing a star formation burst, and it continued to do so for a few billion years.
Around today can be seen the byproduct of this, three large galaxies in our local group (Andromeda, Milky Way &amp; Triangulum Galaxy) and ~50 much smaller ones. When Andromeda and the Milky Way collide in a few billion years, intense star formation will occur. T
he difference between this and the interactions in the early universe is that many more galaxies were merging and interacting 11 billion years ago.
The <a title="Galaxy Classifications: From Dwarfs to Spirals and Beyond" href="https://futurism.com/galaxy-classifications-from-dwarfs-to-spirals-and-beyond/" target="_blank">types of galaxies </a>that were colliding in the early universe compared to today, must also be taken into consideration. Currently, it is mostly dwarf galaxies colliding, many of which are hard to detect. The rest are spiral, and elliptical galaxies. The merger of two spiral galaxies eventually forms an elliptical. Elliptical galaxies have little to no star formation, they are the universal retirement homes- this is why elliptical galaxies are red, they are comprised of older stars.
At the centre of larger, more compact galactic clusters, the interactions between galaxies, not colliding or merging, heats up the vast amounts of hydrogen gas between them. As they pass through the hot hydrogen, these galaxies are then stripped of their gasses. The stars come out unscathed, only ionized and striped of gas.
<h1 style="padding-left: 30px;">To Conclude:</h1>
As the universe ages, giant elliptical galaxies are more likely to form, slowing star formation even more. Far fewer galaxies are left to collide and create star burst galaxies. Star formation is expected to go along for another trillion or so years, but the glory days are over. 11 billion years ago was the peak, it is when galaxies were colliding left, right, and centre. They were more abundant and there were far fewer elliptical types around. Galaxies were more concentrated, and far more likely to have other galaxies to interact with. It was the perfect condition for star burst galaxies to form. These conditions haven&#8217;t been seen since, and will not be seen again. This isn&#8217;t such a bad thing though, if the Milky Way was undergoing a galactic star burst right now, Earth would be in some serious trouble.

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Recently we published an article in regards to the trend of slowing star formation as the universe progresses towards its maturity. In 2012, a research study involving the Subaru Telescope (NAOJ), Very Large Telescope (VLT) and United Kingdom Infrared Telescope (UKIRT) discovered that the universe hit its star formation peak 11 billion years ago, and that it […]

Discussing The Trend of Slowing Star-Formation Rates:

<figure id="attachment_20739" style="width: 298px" class="wp-caption alignright"><a href="https://wp-assets.futurism.com/2014/03/Milky-Way-From-Afar.jpg"><img class=" wp-image-20739" src="https://wp-assets.futurism.com/2014/03/Milky-Way-From-Afar.jpg" alt="How the Milky Way might look from afar (Image credit: NASA / JPL-Caltech / R. HUrt (SSC-Caltech)." width="298" height="295" /></a><figcaption class="wp-caption-text">How the Milky Way might look from afar (Image credit: NASA / JPL-Caltech / R. HUrt (SSC-Caltech).</figcaption></figure>
The Milky Way is the galaxy in which we currently reside. It is classed as a barred spiral galaxy, although this is actually fairly difficult to determine from within the galaxy. It&#8217;s sort of like trying to determine what a house looks like from the outside by looking out of a window when you are inside of it. Below are are of the most important facts that you need to know in order to understand our galaxy:
<ul>
<li>The Milky Way is estimated to have somewhere between 200-400 billion stars. Unfortunately, it is really quite difficult to tell exactly how many there are. Being in one of the outer spiral arms of the galaxy, there is a lot of dust all around us. That dust gets quite thick when you&#8217;re looking towards the galactic center, so this really impedes our view. Not only that, but we can only see our side of the Milky Way. Since the galaxy is disk shaped, and we&#8217;re within the disk, everything on the other side is hidden from us. As for the exact number of stars in the galaxy, it&#8217;s probably not that important to have an exact figure. Indeed, such a figure is likely impossible, as there are constantly stars popping in and out of existence and so many red dwarfs and brown dwarfs that are just too dim to detect.</li>
</ul>
<ul>
<li>The Milky Way <a title="Black Hole Spits Out a Plume of Gas, Forming a Space Slinky" href="https://futurism.com/black-hole-spits-out-a-plume-of-gas-forming-a-space-slinky/" target="_blank">houses a supermassive black hole</a> at its center&#8230;a black hole of about four million solar masses. As supermassive black holes go, this is fairly average (but a bit on the smaller side). As for the actual mass of the Milky Way, it is fairly difficult to determine. One study by McMillan puts the stellar mass of the galaxy at ~64.2 billion solar masses with the total mass being ~1.26 trillion solar masses. Another report puts the galaxy mass [DM included] at ~1 trillion solar masses which is in general smaller than the average galactic mass, 65 billion solar masses. &#8220;COSMOS&#8221; magazine (2009) mentions another study where the mass is thought to be 50% larger than previous studies and to have a faster rotational speed as well.</li>
</ul>
Basically then, from three separate articles, we&#8217;re getting three separate figures. This just goes to show the difficulty of studying the Milky Way while we&#8217;re trapped inside of it.
<ul>
<li>The actual formation of the Milky Way is another hot topic of debate and has been for many years. Unlike some of the previous topics, like trying to constrain the mass of the galaxy or even what we look like from the outside (which are issues because of our location within the galaxy), the problem of galaxy formation is our inability to view our own past. Interestingly, the<a href="https://futurism.com/what-came-before-the-big-bang/" target="_blank"> Big Bounce theory</a> asserts that, before the our universe existed, there were a plathors of other universes that had the same basic physics as out own.</li>
</ul>
<ul>
<li>One truly amazing thing about the universe is that we can actually study what the universe was like in it&#8217;s earlier history as the higher redshift we peer at, the further we&#8217;re looking back into the past. There does become a point though when we can&#8217;t look back any further and it is at this point, where there is no observational evidence, this the two main and opposing theories come into play.</li>
</ul>
<ul>
<li>The <a title="The Formation of the Milky Way: Two Opposing Models" href="https://futurism.com/the-formation-of-the-milky-way-two-opposing-models/" target="_blank">Top-Down Model and the Bottom-Up Model</a>. These two opposing theories govern the way to which all of the first galaxies formed, thus the way that the Milky Way formed.</li>
</ul>
Now that you have an understanding of the fundamental principles, here are some more interesting facts:
<a href="https://wp-assets.futurism.com/2014/04/milky-way-infographic.jpg"><img class="aligncenter size-large wp-image-23398" src="https://wp-assets.futurism.com/2014/04/milky-way-infographic-596x1024.jpg" alt="milky way infographic" width="596" height="1024" /></a>

The Milky Way is the galaxy in which we currently reside. It is classed as a barred spiral galaxy, although this is actually fairly difficult to determine from within the galaxy. It’s sort of like trying to determine what a house looks like from the outside by looking out of a window when you are […]

Infographic of the Most Astounding Facts About The Milky Way:

<figure id="attachment_21551" style="width: 300px" class="wp-caption alignright"><a href="https://wp-assets.futurism.com/2014/04/stars-google.png"><img class="size-medium wp-image-21551" src="https://wp-assets.futurism.com/2014/04/stars-google-300x225.png" alt="Image via 100,000 Stars" width="300" height="225" /></a><figcaption class="wp-caption-text">Image via 100,000 Stars</figcaption></figure>
Question(s):
<blockquote>&#8220;Is there a theoretical limit to how big can a star get, or would it collapse from gravity before? If no, then in theory could we get a star as big as a mini-galaxy?&#8221; Asked By: Théo De Freitas Neto</blockquote>
The main determining factor in all of this is the mass of the star. The mass of the star eventually determines the potential size of a star. So I&#8217;ll start off by mentioning how the mass effects the star, and then explain why you’re not going to get a radius the size of a mini-galaxy.
First, at the beginning  of star formation, you have a molecular cloud that may be 200 thousand solar masses, such as the Orion Molecular Cloud,<a href="http://www.astro.wisc.edu/~dolan/constellations/messier/m42.html" target="_blank"> for example</a>. We can use Jeans Mass and Jeans Length (I’ll explain it a bit later on), to work out the size that a part of a cloud needs to be in order to start to collapse and start star formation. This is dependent on the mass, density, and the temperature of the molecular cloud. When it starts collapsing, you&#8217;re going to end up with parts of the cloud that are denser than other parts, so you get what is called &#8220;Fragmentation.&#8221;
<h1 style="padding-left: 30px;">JEAN&#8217;S MASS:</h1>
Basically, the original Jeans Mass determines the size the initial cloud needs to be in order to collapse. As it&#8217;s collapsing, the Jeans Mass decreases more and more due to pockets of greater density. Thus, instead of a 500 solar mass star from a 500 solar mass cloud, you&#8217;re more likely to end up with 100 smaller stars.
Let&#8217;s say, for example, that one of the stars that forms is 80 solar masses. That&#8217;s 80 times the mass of  our sun, so it&#8217;s pretty huge. It is going to end its life with a supernova, leaving behind a black hole. Of all the main sequence burning cycles in stars, the cycle where hydrogen is burning requires the lowest temperature, it only needs about 4 million degrees. Compare this to the 15 million needed for the CNO cycle (which is still the burning of hydrogen but using Carbon Nitrogen and Oxygen as catalysts). Moreover, helium burning kicks off at about 100 million degrees, carbon burning at 600 million degrees, oxygen burning at about a billion degrees, and silicon burning at 3 billion degrees.
So as you can see, a 80 solar mass star is going to need to be AT LEAST 3 billion degrees in temperature at its core before it can even start burning Silicon to form Iron. Really then, the hotter that the core is, the more energy that is being released&#8211;And that energy has to go somewhere.
Ultimately, virtually all stars are in a state of<a href="http://www.astronomynotes.com/starsun/s7.htm" target="_blank"> hydrostatic equilibrium</a> (<a href="https://futurism.com/hubble-variable-1-the-star-that-changed-the-universe/" target="_blank">variable stars</a> are the exception). This means that the outwards radioactive pressure and the inwards force of gravity are at equilibrium.
<h1 style="padding-left: 30px;">ABSOLUTE RADIUS:</h1>
Now this is where the absolute radius comes in. At some point, gravity is going to be equal to the radiation pressure at the extremity of the star. Taking our star for instance, at ~700,000 km from the core, our Sun&#8217;s outwards pressure is equal to gravity; hence, this is what we would constitute as<a href="https://futurism.com/how-the-sun-works/" target="_blank"> its surface.</a> In larger stars, it does become a little more complicated because the star is producing FAR more energy. As such, there is a very strong solar wind that tends to blow some of the star away (but the basic principles still apply).
As for actually calculating how big a star can get in radius, there is no equation to do that and it is VERY difficult to even get moderately accurate computer models. The <a title="How Do We Know What Planets Are Made Of?" href="https://futurism.com/how-do-we-know-what-planets-are-made-of/" target="_blank">composition of the star </a>plays a huge roll in how large it can get, the more metals that are in the star to begin with make the star less opaque to the emitted radiation which means that the star is going to be a little more puffed up than if it had less metals (radiation pressure becomes more effective). It also depends on how the star has evolved over time, how much of its mass it has actually lost due to its immense solar winds, whether or not it passed through an &#8220;instability strip&#8221; earlier on and became unstable for a time.
<figure id="attachment_501" style="width: 800px" class="wp-caption aligncenter"><a href="https://wp-assets.futurism.com/2013/01/309892_408096662610082_1069106120_n.jpg"><img class="size-full wp-image-501" src="https://wp-assets.futurism.com/2013/01/309892_408096662610082_1069106120_n.jpg" alt="Some of the largest stars in the universe" width="800" height="525" /></a><figcaption class="wp-caption-text">Some of the largest stars in the universe (via <a href="http://en.wikipedia.org/wiki/File:Star-sizes.jpg" target="_blank">Wikimedia Commons</a>)</figcaption></figure>
So basically, the mass is more or less the determining factor as to how large in radius a star can become, but it will never get to the size of a mini galaxy because, eventually, radiation pressure from the star will lose against gravity and gravity will hold it back. Even if that doesn&#8217;t happen, the star will just keep puffing away its outer shell, and that gas will just eventually mix in with the Interstellar Medium, where it is no longer even constituted as being a part of the star.
As a side note, as in the very early universe, at the time of the first stars, there were no metals. It was just hydrogen and helium ;with traces of lithium which were consumed prior to the main sequence anyway so the stars although having a very large mass also for their mass were far smaller than what they would be in our current time in the universe. No metals meant that the stars were more opaque. So radiation could get through the star with less resistance; thus, there was less outwards pressure, meaning gravity was more effective, <a title="What is the Smallest Possible Size for a Star?" href="https://futurism.com/the-smallest-possible-size-for-a-star/" target="_blank">making the stars smaller.</a>

Question(s): “Is there a theoretical limit to how big can a star get, or would it collapse from gravity before? If no, then in theory could we get a star as big as a mini-galaxy?” Asked By: Théo De Freitas Neto The main determining factor in all of this is the mass of the star. The […]

Is There a Limit to How Large a Star Can Be?

<figure id="attachment_7068" style="width: 450px" class="wp-caption alignright"><a href="https://wp-assets.futurism.com/2013/09/tumblr_m2udkn5WHs1r2h5u7o1_5001.jpg"><img class=" wp-image-7068 " alt="The tau particle, you can buy this little guy at the Particle Zoo " src="https://wp-assets.futurism.com/2013/09/tumblr_m2udkn5WHs1r2h5u7o1_5001.jpg" width="450" height="450" /></a><figcaption class="wp-caption-text">The tau particle, you can buy this little guy at the Particle Zoo</figcaption></figure>
We all like hearing about the biggest and the smallest, the heaviest and the lightest, as these categories represent the extremes of the universe&#8230;and they usually they are great for comparisons. Meet the heaviest of all the leptons, the Tauon (also known as the Tau Lepton or Tau Particle). The Tau lepton has a <a href="http://arxiv.org/pdf/0812.1875v2.pdf">mass of 1777 MeV/c^2</a>, to put this lepton into perspective, it is several thousand times heavier than the smallest lepton &#8211; the electron &#8211; and is even heavier than a proton. But before we get into a thorough discussion of these particles, let&#8217;s have a bit of background in how particles function, just for a bit of context.
<h1 style="padding-left: 30px;">TO START:</h1>
Virtually all of the visible matter in the universe is made up of only several particles.  The standard model of particle physics (a theory related to the electromagnetic, weak, and strong nuclear interactions that outlines the dynamics of the known subatomic particles) contains many particles, but only a few of these make matter. It appears that the universe favours the smaller of the particles; the smallest lepton, the electron, and the two smaller of the quarks (the Up and Down). From these three particles you, me, the the Earth, the Sun&#8230; everything in the cosmos is made! The tauon then, being so large, isn&#8217;t favoured.
Unfortunately, this means that this big guy doesn&#8217;t exist for long in the universe. In fact, it only exists for 2.95&#215;10^-13 seconds or 295 femtoseconds (one femtosecond is one quadrillionth [one millionth of one billionth] of a second). But the lepton is actually able to break down into a hadron with the use of a <a href="http://kjende.web.cern.ch/kjende/en/wpath_lhcphysics2.htm">W Boson</a>, or a quark anti-quark pair, or lepton anti-lepton pair, through the use of a <a href="http://kjende.web.cern.ch/kjende/en/zpath_lhcphysics2.htm">Z Boson</a>. But don&#8217;t feel too bad about its paltry existence, the Tau particle does have a few claims to fame. For example, the decay of the Tau particle has been used in the <a href="http://www-d0.fnal.gov/results/publications_talks/thesis/herner/thesis.pdf">detection of the Higgs Boson</a>, but ultimately the Higgs wasn&#8217;t found through this method (alas); however, it was able to determine that the Higgs Boson should lie below the 160 GeV region. As the Tauon is the most unstable of all the leptons, as it is the largest, it doesn&#8217;t occur naturally in the universe like the electron.
<figure id="attachment_21730" style="width: 900px" class="wp-caption aligncenter"><img class="size-full wp-image-21730" alt="The Higgs Boson was discovered at The Large Hadron Collider. For perspective on its size, see the person standing in the middle of the machine. (Credit: Maximilien Brice, CERN) (Source)" src="https://wp-assets.futurism.com/2013/09/The-Large-Hadron-Collider.jpg" width="900" height="600" /><figcaption class="wp-caption-text">The Higgs Boson was discovered at The Large Hadron Collider. For perspective on its size, see the person standing in the middle of the machine. (Credit: Maximilien Brice, CERN) (<a href="http://apod.nasa.gov/apod/ap080225.html" target="_blank">Source</a>)</figcaption></figure>
&nbsp;
The Tauon, which exists for such a short period of time, only exists in places where there is high energy particle collisions like in the core of stars, supernova, black holes, neutron stars, or our humble particle accelerators. As previously mentioned, it has a mass of approximately 1777 MeV/c^2, and as it is a lepton it is negatively charged and has a spin of 1/2. Like other particles, it has an anti-lepton, which is a 1/2 spin but is positively charged (this is known as the antitau). The Tau Neutrino is also the largest of the neutrinos with a mass of <a href="http://nu2000.sno.laurentian.ca/M.Roney/Roney.pdf">~15.5 MeV/c^2</a> which is still some 30 times the mass of an electron and incredibly larger than the electron neutrino.
<h1>So, to Sum the Tauon: </h1>
It is the heaviest lepton in the whole particle zoo at nearly 3,500 times larger than the smallest of the lepton, the electron. The tau neutrino is the heaviest of the neutrino family and is heavier than an electron! Compared to the other neutrinos, the tau neutrino is truly a massive particle. Tauons are large enough that they are actually capable of decaying into hadrons, quark/anti-quark pairs and lepton pairs. Because of how heavy they are they are incredibly unstable and tend to decay very quickly.

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We all like hearing about the biggest and the smallest, the heaviest and the lightest, as these categories represent the extremes of the universe…and they usually they are great for comparisons. Meet the heaviest of all the leptons, the Tauon (also known as the Tau Lepton or Tau Particle). The Tau lepton has a mass […]

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Colin Robson

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