FromQuarkstoQuasars

The Day With No Tomorrow: What Happens When The Sun Becomes A Red Giant?

sun as a red giant
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We all love a good doomsday theory, but never has one been so conceivably real. Based on current variables and conditions, scientists have produced some profound calculations which assert that our planet is in jeopardy.

Fortunately, the apocalypse won’t come for a few billion years.

The metamorphosis of our Sun from a main sequence star to a red giant is a process that all low-mass stars endure at some point in their lives.

It has long been a supported theory that our Sun loses mass due to cool non-dusty solar winds stripping off matter. Indeed, scientists believe that our sun will lose so much mass over the next 5.42 billion years (which is the time needed for the depletion of core hydrogen to occur, at which point the  and the red giant stage will begin) that the sun’s gravitational effect on the Earth will have diminished to the point where Earth’s orbit becomes far larger than it is currently. So large that scientists assert it will be approximately 1.2-3.0 astronomical units farther out than where it is now.

Such a change would allow Earth to drift harmlessly out into the solar system and avoid being consumed by our huge red giant sun. But it seems that theory may be incorrect.

A pair of astrophysicists detailed how the previous theories  have neglected to consider the Habitable Zone and the effects of the sun’s tidal forces.

Firstly, a brief reminder about the habitable zone: Having water in a liquid form is thought to be essential for life as we know it, and also a key element for the formation of stable atmospheres. Based on our knowledge of water, we have found that (depending on the specific variables) it can exist in liquid form between 273K – 373K ( -0.15°C to 99.85°C). The region of space that supports this temperature range around a star is known as a “Habitable Zone.”

Image Credit: Wikimedia
Image Credit: Wikimedia

Of course, there will be slight variations in the range of a star’s habitable zone, based upon a planet’s ability to reflect and absorb solar radiation which must be taken into account on a planet-by-planet basis. The academic paper puts forward some alarming figures on the theorized relocation of the habitable at the various stages of the sun’s period of change. These are :

  • The current HZ of our solar system is 0.95AU – 1.37AU.
  • Once the Sun begins the Red Giant phase, the HZ will be 1.29AU – 1.86AU
  • At peak Red Giant size, the habitable zone will be a staggering 49.4AU – 71.4AU

As you can see, if the Earth remained at 1AU from the Red Giant phase commencing, it would fall on the horribly incinerated side of the habitable zone.

Image Credit: NASA/JPL
Image Credit: NASA/JPL

And the scientists assert that the effects of the cool non-dusty solar winds on the solar mass have been greatly overestimated. They argue that the mass of the sun, and therefore its gravity, will only suffer a tiny variation. This is believed to be somewhere in the region of 0.5%-1% of 1AU. Virtually negligible for this event. This calculation shows that, while the habitable zone will move farther out into our Solar System during the sun’s transition into a red giant, our planet will remain basically where it is today.

The effects of this are pretty straight-forward: Devistation. Earth will be bombarded by solar radiation of such great intensity that the liquid water of the oceans and seas will be boiled away. Not only this, the atmosphere of the entire planet will be burnt from the planet, leaving the planet exposed to deep space.

During even the earliest stages of the red giant phase, the scientists assert that life would be impossible at this point on Earth, and it would get far worse.

As the Red Giant continues its expansion, it will devour Mercury and Venus – they cannot move away from the encroaching Red Giant and are trapped by the Sun’s tidal forces, sapping the planets of their angular momentum.

A rendering of how that might look. (Credit: American Museum of Natural History)
A rendering of how that might look. (Credit: American Museum of Natural History)

It is theorized that, not long after this (several million years), the tidal force of the sun’s Chromosphere will conflict with the Earth’s gravitational and tidal hold on the moon, to the point where the moon will be torn apart. It is believed that these fragments of Moon will mostly stay in orbit as an accretion disk; however, some of the larger pieces will fall into a re-entry with Earth causing untold devastation to the already-scorched planet.

In the final stage of the Red Giant expansion, the Earth will be pulled into a 200-year decreasing spiral orbit, which will destroy the planet as it descended through the layers of the red giant sun.

On a slightly more cheery note, it was calculated that an 8% increase in angular momentum upon Earth’s orbit would allow it to survive all the cataclysmic occurrence. We just need to plan how to deliver that incredible amount of energy to the planet, if we are still here then…

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