FromQuarkstoQuasars

How Do We Know Our Galaxy is Spiral?

The new Ultraviolet Coverage of the Hubble Ultra Deep Field (UVUDF) image laboriously captured by the Hubble Space Telescope shows over 10,000 galaxies hanging in deep space. The image reminds us that galaxies come in a multitude of shapes and sizes, each one uniquely expressing its arrangement of stars. According to most reliable estimations, our own galaxy, the Milky Way, shines with the light of some 400 billion stars. It has inspired countless artists to share their impressions of our galaxy, but how have we come to the conclusion that the Milky Way is that shape we believe it to be?

Through the observations of the night sky we learn a lot. Simply by looking up, on a clear and dark night, any person(who is not hounded by light pollution) can see a band stretching across the darkness. This band is made up of stars that are more densely packed there than in the rest of the skyscape. These are our neighboring stars that travel with us on our journey through space. Because this strip of starlight is not in a more irregular fashion, we can understand the Milky Way as a flat structure within which we are embedded.

On a dark night, a bulge of closely packed stars can be seen intruding on this galactic band tied around the midnight sky. This is the center of our galaxy, and its spheroidal shape is a common feature in other spiral galaxies that we have observed. Like those galaxies in Hubble’s Deep Field image, and the others witnessed cartwheeling through the cosmos, a flat galaxy with a central bulge is indicative of a spiral galaxy. One such galaxy that appears as a typical spiral is NGC 6744 that was photographed by the team at La Silla Observatory. 30 million light-years away, it is often referred to as the Milky Way’s twin.

NCG 6744. Credit: ESO
NCG 6744. Credit: ESO

The Milky Way is understood to be a rather typical spiral galaxy but it also sports a more evasive statement of classification: a central bar. Astronomers had little evidence to support the idea that the Milky Way had a bright, elongated, bar-shaped core until 2005, when the Spitzer Space Telescope gathered some detailed infra-red readings.

After analyzing 400 hours of data captured using Spitzer, cosmologists saw a clearer barred structure to the Milky Way. At 27,000 light-years long, some may think the Galaxy’s bar would be easy to spot but it took a survey of 30 million stars of the innermost region to reach this conclusion. As we understand it today, two-thirds of observed spiral galaxies are barred.

Unless you want to pack a camera and launch with a spaceship to leave our galaxy and get a real bird’s eye view of the immense Milky Way (which is not recommended, it is quite a trek), our observations are the next best thing. However, it was not easy for early astronomers to understand the galaxy as we do now. Interstellar gas and dust is abundant in the Galaxy. Astronomers like R. J. Trumpler of the early 20th Century, while unaware dust was a common characteristic of spiral galaxies, knew it hindered efforts to map space accurately.

Observations of the time were based in visible light wavelengths. The dust obstructed the true brightness of the more distant stars. It was not until the 1950s when telescopes making use of a shorter wavelength, radio, were able to punch through the interference like the modern day Spitzer. This breakthrough let us accurately map the scale of the Milky Way and understand the galactic plane extends a monstrous 100,000 light-years in diameter.

Credit: NASA/JPL-Caltech/ESO/R. Hurt
Credit: NASA/JPL-Caltech/ESO/R. Hurt

Until Edwin Hubble explained his extragalactic concepts in 1920, it was a common notion that every star in the universe was part of an infinite Milky Way. As we developed our technology and enhanced our observations, it became clear we inhabit just one galaxy in a menagerie of 100 billion others. It has been no small doing to compile precise information on the nature of our own galactic home and we are still learning new things to improve our knowledge on it.

Our picture of the Milky Way is likely far from completion.

Credit: Digital Sky LLC
Credit: Digital Sky LLC

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