The Milky Way

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Picture: The Lund Observatory mosaic of the Milky Way



 



Introduction

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If you look up into the sky on a clear dark night, far from the intrusive glow of streetlights, you will see a faint band of light running across the heavens. This is the Milky Way, our own galaxy. It contains about two hundred billion stars and countless other objects besides. The Milky way viewed through binoculars is a magnificent sight, with thousands of stars in each field of view. The image on the left was taken at the Anglo Australian Observatory and shows the centre of the Milky Way, in the constellation of Sagittarius.

Like the constellations and planets, the Milky Way was important in mythology, and many stories were told to explain it.

Our galaxy is only one of billions of other galaxies in the universe, but it is special to us, because it is home to Earth and our solar system. The Milky Way has been studied extensively by astronomers, and today our ideas about its nature are set on a more firm foundation. Here we set out to examine what this faint band of light is, where it came from, and where Earth and the Sun fit into the picture.

 

The birth of our Galaxy

Long before the Sun and the solar system formed, before the galaxy existed, the universe was filled with gas - mainly hydrogen, with some helium. This gas was eventually to be turned into stars, planets and people. But before these things could happen, the galaxy had to form.

Astronomers believe that the galaxy formed out of a large, fairly spherical cloud of cold gas, rotating slowly in space. At some point in time, the cloud began to collapse in on itself, or condense, in the same way that the clouds which formed individual stars also condensed. Initially, some stars may have formed as the gas cloud began to fragment around the edges, with each fragment condensing further to form a star or group of stars. Because the cloud was spherical at that time, we do see some very old stars distributed in a spherical halo around the outside of the galaxy today. At such early times, these stars consisted only of the hydrogen and helium gas which made up the cloud.

The cloud continued to collapase, with more and more stars being formed as it did so. Since the cloud was rotating, the spherical shape began to flatten out into a disc, and the stars which were formed at this time filled the disc regions. Once again we see this shape today in the main body of the galaxy. As the formation of new stars continued, some of those which had been created earlier had enough time to evolve to the end of their active lifetimes, and these stars began to shed their atmospheres or explode in huge supernova events. In the process, these older citizens of the still young galaxy enriched the gas in the cloud with the new, heavier elements which they had formed, and the new stars being created in the disc regions contained the heavier elements. Astronomers call these younger, enriched stars population 1 stars, and the older stars population 2.

This process of star formation, then manufacturing heavier elements inside stars and finally returning these elements into the gas between the stars (called the interstellar medium) continued, as it still does today, all the time enriching the medium so that today the gas within our galaxy consists not only of Hydrogen and Helium, but also the other elements which are needed to form the Earth and the rest of the universe we see around us.

 

The Milky Way today

The Milky Way galaxy in which we now live is a very different place to the cold gas from which it formed over 16 billion years ago. No longer is it a spherical mass of hydrogen; today astronomers with radio telescopes have charted the clouds of gas and have found that the Milky Way is a Spiral Galaxy, one of countless others. Whilst we can't "stand back" and see our galaxy as a whole, we can look out into space and see other galaxies which we think may be similar to our own.



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(TOP) The spiral galaxy NGC2997 (C) the AAO & (BOTTOM) The edge-on galaxy NGC4565 (C) KPNO

 

The images above show two different galaxies. Both are spirals like our own, but whilst the first galaxy is seen "face on", displaying its spiral arms to full effect, the second system is edge on. We cannot see the spiral arms, but instead this galaxy shows the central "bulge" of the nucleus, and the disc structure described above - becoming thinner towards the edge of the galaxy. The image also shows dark streaks, particularly noticeable through the central bulge. These streaks are in fact lanes of dust which absorb light and prevent us from seeing the stars behind. The significance of these images is that they show different aspects of spiral galaxies, and they are very close in appearance to the Milky Way as it must look to an observer in another galaxy.

The photograph below also shows NGC2997, with the size of the Milky way shown, along with the approximate position of the Sun and solar system, which are located about two thirds of the way out from the centre of the galaxy. Since the galaxy rotates, the Sun "orbits" the centre, taking 200 million years to complete one circuit.


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Not shown on these pages is the halo which is a spherical region, centred on the nucleus, with a radius of about 50000 light years. This halo contains very old stars, produced early on when the galaxy was still forming. Most of these stars are in vast collections called globular clusters

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