OUR GALAXY:
THE MILKY WAY
Our own galaxy, the Milky Way, is a quite typical resident of the Universe today (about 14 billion years after the Big Bang). It’s about average size, with a distinct spiral structure, living in a small group of other galaxies while it grows slowly but steadily. But even a late bloomer like the Milky Way began its growth during the Cosmic Dawn, and astronomers are learning more and more about the earliest phases of its evolution.


This infrared image from NASA Spitzer Space Telescope shows hundreds of thousands of stars crowded into the swirling core of our spiral Milky Way galaxy.
Image Credit: NASA/JPL-Caltech
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What is the Milky Way?

The Milky Way is the galaxy in which we live: a collection of more than one hundred billion stars, most of them in a thin disk orbiting the center of our Galaxy. You may have seen it as a faint band of light stretching across the sky (but only if you live somewhere with dark skies – outside of a city), which is the source of its name. Although it appears as a glow, the faint light in this band is actually the result of the many, many stars arrayed inside the disk; we see it as a band because the Sun is inside the disk, about two-thirds of the way from the center of the Galaxy.

Image: The night sky above Paranal, Chile on 21 July 2007.
Credit: ESO astronomer Yuri Beletsky, Wikipedia Commons

What are the components of the Milky Way?

  • The disk: This part contains most of the stars and stretches more than one hundred thousand light years across. But it is only a couple thousand light years thick – that makes it (in relative terms) thinner than a pancake! The disk also contains most of the Milky Way’s gas clouds. These clouds are the starting point for forming stars, so new stars are forming continuously there as the clouds are compressed by spiral waves moving through the disk. These kinds of waves form the beautiful spiral structure seen in many nearby galaxies (and in the Milky Way, if we could view it from above!).
  • The bulge: This a football-shaped structure of stars and gas in the central region of the Milky Way. The stars have a wide range of properties and ages. At the very center of the bulge is a supermassive black hole (several million times more massive than our Sun!).

Image: The barred spiral galaxy M83, also known as the Southern Pinwheel, 15 million light-years away in the constellation Hydra.
Credit: NASA, ESA, and the Hubble Heritage Team

  • The halo: This the least obvious part of the Milky Way: its stars are distributed across a sphere, hundreds of thousands of light years across, that surrounds the disk. But there are so few of these stars that only careful searches can identify them. They are also all very old stars, and no new stars are forming here because the halo is nearly devoid of gas clouds. The most obvious components of the halo are dense clusters of millions of stars, called globular clusters, which also orbit the Milky Way.

Image: The barred spiral galaxy M83, also known as the Southern Pinwheel, 15 million light-years away in the constellation Hydra.
Credit: NASA, ESA, and the Hubble Heritage Team

How did the Milky Way form?

Like that of most present-day galaxies, the history of the Milky Way stretches back more than thirteen billion years, to the Cosmic Dawn. Galaxies form through the growth of very small seeds, so only a small fraction of the Milky Way’s stars formed during those earliest phases. Astronomers believe that these rare stars formed in small, early galaxies and globular clusters, which later merged together as the Milky Way grew. But the system acquired most of its mass – and its stars as well – at much later times. During the Cosmic Noon, the Milky Way’s gas settled into a disk, inside which most of those stars formed. The disk grew both through gas flowing onto it from the surrounding medium and through mergers with other (much smaller) galaxies. (Many galaxies grow through mergers with large systems, but astronomers believe the Milky Way has not yet done so.) During these mergers, some of the older stars were ejected into wide orbits and are now part of the galaxy’s halo, and others were mixed into the Milky Way's bulge. Mergers with such small galaxies are continuing, as we will discuss in the next section.

Image: The Large Magellanic Cloud, a satellite galaxy to our own Milky Way galaxy
Credit: JPL, Spitzer Space Telescope

What signatures of the Cosmic Dawn remain in our Galaxy?

Although the Milky Way started to form during the Cosmic Dawn, the vast majority of stars in it formed much more recently. But careful searches have revealed several aspects that hold clues to the Cosmic Dawn.

  • Halo stars are almost all very old, and they were likely ejected from the more central regions of the galaxy during mergers or other violent encounters. Careful study of these stars has revealed some of the oldest objects in the Milky Way – some stars have even been found to have formed more than thirteen billion years ago! These stars also have far fewer heavier elements than the Sun. Such elements – everything except hydrogen and helium – formed in stars, which then spread them through winds or supernova explosions. The oldest stars we can see may have formed from gas that was only enriched by a single such supernova! Careful study of the abundances of heavy elements in such stars may reveal the nature of the first stars and their supernovae.

Image: Galaxy NGC 4526, with dark lanes of dust near the disk and bright diffuse glow highlighting the halo.
Credit: ESA/Hubble, NASA

  • Globular clusters are groups of millions of stars packed into a region just a few light years across. (In contrast, the nearest star to the Milky Way is still four light years away!) They are also full of very old stars, more than ten billion years old, but the manner in which they may have formed remains elusive. If most globular clusters formed during the Cosmic Dawn, it is possible that most stars in the early Universe formed in such systems! Further study of the history of these systems will help us to understand their importance during the Cosmic Dawn.

Image: Globular Cluster Messier 79, unusually found in the direction opposite of our galaxy's center, evidence that it may have been stolen from a nearby galaxy.
Credit: NASA, ESA STScI, F. Ferraro and S. Djorgovski

  • Dwarf galaxies also provide a window into the past. These are tiny galaxies orbiting the Milky Way – destined eventually to be absorbed into it! Some are large enough to be observed directly (such as the Magellanic Clouds, visible from the southern hemisphere), but most have so few stars that they require careful analysis to identify. In these smaller dwarf galaxies, all the stars are quite old – indicating that their star formation turned off many billions of years ago. Heating from reionization may be the reason these systems stopped forming stars, and studying these ancient systems may help us understand how reionization proceeded near the Milky Way.

Image: NGC 5477, one of these dwarf galaxies in the Messier 101 group.
Credit: ESA/Hubble, NASA

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Content and supervision by Professor Steven Furlanetto, website design by Erika Hoffman, funding and support from NASA NESS, NSF, & UCLA Physics and Astronomy.
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