Image source: apod.nasa.gov/apod/ap090926.html
The Milky Way is the galaxy that we live in. Much of it looks like a whitish glow spread in a wide line across the night sky. This glow is the starlight of stars too numerous and to far away to distinguish one from another, their light just blends together. The galaxy gets thicker in the middle, in the part we call "the bulge" and tapers off to either side. Dark bands of dust obscure parts of the starlight. These dust lanes are huge, and make up an important part of the Milky Way. The Milky Way is more than just stars, it is also the material that goes into making new stars, and the leftover material from stars that have long since exploded. There are also globular clusters, large clusters of stars that reside in the halo and orbit the main galaxy.
Since we reside inside the Milky Way, we cannot really see what it looks like from outside. We have spent quite a bit of time and energy analyzing the light that we see to infer what the shape must be. The image above is a panorama view with 800 million pixels of information stitched together in a large mosaic, part of ESO's Gigagalaxy Zoom Project. Even at that, the information is lacking, since we estimate there are 100 billion to 200 billion stars in the Milky Way galaxy. The large bright spot below the Milky Way is a satellite galaxy, one of the Magellenic clouds.
Image source: apod.nasa.gov/apod/ap151008.html
This is Southern Pinwheel galaxy, seen in the constellation Hydra. Astronomers believe that the Milky Way galaxy would look similar to this galaxy, if it was viewed from this angle. It is designated as a spiral galaxy, due to the spiral arms. Note the elongated bulge in the center. This kind of galaxy is called a "barred spiral." We will learn more about galaxy types in the next chapter.
This is an edge-on view of a spiral galaxy called the Needle Galaxy, in the constellation Coma Berenices. Notice how thin it is, and how much thicker the central bulge is than the rest of the galaxy. Even though it looks thin, it would take roughly a thousand years for light to cross through the thickness of the disk. The diameter is about 100,000 light years, and this galaxy lies about 40 million light years from us.
Image source: zebu.uoregon.edu/~soper/MilkyWay/structure.html
This diagram includes labels for the various parts of a spiral galaxy. The center of the bulge is called the nucleus. The spiral arms extend from the bulge out through the disk, consisting of stars, dust and gas. Globular clusters orbit out of the plane of the disk. The largest structure of the galaxy is called the halo, which is a huge, roughly spherical orb of old stars, globular clusters and dark matter.
The globular clusters and other halo stars orbit the center of the galaxy, with their orbits extending above and below the disk, while the disk stars orbit the center roughly staying in the plane of the disk. They do tend to pass through the plane of the disk, traveling somewhat above and below it, but not nearly to the extant that halo stars do.
One thing that we learned from spectroscopy is that the peak wavelength of a star's spectrum is proportional to its surface temperature. In short, blue stars are hotter than red stars. It is also known that very massive, hot stars have very much shorter lifespans than do smaller, cooler stars. The relationship is roughly
When we look at the color of the stars in the halo and the outer part of the bulge, we see that the stars are mostly red stars. The fact that there are very few blue stars means that all of the hot, massive stars that were formed in these regions no longer exist. There is simply not the dust and gas in these regions that are necessary for star formation. In the inner part of the galactic bulge and in the disk, a mixture of red and blue, and thus old and young stars, are seen. In fact, an abundance of blue stars are found on the edges of the spiral arms, signifying that active star formation is taking place there. This is evidence that in the early stages of the evolution of the galaxy, the dust and gas in the halo fell into the galactic plane from the halo, and that similarly, the dust and gas is gravitationally attracted from the edges of the bulge to its inner region.
Young blue stars are found on the leading edge of a spiral arm
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Red emission nebulae arise where newly forming blue stars ionize gas clouds
This close-up image of the leading edge of a spiral arm in a nearby galaxy illustrates the evidence of ongoing star formation. The appearance of young blue stars on the leading edge, coupled with the emission nebulae (gas clouds) are in close proximity where rich, dark dust lanes provide material for forming stars.
Image source: apod.nasa.gov/apod/ap151015.html
This Hubble telescope image of the region in the Eagle nebula known as the Pillars of Creation is a classic example of star formation. You can see a cluster of bright, newly formed stars above the pillars. Their strong stellar winds are sculpting the material and eroding it, sweeping away the light material and compressing the denser material.
Image source: apod.nasa.gov/apod/ap150107.html
If we zoom in on the top of one of the pillars, we can see the dense nodules that will "soon" be young stars.
Video source: hubblesource.stsci.edu/sources/video/clips/details/orion.php
Astrophysicists and animators in the Hubble telescope group at NASA worked together to create an animation to show what it would you would see if you could fly through one of these star forming regions. As you get closer in, you see structures like gas cocoons, called proplyds, surrounding the nascent stars. Inside an envelope is a disk, which will eventually become a system of planets orbiting the star, with two jets of material shooting off the rotational poles, channeled by the magnetic field of the system.
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The spiral arms sweep around a galaxy, staying relatively intact. If they moved the same speed as the gas, they would wind up, since the gas is differentially rotating. They do not.
It is believed that the spiral arms are density waves, intimately connected with star formation. The waves are huge compression waves, and this compression is what triggers star formation along the leading edge.
Video source: youtube.com/watch?v=1yaqUI4b974&nohtml5=False
What makes the waves? Every object has a natural frequency of oscillation. The experiment shown above uses a Chladni plate apparatus to illustrate this concept. The metal plate is sprinkled with sand, and then made to vibrate at increasing frequencies. At characteristic frequencies, standing wave patterns set in. The sand vibrates off of the waves except at the nodes, allowing you to see the wave pattern. The higher the driving frequency, the more intricate the pattern of standing waves. Also, notice that the edges are important here. The boundaries help to give shape to the wave patterns.
Now, imagine that instead of sand on a plate, that the sand is a huge differentially rotating disk that is held together by gravity. The density structure of the disk, together with the boundaries, allow standing wave patterns to form. The pressure waves are characterized by the density of the material, so they are called density waves. The compression of the density waves can trigger star formation. Also, stars ending their lives spew out matter, which helps to fuel new stars. Shock waves emitted from the supernovas also help to start contraction of the matter to form new stars. Once the matter gets dense enough, the gravitational force begins to pull the matter inward. We will learn more about galaxy formation in general later on.
When you look at spiral arms in a galaxy, you might assume that they are extended groups of stars that move along together. They don't. What is moving is a disturbance.
Check out this time lapse video of clouds. What you see is that the clouds don't just float along, intact, it is like they are continuously forming as they move. They are density waves. The air compresses and rarefies, making the water droplets condense as the disturbance moves through the air. The clouds are not going the same speed as the air. The disturbance is moving through the gas.
Density waves in a galaxy are similar to this. The matter is compressed along huge spiral waves that move along the disk. These waves don't move at the speed of the gas, they stay intact as they go, like the second hand of a clock, sweeping around.
The compression in the density wave triggers star formation to start along the leading edge of the wave, which is also the leading edge of the spiral arm. When a density wave passes through a big cloud of dust and gas, the material compresses. If the material is compressed enough, gravity takes over and the matter contracts under the pull of gravity.