Our Milky Way Galaxy as a whole		Above, you can see pictures of the Milky Way galaxy imaged through different wavelengths of light. Pictures like these help astronomers figure out the shape of the galaxy, enable us to view through gas and dust (infrared and radio are affected less by absorption), enable us to view the regions where stars are forming (molecular clouds), enable us to view cold neutral hydrogen (especially through radio waves), and enable us to view hot gas (X-rays) produced by supernova remnants and bubbles of interstellar gas blown by stellar winds. Below is an image of NGC 6744, a galaxy believed to be similar to our own. The Milky Way is about 100,000 light years across. The Sun is about 28,000 light years from the center. In the center of a spiral galaxy is a bulge. As you can see in the picture below, the stars in the spiral arms are bluer and the stars in the bulge are redder. NGC 6744 has a bar coming out of its bulge, and the Milky Way is believed to have a bar as well. The Milky Way has about 100 billion stars, or 100,000,000,000 stars! The height of the Milky Way is about 1,000 light years. The arms also contain more gas and dust than the bulge. New stars are born mostly in the disk, and not so much in the bulge. (The "disk" can be thought to contain both the spiral "arms" and the stars between the arms. Actually there aren't that many more stars in the arms than between them--it's only that the bright new-born stars are found in the arms.) The stars in the disk have more heavy elements (C, N, O, etc.) than the stars in the bulge, which have larger proportions of pure H and He. In the disk, stars go in orbit around the center (where the bulge is); our own Sun takes about 250 million years to orbit the Milky Way. That means the last time the Sun was in this part of the Galaxy, the dinosaurs were still getting revved up! The stars in the disk also bob up and down while they orbit. That's because when a star is above the central plane of the galaxy, it's pulled down by gravity. But when it's below the central plane, it's pulled up (most of the mass pulling on the star is then above the star.) The Sun bobs up and down every 10 million years or so in its orbit. In the bulge, the stars are more like the comets in the spherical Oort cloud; they orbit all over the place, in elliptical orbits with all sorts of tilts. They don't all go in the same direction. Most galaxies are surrounded by halos of mysterious dark matter (we'll talk more about that soon) and globular clusters. The Milky Way has about 200 globular clusters in orbit around it, with about 100,000 stars in a typical cluster. Globular cluster stars, like bulge stars, tend to be old, red, with low abundances of heavy elements.
Why a Spiral?		Why do spiral galaxies like our Milky Way have spiral shapes? It's important to remember that just because the spiral arms give out so much light doesn't mean that there are many more stars there than elsewhere. There are only more bright stars there! Most galaxies have stars that orbit all at similar speeds, no matter how far they are from the center. If all the stars started out in a spiral shape, then within a billion years or so, the spiral would certainly be so "wound up" it would no longer look like the spiral galaxies we see. This is because the stars near the outside would move a smaller angle around their circular orbits if they moved at the same speed (because they have larger orbits to go in.) Instead, the spiral arms mark places where new stars are born. The low mass stars live long enough to move away from the spiral and into the rest of the disk. The massive bright stars won't get far before dying--they'll stay in the spiral where they're born. We're not entirely sure, but spiral shapes are probably a mixture of 3 effects: Spiral density waves: Stars can go about the center of the galaxy in elliptical orbits that bunch up in spiral density waves. The orbits of stars slightly bunch up, and the extra gravitational pull of that slight bunching up pulls gas in. That gas is then compressed and forms new stars. A galaxy's spiral is a wave phenomenon. The stars don't move at the speed of the spiral, any more than the water in a water wave moves at the speed of the wave. Think about a traffic jam. Say a slow-moving bus is holding up traffic. The cars behind it slow down and eventually pass the bus. But while they're slowed down they create a place where cars are more dense. Looking from a traffic helicopter, you'd see cars bunched up behind the truck. The bunching-up would move ahead at the speed of the truck. But that's not the speed of any of the cars! It's the same thing with a spiral galaxy--the stars are like the cars that get bunched up. This artist's interpretation shows the similarity between water waves and the spiral density waves of a galaxy: Waves of supernovas Another possibility is that given massive stars all in the arms, they'll eventually go supernova and send shock waves passing through interstellar gas. It's thought this mechanism may work in flocculent spirals, spirals in which the arms are not as well defined. Tides from the gravity of nearby galaxies As you saw in the galaxy collisions lab, spiral shapes can sometimes be formed by gravitational tides when galaxies get near each other.
The Black Hole at the Center of the Galaxy		Click here to see a movie of the stars near the center of our galaxy in orbit around the unseen black hole thought to reside there! From the speed of the stars in orbit around the black hole, we can measure its mass to be 2.6 million times the mass of our Sun. (Remember Newton's form of Kepler's 3rd law--you can measure the mass of a central object by the period and semimajor axis of objects in orbit.) Andra Ghez has pioneered the discovery of the black hole at the center of the Milky Way, which is called Saggitarius A (it's the brightest source of radio waves in the constellation of Saggitarius.) Although it is poorly understood, there also appears to be antimatter spewing from the Milky Way's center. Below is an image of the strange happenings near the center of our galaxy, as revealed in radio wavelengths (which get through dust easier!):
Winds and Bubbles and Fountains		Supernovas and planetary nebulas spread into space the elements that make our bodies. But how does this material, the product of fusion, spread throughout the galaxy? That's the story of galactic ecology. A supernova explosion sends gas hurtling through space. But that gas will eventually impact against the gas between the stars, pushing it out and piling it up like a snowplow. The expansion slows down as it radiates and as it picks up more and more interstellar gas. The gas inside becomes very sparse, and reaches high temperatures. The collision (shock wave) of the expanding shell and the outside gas is responsible for raising temperatures. Bright massive stars that go supernova tend to form in clusters of "associations." Eventually a supernova may overlap with another, or with the "bubble" of a hot massive star's stellar wind. Remember, a stellar wind is gas blown off from the surface of a hot star by radiation pressure. It can amount to 10-5 of the Sun's mass every year. The interstellar gas may form a kind of "swiss cheese" of overlapping bubbles. The combined action of supernovas and stellar winds can create a "superbubble" of hot gas expanding and pushing out a cooler shell of gas. These superbubbles may expand until they break through the top layers of the disk of the galaxy. From there on, they encounter less resistance. The hot gas spills out of the galaxy and into the halo, but some clumps cool off by radiating and drop back down, enriching the galaxy with the supernova-created elements. There is observational evidence that something like this really happens!