I. The Milky Way Galaxy (Ch. 22) -- One of the blurred, puffy bright nebulae that can be seen on a very dark night is actually not a nebula from within our galaxy, it is a huge collection of billions of stars, the Andromeda Galaxy. It is so far away that the light we see from it left that galaxy about 2 million years ago! -- A galaxy is not just composed of billions of stars, there is also low-density gas and dust in between the stars, called the interstellar medium. Here is a nice picture of dust in a nearby galaxy. -- The presence of dust makes it impossible to see across the galaxy in visible light, which limited the attempts by early astronomers to study the Milky Way. It was realized that the "milky" swath of light you can see on a dark night away from the city is actually an edge-on view of a flat disk of billions of stars too distant to see individually. This disk is our galaxy, the Milky Way. -- In reality, the disk breaks up into spiral-arm structures, like when you first pour cream into stirred coffee. So the Milky Way is a spiral galaxy, and the milky swath of light we see is actually our neighboring spiral arm. Many distant galaxies are also spirals, like the whirlpool galaxy. -- What holds a galaxy together? The same thing that holds just about everything in astronomy together. Gravity. The gravity from a single star could never hold another star, unless they were very close together like in a binary. So it takes the combined gravity of billions of stars to produce a strong enough force. -- In between stars there is diffuse gas, mostly hydrogen as usual. There is only about one atom in every cubic cm, on average, but it's a big galaxy. These atoms absorb light coming from distant sources (like stars), and produce an absorption feature in the spectrum (an absorption line). Also, hydrogen emits light at 21 cm due to a transition within the lowest energy level, so even very cold hydrogen can emit at this wavelength. This emission line has been observed with radio telescopes, and it is very useful for mapping out our galaxy and even for seeing other galaxies. -- HI and HII regions: when hydrogen clusters together in a huge cloud, it is called an HI region if the cloud is neutral, and an HII region if it is ionized (which means that the electron is stripped from the atom). The energy to ionize HII regions comes from very luminous stars, usually O or B stars, within the cloud. HII regions often glow with a reddish color due to red light emitted as electrons and protons collide. -- Dust plays a role in absorbing visible light, but it also scatters some of the visible light. If the absorption is most important, you see a dark nebula in front of a light source, since the dust is absorbing the light. But if you see a bluish nebela next to a bright source, that means you are seeing the visible light that is scattered by the dust, from the bright source and toward you. So nebula can be red, blue, or dark, depending on the emission within the nebula, the scattering and absorption by dust. A good example of all three is the Trifid nebula, which is a star-forming region seen in Sagittarius and is about 3000 light years away, or the famous Orion nebula in the sword on Orion's belt. -- Globular clusters are groups of hundreds of thousands of stars, each about a light year apart, but they look like a very dense clump. They are stars that formed together in a ball, and are like miniature spherical galaxies of their own. You can see a bunch of them here. By looking at the distribution of globular clusters, and assuming the center of that distribution is the same as the center of the Milky Way, it is estimated that our Sun is about 8.5 kpc from the center. This means that the Milky Way is very large, much larger than was originally understood by astronomers who didn't know about dust absorption.
II. Galactic Rotation and Dark Matter -- If we assume the mass in the galaxy is in the stars, and we have a typical distribution of stellar masses, then we can expect the mass to be proportional to the brightness. Also, the central concentration of brightness in a typical spiral galaxy would imply that most of the mass in the galaxy is near the center. -- When the mass is concentrated in the center, the orbits of gas and stars around the center should obey Kepler's law, meaning that the farther from the center, the slower the speed. -- When the orbital speeds are actually measured, called the "rotation curve," two problems emerge: the speeds are all about the same, so they don't fall with distance from the center, and they are surprisingly fast. -- This implies we either have a mistake in how gravity works, or there is a lot more mass in our galaxy than is in the stars, and it is much more spread out. -- This additional mass does not make light, so it also does not absorb light, it is invisible, but gets called "dark matter." -- Make no mistake, it is very strange that most of the universe is made of something that is invisible to us, it almost sounds like science fiction but we are forced to this conclusion by observations on all scales larger than a galaxy. -- There is not dark matter on stellar and planetary scales, because these objects need to be able to emit light to let gravity win over pressure and hence form them, but dark matter does not emit light so cannot contract into small objects like stars and planets.
III. Is there a Huge Black Hole at the Center of the Milky Way, in Sgr A? --Black holes do not emit light from within their event horizons, but they do emit light from orbiting gas outside the event horizon that is accreting onto the black hole gradually over time. -- Also, the gravity of black holes affects the orbits of stars and gas very near the black hole, in ways that allow us to infer there is a very concentrated mass there which can only be a black hole. -- Using that type of reasoning, we have found supermassive black holes in the cores of most, if not all, galaxies, including our Milky Way. -- It is not known how these supermassive black holes form, but they can be millions or even a billion solar masses of material, so their formation must have released a pretty spectacular amount of energy, even if it played out gradually. In some cases, we can still see that energy release as gas accretes into the supermassive black hole, causing what is called an "active galactic nucleus", or if it is very bright, a "quasar."
Week 12 notes
