Chapters 12 & 13 – Galaxies We want to judge everything, and we’re always at a bad vantage point.
We want to judge ourselves, we’re too close;
we want to judge others, we’re too far away.
- de Fontenelle, 1683
That faint milky band stretched across the night sky has long been called the Milky Way. (The ancient Greeks called it the “kyklos galaktikos,” which means “milky circle.”) In 1610, Galileo used his telescope to see that the Milky Way was in fact a tremendous number of faint stars that seem to circle the sky. We must be in an enormous disc of stars! All the typical stars we see at night are part of the Milky Way.
In the late 1700’s, William and Caroline Herschel counted how many stars they saw in each “section” of the sky, to see how the Milky Way disc is shaped. Astronomers at the time did not know that interstellar dust obscures the view of far-away stars, so when the Herschel’s saw about the same number of stars in every direction of the Milky Way circle, they concluded we live at the center of a disc of stars 15,000 light years in diameter. This was called the grindstone universe, and was the prevailing theory for over 100 years. Early in the 1900’s, Harlow Shapley studied globular star clusters, which are not all in the Milky Way disc (many are above and below, where you can see them easily!), and found that the Milky Way galaxy was far larger than 15,000 light years, and the Sun is not at the center!
Shapley could determine the distances to globular clusters because of Cephied variable stars (and similar RR Lyrae variable stars). Some massive stars pulsate during their giant phase, and how fast one pulsates tells you how luminous the star actually is. Compare this true luminosity to the apparent luminosity, and you can tell how far away the star must be! Henrietta Leavitt discovered this helpful “trick” in the 1910’s.
We now know that the Milky Way Galaxy is ~75,000 light years across, and has a central bulge, a flat disc, and a large halo (where Shapley’s globular clusters are). (See page 256.) We live on the disc, about two-thirds of the way out from the galactic center. (This isn’t surprising, as the bulge and halo consist of mostly old, cool stars.) We can map our galaxy because a lone hydrogen atom in empty space emits 21cm wavelength radio waves, which cut through interstellar dust. We use radio telescopes at the 21cm wavelength to “see” our entire galaxy!
All stars in a galaxy orbit the galactic center. Halo stars orbit in eccentric orbits, while stars in the disc follow parallel, almost-circular orbits. We orbit the galactic center every 240,000,000 years. We can use the distance and speed of the Sun from the galactic center to estimate how much mass must be inside the Sun’s orbit (just like Kepler’s third law); we get about 100 billion solar masses. Using stars farther away from the galactic center, we get larger mass estimates (because such estimates only guess how much mass is inside an orbit). Our mass estimates go up much faster than we’d expect based on the number of stars we see: this led to the idea of dark matter, and we still don’t know what the dark matter is!
As we know from the last classes, stars are giant atom-fusing machines, and they seed the interstellar medium with heavy elements. (Astronomers call any element heavier than hydrogen and helium a “metal,” which is silly, but that’s what they do.) We expect the oldest stars to have few metals, and younger stars have more metals. Our Sun is a population 1, or “metal-rich” star. Like most stars in the galactic disc, it is relatively young. We can use the HR diagram turn-off point in open star clusters to figure out how old the galactic disc is; the oldest clusters in the disc are ~9,000,000,000 years old. The halo of our galaxy has population 2, or “metal-poor” stars, and the globular clusters there are upwards of 13,000,000,000 years old. (See the table on page 260.) The halo of our galaxy is almost as old as the universe, but the disc didn’t start forming until 4,000,000,000 years later!
Our galactic disk has spiral arms lit up by massive, hot blue stars. Any object we can use to see spiral arms (like hot blue stars) are called spiral tracers, and they tend to be very young objects. We conclude that spiral arms are areas of active star formation. The spiral arms are probably rotating density waves that stars and gas pass through, much like cars going through a gaper’s block. As gas gets compressed by the density wave, it triggers star formation, which lights up the density wave as a spiral arm. (This theory is popular, but our understanding of galactic structure formation is not great right now. De Fontenelle’s quote at the beginning of this handout is exactly right: we’re too close to our own galaxy to see its structure well, and we’re too far from other galaxies to get all the information we want!)
Sagittarius A*. Oh boy, this is cool. If you point a telescope towards the Sagittarius constellation, you are looking directly towards the center of the Milky Way galaxy. This area radiates a tremendous amount of radio waves, and one of the sources, called Sgr A*, is about the size of a giant star (about one AU in diameter), and lies smack dab where we think the center of our galaxy is. There seem to be tons of stars and heat right there, and we see some big stars that orbit Sgr A* incredibly quickly. Sgr A* must be a super-massive black hole at the center of our galaxy, containing ~2,600,000 solar masses!!! (Sorry, but it does not seem to be actively eating stars anymore, which makes it a dormant galactic center. We’ll study active galactic centers later. And yes, almost all galaxies have super-massive black holes at their centers.)
Is the Milky Way the only galaxy, or are there others? Back in the 1770’s, a comet hunter named Charles Messier made a list of “fake” comets: objects that look fuzzy like a comet, but are not comets. Most of 110 Messier objects were found to be open star clusters, globular star clusters, or nebula (M1 is the Crab Nebula, the supernova remnant we discussed last class!). But what about the bizarre M31, or M51, or M100? Some, like Kant, suggesting that these are full galaxies just like our own, called the island universe hypothesis. Others believed they were unknown kinds of nebulae or star clusters within our one and only galaxy. If only we knew their distance!
Any object that has a known magnitude can be used to determine distance: just compare the known magnitude to the apparent magnitude! Such objects are called standard candles. Cephied variable stars are one kind of standard candle. Type 1a supernovae are another, and even globular clusters can be used to ballpark distance. In 1924, Edwin Hubble found Cephied variables in M31 (called the Andromeda nebula at the time), and determined that Andromeda was a couple million light-years away! Clearly, Andromeda was an entire galaxy!
Within just a few years, many galaxies were identified, and all showed significant red-shift (they are moving away from us.) Hubble noticed that farther galaxies were more red-shifted, as if the entire universe were expanding. Universal red-shift was the first piece of evidence for the Big Bang theory. Galaxies that are 1,000,000 parsecs away are moving away from us at ~70km/s. Galaxies twice as far are moving away twice as fast. This ratio is known as Hubble’s constant, and it measures of the expansion rate of the universe.
Galaxies come in many shapes and sizes. Most galaxies are shaped like a football (ellipsoid galaxies), and have almost no gas and dust, so they can’t make new stars. Some galaxies are spiral shaped, and have active star formation. If a spiral galaxy has a bar in the middle, like our own Milky Way Galaxy, it is called barred spiral galaxy. There are also bizarre galaxies that don’t fit into any good category, some of which are oddly-shaped due to galactic collisions. In a few billion years, we will collide with our nearest large spiral galaxy, Andromeda! The Milky Way has cannibalized small galaxies before, but Andromeda is huge, and it is unlikely that either spiral galaxy will retain the nice spiral shape after collision.
Andromeda and the Milky Way are the largest two galaxies in our Local Group, though there are also many dwarf galaxies in that group. The Local Group is a small galaxy cluster, while the biggest galaxy clusters have thousands of galaxies! (Small clusters tend to have more spiral galaxies, and spiral galaxies are where star formation occurs, and only young star formations have metal-heavy planets like Earth. So it’s not too surprising that we live in a small cluster!)
All spiral galaxies rotate, and we can use red-shift and blue-shift to determine the speed of rotation. As usual, we use rotational speed to find the mass of the galaxy. Just like the Milky Way, all galaxies seem to contain far more mass than we can see: every galaxy seems to have dark matter! This is confirmed through the motions of galaxies around the center of their galaxy cluster (they move too fast – there must be extra mass we can’t see), and through gravitational lensing (the lensing effect is too strong – there must be extra mass we can’t see).
In 1995, the Hubble Space Telescope was pointed towards a dark patch of sky near the big dipper, and took a photo with a 10-day exposure time. The resulting image, called Hubble deep field, is one of the greatest photos ever taken. The sky is carpeted with galaxies. We estimate that there are as many galaxies in the known universe as there are stars in the Milky Way. Some of the galaxies in the photo are upwards of 13 billion light-years away, so we are truly looking into the deep past. Faraway galaxies tell us about the history of galaxy formation. They also tell us that the universe is very, very, very, very big.
By Robinson Jeffers
The polar ice-caps are melting, the mountain glaciers