The Milky Way.
Almost everything we see in the night sky belongs to the Milky Way. Since we are inside, we only see the one band. From outside, our Milky Way might very look like our cosmic neighbor, the Andromeda Galaxy.
If you dare, read “Galaxy 666” Widely regarded as one of the worst SF novels ever written.
There are a lot of these things out there, but it took us awhile to figure out what they are. The first time someone tried to understand the structure of the galaxy, was in 1785. William Herschel based on optical observations. If we just count stars, assuming that the uniform stellar density, we get a shape a little like a platypus viewed in profile. Herschel’s immediate model is that the galaxy was a flattened disc. The shape of the Milky Way was believed to resemble a Grindstone with the sun close to the center.
We can do better now. The strategy that we use to explore the size and structure of the Milky Way.
1. Select bright objects that you can see throughout the Milky Way and trace their directions and distances. M stars are plentiful, but not bright, so those wouldn’t be good. O stars are bright but rare, so they aren’t good either.
II. Observe objects at wavelengths other than visible, which gets around the problem of dust.
III. Trace the orbital velocities of objects in different positions to our relative position.
We looked at a full sky slide of the Milky Way as viewed from earth.
The structure of our Milky Way is hard to determine because:
1. We are inside
2. Distance measurements are difficult
3. Our view towards the center is obscured by gas and dust.
Measuring Distances:
So, looking at the HR diagram again, with the main sequence, if we look at how higher mass stars evolve off the main sequence, there’s something called ht instability strip. There’s a region in which stars become unstable and pulsate. The structure of the star changes. So you get a pulsation related to the size of the stare. More massive stars are more luminous and larger, so they pulsate slower. Observing the period yields a measure of its luminosity and thus its distance. The thing about this, is that if you can see a Cepheid variable, even if its obscured by dust, all you have to do is measure its period and then you’ll know it’s luminosity. This allows us to measure the distances to star clusters throughout the Milky Way. There are Type I (classical) Cepheid, Type II Cepheid, RR Lyrae
Once we know what kind of star it is, and what kind of spectral shape it has, we can correct for obscuring factors to determine the distance.
Two kinds of star clusters where there are enough stars to find Cepheids. Open star clusters = young cluster of recently formed stars; within the disk of the galaxy. They follow the Milky Way all around the sky. They are young and tend to drift apart as they age.
Globular clusters = We see them all over the sky. In the plane of the galaxy and everywhere else too. They are old, centrally concentrated clusters of stars; mostly in a halo around the galaxy. 100 thousand and a million stars. Their own self gravity is strong enough that they aren’t tidally effected as they move through the galaxy.
We use them as tracers of galactic structures. The globular clusters are clustered around what we have determined is the center of our galaxy. They are everywhere in the sky, but an excess in the center.
Dense clusters of 50,000 – a million stars.
Old (~ 11 billion years) lower-main sequence stars
Distribution of globular clusters is NOT centered on the sun, but on a location which is heavily obscured from direct (visual) observation.
As amateur astronomers, they look for them in the summer, but not the winter. In the summer, we are pointing toward the center of the galaxy at night. In the winter, away.
Looking at the distribution, this is why we think we are ~20 thousand light years out from the center. After we do all these mapping methods. We think it’s about 75,000 light years in diameter. We think there’s a bar across the middle. (Steve: “That’s good, you go that far, you’re going to be thirsty.)
We have the disk component and then in the disk we have Open Clusters and O/B Associations. There’s the Nuclear bulge in the middle. Interstellar dust emits mostly infrared, while absorbing optical light. In the far infrared we see that dust is mostly confined to the galactic plane.
Orbital Motions in the Milky Way. Stars found in the disc have nearly circular orbits, going around the center. Don’t think about it like the solar system at all. They are moving around in relation to each other, not orbiting a black hole in the center. The black hole is unimportant to them.
Halo Stars: Highly elliptical orbits; randomly oriented.
Differential Rotation: Things at different distance move at different speeds. Sun orbits around galactic center at 220 km/s. 1 orbit takes approx. 240 million years. Stars closer to the galactic center orbit faster. Stars farther orbit slower. This can contribute to the spiral structure, but not the cause of the spiral.
Mass Determination from Orbital velocity.: What we can use the distance and the speed which gives you some measure of the gravity, which tells us something about the mass that’s in the orbit. The more mass there is inside the orbit, the faster the sun has to orbit around the galactic center. From the sun’s orbit we can tell mass of the galaxy.
To measure velocity we measure the speed with which something is moving towards or away from us. Just like with the Doppler effect, where the sound changes as an object approaches us. Objects in space shift to higher or lower frequencies of light as they move. (redshift/blueshift)
Total mass in the disk of the Milky Way: Approx. 200 billion solar masses. Additional mass in an extended halo: Total approx 1 trillion solar masses. Most of the mass is not emitting any radiation: Dark Matter.
As we measure the velocity of things farther out, if we follow the Keplerian curve, they should slow down, but they don’t. Which means that there’s more mass out there.
Dark Matter
The Father of Dark Matter was an incredibly brilliant physicist at Harvard that they put int he basement because everyone hated him but he was almost always rate. He called everyone bastards. In 1933 Fritz Zwicky checked out the Coma Cluster. The galaxies were flying around too fast for their visible mass to keep them together, so he proposed dark matter was present.
If you look at the motions of objects in a cluster, and the speed that they are moving, if there’s not enough gravity in the center, they should fly apart. If they don’t there must be other mass.
A few decades later, Vera Rubin pioneered the study of galactic rotation curves. She looked at the spiral galaxy curves. She started measuring the shift of light. She put her spectrometer on top of the galaxy and look at the object. She’d look at the H line and see that there was a blue shift where gasses moved toward us and on the other side there was a red shift where it was moving away. She stared to notice flat rotation curves in spiral galaxies. Which meant that there must be something there.
The Nature of Dark Matter
Can dark matter be composed of normal matter? If so, then its mass would mostly come from protons and neutrons = baryons. In the very early universe things were hot, dense and cooling. The density of baryons right after the big bang leaves a unique imprint in abundances of deuterium and lithium. We get a consistent answer when looking a the abundance. We determine baryonic density and it’s ~4% of critical density, the total matter is 30%.
Critical density is a reference number that’s important in cosmology.
On of the sources of the non-baryonic matter is called the WIMP Weakly Interactive Massive Particles.
Neutrinos? Seem to have mass, but they are too small. They are a component, but not the dominant, there’s not enough mass.
Axions? hypothetical elementary particle. As yet, not detected (axions are predicted to change to and from photons in the pretense of strong magnetic fields and this property is used fo r creating experiemtnst o determine argons.
When a particle goes faster than the speed of light in a given medium, it gives off Cherikoff radiation. Light travels slower through water than air. A neutrino doesn’t change its speed. So you look for blue flashes as neutrinos pass through. It used to be dry-cleaning fluid. The passage of neturinos change it to radioactive argon, so you’d set it up, leave it alone and then see how radioactive it was later.
If not WIMPs how about MACHOs? Massive Compact Halo Object
Just trying to explain flat rotation curves with things like black holes, brown dwarfs, etc.
These are “baryonic” made from conventional stuff on the periodic table. There are some of these out there, but not enough to account for all the dark matter.
If you have a MACHO floating around in the halo, you look for a background star to suddenly increase in brightness. It will suddenly, over the course of a few weeks, suddenly increase in brightness and then decays to its previous luminosity. That’s the effect of gravitational lensing. People doing the MACHO project in the 90s would take pictures of the Magellan cloud and then one out of every hundred photos would do this.
Is the dark matter real? Maybe we don’t understand the law of gravity. It could be MoND = Modified Newtonian Dynamics. The Dark Matter paradigm explained a lot of stuff about the big bang, but the MoND was still viable, until 2006. Light coming out from a quasar is bent around a distance galaxy making two images of the same quasar. Einstein predicted this lensing effect. It lets us not only observed the quasar, but also measure the mass of the intervening object. We can use this to probe the distribution of matter in the cluster. We can map out where things are.
There is a large component of hot gas, which is mass that’s not in the optical light but that we can see in x-ray. It’s more mass than all the starlight in the galaxy. It’s the dominate baryonic mass in the galaxy. This gas remains gravitationally bound, which provides further evidence for dark matter. The gas is still not enough to account for dark matter.
The smoking gun is the “Bullet Cluster” Given what we know about gravitational lensing — it doesn’t car if its baryons on non-baryons, it only cares if its got mass and gravity. Lensing of background galaxies seen in the optical lets this mass distribution be mapped. X-rays trace the hot gas, the dominant source of baryons in this cluster merger. They don’t line up. Why? Dark matter seems to not interact with itself the way diffuse gas does during a cluster collision.
We believe we have two galaxies flying through each other. The dark matter doesn’t care. But the gas will experience drag. The hot baryonic matter slowed down during the collision, the dark matter didn’t. This lines up with what we understand of gravity, but people who want to use MoND would have to change their rules to explain what we observed with the Bullet Cluster — there are other example galaxies.
The History of the Milky Way.
This is the traditional picture: A spherical cloud of turbulent gas gives birth to the first stars and star clusters. the rotating cloud contracts to the equatorial plane. Stars and clusters are left behind in the halo as the gas cloud flattens. New generations of stars have flatter distributions. The disk of the galaxy is now very thin.
There are some problems with the traditional picture. There might be violent cosmic mergers which account for the problems.
Spiral arm structure. Young stars are found in the spiral arm. Older stars tend to be found anywhere. The sun is on the edge of the Orion-Sirius arm. O/B Associations trace out 3 spiral arms near the sun. We can look in the radio, because H emits 21-cm radio waves which reveals neutral hydrogen concentrated in the spiral arms. Shock waves from supernovae, ionization fronts initiated by O and B stars and the shock fronts forming spiral arms trigger star formation. Spiral arms are stationary shock waves that initiate star formation.
A gas clouds flies into a spiral arm, makes stars. The massive stars are highly luminous light up the spiral arm. The most massive stars die quickly. Low-mass stars live long live but are not highly luminous.
Self-sustained star formation in spiral arms. Star forming regions get elongated due to differential rotation.
Grand-design spirals have two dominant spiral arms.
Flocculent (woolly) galaxies also have spiral patterns, but no dominant pair of spiral arms.
The Whirlpool Galaxy is a grand-design galaxy. Self-sustaining star forming regions along spiral arm patterns are clearly visible.
Our optical view towards the Galactic center is heavily obscured by gas and dust. Only 1 in 30 photons reach us from the center. In the radio view, you can see many supernova remnants, shells and filaments. The galactic center contains a supermassive black hole of approx. 2.6 million solar masses. We can measure in the infrared where 1 in a million photons gets out, we can take pictures of stars in the center and we’ve mapped out stars orbiting the black hole.
Distance to Other Galaxies
a) Cepheid method
b) type 1a supernovae (collapse of an accreting white dwarf in a binary system)
Both are examples of “standard-candle” method. (If you standardize a candle, then when you look at another candle you can compare the two to determine how far away it is.)
The Hubble Law: The more distant the galaxy, the faster its moving away from us. The Hubble constant. vr =H0*d (H= 70 km/s/Mpc is the Hubble constant.)
Many galaxies are millions or billions parsecs from our galaxy. Typical distance units. Mpc = megaparsec = 1 million parsecs
Gpc = gigaparsecs
Look-back time. When we observe the universe we are looking back in time, because of how long it takes for the light to reach us.
Supermassive black holes in the center of every massive galaxy.
Clusters of Galaxies. Most galaxy is in a cluster of galaxy.
Rich clusters
1,000 or more galaxies
or Poor clusters.
Could be just a handful of galaxies. We’re in a poor cluster, the Local Group. In our group, the majority of galaxies are dwarf elliptical galaxies. Three spirals.
Particularly in rich clusters, galaxies can collide and interact. Galaxy collisions can produce ring galaxies (such as the Cartwheel galaxy) and tidal tails (NGC 4038/4039 and also The Mice)
They can do simulations and produce galaxies that look much like all of these.
Active galaxies seem to be driven by this merger activity. There’s a class we call “radio loud” with extremely violent energy release in their nuclei. “active galactic nuclei” = AGN Up to many thousands times more luminous than the entire Milky Way.
Taking a spectrum of normal galaxy looks like its just a bunch of normal stars all thrown together. It tells you the population of stars. Elliptical galaxies tend to have older stars.
Seyfert Galaxies are a particular type of AGN which have a core that is unusually bright. Active galaxies are often associated with interacting galaxies, possibly result of recent galaxy mergers.
Cosmic Jets and radio lobes. Material in jets moves with almost the speed of light “relativistic jets” Some of these structures are quite large. Jets visible in radio and x-rays; show bright spots in similar locations. Centaurus A the closest AGN to us.
Radio Galaxies (II) If there’s intergalactic material, it bends the jets back. We believe that the jets are powered by the accretion of matter onto a super massive black hole. Twisted magnetic fields help to confine the material of the jet. Look at the Jets of M87 The jets are about half light sped.
The Dust Torus of NGC4261
Model of Seyfert Galaxies. It’s very similar in nature to the structures we talked about in binary systems but on a much larger scale. There are high energy gas clouds ionized by the accretion disc, which gets raised to extremely hot temperatures that radiate.
Quasars.
They are like extreme Seyfert galaxy but they are so bright that you can’t see the galaxy around them. They show extreme redshifts. Quasars are extremely far away and moving very fast. They don’t exist in our area, which means that they burn out. Quasars have been detected
Dominated by light coming from the accretion discs in all kinds of temperatures. Gas moving in the vicinity of the quasars gets ionized. They have seen quasars redshifted up to z~6. At this level, the ultraviolet lines are showing up in the red part of the spectrum.
Study of Quasars show us
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large scale structures
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early history
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galaxy evolution
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dark matter
We think the biggest galactic collisions happened when the universe was young and things were closer together. The big ones have happened so we only get small ones.
Interesting classification 😉
Ha! That should have been “worst.”
(…shoulda had you ask about Heim-Dröscher Theory — talk about geekin’ the geeks. 😉 )