Launch Pad, Day Five: Mike Brotherton on Summing Up Stars, and Moving on to Galaxies

To see the rest of my Launch Pad posts, click here.OK, let's get back to the destruction of the Earth.The fate of our sun and the end of the earth--earth will be incinerated when the sun becomes a red giant, unless we find a way to move the earth. We have about five billion years before that happens.Now let's talk about massive stars, stars greater than four solar masses.The most massive stars can continue fusing elements and releasing energy all the way to iron. The most massive O stars can burn through that in a million years.A 25 solar mass star will live seven million years, burning hydrogen for 7mil, helium for 500,000mil years, carbon for 600 years, oxygen for six months, and silicon for 1 day."So," says Bud, "You're saying if we can find a star in the oxygen burning phase, we can watch it die in real time.""Yeah," says Mike, "but we have so few of those locally that haven't gone supernova that we have little observational data. Also, we can't see when the star switches to burning the last elements in its core because that's internal, not what's being sent from the surface."The deathsof massive stars: supernovae. The final stages of fusion in high -mass stars. If you try to fuse iron with anything else, it takes energy rather than producing energy. You end up with the core of a red giant or blue supergiant (there's not a lot of data about this, so it's still poorly understood), being iron. When this happens, gravity wins out.We think red and blue giants may try to become planetary nebula (so far we see that happening always, but finding these massive stars in late stages and being able to image around them is very hard), but then they blow up before they can.Supernovas leave remnants, such as the veil nebula."How do they know to look at supernovas?" asks Cecilia."We take pictures of everything all the time," says Mike.A supernova produces comparable luminosity to a galaxy, for a period of time.We used to find supernovae more or less by chance. Now we find them all the time. People are looking for them. There are a couple of different types of supernovae. Type I and Type II.So you cram all the dense star matter together, and you get neutrons, which release neutrinos. Neutrinos can escape the gravity, so they go out flying in an incredibly huge neutrino flux and blast out the outer part of the star. Neutrinos power the explosion. Neutrinos usually go through everything, but there's such an enormous, enormous amount of them that they become significant.The core goes on to become a neutron star or a black hole.Back to the types. We can distinguish them with spectroscopy or with their light curves. Type II is the core collapse of a type two star,where we see hydrogen absorption in the spectrum because the outer parts of the massive star get blown out. Type I is what happens when an accreting white dwarf (in a binary system) exceeds the Chandrasekhar mass limit which is 1.4 solar masses; it keeps collapsing, trying to collapse down to nothing, but as the material gets more and more compact you have the same kinds of reactions as in the other cores, triggering a neutrino flux and an explosion, or a type Ia supernova. Type I supernovae do not produce visible hydrogen in the spectrum because the cores of these things have used up their hydrogen. These supernovae, however powerful they are, do not destroy companion stars. (Type I refers to all supernovae that don't have hydrogen in their spectrums, even if that happens outside the scenario that causes Ia specifically.)Bud: "So these are explosions of particles, not matter?"Mike: "Particles are matter."Bud: The outer parts of the star are still degenerate matter?Mike: No, the outer parts are not. The envelope is mainly hydrogen gas. The core is degenerate.You can see the timescale is slow, you can watch these things for months decaying. At some point, it expands so much that you can't keep studying it, but they do linger for thousands of years.Neutron stars: a degenerate core has collapsed, all the protons and electrons have been forced together, releasing neutrinos, and converting everything into neutrons. Neutrons are a fundamental particle, neutral, about the same mass as a proton. We think neutorn stars exist up to about 3-3.5 solar masses. UP to about 1.4 solar masses, you can have white dwarfs. Up to about 3-3.5 solar masses, we think neutron degeneracy can resist gravitational collapse.Typical size of a neutron star is the size of Laramie, a radius of about 10 kilometers with about 1.4-3 solar masses. Density is 10^14 grams per cubic centimeter, like the density of the nucleus of an atom, just solid neutrons packed together. The gee whiz kinda thing is that a sugar cube made out of neutron star matter would have a mass of about 100,000,000 tons.Discovery of pulsars: You collapse these giant fields that are spinning just a little bit into stars. Stars are spinning more. Collapse that core down even further to the size of Laramie and angular momentum means they're going to spin like mad. They look like they're spinning at the speed of light (though it's actually just a good fraction of the speed of light) with something the size of a city spinning a thousand times per second. They have magnetic fields with maybe 1,000 times the strength of the magnetic field of the sun.Neutron stars in binary systems--x-ray binaries--you get a star paired with a neutron star, which can create powerful pulsing x-ray beams.The first planets in space were not discovered around sunlike stars, but red pulsars. This is weird and not well understood. One thing astronomers do very well is timing. Our clocks are good. And if you have a pulsar giving you these steady pulses, you can measure any shift in frequency of that pulsation.If I'm a pulsar and I have planets moving around me, then the planet is actually orbiting the center of mass. And as the planet orbits the pulsar, that causes the pulsar to wobble, which has a slight effect on when we see the pulsation coming from it.These won't be like our solar system. There won't be terrestrial and jovial planets with temperatures where life can form.Carrie: Do we have planets around binary neutron stars?Mike: These are single neutron stars. The problem is we don't even know why there are planets around neutron stars at all. There were supernovas here. They should have toasted the planets. We don't know why there are planets at all. Maybe some of the debris from the supernova wasn't expelled, stayed in orbit, and formed planets.Carrie: So the planets are young?Mike: ...maybe? We don't know much about these systems.Now we're talking about life on planets around binaries. You could have close stars with a planet far away from them. Or you could have two stars that are in a binary but they're not close to each other and so they would sort of look like discrete solar systems, with planets able to circle each separately.Bud: What about figure eights?Mike: Yeah, but those wouldn't be stable.Ian: How about orbiting their center of mass?Mike: Won't be stable.Bud: There's a chaos equation that does not evolve because ithas two center points.Mike: That would be cool.Rachel's head: ?Black holes:Just like white dwarfs, which have the Chandra limit, there is a mass limit for neutron stars. If you have more stuff than 3 solar masses made out of neutrons, the pressure of neutrons packed against neutrons will not be enough to resist gravitational collapse. Now, our understanding of physics could be wrong. Other things could be going on. But if that's true, we don't know about it yet. So we expect these to collapse into singularities, or black holes.These were initially referred to as frozen stars, the idea being you have this collapsing core, with a time dilation effect as we watch it collapse toward the size of the event horizon, we see time appearing to slow down to an outside observer, and the star appearing to freeze as time appears to become frozen from the outside. There's also a gravitational red shift effect which will fade to black. Frozen stars was not a good name--it implied they were cold and that they were stars. Black hole is better.From an outside perspective, it looks like it takes infinite time for the black hole to form, but you'd have to have really really good radio telescopes to see that.You can still fall in and be destroyed by it, if you want.Ian--but if it appears they take infinitely long for them to form, how have they formed from our point of view?Mike--they still behave like black holes. We can't observe most of these well anyway. In principle if we looked at them with giant radio telescopes with infinite resolution we might have a clue that we're seeing detail down there. This has been used in some nice sf stories, too. Will McCarthy Flies from the Amber.Ian: can you have a black hole with less than 3 solar masses?Mike--Yes, deal with it later.Escape velocity. The tidal forces will rip you apart around a stellar-mass black hole, although tidal forces are limited around the event horizon of a super-massive black hole because they're so big.There is a limiting radius for any given mass where if you can get that much mass within that radius then the escape velocity exceeds the speed of light. This is the schwarzschild radius, or the event horizon.So can you have black holes with less than 3 stellar masses? Yes, if you can crush the mass small enough, to get it within the schwarschild radius. When you're making them out of stars, 3 stellar masses is a reasonable restriction, but if you could do it to other things, then you could get black holes. There are indications that in the primitive universe, there were some conditions that could do that, which means there may be old, tiny black holes around. Also, we could theoretically make them in labs, although we cannot currently.Schwarzschild radius is usually the event horizon. You can't see anything w/in the event horizon, because in order to see it, you'd have to see the light coming from it, and the light can't escape.Bud: You also can't see stars behind black holes.Mike: Yes, you can, because of gravitational lensing. Some of the light will be absorbed, but some of the light will be bent by the black hole around to you. This is like Kevin talking about how they could get Cassini signals when the sun was in the way. They bend around the sun because of gravitational lensing.Cecilia: Do we have speculation about what's going on inside event horizons?Mike: Speculation? Oh yeah. So, I know a lot about these at some level, and there's another level I don't. Greg Egan's story about life in a black hole based on scientific calculations... if you can survive the tidal forces, you can go inside the black hole, and things will look like something. You can see all the light coming in toward you, you just can't communicate outside. And Greg Egan has a story about making electronic copies of people downloaded into the black hole which allows you to live forever with some kind of complicated orbital solution... "The Planck Dive." Also Geoffrey Landis "Approaching Perimelasma."There's a correlation between mass and schwarschild radius. For one solar mass of the sun, it's 3km. So if it's a billion solarmass black hole, then you multiple 3 by a billion. An earth mass black hole would have an event horizon about an inch across. For 3 solar mass neutron stars, the radius is about 9km. So for neutron stars that are about 10km, and if you were able to crush the down just a little more, to 9km, then you'd get a black hole.Black Holes have few properties. Stars have chemical compositions. Black holes...? From the outside it doesn't matter what they're made of. Mass determines the size of the event horizon and gravity. They can have angular momentum and spin. That's preserved & has interesting effects. In the case of a spinning black hole, the vicinity of space outside the event horizon will be dragged along. The space itself will move in conjunction with the spin of the black hole. This has measurable effects if you study x-ray emitting black holes, the stuff outside the event horizon emits the x-rays, but we have theories about how this should happen and be modified if space is dragged, and it does seem to happen. There can also be electrically charged black holes if the black hole ate all electrons, for instance, then there would be electric field lines coming from the black hole. We think, though, that in any reasonable astronomical environment, this effect would be small--the black hole would eat positively charged things, too.Carrie--the mass of a star changes over its life, so the mass of the neutron star would be different from the mass of the red giant because it blew up all the hydrogen. So is the mass of a black hole different from the mass of the neutron star?Mike: Yes. It's important to remember that we have core collapse so in order to get a 3 solar mass star you have to start with a much more massive star.Carrie: The radius is determined by the resulting mass?Mike: Yes.So we have these diagrams that try to explain the stretched out sheet, etc. It's hard for us to imagine, as three dimensional creatures, space warping. But that's where the exotic effects come from, the idea that space is stretched infinitely entering the black hole.Spagghetification--an astronaut descending down towards the event horizon of the black hole will be stretched vertically (tidal effects) and squeezed laterally. However, this approximation is... not really true, the astronaut would actually not just become infinitely stretchy, but will actually be ripped apart and die.Mike: A black hole is not a black body in the sense of other black bodies. A black body is something that absorbs all light and does not reflect it. But it's absorbing light, not preventing light from escaping. The definition of a black hole is that light cannot escape from it. This is one of the reasons I don't like the term black body; it creates opportunities for confusion. Light will orbit inside a black hole; it will not ever get out. A black body has thermal energy and particles bumping against each other that cascade down and project into the environment so you can see them as black body curves.They do, however, radiate, but it's a different process than Hawking radiation.Bud: It's got mass, polar orientation, angular momentum...Mike: But the only way we can determine that is by the way space is dragged around them.General relativity near black holes: time dilation. If you were to drop a clock into a black hole, as it approached the event horizon, and we watched from distance,it would move slower and slower, until ti reached the event horizon, and then it would appear to freeze in time.Ian: So it's similar to moving closer to the speed of light?Mike: It's similar.Bud: The closer to the orbital radius, theslower it's going?Mike: The closer it gets to the event horizon, the slower time moves compared to an observer.Bud: But we know from orbital mechanics that if we take something from high orbit and drop it down, it goes faster, or covers a greater distance in a shorter time. So as it approaches the event horizon, in this case, it become stationery?Mike: It becomes stationery right as it hits the event horizon.Bud: So it looks like it's regressing?Mike: We can see it approach the event horizon.Bud: The orbital velocity at that point is high, and it decreases as it falls.Ian: No it increases.Mike: If we watch the clock's hands as it falls, it would slow. It's an issue of perspective. If you're the clock, you see things going faster and faster, you don't even see the event horizon. You just keep falling in. But from an outside observer, it seems to slow and stop. To see how it works out, how an observer would see you falling, depends on the mass of the black hole.Marjorie: So the observer is watching, and it looks like time is slowing, but time feels normal for them. So the person inside the event horizon, presuming they're functioning, to them it would seem like only a fraction of a moment has passed, but to the person outside...Mike: Yeah, if you're falling in, the corresponding effect, is you see the rest of the universe going faster than you.Marjorie: As you're falling in.There's another effect that prevents us from seeing black holes form. That's gravitational redshift. It's like we're stretching space infinitely. Space itself is getting stretched so a light wave traveling out from the vicinity of the black hole, the wavelength gets stretched as it climbs out, so if you initially shoot a blue light out, eh wavelength gets red shifted, maybe to red or infrared or even radio wavelengths, depending on how much the space gets stretched. When you stretch it to infinity, you essentially suck all the energy out of all the photons, and it goes black, and you can't see it. To infinity, and black.Hawking radiation - in the vacuum of space, particles and antiparticles pop into existence and then recombine before the universe can notice they borrowed energy to do that. We don't observe this directly, but there are macroscopic effects, like the kasmir effect ,if you push two non-magnetic two non-sticky plates together, a force will hold them together, because there's no room for these particles and anti-particles to appear anymore, but you still have particles and anti-particles outside the plates ,exerting pressure. Before you touch these plates together, particles and anti-particles inside are providing counter-pressure. But it's like the kind of deal with suction cups where you create air pressure and a suction cup will hold pressure. The casmir effect, though, operates in vacuum. You would not expect this to happen, but it does, and it gives credence to quantum foam. In the context of the black hole, imagine quantum foam on the edge of an event horizon, you get particle and antiparticle appearing and before they can come together and vanish, one gets sucked into the black hole. The other one is stuck. It's stuck and it can fly away form the black hole. So the details of this depend on the curvature of the event horizon. For a small black hole, this process is relatively easy, to have the quantum foam right at the edge, some get trapped, the others have to stick around. For massive, this rarely happens because the curvature is so small. So Hawking developed this theory and used it to say the mass has to come from somewhere--it looks like the mass is emitting radiation, but it's particle radiation from the quantum foam. It's a negligible process in massive black holes, in tiny black holes, it's relatively strong and can emit a lot of radiation, x-rays that are associated with particles and antiparticles that are coming out.The idea is, then, given enough time, a black hole can evaporate.Ian: But it's not coming from inside the black hole, the radiation?Mike: Yeah, but it's got to steal the energy from inside it. That's why the idea is that if you could make a lab black hole, it would disappear.Me: Can you use that to generate power?Mike: Yes. It will produce energy, but it's this kind of thing where if you have to make the black holes... if you could capture them, then yeah, you could get energy from it.Dave: New Scientist had an article on black hole powered spaceships. Unfortunately, New scientist requires you subscribe before accessing the article, but Dave chased down a paper by Louis Crane.Walter: Has Hawking radiation ever been observed?Mike: No. We do observe some things in space we think are black holes.No light can escape black holes so you can't observe them directly. We have about a dozen black hold candidates. We do estimates of what we think are masses of the system based on the wobbles, the orbital motions we can see. There's some uncertainties in the process about inclination angle and so forth, but we see dark companions in orbit around a variety of stars. The estimates of the masses, divined by watching the orbital dynamics, vary a lot, but are all above the 3 solar mass limit for neutron stars, and we don't see them as pulsars either. These are likely black holes, stellar-mass black holes in binary systems with other stars.Me:What's forming the super-massive black holes at the centers of galaxies?Mike: We don't know. The problem is the most distant objects in the early universe we can see are quasars and they already have a billion solar mass black holes at their centers and we don't know how they're built up. We imagine they were supermassive stars that became black holes that accumulate. But we don't know. Because there's so little time for them to do that early in the universe, so it's an open problem. But we do see supermassive black holes at the center of every galaxy, so it happens a lot.Gamma-ray bursts, the military put up satellites for nuclear tests they were going to use to look at the earth, but from space they kept getting all these hits so they came to the astronomers and asked, "Do you expect to see several bursts of gamma-ray radiation per day from all directions?" and astronomers said "Huh? That's cool," so it took a long time to start identifying optical afterglows. The telescopes would say "there was an afterglow over there... a while ago. last week. or a few minutes." you'd look over and not see anything interesting. but we got better satellites and x-ray- and gamma-ray telescopes that could give us alarms when these things went off and we had optical telescopes that would move within seconds and look and we saw optical afterglows. a few were bright enough to see with the naked eye for a few seconds before they faded. so we started getting an idea about the emissions of these things, and eventually we started to be able to get spectra and many of them were associated with galaxies halfway across the universe, star-forming regions. not all, but some.the picture that developed, we started calling these hypernovas, and we believe it's a particular kind of core collapse into a black hole that gives us a supernova, and in particular, a jet of radiation going along the spin axis. a burst of radiation out the spin axis. so if we're in the right direction, that jet shines at us and we see that gamma ray burst and optical afterglow. if it's in the wrong direction, we don't see it. So a particular kind of supernova blast, and we have seen some supernova remnants in those regions, when we point our biggest telescopes there. that's what we think many of those things are.Nick: is there a correspodning neutrino flux with the gamma ray bursts?Mike: Not that we can detect, but they're so far away. We've only been able to detect neutrinos from one supernova burst, and we only got 11 or 12 of them. That was in 1987, though, and we have better tools now.GALAXIES:We're in the disk of a spiral galaxy, so when we look out from that disk, we see a swath of light.One thing Galileo got in trouble for was looking at the milky way with the telescope and seeing that rather than a diffuse cloud, it was a lot of stars, which was not known before the telescope.How do you figure out where you are in the galaxy? You look around. William Hershel in 1785 did star counts. When he did this, he discovered we live in an amoeba, with the sun not quite in the center, but close. What's actually going on is that he counted more in the direction of the milky way than in other directions, so he thought we were in a flattened distribution of stars with the sun close to the middle.He wasn't able to see every star in the milky way because of galactic reddening, gas, and dust. He just couldn't see it with his telescope in the optical. Neither can we, in the optical; we do better in the radio and infrared.Kelly: Was he counting parallax distance?Mike: No, he was assuming uniform distribution in a direction.Bud: He had no concept of galaxy at this point, right? So he was counting in the universe?Mike: Right.So to do this right you need to start with bright objects that you can see further than stars and then see what directions they're in and getting distance to them is even better. You can also look outside the optical and at orbital velocities.So inside the milky way, we can't see terribly well because we're inside it. Distance measurements are difficult. And our view toward the center is obscured by gas and dust.Measuring distances: The Cepheid method. One of the best tracers is Cepheid variable stars. There's a portion on the H-R diagram called the instability strip in which stars pulsate because conditions are just right to ionize a layer of helium, and then that changes the structure and makes the helium neutral, which causes it to ionize again... these structural changes alter the size and temperature of the star. Some of these stars are extremely luminous. 10,000-100,000 times more luminous than the sun. Polaris is one. What makes this especially useful is there's a relationship between the luminosity (massiveness) of a Cepheid star and its period of oscillation. Big ones have a long period, smaller ones have a shorter period. We're talking days/weeks/months. So if you have a Cepheid variable star you're watching, you can get its intrinsic luminosity just by watching its period. It can be obscured by gas and dust, but you can still figure out its intrinsic luminosity and thus how far away it is.The gas and dust is still a big problem, but there's a trick to get around it. (Cepheid stars are generally observed in optical, though also in infrared.)These are also very useful for figuring out distances to other galaxies because they're bright enough to see as individual stars in other galaxies.A little bit about star clusters. There are two types we see in the milky way. Open clusters with hundreds of thousands of stars like the Pleiadies, not centrally concentrated, not too old, maybe a hundred million years old, generally found within the disk of the galaxy, close to the milky way light. These form in star-forming regions then drift apart over time because they're only loosely gravitationally bound.There's another class, globular clusters, centrally concentrated clusters of stars, hundreds of thousands to millions of stars, not concentrated in disks, billions of years old, and can have Cepheid variable stars, mostly in a halo around the galaxy. These do not drift apart over time because they're strongly bound together.Globular clusters are dense clusters of 50,000-a million stars, old, lower main-sequence stars, there are approximately 200 in the milky way.Monte: Even though these are in a halo around the milky way, they're still considered part of it?Mike: Yes.So we can plot clusters. Some are away from the galactic center. A lot are concentrated around the disk of the milky way, centered on a rotation about 25,000 light years away. This was our first clue that we were not in the center of our galaxy, but about 25,000 light years from it. The distribution of globular cultures is not centered on the sun, but on a location which is heavily obscured from direct visual observation. We can look at the galactic center, but not in the optical.Since then we've been working really hard to build up a picture of the structure of the milky way. This has been done with many techniques, over decades. Now with recent infrared telescopes we have been able to determine that we think there's a bar across the center of the milky way. It's tricky--since we're inside, it's hard to observe.So we're in a spiral galaxy. We know we have spiral arms. We know there's a bulge of older stars in the center. We know there's a halo with globular clusters in it surrounding us. The sun is 2/3 out from the center. The 75,000 light year number, which is how far we are across approximately, is rough... it could be 100,000. It's hard to define the edges. O and B stars do cluster together because they have short lifetimes and they never get to live long past where they were formed. You see them in star forming regions. We see they clump in what we recognize today as spiral arms. The outside is thin, maybe 1,000 light years, whereas the bulge would be a few thousand thick.Infrared view of the milky way... infrared lets us see through the gas and dust more easily. If we look in the far infrared we see the dust itself radiating black body radiation and the milky way changes shape. Something like this, which we can see--Carrie points out--because being on the outside allows us to get a nice view of the disk.Cecilia: so if we were closer to the galactic center we might not know this much?Mike: It would be harder. And if we were more surrounded by dust and clouds, it would be harder.Questions that come up for me: Can stars exist outside clusters? Does being in a cluster or not, and what type of cluster, affect how life would be on a planet? Can there be life around Cepheid variables? Do huge meta-variables like the kidn of galaxy you'r ein matter at all on the planetary level?Orbital motions of the milky way- if all the stars are going to stay in the plane of the milky way that we see, they have to have orbits that stay in that plane. This is more or less true.Halo stars can be all types, but generally speaking they are older stars.The sun moves at orbital velocity around the center of our galaxy, at about 220km/s. One orbit takes 240mil years. What we see is that the speed is not too different with distance. Stars closer in or further out still go at 220km/s, though their orbital distance varies. This effect gives you twisting and spiral structure, but is not the origin of spiral structure in many galaxies.Orbits can cross through arms. A spiral pattern can turn on its own, but it's not made of the same stars all the time.While stars move around the galactic center at the same speed, no matter distance, this is not true for planets around our sun. Mercury moves fast, while Neptune moves very slowly. It's very different on a galactic level.If you know the distance and speed of an orbit, then you should know the mass that's inside the orbit. We can use the orbits of stars around the milky way to map out mass as a function of radius and if we had mass centrally concentrated in the center of the galaxy, as we do in the solar system, then we would expect the orbits to fall on a keplerian curve where things close in move fast, and things far away move slowly. The stars do move more quickly just at the center, and then drop off, but then they stabilize at 200km/s. That means there's a lot of mass at larger scales.We can use this to estimate the mass of the milky way, which has about 200 billion solar masses in the center, plus an additional mass in the extended halo, leading to a total of approximately 1 trillion solar masses. So we know what the mass should be, but we also know we can't see most of that mass, so we assume that most of the mass emits no radiation. We assume there's a large part of the universe that's not made out of stars, gas, and dust: dark matter.Dark matter is not distributed the same way as stars, gas, and dust. It seems to be distributed in a dark matter halo around the galaxy, with uniform distribution.There are other arguments that lead to similar conclusions in many branches of astronomy, physics, and cosmology. They are consistent with this solution, but not with other explanations.Dark matter seems to be a real thing. We have ideas of what it might be, but we're not sure. There are experiments looking for dark matter, and preliminary results indicate we may be able to see it.One way we might be able to detect it is that there may be annihilation between dark matter particles that will produce detectable gamma rays. There may also be some very rare interactions between dark and normal matter that we can detect, just as we detect neutrinos. Neutrinos are basically a form of dark matter. There may be a number of different types of dark matter.The traditional theory of the history of the milky way is that a quasi-spherical mass of gas cloud spins and collapses into one plane, fragmenting into pieces, which forms first, metal-poor stars. The stars that form first keep that nearly spherical geometry, which is what we see in the globular clusters, wherein the stars are almost as old as the universe. Newer generations of stars have flatter distributions, and today the plane is very thin and we see that's where new stars are forming.Population one and population two stars--not super-critical except we know massive elements are formed in massive stars. When I talked about making things up to iron, in the super-nova explosions, you make elements heavier than iron and blow them out into the interstellar medium. This means that you initially start making stars in the spherical config, they are mostly hydrogen and helium, and now newer stars can have the heavier metals in them. It also gives us reason to suspect that terrestrial planets may be easier to form now than they were in the past since they need heavier elements to form.There are modifications of this traditional theory. We see evidence for a ring of material outside the galaxy, with streamers of stars going off in different directions. It seems to be that even in the milky way we've got this interesting, stable spiral structure that would be disrupted if we encountered a big galaxy. We seem to have cannibalized smaller galaxies. Well, we see cannibalize, but they just merge. Tidal forces stretch them out, and may leave remnants of earlier structures.We have spiral structure. But you can see there's a sort of red light along the spiral arms--hydrogen--which is being ionized by hot young stars. OB associations that never live long enough to move past their nurseries trace out the arm structures. You can trace out different spiral arms, with angles relative to our line of sight as we look into the galaxy.We can trace these spiral arms. They show up so brightly because that's where star formation happens, not between arms. If we took pictures in infrared that trace the locations of old stars, you wouldn't see the spiral arms. It's the blue, young, hot stars that trace spiral structure.So star formation happens in spiral pattern. It's not that the stars clump that way, it's that gas densities clump that way.Kelly: I get it that we start with a cloud... but what incites star formation from that?Mike: It could be a supernova.Kelly: But some kind of inciting event.Mike: These clouds are just marginally stable.Alice: What evidence led them to think that the galaxy has one less arm than previously?Mike: I wouldn't say we have a good idea about how many arms the milky way has. It's hard to trace that structure. This is where the gas and dust clump, where the stars can be formed. Radio waves travel well through gas and dust. So we can map the galaxy in radiation. We have to make some assumptions about the dynamics to separate out where the radiation is coming from. But if we take doppler shifts into account, and we know how the stars move, we can map out in the radio. You'll see it's more complicated than how many arms we have.Bud: But why does the gas and dust accumulate in spiral arms?Mike: There are two things going on. One thing is chains of supernovae that propagate star formation and channel where that happens, and differential rotation will twist that out. Some galaxies have a sort of fuzzy spiral structure with that process. The ones with this really clear spiral structure, there's something called density wave theory that describes how a spiral arm forms and propagates over time, the analogy is if you're driving down the highway, you cruise along at the speed limit, and suddenly there are a bunch of cars ahead, and you have to work through them before you can work back to the speed limit. There's a density wave on the highway. Individual cars pass through but that dense clump of traffic can continue to propagate at a different speed from the other cars, and that can happen with stars. Sometimes you need something to start it, like gravitational influence from another galaxy that gives it a little tug.Radio observations-- 21cm radio observations reveal the distribution of neutral hydrogen throughout the galaxy. Distances to hydrogen clouds are determined through radial-velocity measurements--the doppler effect.Neutral hydrogen is concentrated in the spiral arms.There are galaxies in which the central part rotates in the opposite direction from the outer part. This is probably the result of galactic mergers.Stars pass through spiral arms. Spiral arms trace where star formation happens, just like traffic clumps.Star formation in spiral arms--shock waves from supernovae can trigger star formation. So can ionization fronts initiated by O and B stars. Spiral arms are stationary shock waves, initiating star formation.To expand the analogy, the dense parts on the "highway" are where car crashes happen.So, again, spiral arms are stationery shock waves. Kelly: What does that mean? Mike: Coming up. So a cloud drifts along, encounters a trigger in the spiral arm, and this causes it to compress and triggers star formation. While the gas is in the spiral arm, the most massive, short-lived, luminous stars are formed, and when you look at the galaxy, they're easy to spot. Those die off quickly, before they leave the spiral arm. The rest of the stars formed in that cloud will continue to orbit because they have longer lifetimes. So by the time that cloud moves on, you've lost the hot, young stars. The hot, young stars trace the traffic jam.Ian: and as the hot young stars die there, the produce the kick that stars the next formation.Mike: The newborn stars that move on may stay as an open cluster for a while, as they move on, and then drift apart because they're not held together well.So imagine you've got your cloud orbiting through the galaxy and something triggers star formation. We know there's a shorter orbital path close to the center than as you move out. So this means that if you start with a spherical cloud, it will stretch out so there will also be a natural tendency for regions of star formation distributed over some distance to get stretched out in the same direction as spiral features.Supernovas can trigger star formation in other clouds, so you can start star formation across multiple clouds. Combine that with the stretching, and you get clouds at an angle, that are spiral-like.So we have grand-design spiral galaxies with two spiral arms. This kind of galaxy is thought to have a dominant spiral density wave that gives you a spiral structure with star formation in the arms.Flocculent (or woolly) spirals don't have dominant pairs of spiral arms. This mode is associated with extended chains of star formation being stretched.Ian: Is there a difference in age between these two?Mike: Probably not. It's thought that in a lot of instances to start up this kind of spiral density wave, you need gravitational influences from nearby galaxies, and the woolly spirals may not have had that.Kelly: Galactic plane, how big is it?Mike: Different ways of measuring that. Maybe 1,000 light years on average.Kevin: Bulge is about 5,000 light years.Whirlpool galaxy is a nice example of a grand design spiral galaxy.The galactic center is heavily obscured by gas and dust. One out of a trillion photons coming from the galactic center makes it to earth. It's hard to study the galactic center in the optical. We can look in radio; in the radio, we can see through the gas and dust. We see some interesting structures. Lots of supernova remnants, with star formation in the galactic center, and lots of massive stars that blow up there.And one particular object right at the center, saggitarius A star, seems to have a disk of gas around it, and lots of strange features going on around it. We think it's about 4 million solar masses. We've got gas here in orbits and can use keppler's laws to determine the mass. We can see stars if we work in the infrared spectrum.So, over the years, we've mapped out the stars orbiting a dark object at the center--a supermassive black hole. You can see an image of stars orbiting our black holes on this site.Kelly: So does every galaxy have a black hole at the center?Mike: Yes.Ian: They have elliptical orbits.Mike: Yes, some very elliptical, some almost circular.Dave: Can they have planets?Mike: They would probably be pulled off. Also, the stars in these images would have to be very luminous for us to pick them up, so they're younger, hotter stars. This is not a sort of place you'd want to be growing up.Alice: What's the time scale for the orbits?Mike: Years. For the really bit elliptical one there? I think the last 350 years have just finished one orbit. The stars appear to be in different planes. They're not all in a disk.Continuing on with the galactic center, x-rays: there are a lot of x-rays coming out of the galactic center. We have all these supernova remnants. We have x-ray binaries concentrated in the center.Supermassive black hole in the galactic center is unusually faint in the x-rays compared to those in other galaxies. Our black hole is inactive, though occasionally people do see it flare up in the time scale of 30 seconds, in the infrared.More tomorrow.