Review on Stellar Scales		The picture above illustrates the relative sizes of main sequence stars like the Sun and Spica (spectral class B1 V). You can find Spica, about 10 times the size of the Sun, by means of the handle of the Big Dipper. Follow the arc of the handle to Arcturus (follow the Arc to Arcturus!), a red giant (K2 III, similar to the red giant shown above.) Then after you reach Arcturus, "Speed on to Spica!" A typical Red Giant star, such as the Sun will turn into billions of years from now, is about 100 times larger than the Sun, or about 10 times larger (in radius) than the largest Main Sequence stars. The largest Red Supergiants, such as Betelgeuse, reach about 1,000 times the radius of our Sun, or about the size of Jupiter's orbit. At different stages in its life, a massive star may be a red or blue supergiant. Here's an artist's comparison of a blue supergiant with the Sun and Jupiter: What about the smallest stars? The image above shows the other end: the smallest stars, compared with the Sun. The smallest Main Sequence stars have about 10% the radius of the Sun. These stars are called Red Dwarfs--not every red star is a Red Giant! Low mass Main Sequence stars don't have as much weight pushing them down in the center so don't have fusion at such high temperatures as the Sun.
White Dwarfs		What is a white dwarf star? Our Sun will one day leave behind a white dwarf as a corpse. It's degenerate, the pressure is from electrons resisting being squeezed beyond what's allowed by the Pauli Exclusion Principle. So a white dwarf star with the mass of our Sun may have the size of the Earth--very compressed! A teaspoon of matter of a white dwarf would have as much mass as a truck! We observe actual white dwarf stars--for example, the brightest star in the night sky, Sirius, is actually a binary, with a dimmer companion star that's a white dwarf.
When Stars Blow Up		Sometimes stars blow up. This can be good. Ok, we as astronomers like stars. Stars blowing up means fewer to study! But we can study them blowing up! And there are some reasons why stars blowing up are good: When a massive star blows up, it releases the heavy elements its made by fusion back into space--those elements can then go into making up beings like ourselves (calcium in our bones, iron in our blood cells, zinc, copper, etc.) When stars fuse, it takes energy to make elements heavier than Iron. So how are those elements made at all? Many are made in the explosion that blows up a star. Star explosions can lead to the birth of other stars. The spiral shapes you see in many galaxies (like our Milky Way) are not because there are many more stars in the spirals--no, there are just more bright massive stars in the spiral arms. The spiral shapes are density waves created by bright stars going supernova. The shock waves from a star's explosion compress gas ahead of the spiral and help to form new stars. Behind the spiral the gas is already used up--the spiral arm is like a snowplow. There are several kinds of exploding stars: Novas--meaning "new stars." We'll talk about these next class, when we talk about the lives and deaths of binary stars. A nova can reach 105 LSun, or 100,000 times the energy output (luminosity) as our Sun. Supernovas--at first people didn't realize there was a difference between supernovas and novas. But supernovas can be about a million times brighter! They rise to 108 to 109 solar luminosities. Sometimes people would see supernovas in these "spiral nebulas" and thought they were just like novas. But now we know these "nebulas" are actually distant galaxies like our own. Because these "novas" are really much further away, and still appear bright, they must in reality be much brighter. There are two main kinds of supernova explosions: White dwarf supernovas--we'll talk about these next class Massive star supernovas--a massive star (greater than 8 times the mass of our Sun or so) fuses material in its core all the way up to Iron. But then fusion doesn't give back energy--it takes in energy! Remember, this is an eternal struggle between gravity and pressure. The star continually loses energy because it's shining, so it needs fusion to keep it hot--the heat pressure then holds off the gravity crushing everything to the center. But when the massive star's fusion makes iron in the core, all hope for generating energy is gone. The core of the star collapses, and not even the pressure of degenerate electrons can stabilize the core. What happens is that the electrons (negative charge) and protons (positive charge) merge to form neutral neutrons, and the core becomes a neutron star. In the process, neutrinos are formed too. Why does the star explode? We think it is for two reasons. One, the core solidifies at a size of about 10 km, when the neutrons can't be squeezed any tighter (degenerate neutron pressure). So the rest of the star falls in, pulled by gravity, but has to stop when it reaches the solid core (very solid: a trillion kg per cubic centimeter, more mass in a cubic centimeter than all the human beings who have ever lived!) So the rest of the star in fact bounces away from the core, and a shock wave is produced. The second main cause of the supernova explosion, we believe, is that the huge number of neutrinos spewing out can transfer some of their energy to the matter. In fact, 99.9% of the energy of the explosion comes out as neutrinos, not as light! Neutrinos rarely interact with matter, but in this case there is simply a huge number of neutrinos and a huge amount of matter in a small volume. Movies of computer simulations of supernova explosions show that the simulations don't always work! It's a tricky business, and astronomers may be ignorant of some important ways that neutrinos interact with matter. It appears that a spherically symmetric explosion doesn't work--you need convection to move around the gas so that there's a lot of it far away from the neutron star so that the shock wave gets further away. The shock wave creates greater density, which means easier absorption of neutrinos. Being further away means it is easier to escape from the star--an explosion! Gamma ray bursts--these are the most mysterious. We don't even know what kind of stars cause them. Their luminosity is about 1018 times that of the Sun.
What's left after a star goes kablooie		Here are some images of "supernova remnants", the gas that's left over after a massive star blows itself apart: Above is an image of the Crab Nebula, the result of a supernova explosion in the year 1054 that was seen in the sky by Chinese astronomers and Native Americans. In the late 1960s, a graduate student named Joclyn Bell discovered that radio waves from the center of the Crab Nebula were pulsing on and off with a regular period. The source of the radio waves was labelled LGM-1 for "Little Green Men 1", because it was thought that only an extraterrestrial intelligence could create such an exact "clock" in space, as a beacon to other intelligences. However, the Cornell astronomer Thomas Gold showed that pulsars are actually the neutron stars first theorized by maverick Fritz Zwicky. In 1987, a supernova went off in a nearby galaxy, the Large Magellanic Cloud. This is in fact the galaxy closest to our own, only about 160,000 light years away. White dwarf supernovas go off about once every 200 years and massive star supernovas go off about once every 50 years within our own galaxy, but we don't always see them because the gas and dust of the rest of the galaxy can get in the way. Kepler and Tycho both were lucky enough to see supernovas in our own galaxy--not that they knew what they saw! The supernova in 1987 was the first supernova to go off during the era of modern telescopes--so it and its remnant have been observed in ultraviolet, X-rays, radio, infrared, you name it! Even neutrinos from the supernova were observed (all 19 of them! it's hard to find neutrinos because they pass though matter so easily!), confirming our ideas of what happens in the core at the moment of explosion. Above is an image of the region around the supernova--showing 3 still mysterious rings left by the star before it went kablooie. When it was a red giant, the star probably puffed out gas, and the bright ring around its equator probably kept the wind from expanding very fast in that direction.
Gamma Ray Bursts		Gamma ray bursts are the most powerful explosions known in the cosmos. They were discovered in the 1960s by secret military detectors in space that were designed to make sure the Soviet Union was complying with nuclear test bans. The detectors found evidence of explosions all right, but they were coming from space! Later the news was made available to astronomers--it was now their problem! Above is a plot of one of the brightest gamma ray bursts ever seen--it shows that the peak brightness is reached very quickly and then becomes dimmer in a matter of seconds. Click for more information. Gamma ray bursts happen once and then are not seen from the same place again. At first astronomers thought they were mostly from within our own galaxy. (Some even tossed around the idea that they were from comets at the edge of the solar system!) Remember that gamma rays are very short wavelength (high frequency, or energy) photons; they are just like the light we see but at shorter wavelength. A great debate (that's a good site) on the location of gamma ray bursts broke out among astronomers. The debate was ended when a satelite named BATSE monitored the whole sky for several years and found gamma ray bursts were coming equally from all directions: I'm not sure what the colors represent If gamma ray bursts were in the halo of our galaxy then we'd expect a similar excess in the halo of the Andromeda galaxy, for example. There was absolutely no enhancement of gamma ray bursts from directions in our own galaxy. There's really only one distribution in the sky that's so perfectly equal in all directions--the galaxies of the Universe itself. Then later an X-ray telescope (called Bepposax) pinpointed the locations of some gamma ray bursts. They are coming from galaxies with large red shifts--it is true--they are very far away. So gamma ray bursts have to be so incredibly energetic because they are so far away. They might not be quite as energetic as they appear, however. It's possible that their radiation is "beamed" in our direction. There might be other gamma ray bursts that just happen not to point their radiation in our direction. Here is an artist's interpretation of a gamma ray burst: Artwork by Lynette Cook. What causes them? We don't know. It could be colliding neutron stars or black holes. If anything, there are too many theories possible, given what we see. Some gamma ray bursts may be associated with supernova explosions. There's evidence that the gamma ray burst we see causes exploding fireball expanding at nearly the speed of light. At first the radio waves from the gamma ray burst "twinkle" because the expanding fireball makes a very small angle on the sky. Then when the fireball gets bigger, it stops twinkling so much. (For the same reason, planets don't twinkle in the night sky, and stars do.) The real cause of gamma ray bursts??