Star clusters also teach us much about stellar evolution, because as in binary and multiple star systems, all the stars are nearly the same age (since they formed together), and nearly all are at the same distance, so a difference in apparent magnitude corresponds to a difference in luminosity: we need not know the distance to the cluster. We can, however, see how stars of different luminosities (and masses) change, in a given amount of time: the age of the cluster. This is shown by the cluster's H-R diagram. There are two general types of star clusters: Open Clusters (e.g., the Pleiades = M45; the Beehive = M44; M35 in Gemini, M36, 37, 38 in Auriga.) These are young, with hundreds to thousands of stars. Globular Clusters (e.g. M13 in Hercules, observable in summer-fall.) These are very old, most having formed just after the Big Bang; They typically have 105 to millions of stars. Stellar evolution: Low-mass stars, like the Sun, spend about 10 billion years on the main sequence. They run out of nuclear fuel, and swell up, becoming red giant stars, e.g., Aldebaran or Arcturus. They then literally come apart, gently, throwing off planetary nebulae. Their cores, what were once their nuclear furnaces, are left behind, And these cool down, turning into white dwarf stars. Massive stars, with masses over 8-11 times that of the Sun (8-11 solar masses), die more spectacularly. They run out of fuel, become red giants, then become red supergiants, e.g. Betelgeuse. They burn heavier and heavier elements in nuclear fusion reactions: Hydrogen -> Helium Helium -> Carbon Carbon -> Oxygen, Magnesium, and Neon OMgNe -> Silicon Silicon -> Iron Nuclear fusion reactions that convert iron to heavier elements, though, are endothermic: they require more energy than they give off. This cools the massive star's core, which collapses and explodes, in a Type II supernova. (A Type Ia supernova is when a white dwarf is pushed over the maximum mass a white dwarf can have, the Chandrasekhar limit, from mass spilled on it by a binary companion; a Type Ib supernova is when a really massive blue supergiant explodes suddenly.) Whatever the case, when a massive star explodes, it seeds the Galaxy with heavy elements—the stuff you're made of. If you don't believe it, try finding a hotter place! Star formation is among the major unsolved problems of astrophysics. We understand few of the details (e.g. why stars have the masses they do, why so many are binaries), but at least we can see them doing it, so we know that at least the basic picture is OK: Essentially, gravity pulls gas together into a star. Since the cloud is rotating, angular momentum conservation flattens it into a disk, just like the Solar Nebula. Proplyds = protoplanetary disks: over 150 discovered in the Orion Nebula with Hubble Space Telescope. The prospects for improving our understanding of star formation are good, because of improving technology for observing radio and infrared radiation, which has wavelengths longer than the dust grains that obscure our view, so it goes right through the dust. The dust therefore causes reddening of starlight: since only the red light gets through, stars obscured by dust often look much redder than they really are. This is also why sunsets are red: because of dust in Earth's atmosphere, which we look through a lot of, when the Sun is low in the sky. Nebulae (clouds in interstellar space) come in several distinctive physical types: 1) Dark clouds, which are full of dust. This dust comes in a wide variety of types: silicates (rocks), carbon (graphite, soot, diamonds), and many types of molecules, especially organic molecules, including the "brown organic tarry gunk" (polycyclic aromatic hydrocarbons) found all over the Outer Solar System. Dark clouds are very dense, and so are often where stars are forming. This makes observing star formation difficult: the young stars are surrounded by dense dust cocoons. 2) Bright-line, emission nebulae: these are the glowing red clouds, lit up by starlight, just like the fluorescent lights above you. Most planetary nebulae (thrown off when stars like the Sun die) are emission nebulae, lit up by their central stars (which are very hot, because they were once a star's nuclear furnace). Many star-forming regions (e.g. the Orion Nebula) are also emission nebulae, because their, hot, bright, young, O and B stars are also very hot, and light them up. 3) Reflection nebulae: these are the blue clouds. They shine by reflected starlight (like planets), but they become blue because they scatter light. This is the same reason the Earth's sky is blue (or oceans, or glaciers): blue light scatters more than any other color, because air molecules (or interstellar dust grains, or water or ice molecules ) are about the same size as the wavelength of blue light (see CP, p. 279). We can therefore summarize stellar evolution surprisingly easily: 1) Stars form by gravitational collapse. 2) When they start making energy by nuclear fusion in their cores, they settle onto the main sequence. 3) Low-mass stars like the Sun evolve slowly, with enough hydrogen in their cores to fuel them for billions of years. When this runs out, they become red giants, which come apart into planetary nebulae; their cores turn into white dwarfs. In other words: Low-mass main sequence -> red giant -> planetary nebula + white dwarf. 4) Massive stars, more than 8-11 times more massive than the Sun (like Rigel), evolve much faster, shining much hotter and with more luminosity. After "only" a few million years, they become red supergiants, which explode in supernovae, leaving supernova remnants. Their cores become superdense neutron stars, spheres of essentially nuclear matter. In other words: Massive star -> red supergiant -> supernova remnant + neutron star. 5) The rarest, most massive stars (with 50-120 solar masses) evolve even faster, exploding suddenly when they're still blue supergiants (as Supernova 1987A did). Do these leave behind black holes?