You are about to witness the strength of space knowledge.

* As we saw last week, the Hertzsprung-Russell (HR) diagram plots stars according to surface temperature (x-axis) and luminosity (y-axis). * Temperature determines a star’s color, and its luminosity tells us its size. * Stars are not randomly spaced on the HR diagram; there are distinct groups. * A star’s life and death are determined by its mass.

* Stars that didn’t quite make it. * Jupiter masses (m jup ) * Very faint. * Hot enough to fuse deuterium (10 6 K), but not regular hydrogen (4x10 6 K). * They fuse their deuterium over a few tens of millions of years, then fade away. * They can only do steps 2 and 3 of the PP chain.

* Most common type of star. * <0.4 solar masses (m sun ). * Hot, but not too hot, fuse H into He slowly and steadily, via the PP chain. * Lifespan may be to years. * Every red dwarf ever formed is still going. Like Mista Russell.

* 0.4-~2m sun. * These stars slowly heat up as they burn through their supply of hydrogen. * As fusion starts to slow down (after perhaps years), gravity compresses the star’s core. * This heats the core up enough that it starts to fuse helium.

* The triple-alpha process.

* This hotter core means a more powerful solar wind, so the outer layers of the star are pushed outwards – the star expands. * If you let a gas expand it cools down. This shifts its color towards the red end of the spectrum – the star becomes a red giant. * After ~10 6 years, the helium has been fused into carbon & the outer layers are long gone, blasted into space as a planetary nebula.

* Image: IC418

* These stars may just be hot enough to turn a tiny bit of their carbon into oxygen, but no more than that. * What remains is a white dwarf, about the size of Earth, and composed almost entirely of carbon – a giant diamond in the sky. * They take billions of years to cool down. * Would eventually become a theorized object called a black dwarf – but none exist yet.

* Lifespan ~10 9 years. * Above ~2m sun, stars are large enough that their cores heat up enough to fuse carbon, oxygen, neon, and silicon into heavier and heavier atoms. * All these reactions are exothermic – they release heat – so the core just gets hotter and hotter.

* Image: onion-like shells inside a giant star

* Once they begin to fuse iron-56 (element 26) into nickel-60, the star has a few minutes to live. This reaction is endothermic, so it cools the core off. * Without outward fusion pressure counteracting gravity, the core collapses at about 23% of the speed of light. A star with core radius 300,000 km would collapse completely in 0.004s. Wow.

* Some of the core survives, but is squashed down by unimaginable gravity. * The electrons (which have a negative charge) of neighboring atoms normally repel one another (electron degeneracy pressure) * Gravity squashes them down so that the core becomes a ball of neutrons ~20km across, releasing countless neutrinos that blast the star apart as a supernova.

* The insane temperatures involved in a supernova mean that the nuclei of the metals within the star are bombarded with neutrons, which stick to them in a process called rapid neutron capture (r-process). * Many of these neutrons spontaneously become protons (β - decay), generating the heaviest elements. * Gold and platinum are rare because they only form in supernovae. Yeah. You love this class.

* The biggest stars do all the things that group 4 did, but the core has so much mass that even neutrons are crushed when such stars collapse. (Neutron degeneracy pressure can’t save it). * This is a black hole. * Escape velocity > speed of light. * We don’t know exactly how large a star has to be to leave behind a black hole. The smallest known black hole is ~5.5m sun, but the original star would have been bigger.

* With so much drama in the galaxy * It’s kinda hard bein’ a red G-I-A-N-T * But I somehow, some way, * Keep fusing helium to carbon like every single day.