Type Ia Supernovae (note, supernova is abbrevieated "SN", and supernovae (the plural) is abbreviated SNe) Lecture Oct 28 Stars (Astro 203) Rigby was lecturing THE MYSTERY Say you're an astronomer 50 years ago, struggling to understand what supernovae are. You don't know what kinds of stars make supernovae. All you know is, they're really bright. How do you set about making progress? Well, first look at the supernovae. Are they all the same, or are there several types? Looking at the light-curves (how bright a SN is as time pases) and the spectra, you notice trends. THE TRENDS: A peculiar type of SNe has NO HYDROGEN in the spectra. Called "Type Ia" supernova Instead of H, it has lines of heavy elements (Silicon, Oxygen, Magnesium, Sulphur, Calcium) All SNe in elliptical galaxies are Type Ia! By contrast, in spiral galaxies only ~15% of SNe are Type Ia. Spirals are a mix of old & new stars. some pictures: http://antwrp.gsfc.nasa.gov/apod/elliptical_galaxies.html some pictures: http://antwrp.gsfc.nasa.gov/apod/spiral_galaxies.html Most supernovae span a range of ~40 in peak luminosity. But these weird Type Ia SNe all have about the same luminosity! (10^10 Lsol) So what's going on? Why no hydrogen? Why all the same brightness? Why are Type Ias the only SNe in elliptical galaxies? SOLVING THE MYSTERY: Using Clue 1: They're the only kind of SNe that happens in Elliptical Galaxies. What do we know about the ages of stars in elliptical galaxies? Elliptical gals are red. From their colors and their spectra, we know that elliptical galaxies are made up of old stars. There are NO young, massive stars in elliptical galaxies. So if Type Ias happen in Elliptical Galaxies, they're happening on OLD stars. NOT massive stars. So type Ia SNe are NOT CORE COLLAPSE SNe! NOT INVOLVING MASSIVE STARS. Must be old stars blowing up. Using Clue #2: No Hydrogen in their spectra What types of stars have no hydrogen? A red giant has run out of H in the core, but there's still plenty in the envelope. so not a red giant. What about WHITE DWARFS? They don't have any hydrogen. A suspect! Using Clue #3: All Type Ia supernova are about the same brightness. If white dwarfs blew up near the Chandra mass, they'd all be about the same mass blowing up. So there would be a similar amount of energy in all the explosions --> similar brightnesses. THESE PIECES OF EVIDENCE ARE WHY we think Type Ia supernovae are WHITE DWARFS blowing up. THE EXPLOSION Remember the Chandrasekhar mass -- it is the maximum mass that electron degeneracy pressure can support -- so it's the maximum mass a white dwarf can have -- it's about 1.4 solar masses -- a white dwarf can't *really* have the Chandrasekhar mass. At the Chandra mass, the radius goes to zero, so the density is infinite! not possible How this relates to the explosion -- Imagine you siphon mass onto a white dwarf. As it gets ~1% less massive than the Chandra mass, the density in the center gets enormous. -- Spoon just a *little* more mass onto the white dwarf, central density goes up lots more. -- regardless of Temperature, the DENSITY gets high enough to fuse Carbon to heavier elements -- Carbon fusion creates a lot of heat --> raises temperature in the center of the star -- but degeneracy pressure is based on density, not temperature degeneracy pressure ONLY cares about how close the particles are to each other. -- the degeneracy pressure is NOT affected by the increased T from fusion -- but fusion happens easier w/ high T (it's easier to smash carbons together when the T's higher so the particles are moving faster) so fusion happens faster --> center gets hotter so let's summarize: add mass --> raise central density --> fuse carbon --> raise T --> fuse more carbon --> raise T... -- THIS IS A RUNAWAY REACTION! In the sun, this wouldn't have happened: -- if the sun got a little hotter in the center, the H could fuse more easily, so T increases -- higher T, higher P, core expands, which lowers the P -- so the sun regulates its own temperature (like a thermostat), so it's stable. -- by contrast, in the white dwarf, once C fusion starts, T gets hotter and hotter, with no expansion to lower the pressure. so thermal runaway results. RESULTS OF THIS THERMAL RUNAWAY -- half the Carbon and Oxygen in the whole white dwarf fuse all the way to Iron in 1 second! -- a wave of burning passes through the star. -- astronomers don't know yet how fast this wave moves through the star. it may be fast=supersonic=like a bomb, or slow=sub-sonic=like a forest fire -- for our purposes, it's pretty darn fast, though. Wave only has to travel across a star the size of the earth, after all. so quick. -- neutrinos are NOT made in the explosion, NOT important in a Type Ia supernova. -- FUSION of C and O to Fe is the PRIMARY ENERGY SOURCE of a type Ia supernova. (above 2 points are two important differences between type Ias and Core Collapse Sne) -- because fusion of C and O is the erergy source, a Type Ia is called "a thermonuclear supernova". -- NO COMPACT REMNANT IS LEFT! THE WHOLE STAR BURNS/BLOWS UP aMOUNT OF ENERGY MADE IN THE SUPERNOVA -- A core collapse SN has 100 times more energy than a Type Ia SN. -- but remember, 99% of the core-collapse SNe energy was lost as neutrinos. -- so the amount of energy in the explosion (into light, into energy-of-motion) is similar for both types of supernovae; about 10^51 ergs -- but a WD is a much smaller star than a core-collapse SNe star. Same amount of energy-of-motion, smaller mass --> higher velocities -- therefore, Type Ia has velocities of 10,000 to 20,000 km/s (1/15 to 1/30 the speed of light) MAKING LIGHT -- ALL the light you see from a Type Ia SN comes from radioactive decay 56Nickel --> 56Cobalt --> 56Iron -- How does the radioactive decay happen? 56Ni (28 p, 28 n) + e- --> 56Co (27 p, 29 n) + neutrino + energy (6.1 day halflife) 56Co (27 p, 29 n) + e- --> 56Fe + neutrino + energy + e+ (77 day halflife) (what happens in each of these 2 reactions is that, a proton in the nucleus grabs an orbiting electron, the p + e- turn into a neutron, and now there is one extra neutron and one fewer proton in the nucleus. this is more stable.) -- this radioactive decay, which males ALL they light in a Type Ia SN, makes the light in core-collapse SNe starting a few months after the explosion -- A Type Ia SN makes ~0.7 solar masses of Iron. So half the WD got converted to iron. -- A Type Ia SN makes 5-10 times more iron, total, than a core collapse SN -- so even though a type Ia SN has much less total mass to start with, it makes more iron than a core collapse supernova -- Type Ia makes so much iron because the densities are so much higher than in a core collapse SN PROGENITORS: -- so we know the energy source of a Type Ia SN (== fusion of Carbon and Oxygen to Iron) -- we're pretty darn sure it involves the blowing up of a white dwarf -- after all, if a WD gets very close to Chandra mass, it will naturally blow up -- BUT HOW DO YOU ADD MASS TO A WD SO IT WILL BLOW UP?????????????? -- This is an embarrassingly hard question. -- IN ALL THE TYPE IA SUPERNOVAES WE'VE SEEN, WE'VE NEVER OBSERVED THE PROGENITOR STAR BEFORE IT BLEW UP. ALL THESE MODELS INVOLVE BINARY SYSTEMS. A WHITE DWARF, HANGING AROUND BY IT'S LONESOME (LIKE THE SUN WILL EVENTUALLY DO), CAN'T POSSIBLY GAIN ENOUGH MATTER TO BLOW UP. So we're pretty darn sure that type Ias are White dwarfs in binary systems that blow up. How do we *think* we might make a Type Ia supernova? some candidates: 1) igniton of He on the surface of the WD He would be matter accreted from a companion. This one's described in Wheeler. The outside starts burning, then the burning travels through the star toward the inside. this is probably not how Type Ias happen. This model predicts you'd see lots of He in the spectrum early-on in the explosion, and you don't. So unlikely model. 2) Merging WDs? if 2 big white dwarfs are in a binary system, and they merge together, they could make a Type Ia SN. This probably happens, but it's probably too rare to account for all the Type Ias we see. 3) Companion star dumping matter onto a white dwarf We know this happens (remember Cataclysmic Variables, from the binary star section?) But actually making the star blow up is a Goldilocks problem: you need to feed the WD "just right". If you feed WD too slowly, build up H, make a NOVA explosion. You LOSE mass rather than gain. If you feed WD too fast, get common envelope evoln, WD spirals down into the red giant, and the explosion will show hydrogen in the spectrum. That can't happen Turns out the feeding, astronomers think, must be within a factor of 2 of the perfect value. This is hard to do! the roche lobe changes size with time as the stars get closer together, the red giant evolves, things are changing, but the feeding rate needs to stay the same. AS YOU CAN TELL, WE (ASTRONOMERS) HAVEN'T FIGURED OUT HOW TO MAKE ENOUGH WHITE DWARFS BLOW UP! BUT NATURE HAS!!!! WE KNOW THAT NATURE KNOWS HOW TO MAKE LOTS OF WHITE DWARFS MASSIVE ENOUGH TO BLOW UP. WE'RE JUST TRYING TO FIGURE OUT HOW NATURE DOES IT.... WHY DO WE CARE SO MUCH HOW WHITE DWARFS GET ENOUGH MASS TO BLOW UP? -- well, first of all, we're curious about how the universe works -- and it's embarrassing not to know how some of the biggest explosions in the Universe happen But there's another reason, concerning the fate of the universe. We'll talk about it in more detail in Chapter 11, but here's the basics: -- remember how all WDs are about the same brightness? -- that allows us to use them as "standard candles" == objects whose brightness we know, so when we see them far away, we can measure their apparent brightness, and find out how far away they are. -- Distant Type Ia SNe are FAINTER than we expect. This means they're further away than expected at given times. How could they be further away? We're accelerating away from them. -- Not just expanding (we've known for 50 years that the universe is expanding.) The new result is that the universe is expanding ever faster and faster. The rate of expansion is increasing. You would think that the universe's expansion would be slowing down, as all the galaxies in the universe attract one another and resist moving further away. But we don't see the universe slowing down. It's speeding up. "The accelerating universe." -- According to the Type Ia SNe, the universe is ripping itself appart. -- This is a CRAZY result. Yet other approaches (not based on supernove) are getting the same result. -- Are all these experiments messed up somehow? Is there something wrong with the Type Ias? If Type Ias in the Milky Way galaxy are different from Type Ia SNe halfway across the Universe, that could explain away the SNe results. Therefore, we REALLY want to know what the progenitor star systems are for Type Ias SNe, so we can understand how they might be different halfway across the universe compared to nearby. Confused about this? We'll explain it better in Chapter 11, and go into more detail. For now, remember that the Type Ias tell us that the universe is flying apart ever faster and faster, so we want to know how white dwarfs get enough mass to explode as Type Ia supernova, so we can know whehter to believe this result about hte accelerating universe.