Transcript Stars
Stellar Evolution – the Life and Death of a Star… Here’s the Story We’ll Unfold for you… • Formation in giant molecular clouds • Fusion halts collapse, stabilizes star • Low, medium, and high mass star evolution • Stellar corpses • Origin of the chemical elements – stars do it! Star Formation • Stars born in giant clouds of gas and dust • Gravity is what pulls the matter together; requires HIGH density and LOW temperature! • These are also the ideal conditions for the formation of molecules. Giant Molecular Clouds… this is where stars form • Usually gets an assist from a shock wave; from supernovae or from spiral density waves Snake darknebula Dark globules The Formation Sequence… • Shock wave piles up gas/dust to high density • Gravity pulls it together • Center is opaque, trapping the heat and light which can escape only slowly • Raising temperature in the core until… • Hydrogen fusion begins at temperature of 10 million degrees Kelvin • Fusion creates light, who’s pressure fights against gravity, stabilizing the star against further collapse • A star is born! Keyhole nebula Orion disks horsehead OrionNeb+Horsehead Eagle nebula wideangle Shock front, snake Eagle sprite Eagle columns Stars: always born in star clusters! • Low temperature requires shielding from the radiation of other stars; requires dust which requires a lot of mass, since dust is a relatively rare component of interstellar clouds • Star clusters forming in today’s environment are called “open star clusters”, dozens to hundreds of stars foxfur Cone nebula Dark nebula SFR in LMC Bowshock LL Ori cluster and gas Cluster, ha shocks Pelican dust ha Witche’s head rosette New glob and shock front Cluster, gas; tarantula nebula Plieades closeup plieades New cluster, bit of dust left Bright young cluster, little gas/dust m35 m11 m38 m39 Size vs mass for planets, bd’s,stars Stellar Evolution: How stars live and die • Visualize stellar evolution as a path on the H-R Diagram • Remember, it’s a plot of Surface Temperature vs. Luminosity • Where do you suppose stars first appear on the diagram? Ponder…… HR pre main sequence sun Another Quick Overview First… • Stars burn through their hydrogen, evolve off Main Sequence to become Red Giants, then die in various ways • High mass stars evolve fast,… • Low mass stars evolve slowly HR main sequence turnoff The H-R Diagram of a Star Cluster •All Stars born at the same time, only differ in their mass •Stars age at different rates, depending on their mass. More mass = faster evolution •Stellar Evolution web simulator HR of star clusters vs age M55 HR diagram Evolution of Low Mass Stars • Note! I distinguish between low and medium mass stars – the book calls all of them “low mass”. • Begin with H burning in core • When H runs out, core collapses under gravity, releasing grav potential energy, raising star’s luminosity • Core collapse stops when “electron degeneracy” sets in. Electrons are “elbow to elbow” (in a quantum mechanical sense) Layers; main seq vs. giant Medium Mass Star Evolution • H burning until all H is He, then core collapse, releasing gravitational potential energy, raising luminosity and expanding the star ~ x100 times • Core density and temperature rises until 100 million K. Then….. • Well, you tell me – what are the options for further fusion? We have H and He floating around in the core… Helium burning layer Sun to red giant cartoon HR tracks to red giant Sun and red giant side by side Sun’s L vs time We’re all doomed HR with instability and variables The End of the Line for Medium Mass Stars like the Sun… • Added luminosity is so strong, it lifts the red giant’s low density outer envelope completely off the star. • As it expands, its opacity drops and we see to a deeper and deeper and hotter and hotter depth, so the star moves left on the HR diagram • Until… we see the electron degenerate core; the new white dwarf created at the center • This core can now cool, as it can’t collapse further and it is exposed to the cold of outer space. • Thus, it follows the cooling curve of a white dwarf; down and to the right on the HR diagram • So, what we see is a hot stellar corpse surrounded by an expanding and thinning cloud of flourescent gas = a Planetary Nebula HR track to PN stage White dwarf->pN shell w velocity PN misc young Cateye nebula Dumbell Dumbell hst upclose details Egg burst nebula Helix Nebula Ic 4406 P Little Ghost PN NGC 2346 pn Pn abell 39 NGC 2440 pn NGC 6751 PN (blue eye) Ring Nebula Egg nebula (check; pulsar???) PN flying badminton PN misc Spirograph PN Evolution of High Mass Stars – Short and Violent Lives • Have enough mass to heat & compress core to fuse all the way up to iron • Iron – the most stable, most tightly bound of all nuclei • All fusion or fission involving iron will subtract heat from the star’s core, not add to it. “Danger! Danger Will Robinson!” Layers of a pre SN II Eta Carinae Ant nebula Wolf-rayet star The Death of High Mass Stars… • When iron core exceeds about 1.4 solar masses, the temperature becomes high enough to cause nuclear reactions for iron • Nuclear burning causes further core collapse, which raises the density and accelerates the nuclear reactions. • In 0.2 seconds (!) the core collapses, fusing iron into lighter and also heavier elements • This is the ONLY place in the universe that elements heavier than iron are made! • Neutrinos produced, so vast in number that they blow apart the star… Supernova! (SN II) • 99% of energy release, the gravitational potential of the star, goes into neutrinos • 1% goes into the explosion • 0.01% goes into visible light. Still, the light is bright enough to equal the entire galaxy of 100 thousand million stars (Gah!) • SN II are the only place in nature where the elements heavier than iron are produced 3 Possible Ends of a Stellar Corpse! • If mass < 1.4 Msun = White Dwarf • If 1.4Msun < M < 3 Msun = Neutron star • If M > 3 Msun = Black Hole! Let’s look at some ancient supernova remnants… Cass A Cass A colored Cass A upclose Kepler’s snr LMC SNR Another LMC SNR SNR H-alpha Pencil nebula snr Veil Nebula (entire) Cygnus loop SNR Veil closeup1 Crab HST Grav redshift Neutron star layers Crab center w jet Cerenkov radiation diagram Crab center w jet sequence Crab HST center upclos Let’s look at another Pulsar. This one is in the globular star cluster 47 Tucanae… 47 Tuc – ground based 47 Tuc HST Millisecond pulsar How to Detect Neutrinos? • Like, neutrinos from supernova explosions • …or neutrinos from the sun (the strongest source because it’s so close) • - once in a great while a neutrino will hit an electron and deposit its energy, accelerating the electron to almost the speed of light. This rapid acceleration causes the electron to give of photons of light = synchrotron radiation Sudbury neutrino detector The Cosmic Abundances of the Chemical Elements • Due to the nuclear fusion in the cores of stars • And… to supernova explosions • Remember – Supernova explosions are the ONLY place in the universe where heavy elements are created! • All the elements beyond Iron in the periodic table (gold, silver, uranium, copper…) are created ONLY in the core collapse of a supernova explosion. Abundances of all elements Abundances of all elements graph Cosmic Rays… • The blast of a supernova explosion sends out elementary particles at near the speed of light. These get further accelerated by magnetic fields in the galaxy. • When they impact earth, they smash into our atmosphere and create cosmic ray air showers… • Cosmic rays are a significant source of genetic mutations. Cancer odds are higher the higher the elevation you live, in part because of more cosmic ray exposure! Cosmic ray airshower What happens if the stars are in a close binary system? • This happens a lot! Nearly half the stars in our Galaxy are members of binary star systems • Roche lobe defines gravitational “backyard” for each star Mass transfer binary (art) Mass transfer accretion disk X-ray binary art Nova sequence But with all this mass falling onto the white dwarf, there’s another possibility… • … something more ominous… more terrifying… more…. Scary! • What could that BE?! Carbon Bomb Supernova (SN type I) • If the white dwarf is close to the 1.4 solar mass upper limit that electron degeneracy can support… • The added mass could push it past the limit before it gets hot enough to flash off • Then, star collapses under the weight and because it is electron degenerate, energy created will not expand the star and shut off the fusion. • So, entire star (carbon, mostly) undergoes fusion at once. What a star normally takes billions of years to burn, this star burns all at once. BIG explosion! SN Ia sequence Supernova! (SN I) • These are even brighter than SN II’s from massive stars. • Very useful – they’re all the ~same – 1.4 solar mass white dwarfs undergoing nuclear fusion. This turns out to mean they are… • GREAT “standard candles” – objects of known luminosity, on which we can then use simple math to determine their distance. • So, any SN I and its host galaxy, we can find it’s distance, even out to the edge of the observable universe, since they are so bright. • Huge amount of observational effort today is going into discovering and charting the light curve of SN I’s throughout the universe! SN Ia light curves