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Download Stars - Red, Blue, Old, New pt.4
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Red Stars, Blue Stars, Old Stars, New Stars Session 4 Julie Lutz University of Washington Stellar Evolution “Finales” • From formation on, the evolutionary patterns of stars have depended strongly on mass, and the same goes for the final stages of evolution. • Stars do lose mass as they go from the main sequence through other stages. • Recall that the low mass stars are by far the most common. H-R Diagram, 1 Msun For the Lower Mass Stars--about 1 to 8 Solar Masses • The star gets to the point where it has a carbon core. • Core collapses but not hot enough to initiate carbon to oxygen fusion. • Most of star’s mass collapses to “degenerate matter” and star becomes a white dwarf. • Outer layers escape in a “planetary nebula”. Low Mass Stars: Planetary Nebulae • Nothing to do with planets! • First one discovered by Sir William Herschel who discovered Uranus in 1781 looked greenish like the planet. NGC 6720 Starfish Nebula Round PNe NGC 3132 IC 418 IC 4406 Menzel 3 He 2-104 H-R Diagram, 1 Msun What Happens after the PN? • Star settles down in the white dwarf configuration. • No more thermonuclear reactions. Characteristics of White Dwarfs • Matter in WD is “degenerate”. Atoms packed so tightly that electrons move freely between atomic nuclei. • Densities are about a billion particles per cubic centimeter. • The more massive a white dwarf, the SMALLER it is. A Teaspoon of WD Material Would Weigh as Much as… ….a Large Cruise Ship Stellar Old Age • White dwarf stars-up to 1.4 solar masses • Neutron star-1.4 to about 3 solar masses • Black hole-greater than 3 solar masses White Dwarf Stars Sirius B • Sirius A is brightest star in night sky, a main sequence A-type star (T=10,000K) • Sirius B is about 1 solar mass but has a size about that of the Earth. • T = 25,000K 40 Eridani B • • • • • 0.5 solar masses T= 16,500 K 1/70 solar radius 1.5xradius of Earth Part of a triple star system • Home system of Spock of Star Trek Characteristics of White Dwarfs • • • • • • • Maximum mass 1.4 solar masses Those less than 0.5 solar masses are He More massive carbon and oxygen Densities 10,000,000-1,000,000,000 gm/cc Cooling times 10,000,000,000,000,000 yrs Degenerate matter Less massive = bigger size Structure of a C/O White Dwarf • Degenerate matter until just a few meters of the outer part-that’s normal matter, so the white dwarf does radiate according to its surface temp • 70,000-5000 K Why Are White Dwarfs No More than 1.4 Solar Masses? • The gas law obeyed by degenerate matter is such that the more mass, the smaller in radius. • Becomes a point source at 1.4 solar masses. How about Old Stars with > 1.4 Solar Mass? • Will get further than oxygen in the thermonuclear reactions in core. • When collapse of core comes, electrons will be forced into atomic nuclei where they will combine with protons. This produces neutrons. • Core of star becomes neutron star or a black hole Stars with Masses More that 8x Solar on the Main Sequence • Lose a lot of mass as they evolve off the main sequence. More mass=more mass loss. • But they still can’t squeeze into that 1.44 solar mass limit to become a white dwarf as they approach the end of their nucleosynthesis. • The more massive, the closer they get to an iron core towards the end. Characteristics of Neutron Stars • • • • • Mass range 1.44-3 solar masses Densities 100,000,000,000,000 gm/cc Size-few km Predicted mathematically in 1930s First observed in 1967--accidental discovery with radio telescope What’s Beyond Degenerate Matter? • Suppose the energy conditions are sufficient to force protons and electrons together to form neutrons? • Star would be a ball of neutrons (perhaps with a thin skin of regular matter. • Size: few kilometers diameter. • Neutron stars predicted mathematically in 1930s. Rapidly varying radio sources • Periods of seconds or less • Binary?? No, too short • Pulsation?? No, too hard to move the matter that fast • Rapid rotation? First Pulsar: Period = 1.337 seconds Crab Nebula What was known about the Crab Nebula in 1967 • It is the remnant of a supernova that exploded in 1054 AD (a naked eye object) • The gas/dust in the nebula is expanding with velocities of 1000s of km/sec • Exhibits a special radiation called “synchrotron” • Star at center has no features in spectrum Crab Nebula Neutron Star • Observed pulsations in radio waves 33 times a second. • Pulsations occur at all wavelengths--optical, X-ray, etc. • What could it be? Crab Nebula Pulsar in X-rays Pulsar • Rapidly rotating neutron star • “Beaming” of radiation due to very strong magnetic field • Few kilometers in size so it can rotate very rapidly Pulsars • About 1000 discovered • Periods of milliseconds to minutes • Some found inside supernova remnants, many not • Nobel Prize 1974 Supernovas • Final explosion of star which had about 10 solar masses or more when it was on the main sequence • Rare • Star gets iron core and then core implodes • Outer layers lost--heavy elements created • Core becomes neutron star or black hole The Veil Nebula The Gum Nebula Cas A in X-rays Youngest SNR known in Milky Way--150 years Supernova 1987a • Observed Jan 1987 in the Large Magellanic Cloud • Became first magnitude star • Visible with naked eye for about 2 months For the Most Massive Stars • May arrive at the “iron core” stage with more than 3 solar masses. • Can’t make a neutron star with mass more than 3 solar masses. • What comes next? Black Holes Are Out of Sight! • Most massive stars may form black holes • Gravitation so strong that no radiation can escape • How can we study black holes if we can’t see them? • Binary systems with one black hole and one normal star Black Holes Have Event Horizons Bending of Light, Distortion of Space-Time The Black Hole’s Gravitation Warps Space-Time Black Holes • What used to be the stellar mass resides in the singularity. • Don’t know much about the state of that matter except that it has gravitation. • Use General Relativity to deal. Black Holes as Giant Vaccuum Cleaners • If the sun were to suddenly become a black hole, nothing would happen to the Earth’s orbit. • Mass would have to be within 10 miles of black hole sun in order to be sucked in. Do stellar black holes exist? • SS433--first noticed as X-ray source with periodic variations • Normal star B-type • Companion is too massive to be in the neutron star range Black Hole Candidates in Binary Star Systems Name Companion Cygnus X-1 B supergiant LMC X-3 B main seq A0620-00 K main seq G (V404 Cyg) K main sequence GS2000+25 (QZ Vul) K main sequence GS1124-683 K main sequence GRO J1655-40 F main sequence H1705-250 K main sequence Period 5.6 1.7 7.8 6.5 0.35 0.43 2.4 0.52 Mass BH 6-15 4-11 4-9 >6 5-14 4-6 4-5 >4 Massive Black Holes Are found in the Center of Many Galaxies X-Ray Milky Way Center, 2-3 Million Solar Mass Black Hole With Supernova Remnants Often Don’t Know Stellar Result • Could be a neutron star or a black hole. • Can make a black hole at all masses. • Picture shows remnant of 1006 supernova. Bottom Line • Black holes, neutron stars and white dwarfs are all known to exist • Lots of work remains to be done in all areas of stellar evolution. Broad understanding, but details can confound.