Presentation on theme: "Red Stars, Blue Stars, Old Stars, New Stars Session 4 Julie Lutz University of Washington."— Presentation transcript:
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.
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”.
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.
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.
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?
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
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 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 N ame Companion Period Mass BH C ygnus X-1 B supergiant 5.6 6 -15 L MC X-3 B main seq 1.7 4 -11 A 0620-00 K main seq 7.8 4 -9 G (V404 Cyg) K main sequence 6.5 > 6 G S2000+25 (QZ Vul) K main sequence 0.35 5 -14 G S1124-683 K main sequence 0.43 4 -6 G RO J1655-40 F main sequence 2.4 4 -5 H 1705-250 K main sequence 0.52 > 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.