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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.
NGC6720
StarfishNebula
RoundPNe
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 Period Mass BH
Cygnus X-1 B supergiant 5.6 6-15LMC X-3 B main seq 1.7 4-11A0620-00 K main seq 7.8 4-9G (V404 Cyg) K main sequence 6.5 > 6GS2000+25 (QZ Vul) K main sequence 0.35 5-14GS1124-683 K main sequence 0.43 4-6GRO J1655-40 F main sequence 2.4 4-5H1705-250 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.