Transcript Stars - Red, Blue, Old, New pt.4

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.