Chapter 18 The Bizarre Stellar Graveyard. What is a white dwarf?. White Dwarfs. White dwarfs are the remaining cores of dead stars Electron degeneracy pressure supports them against gravity. White dwarfs cool off and grow dimmer with time. Size of a White Dwarf.
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It can’t. It’s not part of a binary system.
What happens to a white dwarf when it accretes enough matter to reach the 1.4 MSun limit?
A. It explodes
B. It collapses into a neutron star
C. It gradually begins fusing carbon in its core
One way to tell supernova types apart is with a light curve showing how luminosity changes with time
In the multiple shell burning stage of a high mass star:
As each element is burned to depletion at the center, the core contracts, heats up, and starts to fuse the ash of the previous burning stage. A new inner core forms, contracts again, heats again, and so on. Through each period of stability and instability, the star’s central temperature increases, the nuclear reactions speed up, and the newly released energy supports the star for ever-shorter periods of time.
For example, in round numbers, a star 20 times more massive than the Sun burns hydrogen for 10 million years, helium for 1 million years, carbon for 1000 years, oxygen for 1 year, and silicon for a week. Its iron core grows for less than a day.
Only a few have been seen with the naked eye in recorded history.
Arab astronomers saw one in AD 1006.
The Chinese saw one in AD 1054.
European astronomers observed two—one in AD 1572 (Tycho’s supernova) and one in AD 1604 (Kepler’s supernova).
In addition, the “guest stars” of AD 185, 386, 393, and 1181 may have been supernovae.Observations of Supernovae
The gas, originally expelled at 10,000 to 20,000 km/s, may carry away a fifth of the mass of the exploding star.
They last a few tens of thousands of years before they mix with the interstellar medium and disappear
Some can only be seen today in X-rays or radio wavelengths
Some, like Cassiopeia-A, show evidence of jets of matter ejected in opposite directions.Observations of Supernovae
Remember – accelerated charged particles emit radiation
In most nebulae
this radiation is in the
radio part of the EM
In the Crab Nebula
it is in the visible
to the date of the
Extrapolation shows it exploded about 9000 B.C.
Evidently, the star was a red supergiant a few thousand years ago but had contracted and heated up slightly becoming smaller, hotter, and bluer before it exploded.SN1987A
At 2:35 AM EST on February 23, 1987, hours before the supernova was first seen, instruments buried in a salt mine under Lake Erie and in a lead mine in Japan, recorded 19 neutrinos in less than 15 seconds.
Trillions must have passed through our bodies
About 1017 neutrinos must have passed through the detectors
They came from the direction of the supernovaSN1987A
A neutron star is the ball of neutrons left behind by a massive-star supernova.
Degeneracy pressure of neutrons supports a neutron star against gravity.
degenerate neutron pressure
Electron degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos
Neutrons collapse to the center, forming a neutron star
They have a radius of about 10 km and a density of about 1014 g/cm3
Mass of about 1-3 M
It should spin many times per second – conservation of angular momentum
Surface many times hotter than the sun – energy from gravitational collapse
Magnetic field a trillion times stronger than Earth’s – field squeezed into a small volumeTheoretical Prediction of Neutron Stars
Observed in gamma radiation, Geminga’s period is seen to be about .24 s.
The Crab’s period is too short to be resolved by the detector.
Being so dense and orbiting quickly, the system should be losing energy to gravitational radiation.
They are spiraling toward each other
May give us another test of Einstein’s theory.Binary Pulsars
Orbital period of only 2.4 hours.
Beams from both pulsars sweep over Earth.
One spins 44 times per second.
The other spins in 2.8 seconds.
Relativity predicts the system should be emitting gravitational radiation and
decreasing their separation
by 7mm/yearDouble Pulsar
Matter falling toward a neutron star forms an accretion disk, just as in a white-dwarf binary
hot enough to fuse He.
than a Nova because
of the stronger gravity
There may also be jets typical of other objects with accretion disks.
The false-color image below is of SS 433 – called a microquasar
Optical and X-ray images of an x-ray source in the globular cluster Terzan 2.
as 0.001 s. We call them
They determined that it was due to small planets orbiting the pulsar.
Four have been found, from smaller than to moon to 4.3 Earth masses.
It seems likely these
planets are remnants
of long gone companion
Cleverly named: Gamma-ray Bursts
Compton Gamma Ray Observatory, put in orbit in 1991, reported several per day.
> 2 secGamma-Ray Bursts
Such eruptions have been called a hypernovae.Gamma-Ray Bursts
Some bursts repeat
May be produced by neutron stars with magnetic fields 100 times stronger than that in a normal neutron star.
Other short bursts may be produced by the merger of two neutron stars
Or even the merger of a neutron star and a black holeGamma-Ray Bursts
A black hole’s mass strongly warps space and time in vicinity of event horizon
That definition leads to a discussion of escape velocity – the velocity necessary for an object to completely escape the surface of a celestial body.
Escape velocity depends on two things:
The mass of the celestial body
How far away the object is from the center of mass of the celestial bodyBlack Holes
known as the Schwarzschild radius.
The event horizon of a 3 MSun black hole is also about as big as a small city
If the Sun shrank into a black hole, its gravity would be different only near the event horizon
3 MSun black hole would be lethal to humans
Tidal forces would be gentler near a supermassive black hole because its radius is much bigger
Light waves take extra time to climb out of a deep hole in spacetime leading to a gravitational redshift
Some X-ray binaries contain compact objects of mass exceeding 3 MSun which are likely to be black holes
One famous X-ray binary with a likely black hole is in the constellation Cygnus
Compact object with > 3 Msun must be a black hole!