White dwarfs: properties - Remaining cores of dead, low mass stars
Electron degeneracy pressure supports them against gravity
Slowly fade with time
Sirius and its hot WD companion
White dwarfs with same mass as Sun are about same size as Earth; slowly cooling
Typical density 10^6 gram / cubic ...
ASTR 1020 Final Exam Study Guide 2024
White dwarfs: properties - Remaining cores of dead, low mass stars
Electron degeneracy pressure supports them against gravity
Slowly fade with time
Sirius and its hot WD companion
White dwarfs with same mass as Sun are about same size as Earth; slowly cooling
Typical density 10^6 gram / cubic cm
Higher mass white dwarfs are smaller
Cannot be more massive than 1.4MSun, the Chandrasekhar limit
WDs in binaries: nova, supernova Type I - Mass falls toward white dwarf from binary
companion
Gas orbits white dwarf in an accretion disk
Friction causes heating and accretion onto white dwarf
neutron stars, pulsars, Crab Nebula pulsar - Ball of neutrons left behind by a massive-
star supernova (10 km radius); density 10^14 g/cm^3; Degeneracy pressure of neutrons
supports it against gravity (maximum 3 solar masses)
Radiation beams along a magnetic axis that is not aligned with the rotation axis
Pulsar at center of Crab Nebula pulses 30 times per second
X-ray binary stars, SS433 - Neutron Stars in Close Binaries
Hot gas in accretion disk forms X-rays: X-ray Binaries
Accretion may cause episodes of H fusion on the surface, leading to X-ray bursts
black holes, escape velocity, event horizon, Schwarzschild radius - A black hole is an
object whose gravity is so powerful that not even light can escape it
GSU ASTR 1020 Final Exam
, GSU ASTR 1020 Final Exam
Initial Kinetic Energy = Final Gravitational Potential Energy
(escape velocity)^ = G x (mass) / (radius)
For escape velocity = speed of light (fastest possible), need small radius and/or large
mass
Can occur in the collapse of a massive star
"Surface" of a black hole is the radius at which the escape velocity equals the speed of
light = the event horizon
Nothing can escape from within the event horizon because nothing can go faster than
light
The radius of the event horizon is known as the Schwarzschild radius: 3 (M/MSun) km
(shrink Earth to size of a dime)
Cygnus X-1 binary: O-type supergiant plus black hole - Black holes cannot be seen
directly BUT we can search for evidence of their gravitational tug on nearby stars and/or
the emission of X-rays from the surrounding got gas
First direct evidence from the X-ray binary system Cygnus X-1
First X-ray satellites flown in 1970s led to the discovery of many X-ray sources
Brightest source in constellation Cygnus named Cygnus X-1
Very luminous and rapidly variable (suggesting a small size)
Accurate position not known until a sudden change occurred in X-ray and radio
brightness
gamma ray burst sources: merging NS and collapse of massive star - Brief bursts of
gamma-rays coming from space were first detected in the 1960s
Observations in the 1990s showed that many gamma-ray bursts were coming from very
distant galaxies
They must be among the most powerful explosions in the universe - could be the
formation of a black hole
Two models for gamma-ray bursts:
GSU ASTR 1020 Final Exam
, GSU ASTR 1020 Final Exam
(1) merging neutron stars or (2) a hypernova
In both models the energy is restricted to narrow jets of emission (like pulsars)
Hypernova: explosion of a very massive star that leads to the birth of a black hole
gravity waves: LIGO discoveries of merging BH+BH and NS+NS - Energy lost in GW
causes stars to spiral inward
First detection 2015: 36 + 29 solar mass BHs combine to make single 62 solar mass
BH: 3 solar masses converted to GW energy
LIGO didn't watch the whole many-year-long dance of the black hole duo, but it did see
the last few cycles of the death spiral, the merger itself, and the "ringing" effect as the
merged black hole settled into its new form
Accompanied by a gamma-ray burst
Optical counterpart found in distant galaxy: "kilonova"
Neutrons liberated create many heavy elements (10x mass of Moon in gold)
final outcomes versus initial mass: - M < 0.08 M sun Star cools as brown dwarf
• 0.08 < M < 10 M sun White dwarf remnant
• 10 < M < 18 M sun Neutron star remnant
• 18 < M < 140 M sun Black hole remnant
• M > 140 M sun No remnant?
Milky Way: appearance in visible and other wavelengths - From Earth, see few stars
when looking out of galaxy, many when looking in. Milky Way is how our Galaxy
appears in the night sky
Primary features:
Disk, bulge + bar, halo, globular clusters
1 parsec = 3.26 light years
Diameter is approximately 30,000 parsecs
GSU ASTR 1020 Final Exam
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