Stellar remnants Flashcards
What are stellar remnants?
What’s left after the death of a star
What is the remnant of a low/intermediate mass star?
White dwarfs
What are the remnants of massive stars?
Neutron stars
What are black holes the remnant of?
A massive star in which the resulting neutron star was massive enough to collapse further
What is a white dwarf?
The exposed core of a star that has died
Describe white dwarfs
•may shine brightly in x rays or uv
•isn’t very luminous
•supported by electron degeneracy pressure-closely packed electrons
What happens to electron speeds as white dwarf mass increases?
Increases
When can a white dwarf gain mass?
If in a close binary system and companion is a main sequence or giant star
Explain white dwarf accretion
•accretion disc formed
•can provide a dead star with new energy source
•may occasionally flare up
•gas accreting is predominantly hydrogen
•gravity of white dwarf compressed hydrogen into thin surface layer
•hydrogen fusion ignites causing star to shine brightly temporarily
What are differences between novae and supernovae?
•novae:-as bright as 100000 suns
-may repeat
•supernovae:-death of a massive star
-as bright as 10 billion suns
What happen each time a nova occurs and after each nova to white dwarfs?
Ejected some mass when occur and accrete matter again after nova
What happens to white dwarfs once Chandrasekhar limit exceeded?
White dwarf supernova with no leftover remnant
What do all white dwarf supernovas have in common?
Same luminosity
What might neutron stars have in their cores?
Quark gluon plasma
Where do we find pulsars?
In centre of supernova remnants (they are neutron stars)
How many pulsars do we know of in our galaxy?
Over 3000
Why must pulsars be neutron stars?
No other massive object could spin this fast
What do pulsars have?
Intense magnetic fields that direct beams of radiation out along magnetic poles
Why is binary pulsar PSR B1913+16 orbit getting smaller?
Emission of gravitational waves
What can create a hot accretion disc around a neutron star?
Gas overflowing from companion main sequence or giant star
Why does the accreting matter if a neutron star release enormous amount of energy?
Due to strong gravity of neutron star
What does the large amount of energy released mean for neutron star disc?
Much hotter and more luminous
What does high temperature of neutron accretion disc mean?
Radaiates in the Xray and is 10000 times more energetic than sun. Sometimes called xray binaries
How is energy primarily released by neutron stars?
In X-rays
Why is energy released from neutron stars?
Result of sudden ignition of nuclear fusion
What are the most energetic events of the known universe?
Gamma ray bursts
How do gamma ray bursts differ from xray bursts?
Appear to come from random directions and not concentrated in the disk of our galaxy. Intensity of bursts have no patterns
Where do some gamma ray bursts come from?
Powerful supernovae (hyper nova) in which stellar remnant is a black hole (>40m)
What are long gamma ray bursts associated with?
Death of high mass stars
What are short gamma ray bursts associated with?
Neutron star mergers
What is the mass limit of a white dwarf?
1.44 solar masses
What is the mass limit of a neutron star?
2-3 solar masses (more and neutron degeneracy pressure can no longer support neutron star)
Can any force keep neutron core from collapsing into black hole?
No known force can
What star can directly generate a black hole after supernova explosion?
More massive than 18 solar masses
What does escape velocity depend on?
An objects mass and size
As an object becomes more compact what happens to its escape velocity?
It increases
Why can not even light escape a black hole?
It is so compact
What does gravity act to do in a black hole?
Curves spacetime
What is the event horizon?
The boundary between the inside of a black hole and the universe
What is the radius of the event horizon known as ?
Schwarzschild radius
Why don’t we know of anything which would stop the collapse of a black hole?
Singularity
Why can’t we know anything about the interior of a black hole?
•no information can emerge from within event horizon
•if we added mass to a black hole it would make no difference what type of material we added
What are the only measurable properties of black holes?
Mass,electric charge, and spin
How can half of the rest mass energy be released?
Material falling into a black hole
What does quantum mechanics predict about black holes?
Particle-antiparticle pairs can come into existence in a vacuum which could result in energy extraction from a black hole
What is the second law of thermodynamics?
Entropy will only increase
When will the universe end?
At maximum entropy (heat death: we can’t extract energy out of a universe at max entropy)
When do we estimate the last stars will die?
100 trillion years
When might protons decay?
10000 trillion trillion years
What will the last objects shining in our universe be?
Supermassive black holes
