3.9.2 Classification of stars; Stellar Evolution Flashcards
First stage - protostar to main sequence
Stars are born in a cloud of dust and gas, most of which was left when previous stars blew themselves up apart in supernovae. The denser clumps of cloud contract very slowly under the force of gravity.
When these clumps get dense enough, the cloud fragments in to regions called protostars, that continue to contract and heat up.
Eventually the temperature at the centre of the protostar reaches a few million degrees and hydrogen nuclei start to fuse together to form helium.
This releases enormous amounts of energy and creates enough radiation pressure to stop gravitational collapse.
The star has now reached the Main Sequence and will stay there, relatively unchanged, while it fuses hydrogen into helium.
Second stage - Main sequence to red giant
Stars spend most of their life as Main Sequence stars - the pressure produced from hydrogen fusion in their core balances the gravitational force trying to compress them.(this stage is called core hydrogen burning).
When all the hydrogen in the core has fused into helium, nuclear fusion stops, and with it, the outward pressure stops. The helium core contracts and heats up under the weight of the star. The outer layers expand and cool, the star becomes Red Giant.
The material surrounding the core still has plenty of hydrogen, so heat from the contracting helium core raises the temperature of this material enough for the hydrogen to fuse (called shell hydrogen burning).
The helium core continues to contract until eventually, it gets hot enough and dense enough for helium to fuse into carbon and oxygen (core helium burning). This releases a huge amount of energy, which pushes the outer layers of the star further outwards.
When the helium runs out, the carbon-oxygen core contracts again and heats a shell around it so that helium can fuse in this region (shell helium burning).
What happens at the end of a low mass star’s life?
In low-mass stars, the carbon-oxygen core isn’t hot enough for any further fusion, so it continues to contract under its own weight. Once the core has shrunk to about Earth size, the electrons exert enough pressure (electron degeneracy pressure) to stop it collapsing anymore.
The helium shell becomes more and more unstable as the core contracts. The star pulsates and ejects its outer layers to space as planetary nebula, leaving behind the dense core.
The star is now a very hot, dense solid called a white dwarf, which will cool down and fade away.
What happens at the end of a high mass star’s life?
Stars with a large mass use up their fuel more quickly, so don’t spend as long being main sequence stars. When they are red giants, the ‘core burning to shell burning’ process can continue beyond the fusion of helium, building up layers in an onion-like structure to become red supergiants. For really massive stars this can go all the way up to iron.
Nuclear fusion beyond iron is not energetically favourable, though, so once an iron core is formed, then very quickly it dies.
The star explodes cataclysmically in a Supernova, For some very massive stars, bursts of high energy gamma rays are emitted. The gamma burst can go on for minutes, or hours (rarely).
Either a neutron star or a black hole (if the star is massive enough) is left behind.
How is a neutron star formed?
When the core of a star runs out of fuel, it starts to contract.
If the star is massive enough, electron degeneracy can’t stop the core contracting. This happens when the mass of the core is more than 1.4 times the mass of the Sun.
Electrons get ‘sqashed’ onto the atomic nuclei, combining protons to form neutrons and neutrinos.
The core suddenly collapses to become a Neutron Star, which the outer layers fall onto.
When the outer layers hit the surface of the neutron star, they rebound, setting up huge shockwaves, ripping the rest of the old star apart in a supernova. The absolute magnitude rapidly increases, meaning light from a supernova can briefly outshine a galaxy.
What are the properties of neutron stars?
Incredibly dense (about 4*10^17 kgm^-3)
Made of neutrons
Very small and rotate very fast (around 600 times a second)
Some neutron stars emit radio waves in two beams as they rotate. These beams sometimes sweep past Earth and can be observed as radio pulses - these rotating neutron stars are called Pulsars.
How is a black hole formed?
If the core of the star remaining after a supernova has a mass of more than 3 times that of the Sun, the neutrons can’t withstand the gravitational forces. There are no known mechanisms left to stop the core collapsing into an infinitely dense point called a singularity.
Up to a certain distance away (Schwarzschild radius), the gravitational pull is so strong that not even light can escape. This is called the event horizon.
Define Schwarzschild radius
The distance at which the escape velocity is the speed of light.
R_s = 2GM/c^2
Describe the light curve of a type 1a supernova
Sharp initial peak
Gradually decreases over time
Y-axis - absolute magnitude, decreases going up
X-axis - time, days, increases to the right
Why are type 1a supernova used as standard candles?
Every type 1a supernova occurs at the same critical mass, hence they all have consistent light curves.
They are also bright for distances up to 1000Mpc.
What are the properties of supernovae?
They release bursts of high energy gamma rays - they are dangerous to Earth because they could destroy the ozone layer and cause extinction.
They release a lot of energy (around 10^44)
What are standard candles?
Bodies whose absolute magnitude is known and whose apparent magnitude can be measured
