Stellar Evolution Flashcards
The Hertzsprung-Russell (HR) Diagram
What is the sun?
The sun is a main sequence star, its spectral class is G and its absolute magnitude is 4.83.
The path of the main sequence star
Once main sequence star uses up all hydrogen in its core, it will move up and to the right on the HR diagram as it becomes a red giant. A red giant is brighter and cooler than main sequence star.
Once the red giant uses up all helium in its core, it will eject outer layers and move down and to the left on the HR diagram as it becomes a white dwarf. A white dwarf is hotter and dimmer than main sequence star.
What is one solar mass?
mass of the sun and is equal to 2 × 10^30 kg.
Stages of stellar evolution (order)
Protostar
Main sequence
Red giant/white dwarf
Red supergiant/ supernovae
Neutron star/ black hole
Protostar
Clouds of gas and dust (nebulae) have fragments of varying masses that clump together under gravity.
○ irregular clumps rotate and conservation of angular momentum spins them inwards to form denser centre – a protostar.
○ protostar surrounded by disc of material (a circumstellar disc).
○ When protostar gets hot enough, begins to fuse elements, producing strong stellar wind that blows away surrounding material.
Main sequence
○ The inward force of gravity and outward force due to fusion are in equilibrium – star is stable.
○ Hydrogen nuclei are fused into helium.
○ The greater mass of star, shorter its main sequence period because it uses its fuel more quickly.
Red Giant (for a star < 3 solar masses)
○ Once hydrogen runs out, temperature of the core increases and begins fusing helium nuclei into heavier elements (E.g.Carbon, Oxygen and Beryllium).
○ outer layers of the star expand and cool.
White Dwarf (for a star < 1.4 solar masses)
○ When red giant has used up all fuel, fusion stops and core contracts as gravity is now greater than outward force.
○ outer layers are thrown off, forming planetary nebula around remaining core.
○ The core becomes very dense (around 108 - 109 kg m- 3) .
○ A white dwarf will eventually cool to a black dwarf.
Red Supergiant (for a star > 3 solar masses)
○ When high-mass star runs out of hydrogen nuclei, same process for a red giant occurs, but on larger scale.
○ The collapse of red supergiants in a supernova causes gamma ray bursts.
○ Red supergiants can fuse elements up to iron.
Supernova (for a star > 1.4 solar masses)
○ fuel runs out, fusion stops ,core collapses inwards suddenly and becomes rigid (matter can’t be forced closer together)
○ outer layers of star fall inwards and rebound off of core, launching out into space in shockwave.
○ shockwave passes through surrounding material, elements heavier than iron fused and flung out into space.
○ remaining core depends on the mass of star.
○ A characteristic of supernova is its rapidly increasing absolute magnitude.
○ Supernovae may release around 104 4 J of energy, which is same amount of energy as sun outputs in its lifetime.
Neutron Star (for a star between 1.4 and 3 solar masses)
○ When core of large star collapses, gravity is so strong that it forces protons and electrons together to form neutrons.
○ A neutron star is incredibly dense – about 10^17 kg m- 3
○ Pulsars are spinning neutron stars that emit beams of radiation from magnetic poles as they spin
Black Hole (for a star > 3 solar masses)
○ When core of a giant star collapses, neutrons unable to withstand gravity forcing them together.
○ gravitational pull of black hole is so strong that not even light can escape.
○ event horizon of a black hole is the point at which escape velocity becomes greater than speed of light .
Schwarzchild radius
radius of the event horizon
Rs = 2GM/c^2
Where G is the gravitational constant, M is the mass of black hole and c is the speed of light in a vacuum.
binary system
one where two stars orbit a common mass.
Types of supernovae: ➔ Type I
When a star accumulates matter from its companion star in a binary system and explodes after reaching a critical mass.
➔ Type II
The death of a high-mass star after it runs out of fuel.
What is a type 1a supernova?
-A Type 1a supernova is a Type I supernova with a white dwarf.
-When companion star in the binary system runs out of hydrogen, it expands, allowing white dwarf to begin accumulating some of its mass.
-When white dwarf star reaches critical mass, fusion begins and becomes unstoppable as mass continues to increase, eventually causing white dwarf to explode in supernova.
All types of supernovae occur….
at the same critical mass, meaning they all have a very similar peak absolute magnitude (about -19.3) -produce very consistent light curves, allowing astronomers to use them as standard candles to calculate distances to far-off galaxies
Light curve for 1a supernova
supermassive black holes
-scientists believe that they form at the centre of every galaxy.
-this is because stars and gas near centre of galaxies appear to be orbiting very quickly.
-concluded that there must be supermassive object at the centre with very strong gravitational field attracting them.
Supermassive black holes can form from:
-collapse of massive gas clouds while the galaxy was forming
-normal black hole that accumulated huge amounts of matter over millions of years
-several normal black holes merging together
Expansion of the universe explanation
-if expansion of universe was slowing down, more distant objects observed to be receding more quickly, since expansion was faster in past.
-light from more distant objects would take longer to reach us so would appear to be in past.
-objects also appear brighter than predicted as they would be closer than expected.
-however type 1a supernovae seen to be dimmer than expected meaning more distant than Hubble’s law predicted.
-suggests that expansion of universe is accelerating and is older than Hubble’s law estimates.
Dark energy
-dark energy thought to be reason behind universe accelerating.
-described as having overall repulsive effect throughout whole universe.
-gravity follows inverse square law,decreases with distance.
-DE remains constant throughout universe, meaning it has greater effect than gravity therefore causing expansion speed to increase.
-DE controversial because there’s evidence for existence but no one knows what it is or what is causing it
