Chapter 5 Flashcards
How are Stars formed?
Cloud of dense, cold gas is compressed via gravity
It eventually becomes so dense that nuclear fusion starts to occur
How cold are the gas clouds?
30-100K
How hot is the Sun’s surface?
5800K
What colour is the Sun?
Green-yellow
Why do we see the Sun as white?
It is so bright that the green-yellow color is masked
How hot is the Sun’s outer atmosphere
1,000,000K
What’s keeping our atmosphere from sinking?
Hydrostatic equilibrium
What is Hydrostatic Equilibrium?
The weight of the uppermost layers crush the lower layers
Once the pressure gets to a high enough level, then the lower gases resist and push back
Where on stars do nuclear reactions occur?
At the core
What is the temperature of thee sun at its core?
1.5x10^7K
How old is the sun?
5bn y/o
What is the lifespan of a star like the Sun?
around 10bn years
What determines what happens to the star?
Its mass
What happens to less massive suns like the Sun?
They slowly die out as white dwarves
What happens to more massive stars than the Sun?
Some end up as neutron stars, others end up in massive cataclysmic explosions called supernovae
What is the raw material from which stars form?
Interstellar gas
What are the clouds of interstellar material called?
nebulae
Why are dust grains important in the formation of stars in the nebulae?
They radiate away energy, helping to cool the cloud to 30-100K
What is the relationship between temperature and the collapse of the gas clouds?
The colder the gas, the greater the chance the gas has of collapsing before the pressure to resist the collapse can build up again
How does the nuclear fusion within the protostar occur?
The gas heats up when it is compressed
Hydrogen is ionised so that it loses its single electron. The more electrons that are liberated, the more opaque the protostar becomes. This means that the photons trap energy within the star and the energy can cause nuclear fusion
What is the minimum mass for star formation?
0.08M.
What are red dwarfs?
Low-mass stars that are cool, glowing with only a faint red light
What are brown dwarfs?
Objects just below the mass for stellar recognition. They tend to be extremely dim, even to infrared light
What are stars primarily made out of?
Hydrogen gas
What was the original belief as to what powered the stars?
Gravity powered the stars
Compression of the gas would cause the gravitational energy to convert into heat and light
What discovery changed the tact from the belief that the Sun was powered by gravity?
The discovery of radioactivity
What is the condition of the gas inside of the Sun?
They are the nuclei: their electrons have been stripped
What is the relation between the size of a star and its central pressure?
The larger the star, the greater the central pressure
What are the 2 factors that control the nature of a star?
Mass, composition
Why do the protons in the core of a star stick together?
They are attracted to each other via nuclear force.
What is the Coulomb barrier?
The gap created when protons repel each other
What is key about the nuclear processes?
They provide the energy necessary to maintain the hydrostatic equilibrium
What is the relationship between the luminosity of a star and its mass?
Luminosity = Mass cubed
Who came up with the diagram relating luminosity and spectrum released by the star?
Hertzsprung Russell
What is the main sequence?
The main line in the Hertzprung-Russel diagram
For how long is the star is in the Main Sequence?
For as long as the fusion of hydrogen to helium dominates
In the main sequence, what is the relationship between luminosity and mass?
Luminosity = Mass ^3.1
What happens at the core when the hydrogen stops fusing with helium? (3)
The pressure drops, causing the force of gravity to make it contract.
As the core contracts, the temperature increases
When the temperature reaches a certain point, helium will begin to fuse into carbon in the deep interior whilst the hydrogen continues to burn normally in the shell surrounding the core
How is a red giant formed?
When the core contracts after Hydrogen stops fusing with helium, the temperature goes up as the helium burns into a carbon core
This heat causes the outer regions to expand and thus, cool.
What happens to large stars when the usable fuel has been converted into carbon?
The core contracts again. Under this pressure electron degeneracy occurs
What is electron degeneracy pressure?
As the body that the electron resides in shrinks, the electron gains energy
Who developed the idea of electron degeneracy pressure?
Chandresekhar
As the core of the star compresses into a white dwarf, what happens to the rest of the star?
The star ejects its swollen outer layers and forms a planetary nebula
What is the relationship between size and mass of White Dwarves
The more massive the WD gets, the smaller it gets
What happens to the WD as it cools?
It crystallises
What happens in a Type 1a Supernova?
A white dwarf “steals” matter from a companion star; the WD’s mass slowly increases until the WD’s mass exceeds 1.4 M¤ and it explodes.
When can a White Dwarf be regenerated?
When it is in distance with a companion star to ‘steal’ matter from it. The mass slowly increases until it reaches around 1.4M and explodes
What happens in the core of very heavy stars?
Increasingly heavy elements are subsequently burned at the core, fusing all the way to Iron
What is the issue when Iron forms at the core?
Iron cannot provide the energy the star needs in order to support its weight.
What is the Chandresekhar limit?
This is the limit of the mass that electron degeneracy pressure can support. When a white dwarf exceeds this, it collapses
What is the maximum original mass that is required for a white dwarf to form from a star?
10M
What happens if the electron degeneracy pressure falls short and can no longer support the star?
Massive explosion calles a Supernova
Characteristics of a supernova
- High energy (can have more than the other 100BN stars combined)
- Creates heavier elements in the explosion
- Ejects material at speeds of around 10,000km/sec
What is a type 2 Supernova?
A massive star (> 10 M¤) explodes when its core starts to produce iron.
How can a white dwarf explode?
When it accrues more than 1.4M worth of mass, it explodes as it has surpassed the Chandresekhar limit. The carbon in it ignites, which ignites all of the matter in the WD simultaneously and BOOM
What is Neutron Degeneracy Pressure?
Neutrons, when compressed, can give significant amounts of energy (more than electrons)
How are neutron stars formed?
As the star collapses into greater and greater compaction, the protons and electrons merge to form neutrons.
These neutrons exert neutron degeneracy pressure. this forms a massive neutron nucleus known as a neutron star
What would the radius be of a neutron star with the density of the sun be?
10km
What do Pulsars emit?
They emit highly regular bursts of electromagnetic radiation, generally as radio waves
Why do neutron stars emit pulsars in the way that they do?
They are spinning very quickly due to the conservation of angular momentum
They rotate around 30x per second and are strong enough not to break up even when rotating at such great speeds
They have one specific beam of radiation. It appears to be flashing this radiation as we are detecting it when it spins towards us
What is the lighthouse model
The theory that pulsars have one specific beam of radiation. It appears to be flashing this radiation as we are detecting it when it spins towards us
Radiation occurs at the north and south poles of the magnetic field
Who is Annie Cannon?
She analysed and catalogued 350,000 spectra of Stars at Harvard and organised them in terms of how much hydrogen they had in them
How did Cannon reorganise the orders of the stars?
OBAFGKM
Why were the orders of the stars reorganised?
Because many of the stars had the same chemical make-up, but emitted different colours. Thus, they were reorganised according to temperature
