Module 5.4 - Astrophysics and Cosmology

Question 1

How do protostars form?

Answer 1

Stars are born from nebula - clouds of gas and dust. Denser parts of the cloud contract under gravity. When these sections clump together enough the cloud fragments into regions called protostars.

Question 2

What is a nebula?

Answer 2

A nebula is a giant cloud of gas and dust in space

Question 3

How do main sequence stars form from protostars?

Answer 3

Protostars continue to contract, heating up whilst doing so. As its volume decreases the gas pressure inside of it increases. Eventually the temperature at the center of the protostar becomes high enough for hydrogen to fuse together to make helium, providing radiation pressure exerted outwards. Together with the gas pressure counteracts the force these counteract the gravitational attraction. This prevents the star from shrinking further, becoming a main sequence star.

Question 4

What is the ‘core hydrogen burning’ stage?

Answer 4

The core hydrogen burning stage is the stage in a stars life cycle where the pressure produce from hydrogen fusion in their core balances the gravitational force trying to compress them.

Question 5

How does a red giant form from a main sequence star?

Answer 5

1) Hydrogen in the core runs out - hydrogen fusion stops and core contracts and heats up under weight of the star
2) Outer layers cool and expand outwards
3) Material surrounding the core has plenty of hydrogen and becomes hot enough for shell hydrogen fusing to occur
4) Core continues to contract till it becomes hot and dense enough for helium to fuse into carbon/oxygen, releasing huge amounts of energy pushing the outer layers outwards

Question 6

What mass does a star need to become a red giant?

Answer 6

0.5 M - 10M

Question 7

How are white dwarfs formed from low mass stars?

Answer 7

1) Core runs out of helium to fuse and contracts again under the stars weight
2) Carbon oxygen core continues to contract under its own weight as its not hot enough for further fusion to occur
3) Helium shell becomes increasingly unsuitable as core contracts. Star pulsates and ejects its outer layers into space as a planetary nebula, leaving behind the dense core.
4) Once the core has shrunk to about earth size electrons exert enough electron degeneracy pressure to keep the core from collapsing further
5) This core is a hot dense solid called a white dwarf that just cools down and fades away

Question 8

What mass is needed to form a white dwarf?

Answer 8

< 1.4M

Question 9

What is the Chandrasekhar limit?

Answer 9

Chandrasekhar limit - the maximum mass for which the electron degeneracy pressure can counteract the gravitational force

Question 10

How does a super red giant form from a red giant?

Answer 10

Stars with a large mas have a lot of fuel but burn it up quickly and spend less time as main sequence stars. When they are red giants the ‘core burning then shell burning’ process repeats beyond helium building up layers in an onion like structure to form a super red giant. This can go up to iron.

Question 11

Why does a supernovae occur when a super red giant runs out of fuel?

Answer 11

1) Stars core is larger than Chandrasekhar limit so electron degeneracy pressure can’t stop it from contracting
2) As the core continues to (suddenly) contract the outer layers fall in and rebound off of the core setting up huge shock waves that cause the star to explode in a supernova, leaving behind a neutron star or black hole

Question 12

How does a neutron star form a red giant?

Answer 12

As the core contracts electrons get squashed onto atomic nuclei where they combine with protons to form neutrons and neutrinos. This happens suddenly causing a supernova to occur

Question 13

What core mass is needed to create a neutron star?

Answer 13

1.4 - 3 M

Question 14

What core mass is needed to create a black hole?

Answer 14

> 3 M

Question 15

What are some properties of neutron stars?

Answer 15

• Super dense and very small (~20km diameter)
• Can rotate super fast (600 rotations per second)
• Emit radio waves in two beams as they rotate. These rotating neutron stars are called pulsars

Question 16

How does a black hole form a neutron star?

Answer 16

The neutrons can’t withstand the gravitational forces so the star continues to collapse. There are no known mechanisms left to stop the core collapse so it collapses to an infinitely dense point called a singularity.

Question 17

What is the event horizon of a black hole?

Answer 17

The event horizon of a black hole is the boundary of the region where the black holes gravitational pull is so strong not even light can escape.

Question 18

Why do you not see any stars in the transitional period of a HR diagram?

Answer 18

Transitional stars are unstable and the transitions happen quickly (compared with the life of the star)

Question 19

What is the general trend for main sequence stars on a HR diagram?

Answer 19

Lower temperature, lower luminosity

Question 20

Where can white dwarfs be found on a HR diagram?

Answer 20

Bottom left corner, They are the hottest but also the least luminous

Question 21

What is the equation to find wavelength from a diffraction grating?

Answer 21

d sin(θ) = nλ

d = a

θ = x/D

Question 22

What would you observe if white light is shone through a diffraction grating?

Answer 22

• The different wavelengths within the white light are diffracted by different amounts
• Each order within the pattern becomes a spectrum with red on outside and violet on inside
• Zero order line stays white

Question 23

How are emission spectra produced by hot gasses?

Answer 23

1) If you heat a gas to high temperature many of its electrons move to higher energy levels
2) When they fall back down to ground state they release this energy as a photon of a specified wavelength (energy)
3) This creates characteristic line spectra when split with a diffraction grating

Question 24

How can we use line spectra to identify elements?

Answer 24

Different atoms have different electron energy levels so different sets of emission spectra. You can identify elements in a gas from their emission spectra.

Question 25

How are absorption spectra produced?

Answer 25

1) light of a continuous spectrum of energy passes through a cool gas
2) Most of the electrons in the gas atoms are at ground state
3) Photons of correct wavelength are absorbed by the electrons exciting them to higher energy levels
4) These wavelengths are missing from the spectrum coming out of the gas

Question 26

What does an emission spectra look like?

Answer 26

A continuous spectrum with black lines corresponding to the wavelengths of light absorbed

Question 27

Why do you get absorption line spectra from stars?

Answer 27

1) Stars can be assumed to emit radiation in a continuous spectrum.
2) This radiation has to pass through a large amount of gas at the surface of the star
3) The gas absorbs wavelengths of light depending on the elements it consists of

Question 28

What is luminosity?

Answer 28

Luminosity is the total energy emitted per second by a light source.

Question 29

What is Stefan’s law?

Answer 29

The luminosity of a star is proportional to the fourth power of its temperature AND directly proportional to its surface area

Question 30

What is luminosity measured in?

Answer 30

Watts

Question 31

State Wien’s displacement law?

Answer 31

For any star the peak wavelength it emits is is directly proportional to 1/ its temperature in kelvin

λₘₐₓ ∝ 1/T

Question 32

Why is Wien’s displacement law useful?

Answer 32

as λₘₐₓ ∝ 1/T
T * λₘₐₓ = k
Using this you can estimate temperature of one star if you know its peak wavelength and the temp+peak wavelength of another star. As k is the same for similar stars

Question 33

What is peak wavelength?

Answer 33

The peak wavelength a hot object emits is the most common (highest intensity of) wavelength it emits.

Question 34

How does the peak wavelength of a star vary with surface tempersature?

Answer 34

Peak wavelength becomes shorter as the surface temperature of a star increases. Same for all hot objects