Life cycle of stars

Question 1

What conditions must be present for nuclear fusion to occur

Answer 1

• both nuclei must have sufficiently high kinetic energy to overcome the electrostatic repulsion between protons
• The conditions required to achieve this are:
  Very high temperature (on the scale of 100 million Kelvin)
  Very high pressure and density

Question 2

What occurs during nuclear fusion

Answer 2

• Four hydrogen nuclei (protons) are fused into one helium nucleus, producing two gamma-ray photons, two neutrinos and two positrons
• Massive amounts of energy are released
• The momentum of the gamma-ray photons results in an outward acting pressure called radiation pressure

Question 3

radiation pressure def

Answer 3

During nuclear fusion, the momentum of the gamma-ray photons results in an outward acting pressure called radiation pressure

Question 4

Describe how stars can reach equilibrium

Answer 4

Once the core temperature of a star reaches millions of degrees kelvin and the fusion of hydrogen nuclei to helium nuclei begins:
- The protostar’s gravitational field continues to attract more gas and dust, increasing the temperature and pressure of the core
- With more frequent collisions, the kinetic energy of the particles increases, increasing the probability that fusion will occur
- Eventually, when the core becomes hot enough and fusion reactions can occur, they will begin to produce an outward radiation pressure which balances the inward pull of gravity
- The star reaches a stable state where the inward and outward forces are in equilibrium
- As the temperature of the star increases and its volume decreases due to gravitational collapse, the gas pressure increases
- The gas pressure and the radiation pressure act outwards to balance the gravitational force (weight, F = mg) acting inwards

Question 5

When will a star expand/contract

Answer 5

• If the temperature of a star increases, the outward pressure will also increase
  If outward pressure > gravitational force, the star will expand
• If the temperature drops the outward pressure will also decrease
  If outward pressure < gravitational force, the star will contract

Question 6

What does a star’s evolution depend on?

Answer 6

its initial mass

Question 7

What are the first 3 stages of stellar evolution (which are the same for all masses of stars)

Answer 7

1. Nebula
   - All stars form from a giant cloud of hydrogen gas and dust called a nebula
   - Gravitational attraction between individual atoms forms denser clumps of matter
   - This inward movement of matter is called gravitational collapse
2. Protostar
   - The gravitational collapse causes the gas to heat up and glow, forming a protostar
   - Work done on the particles of gas and dust by collisions between the particles causes an increase in their kinetic energy, resulting in an increase in temperature
   - Protostars can be detected by telescopes that can observe infrared radiation
   - Eventually, the temperature will reach millions of degrees Kelvin and the fusion of hydrogen nuclei to helium nuclei begins
   - The protostar’s gravitational field continues to attract more gas and dust, increasing the temperature and pressure of the core
   - With more frequent collisions, the kinetic energy of the particles increases, increasing the probability that fusion will occur
3. Main Sequence Star
   - The star reaches a stable state when the inward and outward forces are in equilibrium
   - As the temperature of the star increases and its volume decreases due to gravitational collapse, the gas pressure increases
   - The star joins the main sequence when fusion reactions begin in the star’s core (the thermonuclear fusion of hydrogen nuclei into helium nuclei)
   - The balanced inward and outward forces will remain that way for most of a star’s life

Question 8

What are the following stages of stellar evolution (for low mass stars)

Answer 8

4. Red Giant
   - Hydrogen fuelling the star begins to run out
   - Most of the hydrogen nuclei in the core of the star have been fused into helium
   - Nuclear fusion slows
   - The energy released by fusion reactions decreases
   - The star initially shrinks and compresses the core until fusion can continue in the shell around the core
   - Once fusion reactions start again, the outer layers expand and cool as a red giant forms
   - A red giant is a large, low-temperature, luminous star in which helium nuclei are fused into more massive nuclei such as beryllium, carbon and oxygen
5. Planetary Nebula
   - The outer layers of the star are released
   - Core helium burning releases massive amounts of energy in fusion reactions
6. White Dwarf
   - The solid core collapses under its own mass, leaving the remnant of the core called a white dwarf
   - A white dwarf is an extremely dense, hot star, powered by the gravitational potential energy released as it contracts, rather than by nuclear fusion

Question 9

What are the following stages of stellar evolution (for high mass stars)

Answer 9

4. Red Super Giant
   - The star follows the same process as the formation of a red giant
   - The shell-burning and core-burning cycle in massive stars goes beyond that of low-mass stars
5. Supernova
   - The iron core collapses
   - The outer shell is blown out in an explosive supernova
6. Neutron Star (or Black Hole)
   - After the supernova explosion, the collapsed neutron core can remain intact having formed a neutron star
   - If the neutron core mass is greater than 3 times the solar mass, the pressure on the core becomes so great that the core collapses and produces a black hole

Question 10

supernova def

Answer 10

An object which exhibits a rapid and enormous increase in absolute magnitude

Question 11

what is a Type II supernova and a type 1a supernova

Answer 11

a Type II supernova - a supergiant star collapses and then explodes
a Type 1a supernova - a white dwarf accrues matter and explodes

Question 12

gamma-ray burst def

Answer 12

A short, extremely high energy burst of gamma radiation emitted by a collapsing supergiant star

Question 13

Describe a gamma-ray burst

Answer 13

This energy is usually highly focused, or collimated, as narrow beams are ejected from the poles of the exploding star

Question 14

standard candle def

Answer 14

An astronomical object of known brightness that can be used to calculate galactic distances

Question 15

What are the two most common types of standard candles

Answer 15

• Cepheid variable stars
• Type 1a supernovae

Question 16

How is a type 1a supernova used as a standard candle

Answer 16

• They reach the same peak value of absolute magnitude each time
• They are extremely bright and this means they can be used to measure the distance to the furthest galaxies

Question 17

Neutron star def

Answer 17

An extremely dense collapsed star made up of neutrons

Question 18

What mass must a star have to become a neutron star

Answer 18

a mass between 1.4 and 3

Question 19

Describe neutron stars

Answer 19

• Neutron stars are objects which form after a supernova has ejected the outer layers of a star into space
• They are extremely small and dense
• The immense gravitational forces acting on the core crush the electrons and protons until they combine into neutrons, via reverse beta decay
• Further collapse is prevented by neutron degeneracy pressure

Question 20

Neutron degeneracy pressure def

Answer 20

Question 21

What is a pulsar

Answer 21

a fast-rotating neutron star

Question 22

Why are pulsars easier to identify than neutron stars

Answer 22

because they emit radiation periodically which makes them easier to detect
In particular, they emit radio waves strongly, and sometimes X-rays and gamma rays

Question 23

Describe black holes

Answer 23

• After a supernova has ejected the outer layers of a star into space, the most massive cores can collapse into an infinitely dense point called a singularity
• A core which has a mass greater than 3 solar masses will become a black hole
• has an extremely large gravitational field; not even light can escape it
• The boundary at which light and matter cannot escape the gravitation pull of the black hole is called the event horizon
• The escape velocity beyond the event horizon is greater than the speed of light
• This is why black holes cannot be seen directly, as photons cannot escape beyond the event horizon

Question 24

What is the Schwarzschild radius

Answer 24

The radius of a black hole’s event horizon

Question 25

What do the components of the Schwarzchild radius equation mean

Answer 25

Question 26

event horizon of a black hole def

Answer 26

• boundary where the escape velocity = c
  (The boundary at which light and matter cannot escape the gravitation pull of the black hole)

Question 27

Draw a rough sketch of the Herzburg-Russel diagram and label the axes scales

Answer 27

Question 28

What does the Hertzburg-Russel diagram show

Answer 28

• stars are clustered in distinct areas
• Most stars are clustered in a band called the main sequence
• For main sequence stars, luminosity increases with surface temperature
• Red giants and Red super giants show an increase in luminosity at cooler temperatures. The only explanation for this is that these stars are much larger than main sequence stars
• white dwarf stars are hot, but not very luminous
  Therefore, they must be much smaller than main sequence stars

Question 29

What types of stars does the Hertzburg-Russel diagram show AND which does it not show

Answer 29

• It only shows stars that are in stable phases
• Transitory phases (e.g supernovae) are not shown
• Black holes cannot be seen since they emit no light

Question 30

Draw a diagram showing the Evolutionary path of a solar mass star on a Hertzburg-Russel diagram

Answer 30

Question 31

Describe the lifetimes of stars

Answer 31

• The brightest stars have very short lifetimes (a few million years). These stars use up nuclear fuel at a much higher rate
• The dimmest stars have extremely long lifetimes in comparison (~1012 years). These stars use up nuclear fuel at a much slower rate
• Stars on the main sequence with high luminosities are massive and very bright
• A star that is 106 times brighter than the Sun will use up its nuclear fuel 106 times faster than the Sun

Question 32

What is a cepheid variable star

Answer 32

a type of variable star that pulsates radially, varying in both diameter and temperature.

Question 33

How is a type 1a supernova formed

Answer 33

This type of supernova involves an exploding white dwarf in a binary star system
- The white dwarf increases in mass as it attracts material from its binary pair
- Eventually the white dwarf reaches a critical mass, known as the Chandrasekhar Limit
- This critical mass means the explosion is the same each time, hence it produces a very consistent light curve