Life cycle of stars Flashcards
What conditions must be present for nuclear fusion to occur
- 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
What occurs during nuclear fusion
- 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
radiation pressure def
During nuclear fusion, the momentum of the gamma-ray photons results in an outward acting pressure called radiation pressure
Describe how stars can reach equilibrium
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
When will a star expand/contract
- 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
What does a star’s evolution depend on?
its initial mass
What are the first 3 stages of stellar evolution (which are the same for all masses of stars)
- 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 - 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 - 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
What are the following stages of stellar evolution (for low mass stars)
- 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 - Planetary Nebula
- The outer layers of the star are released
- Core helium burning releases massive amounts of energy in fusion reactions - 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
What are the following stages of stellar evolution (for high mass stars)
- 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 - Supernova
- The iron core collapses
- The outer shell is blown out in an explosive supernova - 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
supernova def
An object which exhibits a rapid and enormous increase in absolute magnitude
what is a Type II supernova and a type 1a supernova
a Type II supernova - a supergiant star collapses and then explodes
a Type 1a supernova - a white dwarf accrues matter and explodes
gamma-ray burst def
A short, extremely high energy burst of gamma radiation emitted by a collapsing supergiant star
Describe a gamma-ray burst
This energy is usually highly focused, or collimated, as narrow beams are ejected from the poles of the exploding star
standard candle def
An astronomical object of known brightness that can be used to calculate galactic distances
What are the two most common types of standard candles
- Cepheid variable stars
- Type 1a supernovae
How is a type 1a supernova used as a standard candle
- 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
Neutron star def
An extremely dense collapsed star made up of neutrons
What mass must a star have to become a neutron star
a mass between 1.4 and 3
Describe neutron stars
- 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
Neutron degeneracy pressure def
What is a pulsar
a fast-rotating neutron star
Why are pulsars easier to identify than neutron stars
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
Describe black holes
- 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
What is the Schwarzschild radius
The radius of a black hole’s event horizon
What do the components of the Schwarzchild radius equation mean
event horizon of a black hole def
- boundary where the escape velocity = c
(The boundary at which light and matter cannot escape the gravitation pull of the black hole)
Draw a rough sketch of the Herzburg-Russel diagram and label the axes scales
What does the Hertzburg-Russel diagram show
- 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
What types of stars does the Hertzburg-Russel diagram show AND which does it not show
- 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
Draw a diagram showing the Evolutionary path of a solar mass star on a Hertzburg-Russel diagram
Describe the lifetimes of stars
- 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
What is a cepheid variable star
a type of variable star that pulsates radially, varying in both diameter and temperature.
How is a type 1a supernova formed
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
