Astrophysics Flashcards
Define the term black body.
A body that absorbs all wavelengths of electromagnetic radiation
And can emit all wavelengths of electromagnetic radiation
State what is meant by luminosity of a black body radiator and what 2 things it relies upon.
The total energy it emits per second
Black body radiator’s temperature and surface area
Outline how parallax measurements are used to determine the distance to nearby stars and explain how the use of a standard candle enables the distance to more distant objects to be determined.
- View star from 2 positions with a 6 month interval
- Measure change in angular position relative to fixed background stars
- Use trig to calculate distance to star using known radius of Earth’s object around the sun
- Standard candles are astronomical objects which have a known luminosity due to some characteristic property of the object
- The intensity of the radiation can be measured on Earth
- Use the inverse square law I=L/(4πd^2)
Suggest why trigonometric parallax is only suitable for measuring distance to nearby stars.
Angular displacement is too small to determine/ uncertainty on angular displacement too large
What is a lightyear?
The distance that electromagnetic waves travel in one year
What are we seeing when we see light from a star 10 ly away and what happens the further away a star is?
We are seeing the star as it was 10 years ago
The further away a star is, the further back in time we are seeing it
The parsec is a unit of distance used for nearby stars. It is calculated by seeing how far stars move relative to very distant stars when Earth is in different parts of its orbit. State what is meant when a star is 1 parsec (pc) away from Earth.
The angle of parallax is 1 arcsecond
Where 1 arcsecond= 1/3600 degrees
Stars are born in a cloud of dust and gas, most of which was left when previous stars blew themselves apart in supernovae. The denser clumps of the cloud contract under gravity and heat up. When these clumps are dense enough, the cloud fragments into protostars. Describe the process from protostar to main sequence star referring to energy/ force balance.
Temperature at centre of protostar high enough for hydrogen nuclei to begin fusing together into helium nuclei
Protostars become main sequence stars. Pressure produced from hydrogen fusion balances out the gravitational force trying to collapse the star.
Describe the main sequence star position on H-R diagram.
Position on main sequence part of H-R diagram depends on the stars size: the more massive a star, the brighter and hotter it is so the more towards the top left it is found
As a gas cloud contracts the internal energy of the system increases. Explain how energy conservation applies to the system during this period of contraction.
GPE decreases as cloud collapses
Decrease in GPE= Increase in internal energy
Explain how the fusion of hydrogen into helium in the core enables large amounts of energy to be released.
The helium has a smaller mass than the hydrogen nuclei that have fused to form it so there is a mass deficit
This mass deficit is converted to energy according to E=mc^2
Although energy released per fusion is small, fusion rate is large so lots of energy is released
Explain why the most massive stars are found in the top left of the H-R diagram.
The most massive stars have the highest temperature and are the most luminous
This is because they fuse hydrogen into helium at the highest rate
Because they have the largest gravitational forces
Describe what occurs in the core of a low mass star as it evolves to become a red giant.
- Once the hydrogen in the core runs out, the core of the star starts to collapse
- The outer layers of the star expand and cool, and the star becomes a red giant
- Red giants have a higher luminosity than main sequence stars and a lower surface temperature
- The star moves off the main sequence to the top right part of the H-R diagram
Describe what happens to a low mass star at the end of the red giant phase of its life cycle.
- Helium core contracts enough so that it becomes hot and dense enough for helium nuclei to fuse releasing lots of energy
- Small stars can’t get up to high enough temperatures for any more fusion so will continue to contract and outer layers become more and more unstable
- Outer layers are ejected as a planetary nebula leaving behind a hot, dense solid/ core called a white dwarf
- White dwarfs have a low luminosity but a high temperature so are found in the bottom left corner of the H-R diagram
Massive stars have a lot of fuel. They spend less time in the main sequence back they use up their fuel more quickly. Describe what happens to a massive star after it becomes a red supergiant.
- Really massive stars can keep fusing elements until their core is made up of iron (no fusion beyond iron- not energetically favourable)
- Star explodes in a supernova, leaving behind a neutron star or a black hole if the star is massive enough
