1. Describe how stars are formed.
show answer
- Stars are formed from huge clouds of gas and dust called nebulae
- Found mainly in spiral arms of Galaxy
- e.g. Orion’s nebula
- Gas cloud begins to collapse and breaks up into protostars
- Rise in temperature and pressure as GPE (gravitational potential Energy) KE (kinetic energy).
- Temperature at core reaches 15 million k.
- Outward pressure from radiation prevents any further collapseMain sequence star is formed.
2. Describe a stars death.
show answer
- Star runs out of Hydrogen
- No radiation countering gravitational collapse
- Star collapses
- Temperature rises further
- Fusion restarts in a shell surrounding the core which is now rich in helium.
- Outer layers of the star expand and cool to form red giant.
- Temperatures at core rise to 100 million kelvin
- Helium nuclei fuse to form carbon.
- Helium is depleted
- Red giant looses it’s outer layers in an expanding shell of gas.
- Core collapses to form White Dwarf.
- Cools eventually to brown and then black.
- Mass of star affects how star dies
- Super giants can form iron through fusion
- Supernova!
- Then Neutron star or Black Hole.
3. What is a supernova?
show answer
- A massive explosion at the end of a supergiant star’s life. (greater than 8 solar masses)
- Outer layers blown away at speeds of 5000km/s
4. What is a Neutron star?
show answer
- A remnant of a supernova
- Mass of sun collapsed into a sphere of 20km diameter.
- Intense gravitational fields cause them to spin.
- Emit radio waves from their polar regions
5. How can we deduce the chemical composition of a star?
show answer
- Each element has it’s own unique set of spectral line wavelengths.
- Analysis of starlight can tell us what the star is made of.
6. How can we deduce the temperature of a star?
show answer
- The temperature of a star determines it’s colour so spectral analysis can also be used to determine its temperature.
7. How can we determine the radial velocity of a star?
show answer
- Analysis of the Doppler shift of spectral lines.
8. Describe the Harvard classification of Spectral type.
9. What is a HR diagram?
show answer
- This is a plot of luminosity against spectral type.
10. Draw a HR Diagram
11. Which stars will run out of fuel sooner?
show answer
- Massive stars (top left of HR Diagram).
- 61 Cygni is a low mass star and will spend 100’s of billions of years on the main sequence.
12. Give evidence for the existence of Neutron stars.
show answer
- Regular pulsating radio signals from Pulsars.
13. Give 3 pieces of evidence for the existence of black holes.
show answer
- Gravitational lensing
- Light bent by black hole’s gravity (click here for demonstration)
- Speed of rotation of our galaxy and its size hints that the mass at its centre is great enough to be a black hole.
- Charged particles spinning around a Black hole produce x rays which we can detect.
