Topic 3.4 - Evolution Of Stars Flashcards
How are stars formed?
Stars are formed by the gravitational collapse of huge clouds of gas and dust that are found mainly in the spiral arms of galaxies. These clouds can be up to about 15kpc across and may contain enough raw material to form several thousand stars. Once a gas cloud has began to collapse it breaks up into smaller collapsing knots called protostars. These rise in temperature as gravitational potential energy is connected to kinetic energy. Eventually the central temperature of a protostar reaches 15 million K, hot enough for nuclear fusion of hydrogen nuclei to helium to begin. Once this occurs, outward pressure from the radiation generated in the star’s core halts any further collapse and the star settles to a stable size. It then begins to radiate energy from its hot surface and becomes a main sequence star
Name the order of the stellar life cycle.
- Stellar clouds with protostars
2a. Average/small main sequence star
3a. Red giant
4a. Planetary nebula
5a. White dwarf
2b. Masive star
3b. Red super giant
4b. Supernova
5b. Neutron star
5c. Black hole
How is a red giant formed?
Eventually the hydrogen which fuels a star’s core runs out. The outward radiation pressure in now no longer present and the star collapses once more under its own gravity. This causes further temperature rise within the star, until hydrogen can undergo fusion in a shell surrounding the core which is now rich in helium. Te nuclear reactions which occur cause the outer layers of the star to expand and cool to form a red giant or supergiant
How is a planetary nebula formed?
With further hydrogen depletion and contraction, the temperature at the centre of a red giant can rise to as hot as 100 million K. This is hot enough for helium nuclei to fuse to carbon. When the helium is depleted the red giant loses its outer layers in an expanding shell of gas
How is a white dwarf formed?
The inner part of the star collapses to form a dense, hot white dwarf star with a mass about the same as the Sun but with a size of more like that of Earth! Eventually, white dwarf stars cool to become red, brown and eventually black dwarfs
What is a supernova?
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
How is a supernova formed?
In a larger-mass supergiant star, the temperature of the central core is hot enough for further fusion reactions involving nuclei of elements up to iron to occur. Once these are depleted, a violent explosion (a supernova) occurs at the outer part of the core
What is a neutron star?
A remnant of a supernova mass of sun collapsed into a sphere of 20km diameter. Intense gravitational fields cause them to spin. They emit intense radio waves from their polar regions allowing astronomers to detect neutron stars as pulsars
How is a black hole formed?
Formed when more massive (greater than about 3 solar masses) supernova remnants are formed
Give 3 pieces of evidence for the existence of black holes.
- Gravitational lensing Light bent by black hole’s gravity
- 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
How can we deduce the composition of a star?
Each element has it’s own unique set of spectral line wavelengths. Analysis of starlight can tell us what the star is made of.
How can we deduce the temperature of a star?
The temperature of a star determines it’s colour so spectral analysis can also be used to determine its temperature.
How can we determine the radial velocity of a star?
Analysis of the Doppler shift of spectral lines.
Describe the Harvard classification of spectral type.
Havard scheme uses the letters O, B, A, F, G, K, M
0 type stars are the hottest and M type stars are the coolest
What’s an HR diagram?
A plot of luminosity against spectral type.
