Astronomical Objects
Physics: AQA A Level, topic 9.2.6
Supernovae
Stars very much more massive than the Sun undergo a much more violent fate.
Massive stars
- Stars with masses significantly heavier than the Sun will explode in a supernova.
- The star undergoes catastrophic gravitational collapse once fusion at the core stops. The collapsing material produces a large emission of gamma rays and then rebounds away from the core in a colossal explosion.
- At this point, the luminosity of the exploding star is many millions of times larger than that of the Sun.
Type 1a supernovae
- Type 1a supernovae occur when a white dwarf orbits another, much larger star.
- The white dwarf attracts material from the other star and grows in mass.
- Once the mass goes above a certain value, the white dwarf explodes in a supernova.
Standard candles
- It is believed that type 1a supernovae give off roughly the same amount of energy and so can be used as a standard candle, much like cepheid variables.
- As these events are so bright, they serve as useful tools in the estimation of the distances to far-off galaxies.
Light curve
- The light intensity against time graph for a typical type 1a supernova is shown.
- The initial peak luminosity is due to the radioactive decay of nickel and is later dominated by the radioactive decay of cobalt.
- Nickel and other iron-like elements were most likely formed in the supernova itself.
Evidence for expansion accelerating
- Measurements of type 1a supernovae in the most distant galaxies provided the controversial evidence that the universe’s expansion is accelerating rather than slowing down.
- This is second hand evidence for the existence of dark energy.
Remnants of Supernovae
The remnant of a supernova can either be:
A neutron star
- Electrons and protons have been pressed together into an extremely dense clump of neutrons which rapidly spins (giving a pulsar).
A black hole
- This occurs for the most massive stars.
- The escape velocity for a black hole is larger than the speed of light, implying that once light has crossed the event horizon, it cannot escape the black hole.
- Scientists currently think there are supermassive black holes at the centre of galaxies.
Schwarzchild Radius
The radius of the event horizon of a black hole is known as the Schwarzschild radius.
Schwarzchild radius formula
- If the mass of the stellar remnant is M and the radius of the object is Rs, then the escape velocity can be equated to the speed of light:
Black hole
- Once the event horizon is larger than the radius of the object itself, then the object has become a black hole.
Test yourself on Astronomical Objects
When does a type 1a supernovae happen?
Which of these statements about supernova are true?
Which of these are true of black holes?
Which of these is NOT a possible supernova remnant?