Astronomical Objects

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Stars very much more massive than the Sun undergo a much more violent fate.

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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.
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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.
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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.
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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.
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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:

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A neutron star

  • Electrons and protons have been pressed together into an extremely dense clump of neutrons which rapidly spins (giving a pulsar).
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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.

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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:
    • 12mv2=GMmRs\frac12 m v^2 = \frac{GMm}{R_s}
    • v2=c2=2GMRsv^2 = c^2 = \frac{2GM}{R_s}
    • Rs=2GMc2R_s = \frac{2GM}{c^2}
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Black hole

  • Once the event horizon is larger than the radius of the object itself, then the object has become a black hole.

Jump to other topics

1Measurements & Errors

2Particles & Radiation


4Mechanics & Materials


6Further Mechanics & Thermal Physics (A2 only)

7Fields & Their Consequences (A2 only)

8Nuclear Physics (A2 only)

9Option: Astrophysics (A2 only)

10Option: Medical Physics (A2 only)

11Option: Engineering Physics (A2 only)

12Option: Turning Points in Physics (A2 only)

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