Flamsteed Astronomy Society

‘Cosmic Explosions, Dark Energy and the Fate of the Universe’ by Dr Mark Sullivan - February 13, 2012

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Mark showed his Toy Universe of Galaxies where an image is stretched and overlaid on the original image. Moving from the centre, the more distant objects move faster; the distance between the closer objects is smaller than distance between the more distant galaxies. Galaxies further away are moving faster and there is no unique centre to the expansion.



The universe expands as a result of the big bang. There are then two possibilities; either we have a 'closed universe' where eventually the expansion energy is overcome by the gravitational pull of matter; or the universe expands forever ('open universe') because the expansion energy is always greater than the gravitational pull of matter.



Fritz Zwicky coined the term “supernova” and discovered around 120 supernovae, suggesting a type Ia supernovae as the standard candle for measuring distances in space. A type Ia supernovae is an explosion of a white dwarf star. The gravitational pull of the white dwarf star pulls sufficient matter from a nearby star to increase it's mass beyond the Chandrasekhar limit.

This means that there is a standard amount of fuel to power the explosion, leading to a luminosity which is the same for all type Ia supernovae. From this standard luminosity, we can determine the distance to the supernova.

On 23 August 2011 a supernova was discovered in M101, 20 million light years away. The supernova went from non-detection to 17th magnitude in 24 hours, and then further increased in brightness every three days.

On the same evening, the Hubble Space Telescope was triggered, and the supernova soon reached magnitude 10. It was visible from the UK with small telescopes. This was the nearest type Ia supernova since 1986.

What did this supernova tell us? The supernova was found only 12 hours after the explosion (it actually happened over 20 million years ago!). Its luminosity tells us the size of the star which blew up; the luminosity measurement only works a few hours after the explosion. It was measured as less than 5% of the size of the sun, and confirms the theory that type Ia supernova originate from white dwarf stars.

No companion star was detected from Hubble Space Telescope imaging before the supernova exploded. This seems to rule out many of the single degenerative systems; it was not a red giant that caused the explosion.


In an expanding Universe, the further a galaxy is away from us, the faster it appears to have moved in the same amount of time.

Tycho's Supernova (SN 1572) was a supernova of Type Ia

SN2011fe type Ia supernova event in M101 in August 2011

Subrahmanyan Chandrasekhar, who determined the maximum mass of a white dwarf star