Supernova SN 2007sr was discovered in the southern tidal tail of NGC 4038 on December 18th, 2007 and first reported by Drake et al. SN 2007sr was a rare Type 1a SN which are used by astronomers to measure distances to host galaxies with high precision. This has had important implications for cosmological theory. The new distance to the Antennae was calculated from the light curves of SN2007sr to be 22 Mpc +/-3 or 72 million light years.
A Type 1a supernova results from the cataclysmic explosion of a white dwarf star. White Dwarfs are the end stage of intermediate mass stars ( like our sun) which have ceased nuclear fusion. White Dwarfs are limited to masses less than 1.38 solar masses. This is known as the "Chandrasekhar limit" beyond which the White Dwarf cannot support its own mass. If the White Dwarf acquires enough mass over time from a nearby rotating companion star (known as accretion) and begins to approach the Chandrasekhar limit it will begin to collapse under its own weight. The resulting pressure of collapse raises internal tempertures to billions of degrees which ultimately destabilizes the core of the white dwarf and causes the star to "unbind" and violently expel all its matter in a sudden cataclysmic explosion. At the instant of explosion an enormous shock wave is released ejecting matter at up to 3% the speed of light. Simultaneously there is an extreme increase in luminosity, typically 5 billion times the brightness of our sun, surprisingly with very little variation.
As mentioned above the light curves of all Type 1a supernove are extremely similar which has led to their use as a "standard candle" to measure distances in extragalactic astronomy. This in turn has helped astronomers to refine the rate of expansion of the universe which they now believe is undergoing accelerated expansion.
To see an image of SN 2007SR click Here