Type 1a supernovas are important in the study of the universe. Each of these white dwarfs— dead stars that no longer have fusion reactions—accumulates material from a companion star until the white dwarf reaches a certain mass limit, then re-ignites violently in a supernova. The predictable brightness of the explosions enables cosmologists to use them as “standard candles,” to compare with other objects in the universe. But mysteries abound about what makes up the companion star, which is destroyed by the explosion.
Astronomers recently used NASA’S Swift gamma-ray observatory to watch a supernova just as it ignited and smashed into its companion, giving off a signature ultraviolet light indicating that the companion was another white dwarf.
Coincidentally, another supernova’s companion, recently found on pre-explosion Hubble images, was identified as a red giant. Said Swift study astronomer Andrew Howell, “No wonder we’ve been so confused.... Apparently you can blow up stars in two different ways and still get nearly identical explosions.”