In a paper published in the October 28, 2004 issue of Nature, researchers say that they have found the likely binary companion to the white dwarf that created the Tycho supernova remnant. The significance of this result is that it gives the first observational support to the theory that type Ia supernovae are caused by the thermonuclear explosion of white dwarf stars when they exceed a critical mass called the Chandrasekhar mass. In practice, the white dwarf is born with a mass below this limit, so for it to reach the Chandrasekhar mass, it must pull in more material. If it is in a binary system with a moderately-sized main-sequence star, then this occurs through a process called Roche-lobe overflow, where the main-sequence star slowly expands and dumps its outer layers onto its companion. When the white dwarf reaches the Chandrasekhar limit, it collapses, its density and temperature rises, and its oxygen and carbon fuse into iron and other heavier elements, releasing energy that blows the star apart in a supernova.
The companion main-sequence star would suddenly find itself free, and it would move away at a velocity equal to its orbital velocity, which would be much greater than the average velocity of the neighboring stars. The research published in Nature finds such a high-velocity star. This star has a velocity that is three-times the velocity of surrounding stars. The alternative explanation for a high-velocity star, that it is an old star from the galactic halo, is eliminated because of the star's high metalicity, as expected for a relatively-young star created in the galactic disk. The its velocity is also the correct value for placing the star at the center of the Tycho nebula on November 11, 1572, the date that Tycho Brahe saw the supernova at the neubla's position.