Stargazing - 04.10.12
Sirius, the “Dog Star,” is dropping from the evening sky.
It is fairly low in the south-southwest at sunset and sets in mid evening.
Sirius is the brightest star in the night sky.
It looks like a brilliant white gem.
Some white-dwarf stars are galactic thieves—they steal gas from companion stars. But others appear to be victims instead of culprits—other stars steal from them. The victims form a class of white dwarfs that was only recently discovered.
A white dwarf is “born” when a normal star can no longer produce nuclear reactions in its core. The star casts its outer layers into space, leaving only its dead core. The core is extremely hot, though, so it continues to shine—as a white dwarf.
Most white dwarfs are at least about 40 percent as massive as the Sun. There shouldn’t be any white dwarfs much less massive than that because the stars that would give birth to them are still shining.
Even so, astronomers have recently discovered white dwarfs that are as little as one-fifth as massive as the Sun. These stars all have companions in tight orbits. Some of the stars are so close together, in fact, that they’re orbiting at speeds of up to one or two percent of the speed of light.
The low-mass white dwarfs in these systems must have lost much of their material to their companions—it was siphoned away by the other star’s gravity. With less material, the white dwarf has lower surface gravity, so it’s much puffier than a normal white dwarf.
Astronomers are studying these stars in more detail to learn more about how they formed, how they differ from other white dwarfs, and what will happen to them in the future.