It’s generally thought that SN2007bi is a clear example of a pair instability supernova due to its intensity and long brightness period, but now new supernova observations suggest that SN2007bi wasn’t a pair-instability supernova after all. But if it wasn’t, then how could it remain so bright for so long?
A supernova is a stellar explosion. They can occur when a large star exhausts its ability to fuse hydrogen into higher elements, and its core collapses. The resulting rebound rips apart the outer layers of the star, creating a supernova while the remains of the core collapses into a neutron star. Another type of supernova, known as a thermal runaway or type Ia, occurs when a white dwarf is a close companion with another star. As outer layers of the companion are captured by the white dwarf, it can trigger a runaway nuclear reaction that rips apart the white dwarf. This latter form always has about the same absolute brightness, which is why they are used to measure the distances of far galaxies.
Type Ia supernovae are brilliant stellar explosions that can outshine an entire galaxy. They also have the useful property of always exploding with a similar brightness. This makes them useful in determining the distances of galaxies. By comparing the observed brightness of a type Ia supernova to its standard brightness, we can calculate the distance of the supernova, and the galaxy in which it resides. But while we know type Ia supernovae have a consistent brightness, we aren’t entirely sure why.
Type Ia supernovae have property that is very useful for astronomers. When they explode, they have a maximum brightness that is fairly consistent. This means they can be used as “standard candles” to determine the distance of galaxies billions of light years away. One challenge we have with this type of supernova is that we haven’t confirmed what causes them.
The image above was taken by the Spitzer infrared space telescope in 2004. It shows what appears to be a red supergiant star in the galaxy known as M74, about 30 million light years away. It isn’t a particularly good image, because in 2004 it was just a red supergiant in a typical galaxy. There are billions of such stars in the universe, and its image was only captured because we do survey images of galaxies and such.
Last time I talked about how large stars can become a supernova through a collapse of their core. But this only occurs in stars much larger than our Sun. So how can a solar mass star become a supernova? For that, it has to dance with another star.
A star is driven by two basic forces: gravity and pressure. Gravity tries to squeeze a star as tightly as possible. This causes tremendous heat and pressure in the center of the star, which is great enough to ignite fusion in the star’s core. For most of a typical star’s life hydrogen and helium are fusing in the core, which creates enough pressure to balance the weight of gravity.
In the constellation Sagittarius is a small, dim supernova remnant. There’s nothing particularly unusual about it until you begin to look a bit closer.
The traditional view of Type Ia Supernovae is that they are caused by the explosion of a white dwarf after it reaches the Chandrasekhar limit. But new data shows that these supernovae are actually due to the collision of two white dwarfs.