Supernova 1994D. The supernova is the bright point in the lower-left. It is a type Ia thermonuclear supernova like those described by Howell. The supernova is on the edge of galaxy NGC 4526, depicted in the center of the image. NASA/Hubble Space Telescope
“Zombie” stars explode like bombs as they die, only to revive themselves by sucking matter out of other stars. According to an astrophysicist at the University of California, Santa Barbara (UCSB), this isn’t the plot for the latest 3-D blockbuster movie. Instead, it’s something that happens every day in the universe — something that can be used to measure dark energy.
This special category of stars, which explode as type Ia supernovae, helps probe the mystery of dark energy, which scientists believe fuels the expansion of the universe. Supernovae have been observed since at least A.D. 1054, when an exploding star formed the Crab Nebula, a supernova remnant.
Andy Howell from UCSB calls stars that have undergone a type Ia supernova “zombie” stars, because they’re dead with a core of ash, but can come back to life by sucking matter from a companion star. Over the past 50 years, astrophysicists have discovered that such stars are often part of binary systems — two stars orbiting each other. The one that explodes is a white dwarf star. “That’s what our Sun will be at the end of its life,” Howell said. “It will have the mass of the Sun crammed into the size of the Earth.”
The white dwarf stars that tend to explode as type Ia supernovae all have approximately the same mass. This was considered a fundamental limit of physics. However, in an article about 5 years ago, Howell reported stars that go beyond this limit. These previously unknown objects have more than the usual mass before they explode — a fact that confounds scientists.
Howell presented a hypothesis to understand this new class of objects. “One idea is that two white dwarfs could have merged together; the binary system could be two white dwarf stars,” he said. “Then, over time, they spiral into each other and merge. When they merge, they blow up. This may be one way to explain what is going on.”
Astrophysicists are using type Ia supernovae to build a map of the history of the universe’s expansion. “What we’ve found is that the universe hasn’t been expanding at the same rate,” said Howell. “And it hasn’t been slowing down as everyone thought it would be due to gravity. Instead, it has been speeding up. There’s a force that counteracts gravity and we don’t know what it is. We call it dark energy.”
Howell said that dark energy is probably a property of the fabric of the universe. “Space itself has some energy associated with it,” said Howell. “That’s what the results seem to indicate, that dark energy is distributed everywhere in space. It looks like it’s a property of the vacuum, but we’re not completely sure. We’re trying to figure out how sure we are of that; if we can improve type Ia supernovae as standard candles, we can make our measurements better.”
Throughout history, people have noticed a few supernovae so bright they could be seen with the naked eye. With telescopes, astronomers could discover supernovae farther away. “Now we have huge digital cameras on our telescopes, and really big telescopes,” said Howell. “We’ve been able to survey large parts of the sky, regularly. We find supernovae daily.”
“The next decade holds real promise of making serious progress in the understanding of nearly every aspect of these phenomena, from their explosion physics, to their progenitors, to their use as standard candles,” said Howell. “And with this knowledge may come the key to unlocking the darkest secrets of dark energy.”