MessageToEagle.com - Type Ia supernovae are violent stellar explosions but astronomers know very little about the
stars they come from and how the explosions happen.
Observations of their brightness are used to measure the expansion of the universe and have shown scientists that the
universe is expanding at an accelerating rate.
Type Ia supernovae are believed to be thermonuclear explosions of a white dwarf star that’s part of a binary system--two
stars that are physically close together and orbit around a common center of mass.
White dwarf stars are one of the densest forms of matter, second only to neutron stars and black holes. Just a teaspoon of
matter from a white dwarf would weigh five tons.
Because white dwarf stars are so dense, their gravity is particularly intense.
The white dwarf will begin to pull material off its companion star, adding that matter to itself.
When the white dwarf reaches 1.4 solar masses, or about 40 percent more massive than our Sun, a nuclear chain reaction occurs,
causing the white dwarf to explode. The resulting light is 5 billion times brighter than the Sun.
Because the chain reaction always happens in the same way, and at the same mass, the brightness of these Type Ia supernovae
are also always the same. The explosion point is known as the Chandrasekhar limit, after Subrahmanyan Chandrasekhar,
the astronomer who discovered it.
There are two different possibilities for how Type Ia supernovae are created from this type of binary system.
Multiwavelength X-ray / infrared image of SN 1572 or Tycho's Nova, the remnant of a Type Ia supernova (NASA/CXC/JPL-Caltech/Calar Alto O. Krause et al.)
In the so-called double-degenerate model, the orbit between two white dwarf stars gradually shrinks until the lighter star
gets so close to its companion that it is ripped apart by tidal forces.
Some of the lighter star’s matter is then absorbed into the primary white dwarf, causing an explosion.
In the competing single-degenerate model, the white dwarf slowly accretes mass from an ordinary, non-white dwarf star,
until it reaches an ignition point.
"Previous studies have produced conflicting results. The conflict disappears if both types of explosion are happening,”
explained lead author Ryan Foley of the Harvard-Smithsonian Center for Astrophysics.
Merging white dwarfs appear to cause Type Ia supernovae in elliptical galaxies. Sun-like stars eventually evolve
into white dwarfs; one can imagine a binary star system with two Sun-like stars one day leaving behind a pair
of white dwarfs that over millions of years slowly spiral towards one another before merging and exploding.
Image: NASA/CXC/M Weiss.
The research team studied 23 Type Ia supernovae to look for signatures of gas around the supernovae, which should be
present only in single-degenerate systems.
They found that the more powerful explosions tended to come from “gassy” systems, or systems with outflows of gas.
However, only a fraction of supernovae show evidence for outflows--the remainder likely come from double-degenerate systems.
This finding has important implications for how astronomers use supernovae to measure the universe’s expansion.
“To maximize the accuracy of our measurements we may have to separate the two kinds of Type Ia supernovae,” Simon said.
“This study gives us one potential way to tell them apart.”
The Nobel Prize in Physics was awarded December 10, 2011, to three astronomers for their "discovery of the accelerating
expansion of the Universe through observations of distant supernovae.”
New research from a team led by Harvard University and including Carnegie’s Josh Simon, Chris Burns, Nidia Morrell,
and Mark Phillips examined 23 Type Ia supernovae and helped identify the formation process for at least some of them.
Their work will be published in The Astrophysical Journal and is available online.
@ MessageToEagle.com via Carnegie Institution of Washington
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