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A second type of supernova can happen in systems where two stars orbit one another and at least one of those stars is an Earth-sized white dwarf. A white dwarf is what's left after a star the size of our sun has run out of fuel. If one white dwarf collides with another or pulls too much matter from its nearby star, the white dwarf can explode. Kaboom!

Not very. Astronomers believe that about two or three supernovas occur each century in galaxies like our own Milky Way. Because the universe contains so many galaxies, astronomers observe a few hundred supernovas per year outside our galaxy. Space dust blocks our view of most of the supernovas within the Milky Way.

NASA scientists use a number of different types of telescopes to search for and then study supernovas. One example is the NuSTAR (Nuclear Spectroscopic Telescope Array) mission, which uses X-ray vision to investigate the universe. NuSTAR is helping scientists observe supernovas and young nebulas to learn more about what happens leading up to, during, and after these spectacular blasts.

Every 50 years or so, a massive star in our galaxy blows itself apart in a supernova explosion. Supernovas are one of the most violent events in the universe, and the force of the explosion generates a blinding flash of radiation, as well as shock waves analogous to sonic booms.

Supernovas were originally classified on the basis of their optical properties. Type II supernovas show conspicuous evidence for hydrogen in the expanding debris ejected in the explosion; Type Ia explosions do not. Recent research has led to a refinement of these types, and a classification in terms of the types of stars that give rise to supernovas. A Type II, as well as Type Ib and Type Ic explosion, is produced by the catastrophic collapse of the core of a massive star. A Type Ia supernova is produced by a sudden thermonuclear explosion that disintegrates a white dwarf star.

Type II supernovas occur in regions with lots of bright, young stars, such as the spiral arms of galaxies. They apparently do not occur in elliptical galaxies, which are dominated by old, low-mass stars. Since bright young stars are typically stars with masses greater than about 10 times the mass of the sun, this and other evidence led to the conclusion that Type II supernovae are produced by massive stars.

Some Type I supernovas show many of the characteristics of Type II supernovas. These supernovas, called Type Ib and Type Ic, apparently differ from Type II because they lost their outer hydrogen envelope prior to the explosion. The hydrogen envelope could have been lost by a vigorous outflow of matter prior to the explosion, or because it was pulled away by a companion star.

The general picture for Type II, Type Ib and Type Ic supernovas - also called core-collapse supernovas - goes something like this. When the nuclear power source at the center or core of a star is exhausted, the core collapses. In less than a second, a neutron star (or a black hole, if the star is extremely massive) is formed. The formation of a neutron star releases an enormous amount of energy in the form of neutrinos and heat, which reverses the implosion. All but the central neutron star is blown away at speeds in excess of 50 million kilometers per hour as a thermonuclear shock wave races through the now expanding stellar debris, fusing lighter elements into heavier ones and producing a brilliant visual outburst that can be as intense as the light of several billion Suns.

Type Ia supernovas, in contrast, are observed in all kinds of galaxies, and are produced by white dwarf stars, the condensed remnant of what used to be sun-like stars. A white dwarf star, a dense ball primarily composed of carbon and oxygen atoms, is intrinsically the most stable of stars, as long as its mass remains below the so-called Chandrasekhar limit of 1.4 solar masses.

Because Type Ia supernovas all occur in a star that has a mass of about 1.4 solar masses, they produce about the s


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