A supernova is the explosive death of a star, which unleashes a burst of light through the cosmos. Supernovas happen in two different ways:
- When massive stars run out of fresh nuclear fuel, there is no more pressure to sustain them against their own weight. The central part of such a star then collapses. The outer layers of the star fall in on the core and then rebound in a tremendous explosion.
- Matter piling up on the compressed core of an already-dead star, known as a white dwarf, can reach sufficient density to trigger a thermonuclear explosion.
These violent deaths occur about once a century in a typical spiral galaxy like our Milky Way. Every 200-300 years we discover a supernova that happens to be bright and close enough to be visible to the unaided eye. The last supernova seen in our galaxy was discovered in 1604. When visible to the eye they appear in the sky as a "new" (Latin: "nova") star. A supernova observed in the "nearby" Large Magellanic Cloud (an irregular galaxy outside of our Milky Way galaxy) in 1987 was the first one visible to the unaided eye since 1604. The Hubble Space Telescope (HST) was not yet in orbit when the explosion was observed, but HST has made dramatic observations of the expanding gaseous remains of that star. Although supernovas are rare in our galaxy, there are many galaxies, and HST has observed stellar explosions elsewhere in the universe.
Supernovas blaze so brightly that they can be seen at distances of up to 10 billion light years. Light from these distant supernovas can tell us how the behavior of the universe has changed during the several billion years of the light’s journey to Earth.
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