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A supernova is when a very big star explodes. This happens when a star runs out of energy to make heat and light, so it collapses, then explodes. The biggest of these stars we know of are called hypergiants and smaller ones are called supergiants. They are very big and because of gravity they press on their centers very hard and use up their energy very quickly, so they usually only live for a few million years. After exploding, the remnant becomes a black hole or a neutron star. Most stars are small and do not explode. They cool and shrink down into a white dwarf star.
Supernovas are very big explosions. When the star explodes, it will be brighter than all other stars around it. If a supernova explosion happened near the Earth, we could see it in the sky even during the day.
Supernova explosions happen rarely. In our own galaxy, the Milky Way, the last supernova happened in the year 1604. We can see supernovas in other galaxies too. Every year we see 300 supernovas in other galaxies, because there are so many galaxies. Sometimes they are brighter than the whole rest of the galaxy.
Supernovas are usually sorted into Type I and Type II supernovas.
Type I supernovas have absorption lines that make them look like they do not have hydrogen in them. One important kind of Type I supernovas are Type Ia supernovas. These supernovas are very bright for a short amount of time. Then they get less bright very quickly. Type Ia supernovas happen when a white dwarf star is orbiting a big star. Sometimes, the white dwarf star sucks matter off of the big star. When the white dwarf gets to be about 1.4 times the mass of the sun, it collapses. This makes lots of energy and light, which is why supernovas are very bright. Type 1A have mostly the same brightness.
Type II supernovas have absorption lines that make them look like they do have hydrogen in them. They get very bright, and then they stay very bright for a long time. In a star, nuclear fusion turns hydrogen into helium. In very large stars, helium gets turned into oxygen, and so on. When nuclear fusion happens, it makes lots of energy and heat. This makes the star stay big. However, when iron is formed, it cannot be turned into any other elements. When there is enough iron, there is less heat, so the star gets smaller. Then the star collapses which makes lots of energy and light.
Supernovas and life
Without supernovas there would be no life on Earth. This is because many of the chemical elements were made in supernova explosions. These are called "heavy elements". Heavy elements are needed to make living things. The supernova is the only way heavy elements can be made. Other elements were made by fusion in stars. Heavy elements need very high temperature and pressure to form. In a macho supernova explosion the temperature and pressure are so high that heavy elements can be made. Scientists call this supernova nucleosynthesis.
It could be dangerous if a supernova explosion happened very close to the Earth. The explosion is very big and many kinds of dangerous radiation are formed. But we do not have to be afraid. Only very big stars can explode as supernovas. There are no stars big enough near the Earth and if there was it would take millions of years for it to happen.
Supernovas and science
SN 1572 was seen by Tycho Brahe. This supernova helped astronomers learn that things in space could change. SN 1604 was seen by Johannes Kepler. It was the last supernova close enough to be seen from Earth's northern hemisphere without a telescope. SN 1987A is the only supernova so close that scientists could find neutrinos from it. SN 1987A was also bright enough to see without a telescope. People in the southern hemisphere saw it.
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