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Our friends at the Golden Pond Planetarium in Golden Pond, Kentucky, asked, "How are supernovas formed and are there any getting ready to form now?"
Supernovas are among the most powerful and spectacular events in the universe. Most of the changes that take place in the universe happen very, very slowly in human terms. For example, it took millions of years for our solar system to form and another 4.5 billion years more for intelligent life to evolve on one of its planets. Our Sun is still only about half-way through its expected lifetime. A supernova, though, happens in only about 15 seconds!
A supernova is the biggest explosion you can imagine, the brilliant, dying gasp of a star that is at least five times more massive than our Sun.
You see, a star is a balancing act between two huge forces. On the one hand, the crushing force of the star's own gravity tries to squeeze the stellar material into the smallest and tightest ball possible. But on the other hand, the force of the tremendous heat and pressure from the nuclear fires burning at the star's center tries to push all that material outward. When the star has used up all of its nuclear fuel, the outward pressure is no longer able to counteract the gravity, and the star suddenly collapses. Imagine something one million times the mass of Earth collapsing in 15 seconds! The collapse of the core happens so fast that it makes enormous shock waves that blow the outer part of the star into space at 20,000 kilometers per second (50 million miles per hour)!
Usually a very dense core is left behind, along with an expanding cloud of gas, called a nebula. Stars that are more than about 10 times the size of our Sun may leave behind the densest objects in the universe—black holes.
Supernovas are not very common. Astronomers believe that in galaxies like our own Milky Way, about 2 or 3 supernovas occur each century. From Earth's location within the Milky Way galaxy, interstellar dust blocks our view of many of them. Because the universe contains so many galaxies, astronomers observe a few hundred supernovas per year outside our galaxy. They can be seen virtually to the edge of the universe, as these fantastic events can be so bright they outshine their entire galaxy for a few days or even months. The aging of stars is well enough understood that astronomers can predict when a star will become a supernova, and we know that none of the stars in our neighborhood of the Milky Way will put on such a spectacular display soon.
It is fascinating to realize that the massive stars that become supernovas are factories for producing and distributing all the raw materials needed to make everything else. Inside their cores, the nuclear fusion reactions create nearly all the atoms that make up planets, moons, asteroids, and us. The carbon in your proteins, calcium in your bones, oxygen you breathe, iron in your blood, and almost all the other atoms in your body were manufactured inside a star! But ordinary stars don't get hot enough to make any atoms heavier than iron. To make heavier elements like gold, silver, lead, and mercury requires the very special conditions of pressure and heat that exist inside a supernova during those few seconds of the collapse. Then the rebound explosion as the star blows itself apart flings all those elements into space.
Eventually the material that is dispersed through space collects and forms a new star and new planets. That new solar system then is fully supplied with all the resources it needs for making planets like Earth, with the iron for the core, the materials for the rocky surface and for the air, as well as the all the ingredients that can make plants and animals—all of it having been created in stars or at the moment of a supernova and all of it released from the star during the supernova explosion.
Visit The Space Place image gallery to see images of beautiful nebulas, which are the glowing, colorful clouds left over from supernovas.