Novas and Supernovas

¡SkyCaramba! Weekly astronomy blog for the week ending August 24, 2013

Several times a year, somewhere in the universe, a star suddenly becomes much brighter. Then it fades to its usual brightness. This is called a nova. Every few centuries, a star suddenly becomes much brighter and then fades out of existence. That’s called a supernova. Let me tell you more about what makes them different.

The word nova is Latin for new. The first known novas were so named because astronomers thought they were seeing new stars. Many of them had been too dim to see with the naked eye or even a telescope. So it’s no wonder they’d be mistaken for new stars when they couldn’t be seen before brightening up a lot.

Only some stars can erupt in brightness to become novas. They are white dwarfs, which are very old stars that have burned off their outer layer of hydrogen. And they must also be orbiting other stars. When a white dwarf is close enough to a companion star long enough to gravitationally draw off some of the other star’s hydrogen, the white dwarf is temporarily refueled.

There’s no set rule for how long the increase in brightness lasts. It may be just a night or two. Or it may take months to dim back down to the level it was. There’s also no guess as to how bright it will become.

Some stars that are sometimes even older explode when their lives end. These are supernovas. There are two kinds. Type I draws matter from another star much like a regular nova does. Except a Type I supernova doesn’t just burn off the excess and go back to being its old self. It takes on so much stellar matter and starts burning it so fast, it creates a stellar flash fire. Type I supernovas are also thought to be white dwarfs like novas.

Type II supernovas don’t take material from other stars. They happen when their own material separates as if in a centrifuge. Heavier molecules accumulate in the center and lighter ones are bobbed to the outside. This keeps happening until gravitational forces from the star’s core are so strong the star collapses. When atoms get close enough to each other, they repel each other through atomic forces far more strongly than they attract each other through gravitation. In a Type II supernova, the stellar material bounces back from the core like a ball off a gymnasium floor. It bounces off so fast, it never comes back.

Most novas and supernovas that astronomers know of happened so far from Earth, it took big telescopes in dark skies to see them. Many have happened in other galaxies. If one were to happen somewhat close to Earth, it would probably be bright enough to see in the daytime. A supernova that happened in 1604 about 20,000 light years from Earth was visible in the daytime for three weeks!

On August 14, 2013, an astronomer in Japan noticed a suddenly bright spot in the northwest corner of the constellation Delphinus. The star is now called Nova Delphini 2013. The astronomer had photographed the same piece of sky the night before and there was no star visible where Nova Delphini 2013 showed the next night. The photograph had showed stars as dim as 13th magnitude, meaning the nova must have been dimmer than that before it peaked at about 4.5 about two nights later. The higher the magnitude number, the dimmer the star. Magnitude numbers of less than 6 are naked eye objects.

Here are some links to more information about novas and supernovas. ¡SkyCaramba!