Greetings, stargazers.
Last month, I talked about the nova in Delphini. The “new star” was not one being born, but a star near the end of its life gasping “not dead yet!” The Delphini nova is now fading, but is still an easy target for binoculars. It is very possible that this star will “go nova” again in a few years or decades, we just have to wait and see.
A nova is a star that suddenly increases in brightness by tens of thousands of times. A supernova, in contrast, increases in brightness by several billion times. Like a nova, it appears to us as a new star, at least for the few weeks that we can see the explosion.
One way to get a supernova is when a white dwarf star happens to be a little bit too massive for the “regular” nova process. For novae like the one in Delphini, a white dwarf in a binary star system pulls in hydrogen gas from a companion star until fusion briefly begins on the surface. The brief flash is the nova.
But a white dwarf can only be so big, or it will be too massive for the nova process. This mass limit, called the Chandrasekhar limit, is about 1.4 times as massive as our sun. If the mass is slightly less than this, the gravitational pressure at the center, while very high, is not quite high enough to cause carbon fusion.
But a nearby companion star can provide the mass needed to push the white dwarf over the limit. If, while pulling matter from its companion, the white dwarf exceeds the Chandrasekhar mass limit, fusion can, and will, occur. Quickly. And it will occur throughout the star all at once.
This sure way to blow the entire star apart is a supernova of Type I. There is nothing left of the star after this.
Supernovae also can form when the core of a very massive star collapses (a Type II), but that is a different process, and has a different remnant.
Because supernovae are so much more luminous than novae, we typically detect them in distant galaxies. Sometimes it is even comparable in brightness to the rest of the galaxy it is in. On average, they occur about once per century per galaxy. The last one seen in the Milky Way was in 1604, so we are overdue for a nearby supernova. If one happens to go off in the Milky Way, we might be able to see it in broad daylight.
The list of supernovae in the link below, shows every one ever discovered. More than half the list is from the last decade, since modern, robotic search techniques have greatly improved the chance for discovery in those distant galaxies. A few of these every year can be seen with small telescopes but most would need photographic equipment on those small scopes.
This month
Mercury, Venus and Saturn are all low in the west for a short time after sunset, and Jupiter rises with Gemini after midnight.
The Four Corners Stargazers are planning a dark-sky star party at Andrews Lake on Saturday. Other planned activities, include events with Durango Nature Studies and the Mancos Library.
hakes_c@fortlewis.edu. Charles Hakes is an assistant professor in the physics and engineering department at Fort Lewis College and is director of the Fort Lewis Observatory.
Useful links
List of supernovae: www.cbat.eps.harvard.edu/lists/Supernovae.html
An astronomer’s forecast for Durango: www.cleardarksky.com/c/DrngoCOkey.html?1
Four Corners Stargazers: groups.google.com/group/Four-Corners-Stargazers
