Greetings, stargazers.
We all have the rare opportunity this month to see a nova that is, as of this writing, visible to the naked eye.
The name nova literally means “new” star. (No, not a vintage model Chevrolet!)
This particular new star is designated Nova Delphini 2013. Delphini, the dolphin, is a small, dim constellation in the part of the sky near the summer triangle, so there are numerous nearby pointer stars that can aid finding the nova. Use the link below for a good star map of the region.
Although no one has observed a nova up close, there is a good model as to how a star such as this one can increase in brightness by a factor of 25,000 in a single day. A nova occurs when hydrogen gas accumulates on the surface of a white dwarf star and undergoes rapid fusion. But several circumstances are needed for this event. First, you need a white dwarf star, and second, you need a nearby companion star to provide hydrogen.
A white dwarf is the remnant core of a medium-sized star that is at the end of its life. This is what is expected of the sun after it has exhausted all the hydrogen and helium in its core, and fusion is no longer taking place. The core will be made of carbon, be about the size of the Earth and have a mass that is comparable to the sun. (That’s pretty dense!) When this carbon core first forms, it will be hot enough to drive off the outer layers of hydrogen and helium that remain, but not hot enough to fuse the carbon into some heavier element.
For a star like the sun with no companion star, a white dwarf will be the end of the line. The surface just gets cooler and cooler, as the heat radiates into space.
However, if a white dwarf is in a binary system, it will begin to attract hydrogen from the second star onto its surface. Eventually, enough hydrogen collects that gravity compresses and heats this surface layer until it restarts the hydrogen fusion in a flash. The flash will blow most of the hydrogen layer back into space, and it is this rapidly expanding sphere of hot gas that makes the brightness increase so quickly.
Eventually, the process repeats – hydrogen collects, condenses and heats until it fuses, then gets blown back into space.
Supernovae are much brighter and more catastrophic than novae such as this one, but I will have to save that discussion for another column.
This month
Look for the nova in Delphini. It will be near your zenith around 11 p.m. As of this writing, the star was still visible. Because the star is fairly dim, it will be easier to find before the moon rises. Binoculars will help with the identification, too.
Other nearby objects include the colorful double star Alberio in Cygnus, easily resolvable in a small telescope. Because the Milky Way passes through this part of the sky, it is a pleasure to scan with binoculars to see the rich star field and the numerous open star clusters.
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
Nova Delphini 2013: apod.nasa.gov/apod/ap130816.html
Four Corners Stargazers: groups.google.com/group/Four-Corners-Stargazers
