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Dr. Neal Sumerlin keeps us abreast of happenings in the night sky and the progress of the new Belk Astronomical Observatory.

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New Stars that are Really Old

Posted on 03/14/2008

One of the real pleasures of being the unofficial "astronomy person" in Lynchburg is hearing from so many people who have questions about astronomy. Anyone who knows me is aware that I love explaining things to people, and I can only hope that people learn something new as a result. I had the tables turned on me a couple of weeks ago. A good friend of mine revealed a heretofore unknown (to me at least) Lynchburg connection to a well-known astronomical phenomenon.

First, the phenomenon. The word "nova" literally means "new", and the astronomical use of it implies that the object is a new star. In fact, the only new thing about it is the temporarily increased visibility of a relatively old star.

Novae occur in binary star systems, where two stars are gravitationally bound and orbit very close to one another. Imagine these two stars, born at the same time with slightly different masses. The mass of a star sets its lifetime, so the more massive of these two will cycle through its various life stages and eventually wind up as a white dwarf star, just as our sun will in about five billion years.

A white dwarf is incredibly dense (it may have all of the mass of the Sun packed into a sphere the volume of the Earth), but it has ceased to combine hydrogen to helium in nuclear reactions at its core. It is this process that drives the energy release of a star like our Sun. In a white dwarf, all the nuclear fuel has been used up, and the star has collapsed to a much smaller and much denser sphere. This collapse itself, however, heats the remaining material to very high temperatures ("white-hot"), hence the term "white dwarf". But with no energy production in its core, the white dwarf is destined to slowly cool over millions of years as the energy radiated away into space is not replaced.

Unless it is one member of a close binary star system, that is.

The gravitational field of the white dwarf in such a system is strong enough to draw material away from its lower-mass companion which has not yet evolved to the white dwarf stage. This material is mostly hydrogen, the best nuclear fusion fuel there is, and it is being accumulated, heated, and compressed on the surface of the white dwarf. The astronomy textbook I currently use likens this to gasoline pooling on the floor of a match factory. When the temperature at the base of this layer of hydrogen reaches a critical temperature, there begins a runaway thermonuclear reaction. There is a tremendous explosion that blows this layer away from the star; we see this as a very sudden and very dramatic brightening of a star that may have been barely visible before. After a few days or weeks, the star fades, although it may take centuries for it to completely return to its pre-outburst state.

Now the Lynchburg connection. Amateur astronomers become very familiar with the pattern of stars in the sky-constellations are as familiar to them as your own backyard is to you. So when a "new" star appears, one that isn't part of that familiar pattern, it is as obvious as an unfamiliar dog would be in your yard.

On Friday night, August 29, 1975, Conrad Richardson of Lynchburg (my friend's stepdad) noticed an extra star in the constellation Cygnus, the Northern Cross. He quickly reported his finding to the appropriate authorities, and was preceded only by one Japanese observer who had the advantage of a night sky when the nova first flared up. The phenomenon is designated Nova Cygni 1975.

You can read more about novae here: http://en.wikipedia.org/wiki/Nova. There you will find that since 1890, there have only been 30 novae bright enough to be seen with the naked eye, and none brighter than Nova Cygni 1975 since Mr. Richardson first laid eyes on it from a Lynchburg patio. Sadly, Mr. Richardson passed away a few weeks ago, and I never had the chance to meet him. I wish I had.

But even before I knew of his role, I used Nova Cygni 1975 in my classes as THE classic example of a periodic nova. Now that I know of our local connection, I will be sure to mention it at every opportunity!

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