Algol

Algol in the Optical

This is a 3600 second, IIIa-J exposure with a GG395 filter from the Digitized Sky Survey. Like many astronomical pictures which look pretty, this image of Algol is of no practical use. Algol is bright, varying from 2nd magnitude to 3rd magnitude. In this long exposure, the light from Algol has used up all the chemicals on the photographic plate and started spilling over. It would be impossible to measure the brightness or postion of Algol using this plate. The exposure is long because the Digitized Sky Survey is more interested in those little stars in the background, which are much much too faint to be seen with the naked eye. This images has been digitized from a photographic plate and false-colored-- brighter pixels are colored yellow, intermediate pixels are colored blue, etc. so this is not Algol's real color.
I nabbed this image from SkyView. Here's the copyright info.

Algol has a long and important history. It was known to ancient astronomers because it is one of the few variable stars which changes brightness enough that you can notice the change with your naked eye. Its name is derived from the same Arabic root as "ghoul" and it is called the Demon Star because of its red color and its changablility. It goes from bright to dim and back to bright again every couple of days.

The advent of telescopes and sophisticated photometry (measurement of the brightness of stars) allowed astronomers to study the light curve of Algol and determine that it is in fact a tertiary, or three-star, system. Two stars are in a close binary. The stars are so close together (4 solar radii apart) and so far from the earth (29 parsecs away) that they appear to be only 2.28 milliarcseconds or one millionth of a degree apart. That means that the two stars look all mooshed together into one star to even the strongest optical telescopes (like in the SkyView image). This is a problem of resolution.

One star, a B star, is about three-and-a-half times the mass of the sun. The other, a K star, is about .8 times the mass of the sun. The lighter star, the K star, is nearly four times the radius of the sun and is so bloated that it sometimes spills material onto its heavier partner, which is ony 2.89 times the radius of the sun. The lighter star orbits the heavier star about once every 2.86 days. Algol seems to get dimmer when the K star passes between the B star and the earth, blocking the B star's light. It also gets dimmer when the K star ducks behind and the B star blocks it. Algol is brightest, of course, when you have an unobstructed view of both stars. This type of variable star is called an eclipsing binary system, and Algol was the first system of this kind to be discovered. Algol also has a third star 1.5 times the mass and radius of the sun which orbits once in 1.86 years, and, as Kepler was clever enough to point out, the longer the orbit, bigger the orbit, so it's kind of out there doing its own thing.

My project is to look at Algol in radio wavelengths using the Very Long Baseline Array. The VLBA (which is run by the NRAO which is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.) is a system of 10 radio telescopes scattered across the United States from Mauna Kea to St. Croix in the Virgin Islands plus the Very Large Array (which is on the Plains of San Augustin, New Mexico). The idea behind Very Long Baseline Interferometry is aperature synthesis. If you hook up two antennas 1000 miles apart, they will give you the same resolution as one telescope 1000 miles wide. The VLBA has resolution that would let you stand in New York City and read a newspaper which was being kindly held up by a person in San Francisco. Of course, they don't print newspapers whose the words emit radio waves, but you've got to admit that that sort of resolution is pretty impressive.

So impressive, in fact, that you can actually resolve the Algol system. You can't actually see the B and K stars, though, because they don't emit in the radio. What I do observe is the synchrotron emmision region surrounding the K star.
Algol in the Radio

This is a radio map of Algol made using the Very Long Baseline Array. This is X-band data. The double lobe is an artifact of the imaging process. We're trying to get rid of it and find out more about the actual structure of the source. We've done a lot of work on the orbits of the system. Because we resolve the system, we can actually watch the K star moving back and forth across the sky. There were lots of contradictory descriptions of the three star's orbits in the literature, and we think, that combining our data with other recent VLBI observations, we have the best answers so far. Dr. Molnar's page on The Dynamical Evolution of the Algol Triple System is pretty technical, but it features a nice cartoon of the close binary.

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brs@astro.cornell.edu
Last modified: Tue May 22 14:24:59 EDT 2001