Fall offers night views

Posted: Sunday, September 07, 2003

Mars, Uranus and Neptune are the planets to watch this month, appearing in the south to southeast in the late evening. I described how to view Mars' motion across Aquarius and how to find Uranus in last month's column and will add about Neptune next month.

Also, around and near Sept. 27, look due east between 6:30 and 7:30 a.m. to first find bright Jupiter very low (beneath bright star Regulus), then Mercury even closer to the horizon.

This month, I will focus on a special star: Algol, the second brightest in the constellation Perseus. Many stars are classified as variables that is, varying in brightness. A variation in brightness can be caused by a star's intrinsic structure, for example it can pulsate. Or it can be caused by a companion star literally eclipsing it.

Actually half of the stars we see are binary or multiple star systems, whose stars consequently orbit each other as dictated by the law of gravitation.

But their orbits may be quite tilted with respect to our vantage point in our solar system; except for a few binary systems like Algol where we see their orbits almost edge-on. And that's why Algol, the brighter component of such a binary star system, and its companion regularly eclipse each other.


Diagram courtesy of Andy Veh

To first orientate yourself in the starry skies, find the Big Dipper low in the north, then extend the dipper's last two stars high into the north to find Polaris, the North Star, and the Little Dipper. With the Big Dipper on one side of Polaris, find the constellation Cassiopeia, a nice-sized W, on the other side of Polaris high in the east.

From here follow down toward the eastern horizon to find the double cluster in Perseus a medium-sized right triangle consisting of Algol, Almaak and Mirphak (the latter is embedded inside quite a few yet-fainter stars). All three exhibit almost the same brightness. As a further landmark use Pegasus, the Great Square to the right (off the chart), which is also in the east. If you need a larger finder chart, you can find one on my Web site at chinook.kpc.alaska .edu/~ifafv. Click on "Lecture Notes," then "The Monthly Sky."

Algol and its companion star are in a tight orbit, circling each other almost every three days (this compares with a 90-day orbit of our Sun's most speedy and close companion, the planet Mercury). In such a tight orbit, the two stars easily eclipse each other. The deeper of the two eclipses happens when the brighter component, Algol, becomes gradually obscured for about eight hours and its brightness diminishes appreciably. Algol's eclipse of its fainter companion a day and a half later is hardly visible because much less light is masked.

The next times to see these ever-repeating eclipses in Alaska are Tuesday-Wednesday from 11 p.m. to 3 a.m., Friday-Saturday from 11 p.m. to 3 a.m., Sept. 29-30 from midnight to 4 a.m., Oct. 2-3 from 10 p.m. to 6 a.m. and Oct. 5-6 from 10 p.m. to 2 a.m.

Algol is usually of almost the same brightness as Mirphak and Almaak, the other two stars in this right triangle. But during an eclipse it fades to the same faintness as the star just below, boxed in in the diagram.

During the five nights just listed, Algol would fade, brighten, fade, fade and brighten, brighten, respectively.

Another eclipsing binary star is Sheliak, the second brightest star in the constellation Lyra, next to very bright Vega, high in the fall sky. Algol has the largest change of brightness, thus it is the one described here at length.

Andy Veh is the physics and astronomy instructor at Kenai Peninsula College. This column appears on the first Sunday of each month. He can be reached at aveh@uaa.alaska.edu.

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