Save our skies

Be a considerate neighbor: Reduce nighttime glare. Shield all your outside lights downward (or turn them off completely) and enjoy the beautiful, starry night sky.

Night Sky—Winking stars

—CHARLIE CHRISTMANN
Like clockwork, Algol, in the constellation Perseus, winks for two hours every 2.87 days. Algol was the first of its class, known as eclipsing variable stars, to be discovered, and it’s still the most famous one. Check on it daily, and you to will notice a distinct change in brightness compared to its neighbors.

The name Algol means “demon star,” from “the ghoul.” It probably received its name from its peculiar behavior. In the constellation Perseus, it represents the eye of the Gorgon Medusa.

The dimming wink is a result of one dimmer star passing in front of another brighter companion. For this to happen, the stars must be in orbit around each other. In Algol’s case, the orbit takes 2.87 days. As the dimmer star, Algol B, passes in front of the brighter star, Algol A, the amount of light reaching Earth temporarily decreases. The normal magnitude is a fairly bright, constant 2.1, but dims to 3.4 magnitude for two hours centered on the time of the eclipse.

Actually, Algol happens to be a triple-star system. The two stars involved in the eclipsing are separated by only 5.8 million miles, while the third star, Algol C, is at an average distance of two hundred seventy-five million miles from the pair, and orbits them every 1.86 years.

Currently, the Algol trio is 92.8 light years from Earth. But, about 7.3 million years ago, it passed within 9.8 light years of us. At that distance, it would have been much brighter than Sirius, at approximately 2.5 magnitude. At that close distance, the gravity may have distorted the Oort cloud of debris orbiting at the very edge of our solar system. That distortion may have caused some of the objects there to fall into the inner solar system as new comets.

The best time to see Algol this time of year is before sunrise. It will be low in the northeast around 5:00 a.m. in May. Mid-month will be best around the time of the New Moon. A good comparison star is Almaak, magnitude 2.1, in Andromeda. You can compare Algol’s brightness with them at a glance.

Algol is not the only eclipsing variable in the sky that can be discerned by the naked eye. There are three others besides Algol that we can see in the northern hemisphere.

Elthor, the lambda star in Taurus, is another eclipsing binary. Its smaller range in magnitude, 3.4 to 3.9, and fourteen-hour eclipse makes it more difficult to observe. This, too, is a triple-star system. The two eclipsing stars orbit each other every 3.95 days. The third, dimmer star has an orbital period of thirty-three days. The Lambda Tauri system is approximately three hundred-seventy light years from Earth.

Sheliak, the beta star in Lyrae, is another eclipsing binary, but it appears different than Algol. The two stars of Beta Lyrae are distorted by each other’s gravity and have become elongated into an ellipse. As the stars orbit every 12.94 days, a continuous change from bright to dim can be observed. The magnitude range is between 3.3 and 4.4. There may be a third, very dim star in this system as well.

Eta Geminorum, also called Tejat Prior, Propus or Praepes, is a multiple-star system whose brighter component is a semi-regular red giant. Most of the time, it varies very little from its normal magnitude of 3.2. But every 8.4 years, the red giant is eclipsed by its dimmer star, causing the pair’s total brightness to drop to about 4.0. Again, there is a third companion star in this group. This system is located three hundred-fifty light years from the Sun.

So, find a nice, dark location and see if you, too, can see Algol, Elthor, or Sheliak give you a wink.

THE PLANETS AND THE MOON
• Mercury will be paying a visit to the evening sky shortly after sunset, low in the west, during the last half of the month. On May 17, look for the Moon to join Mercury about forty-five minutes after sunset in the northwest. The Moon will be a thin one-day-old crescent. You may need binoculars to find Mercury.
• Venus will still be shining brightly in the west. It is bright enough to find before dark.
• Mars is an early morning target, rising from 4:00 a.m. early in the month, to 3:00 a.m. late in the month. On Mother’s Day, May 13, you can find the Moon and Mars above the eastern horizon an hour before sunrise.
• Look for Jupiter to rise from about 11:00 p.m. to 8:30 p.m. as the month progresses. One hour after sunset on May 31, you have the chance to see the Full Moon and Jupiter together. Antares is also in the mix, just above the moon.
• Saturn is forty-five degrees above the western horizon around 9:30 p.m. Saturn and the Moon join up on May 22, about two hours after sunset. Saturn will be to the lower right of the Moon, and Regulus will be to the upper left of the Moon. Procyon will be on the horizon almost due west.
• The Moon is full at 4:09 a.m. MDT on May 2. There will be a “Blue” Moon, the second Full Moon in the same month, on May 31. The New Moon is on May 16 at 1:27 p.m. MDT. Lunar perigee at 223,315 miles from Earth is on May 15, and apogee at 251,939 miles is on May 27.

If you have a question or comment for Charlie, you may email him at k5cec@yahoo.com.