Seeing Double: The Stars of Autumn
At 9 p.m. on November 4, the bright hunter’s moon won’t interfere with observing five beautiful double stars.
Credit: Starry Night® Software

When the moon is close to full, amateur astronomers traditionally watch television. That?s because details on the moon are washed out by the high overhead sun, while faint objects in the sky are washed out by the bright moonlight.

The dedicated astronomer finds opportunities where others see disadvantages. If all we can see are a few of the brightest stars, let?s look at some of those stars.

Remember the scene in the first "Star Wars" movie where the young Luke Skywalker stands in front of a setting double sun? Such double stars are extremely common in the universe, and make wonderful observing targets in even the smallest telescopes.

Let?s start with a very wide double star, so wide that you can split it with binoculars. Enif is the brightest star in Pegasus, but it isn?t part of the Great Square of Pegasus.

If the Square makes up Pegasus? wings, Enif is the winged horse?s nose, almost half way from the Square to the bright star Altair in Aquila. The two components of Enif are of very different magnitude, 2.4 and 8.4, but are more than 2 minutes of arc apart, or almost one tenth of the diameter of the moon. They are too close together to split with the naked eye, but easy to see in any binocular. The 8th magnitude companion appears as a tiny speck just to the right of bright Enif.

Delphinus is a small constellation which hovers on the border between the summer constellations, like Cygnus and Aquila, and the autumn constellations, like Pegasus and Andromeda. In fact, it?s about half way between Enif and Altair, and fits perfectly in the field of a 7x50 or 10x50 binocular. It?s rare among constellations in that it actually looks like its namesake: a pretty little dolphin. Find the dolphin, and then take a close look at Gamma, the star in its nose. Just about any telescope will show this to be a pretty double star, two stars of almost equal brightness separated by about 10 seconds of arc. A magnification of about 100x should be just right for this star.

Cassiopeia is one of the best known circumpolar constellations, located just across the north celestial pole from the Big Dipper. Its five brightest stars form the familiar ?W? shape, and the star we are looking for, Eta, is just above the right downward point of the ?W,? part of the way towards the central upward point. This pair is rather unequal in brightness, 3.4 and 7.5 magnitude, and separated by 13 seconds of arc. Again 100x in any telescope should split it.

The three brightest stars of Andromeda form a chain stretching from one corner of the Square of Pegasus in the direction of Perseus: Alpheratz (Alpha), Mirach (Beta), and Almach (Gamma). It?s this last star, Almach, which is the double star. Again, the two components are separated by 10 seconds of arc, and are of somewhat unequal brightness, 2.3 and 5.5 magnitude. This is one of the prettiest double stars in the entire sky.

The final double, Theta Aurigae, is more of a challenge. Although it?s easy to find, as part of the bright pentagon of stars that make up Auriga, the Charioteer, it?s two stars are of different brightness, 2.6 and 7.1 magnitude, and rather closer than the other stars so far, slightly less than 4 seconds of arc apart. If you can?t spilt them at 100x, try a higher magnification.

If you find these double stars intriguing, you might want to observe more double stars on the list from which they come, the 100 double stars of the Astronomical League?s Double Star Club.

This article was provided to SPACE.com by Starry Night Education, the leader in space science curriculum solutions.