Astronomy for Everyone

Northeast of the giant red star Aldebaran, (see column, Taurus, the Bull) you can see the bright yellow star, Capella (pronounced cuh-PELL-uh). This star forms the northeast corner of a large irregular hexagon of stars. This hexagon represents Auriga, the Charioteer.

Auriga (pronounced oh-RYE-guh), and the star Capella, were identified in Greek mythology with the goat that nursed the infant god Zeus. According to the myth, while playing with the animal, the baby god broke off one of its horns, to which he later gave the magical capability of dispensing great quantities of food and drink to whoever desired them - the cornucopia. The charioteer in the constellation is usually represented carrying two, or three, goat kids on his shoulder. Capella comes from the Latin word for "little she-goat".

Capella is the sixth-brightest star in our sky and among the most famous. It is located halfway between the celestial equator and the north celestial pole, making it visible at least some of the night every month in mid-northern latitudes. This, in turn, made it very important for navigation. It is a bright yellow star, similar to our sun in color, but much larger. Capella actually consists of two giant yellow stars, in close orbit around each other. Because they are so close together, we see Capella from Earth as a single bright star.

This double star system has some other characteristics that make it interesting to astronomers. Not only does it consist of two yellow giant stars revolving around each other every 104 days, there are two red dwarf stars that tag along with the larger yellow pair. They are quite far away from their larger siblings (at least .15 light-year) but astronomers suspect that the red dwarf stars are linked to the yellow ones because the stars have similar motions through space. If they are indeed traveling through space together, this would make Capella a quadruple star system.

Because the two giant yellow suns are so close together astronomically speaking - about 70 million miles apart, or the distance from our Sun to the planet Venus - they cannot be split visually by ground-based instruments. However, astronomers were able to determine that Capella was a double star by measuring the gravitational pull each star had on the other. The one star is of spectral type G1 (our own sun is of spectral type G2) and has a temperature of about 5,700 kelvins; the second star is cooler (type G9) and has a temperature of around 4,900 kelvins. After the separation of the stars was determined by a technique called interferometry, and the velocities and orbital period were calculated, it was possible to extrapolate each star’s size and mass. The G1 star is nine times the sun’s size and 2.6 solar masses; the G9 star is twelve times the sun’s size and weights about 2.7 solar masses.

There are several beautiful open star clusters within the constellation Auriga that can be viewed with binoculars. The first is M-36, a large open cluster containing about 60 stars. Draw a line between the two "bottom" stars of the hexagon. About halfway along this line, and above it, lies M-36.

Just above M-36, and slightly to the north, is M-38. This cluster is easily visible in binoculars and contains about 100 stars. Several of the brighter stars are arranged in pairs.

Go back to the line that forms the bottom of the hexagon. A little more than halfway along this line, and slightly below it, is the spectacular open cluster M-37. Often referred to as "a ruby in a field of diamonds", this cluster consists of about 150 stars that appear white. In the center of the cluster is a red star, a real standout against the "diamonds". A pair of 7 x 50 binoculars gives a lovely view of M-37; if you have trouble holding them steady, try propping your elbows on a table top, or the arms of a lawn chair.

The star located at the bottom corner, on a diagonal line from Capella, is really the star that forms the tip of the northernmost horn of Taurus, the Bull. But even though it is in a different constellation, it is a useful reference point in finding these three clusters. A friend with a telescope is also useful; these clusters are some of the most beautiful telescopic objects in our night sky. A closer look through a telescope at M-37, for instance, reveals that some of the "diamonds" have a faint yellow color to them.

Just south of M-38 is another, smaller open cluster called NGC 1907. This is a lovely sprinkling of stars that are mostly white or blue-white in color.

And while you’re peering at Auriga, don’t forget to check out the many double and multiple stars in this constellation. Epsilon Aurigae, for instance, which is just south and slightly above Capella, is a quadruple star system. There are seven double and multiple stars in Auriga that are well worth a look through a small telescope, as well as diffuse nebulae like IC 405 and IC 410, both located almost due south of M-38.

Auriga is located near the northern part of the Milky Way, and is full of interesting objects to observe. There are several other smaller star clusters and a wealth of other objects. Get out your binoculars, pan through the constellation, and see what you can find. And remember - when it gets dark, don’t forget to look up!