Capella and Auriga: Winter's Bright Beacon

On winter evenings, there's a brilliant golden-yellow star that sits high in the northern sky, almost directly overhead from mid-northern latitudes. That's Capella, the alpha star of the constellation Auriga (the Charioteer), and the sixth brightest star visible from Earth. It's so far north that it's circumpolar from most of Europe and the northern United States, visible every clear night of the year, though it's at its highest and best during the winter months.

Capella: Not One Star, But Four

What appears to the naked eye as a single bright star is actually a system of four stars in two pairs. The main pair consists of two yellow giant stars, Capella Aa and Capella Ab, orbiting each other at a distance of about 100 million kilometers (roughly the distance from the Sun to Venus) with a period of 104 days.

Both stars are G-type giants, similar in spectral class to our Sun but far larger and more luminous. Capella Aa is about 79 times more luminous than the Sun with a radius roughly 12 times larger. Capella Ab is slightly smaller but still impressively large. Together, they produce the warm, golden-yellow light that makes Capella so visually distinctive.

A second pair of faint red dwarf stars orbits the main pair at a much greater distance. These are too faint to see visually, you'd need specialized equipment to detect them.

Finding Auriga

Capella makes Auriga easy to find. On a winter evening, look high in the sky for the brightest star that isn't Sirius (which is lower and to the south). Capella has a distinctly golden-yellow color compared to the blue-white of Rigel and Sirius.

The constellation itself forms a rough pentagon of stars. Capella marks the northern apex. Below it, a chain of stars outlines the charioteer's body, and a small triangle of fainter stars on the western side is traditionally called "the Kids" (baby goats, Capella means "little she-goat" in Latin).

Auriga sits in the Milky Way band, which means it's rich in star fields and clusters. Even sweeping through the constellation with binoculars reveals a stunning tapestry of background stars.

The Star Clusters of Auriga

Auriga's prime attraction for telescope users is its trio of Messier open clusters, all visible in the same low-power eyepiece field or binocular view:

• M36: A compact cluster of about 60 stars. At 100x, it shows a nice collection of moderately bright stars in a concentrated group. Resembles a smaller version of the Pleiades.
• M37: The richest and most impressive of the three. Contains over 500 stars, with a striking orange star near the center contrasting against the blue-white members. At magnitude +5.6, it's faintly visible to the naked eye under dark skies.
• M38: A large, scattered cluster with a distinctive cross-shaped pattern that some observers see in its brightest members. Just south of M38 lies the smaller NGC 1907, making for a nice contrasting pair.

All three clusters sit within a few degrees of each other, making Auriga's cluster trio one of the best binocular sweeps in the winter sky. For more on what to look for, see our guide to the best Messier objects for beginners.

Epsilon Aurigae: The Mystery Star

One of the most unusual stars in the sky sits in Auriga: Epsilon Aurigae, a supergiant star that undergoes a dramatic eclipse every 27.1 years. During the eclipse, which lasts about two years, the star dims from magnitude +3.0 to +3.8, a change easily visible to the naked eye.

What makes Epsilon Aurigae truly strange is the nature of the eclipsing body. Based on decades of study, astronomers believe the companion is a large, dark disk, possibly a dusty disk surrounding a hidden pair of B-type stars. The disk is so large that it takes two years to pass in front of the supergiant. The last eclipse ran from 2009 to 2011. The next begins around 2036.

If you're observing Auriga, take a moment to look at Epsilon Aurigae and consider what a remarkable system it is. Right now it's at its normal brightness, quietly ticking through its 27-year cycle before the next great dimming.

Best Time to Observe

Auriga is at its best from November through March, when it's high in the sky during evening hours. Capella is circumpolar from latitudes above about 44 degrees north, meaning it never sets from cities like Minneapolis, Montreal, or Paris. But it reaches its highest elevation, and the clusters are best positioned, during the winter months.

If you're planning a winter observing session, pair Auriga with nearby Taurus for a full evening of targets. See our Taurus constellation guide for details on what to observe next door.