Aldebaran

How to find it

Start with Orion's Belt. Follow the line of the three stars up and to the right, as seen from the Northern Hemisphere, for about 20 degrees, two fists at arm's length. The first bright star you meet is orange Aldebaran. Continue roughly the same distance again and you reach the Pleiades, the tiny misty dipper of stars.

Aldebaran sits at one tip of a V of fainter stars, the Hyades, which outlines the bull's face. From 40°N it rises in the east on November evenings and stands high in the south, about 65 degrees above the horizon, in late December and January. It is also one corner of the Winter Hexagon of bright stars.

What kind of star it is

Aldebaran is an orange giant of type K5. Like Arcturus, it started out similar to the Sun, with only slightly more mass, about 1.1 to 1.2 Suns. After using up the hydrogen in its core it swelled to about 44 times the Sun's width.

Its surface is about 6,500°F, cooler than the Sun's, which is why it glows orange. Because it is so large it gives off about 400 to 500 times as much light. Its brightness changes slightly and irregularly, by up to about a quarter of a magnitude, too little for most people to notice.

Notable features

Aldebaran looks like the brightest member of the Hyades, but this is a line-of-sight coincidence. The Hyades cluster is about 150 light-years away, more than twice Aldebaran's distance. Aldebaran simply happens to lie in front of it.

Because it lies close to the path the Moon follows across the sky, Aldebaran is one of the few first-magnitude stars the Moon can cover, an event called an occultation. These come in series lasting a few years. The most recent series ran from January 2015 to September 2018, and the next is not expected to begin until 2033. When it does, watching the star blink out at the Moon's dark edge in a small telescope is unforgettable.

In 1972 NASA launched Pioneer 10, the first spacecraft to cross the asteroid belt and fly past Jupiter. Contact was lost in 2003, and the silent probe is coasting in the general direction of Aldebaran. At its speed it would need more than two million years to cover the distance. Astronomers have also reported signs of a giant planet around Aldebaran, but the signal may instead come from the star's own activity, so the planet remains unconfirmed.

Name and history

The name comes from the Arabic al-dabaran, 'the follower'. The star rises shortly after the Pleiades and seems to chase them across the sky every night. Ottoman astronomy texts recorded it as ed-Deberân, described as the bright red star in the Bull's eye.

Aldebaran's place in the Bull has long roots. The figure of a bull in these stars goes back to ancient Mesopotamia, and Greek and Roman writers kept it. In English the star is often simply called the Bull's Eye, and its red-orange color helped make it one of the best known stars of the zodiac.

Observing tips

Aldebaran's orange color is clear to the eye, but it is easiest to see side by side with other stars. Compare it with Betelgeuse in Orion, which is redder, and with blue-white Rigel, and you have three steps on the scale of star temperatures.

Binoculars are the best tool for this part of the sky. At low power they frame Aldebaran together with the many stars of the Hyades; notice how its warm glow stands out against the cluster's mostly white and yellow stars. Then sweep up to the Pleiades for a contrast in young, blue stars.

Planets often pass near Aldebaran, because it lies close to the ecliptic. When Mars is nearby, the two make a good color comparison; astronomy apps list the dates of such close pairings.