Astro Bob: Tag along with Mars to Hyades
With a little help from the red planet, we'll find the Hyades star cluster and its striking double star, Theta Tauri.
As Earth speeds ahead of Mars in its orbit, the two planets are parting. One sure sign of such sweet sorrow is seeing Mars fade. While still bright, it's dimmed to magnitude -0.9 and now stands in fourth place behind Venus, Jupiter and Sirius. Only a couple weeks ago, it bested Sirius. Now, it's just a tad brighter than Canopus , the second-brightest nighttime star.
Mars remains easy to find at nightfall. Just look high in the southeastern sky well above Orion for a distinctive, golden-red "star." The dipper-shaped Pleiades cluster twinkles just 8° to its upper-right. The same distance below the Red Planet you'll see another stellar gathering similar to the Pleiades but more spread out and shaped like the letter "V." That's also a star cluster: the Hyades (HYE-uh-deez).
The Hyades were the sisters of the Pleiades and called the "rainy ones" by the ancient Greeks. Their rising and setting in fall and spring were thought to bring seasonal rains. In our time, we know the group as the nearest star cluster to Earth (only 153 light years away) and also the brightest. No optical aid is needed to see it, but binoculars will definitely enhance the view, making its primary stars appear even brighter and revealing numerous fainter members.
You'll notice Aldebaran first, a bright, orange gem that looks like a fainter version of Mars. Despite appearances, it's a foreground star (65 light years away) that just happens to lie along the same line of sight as the real Hyades. All the smaller stars in the V are genuine cluster members; their gravitational tugs on one another keep them together in a bunch. They travel through space much like a swarm of gnats or flock of blackbirds.
The cluster's core — what we see with our eyes — spans some 18 light years or about two-thirds the size of the Orion Nebula . The group recently celebrated its 625 millionth birthday. And while that sounds like an old age, the Hyades are actually seven times younger than the sun.
On a dark, moonless night, most of us might see a dozen Hyades with the naked eye and 50 or more in binoculars, but astronomers have identified 501 full-fledged members . There are also several hundred additional stranglers in the neighborhood that once belonged to the cluster.
Loose star clusters (called open clusters) like the Hyades "evaporate" over time as the overwhelming gravity of our Milky Way Galaxy slowly disassembles them. Even on the vast time scale of the universe, the maxim "enjoy it while you can" still applies.
One of the coolest sights in the Hyades is the stellar duo of Theta-1 and Theta-2 Tauri , one of the few double stars you can split without optical aid. Located a short distance to the right of Aldebaran, it's comprised of two third magnitude stars.
I see twin beads of light like two tiny planets in a very close conjunction. They're 1/10° apart, equal to about 1/5 of a full-moon diameter, and look absolutely exquisite. If you're unsure of seeing both, train binoculars on them to confirm their appearance. Then try seeing them with just your eyes. When using binoculars, take a moment to check out the stars' contrasting colors. Theta-2 is white and Theta-1 pale orange.
As with so many of the naked-eye stars appearance isn't necessarily an indication of reality. Theta-2 is about four times larger than the sun and shines 60 times brighter, while its companion, Theta-1, is 10 times as big and some 70 times more luminous. Their distance of 157 light-years greatly diminishes the pair's true brilliance. The light we see left the duo around 1866 just after the Civil War ended.
While you're in the area, be sure to give a peek at the Pleiades in binoculars. With all the added stars, it's one of the sky's finest sights.