In my last stargazing story, we looked at the Winter Hexagon, an asterism of six stars spanning six constellations. Today we’ll take a closer look at two of these constellations, Taurus (the bull) and Auriga (the charioteer).
Taurus is the more interesting of the two. Situated above Orion’s head, it is marked by the orange star Aldeberan and plays host to two naked-eye star clusters. If you have a telescope or even binoculars and reasonably dark skies, you’ll also find the famous Crab Nebula there, the remnant of a bright supernova — a dying star that literally blew itself apart — observed by Chinese astronomers in A.D. 1054.
Let’s start with Aldeberan. Stars, as you probably know, come in a range of sizes from white dwarfs no larger than the Earth to supergiant stars so huge they might fill the orbit of Saturn. Aldeberan is an orange giant that lies 65 light years away. I’m 60 years old now, so the light we now see from Aldeberan left that about star five years before I was born. (Astronomy is time travel!) It has a radius 44 times that of the sun. Its magnitude is about 0.85.
If you don’t remember our discussion of star brightness, here’s the explanation:
Usually the fourteenth brightest star in the night sky, Aldeberan’s brightness varies slightly over time, about two tenths of a magnitude. That’s a small enough variation that you probably won’t notice. To date, we’ve detected one planet orbiting Aldeberan, a gas giant several times the size of Jupiter.
If you have reasonably dark skies, or even if you look carefully under fairly light-polluted skies, you’ll notice that Aldeberan doesn’t sit alone in the sky. It lies at the wide end of a “V” of stars. (See the chart above.) This “V” is an open star cluster called the Hyades. An open cluster is a group of stars that were born together and which are still close enough to each other to be gravitationally bound. Most stars, our sun included, are probably born in such clusters, but over time they drift apart from each other. The Hyades consists of a spherical collection of hundreds of stars, the brightest of which form the “V”. Curiously, Aldeberan isn’t one of them. It just happens to sit along the same line of sight, only much closer to us. The cluster itself is 153 light years distant, over twice as far as Aldeberan. The Hyades are “only” 625 million years old, compared to our sun’s 5 billion years.
To the west of the Hyades is Taurus’ second naked-eye open cluster, the famous Pleiades, or Seven Sisters. The Pleiades looks much more compact and brighter than the Hyades. It is 444 light years distant, rather farther than the Hyades, which is why it looks more compact. So why is it brighter? Because it’s stars are younger and hotter than those of the Hydes. They are in the vicinity of 75 to 150 million years old.
To the north of Taurus, the Constellation of Auriga looks like a large hexagon of stars ranging from bright to modest. (See the above chart.) In a curious quirk, the star Elnath, fairly bright at magnitude 1.65, is shared by Auriga and Taurus as the figures are normally drawn. Technically it’s in Taurus.
The gem of Auriga, though, is the brilliant star Capella, which at magnitude 0.05 is the sixth brightest in the night sky and third brightest in the northern hemisphere. At 42.9 light years away, it’s relatively close. It’s also not one star, but four. Two of them are yellow giants, 2.5 times as massive as our sun, and the other two are red dwarfs.
Multiple stars are very common. About half of all sun-like stars are multiples, as are as many as 80 percent of massive stars. Red dwarfs are most often solitaries, but there are exceptions. Sometimes we can split (see) the components of a multiple star system with our eyes, but more often we need a telescope, and in some cases the separation between the components is so small that even the largest telescopes cannot split them. We only know such stars are multiples by analyzing their light.
That’s a subject for another day. Until then, keep looking up!