Check out the evening sky above Lyons on a clear moonless night in January and February, and you’ll see what are probably the two most eye-catching clusters of stars to be found in our heavens. Both the Pleiades and the Hyades pass high overhead adjacent to each other in the constellation of Taurus the Bull. If you’re unsure where to look, follow the line of the three Belt Stars of the familiar constellation Orion towards the west to where the clusters hover in the sky.
Most obvious is the Pleiades cluster: the bright, blue, compact group of stars can be immediately picked out with the naked eye as a fuzzy patch, but a little scrutiny breaks it up into six or more distinct stars depending upon how keen your eyesight might be. It’s frequently (and mistakenly) called the “little dipper” due to the arrangement of its brightest members. While early astronomers couldn’t explain why stars would form such groups, in 1767 John Michell calculated that there were only 2 chances in a million that the Pleiades could appear clumped together as they do by random chance.
The Hyades, by contrast, is a large, rather loose and amorphous collection of stars situated apparently next door. The bright and distinctly reddish star Aldebaran (the Eye of the Bull) marks its location – although it’s actually a foreground star not associated with the cluster. While only a handful of stars can be seen by naked eye and form a V-shaped pattern (the head and horns of the Bull), binoculars or a small telescope reveal more than a hundred or more. (Incidentally there is also another faint and more distant cluster, too dim to be seen with the without optical aid, lying just to the northeast.)
All in the Family
The Pleiades are known in mythology as the “Seven Sisters”, although only six are easily seen. The Sisters are accompanied by their parents, Atlas and Pleione, at the edge of the grouping as the next brightest stars in the group (but are overshadowed by their offspring). Interestingly, the stars of the Hyades were also seen to be sisters to each other, and in fact were claimed to be half-sisters to the Pleiades, sired by the rather prolific Atlas (apparently he didn’t spend all of his time holding up the world).
But in a very real sense, the stars in a cluster are sisters to each other: they are conceived from the same common cloud of nebular gas, all roughly at the same time and in the same region of space. After a few million years of gestation, the stars light up their furnaces and are born, blowing the remaining gas and dust away … and then eventually, slowly, leave home and drift apart as they escape the gravitational bonds of their nursery to live independent lives. It is thought all stars are born this way, including our own Sun – but too much time has passed in its 5 billion year lifetime, we’ve lost track of our siblings.
Because stars in a cluster are all about the same distance from us (give or take a few tens of light years, and are all of similar ages, they have been extraordinarily useful tools for astronomers in the last 100 years in figuring out the distances to, and evolution of, the stars that comprise them … and by extrapolation, the nature of all stars in general.
From visual inspection, which of the clusters appears closest to us? Conceptually, it’s not rocket science, it’s simple common sense: close things tend to be big and bright, distant things look small and faint. So, if you picked the Hyades as the nearer, you would not only have chosen logically, but also correctly.
The Hyades is, in fact, the very closest open cluster to us. So close, in fact, that it is actually difficult to tell where the cluster begins and ends: it’s “in our face”, so to speak; you almost “can’t see the forest for the trees”. A detailed analysis of the stars and their motions within the Hyades has provided astronomy with one of the earliest and most fundamental “yardsticks” for gauging stellar distances. It is now known that the Hyades cluster (and the individual stars contained therein) all lie at about 150 light years away from us in a spherical region of space that spans about 15 light-years across.
Since the Pleiades appears to be about three times smaller, we can also logically assume that it’s about three times further away than the Hyades. In fact the Pleiades are 440 light years distant, with only one other stellar group (in Coma Berenices) lying closer. Common sense and intuition frequently apply in the sky, helping the observer envision the relative placement of things in the heavens.
Somewhat less obvious, but still within the realm of common sense (given some background in how stars evolve), are the relative ages of the clusters. Which of the two clusters would you think is the youngest? Considering the stellar-nursery concept, it’s reasonable to assume that the tightness of the clumping of stars is a giveaway, since as a cluster ages, the populations slowly drift away from each other. If you assumed the Pleiades to be the younger, you are correct.
But there is an even more telltale method for determining cluster age, and this one is somewhat counter-intuitive: the color of its brightest stars. Intrinsically bright, massive blue stars are always very young; they must be, because they consume their nuclear fuel so fast, and glow so brightly, that they burn themselves out after only a few tens of millions of years. Although more massive stars have more stuff to burn, they “burn their candle at both ends” … so ironically, the more massive the star, the shorter its lifetime. And when their supply of hydrogen fuel has been all used up, these stars swell up, cool down, and become red giants or supergiants.
A young cluster will not have reached the age where any of its bluest, hottest, most massive stars will have become red giants: and we see that all of those stars in the Pleiades are still quite blue-hot. By comparison, no bright blue stars still remain in the Hyades, while it does have a number of bright reddish giants – a sure sign of fairly old age.
Of course, the real complexity comes with trying to pin down the actual time since birth. It turns out, the Pleiades are only about 80 million years old; dinosaurs ruled the Earth when this cluster was still a nebula. On the other hand the Hyades is pretty ancient for an open cluster that hasn’t completely fizzed away: those stars are about 600 million years of age, but still only about 15% as old as Old Sol.
A Chance Encounter
Finally, a personal “gotcha”. I’ve spent a good portion of my life telling students that the faint blue wisps of the reflection nebula surrounding the Pleiades (best seen in photographs) were the last remnants of that nursery out of which the stars were born – and used it as yet another indication of its youth. Recent spectroscopic studies, however, have shown that the interstellar dust, which is simply reflecting the blue light of the stars, is traveling through space headed in one direction while the stars themselves are all moving the opposite way. The Pleiades, as it turns out, broke free from their nursery millions of years ago. What we see is a random, chance encounter of stars passing through a leftover dust cloud in space… and for a very brief time (a few million years) the two are seen together to put on a beautiful show for us.