Month after month, Venus keeps blazing in the western sky at dusk. After displaying itself as the Evening Star all winter, Venus is now becoming as high and bright as it can ever appear.
And if you look west at nightfall for the next several days, you will spot a delicate little apparition that has crept out from behind it.
Venus is so bright that you can pick it out of a clear blue sky even before sunset. As twilight fades and night comes on, you will see something odd emerging into view a little to the lower right of Venus, the Pleiades star cluster, sometimes called the Seven Sisters. A few days ago Venus was practically on top of it. Keep watching day by day, and you will see these two celestial attractions continuing to creep apart.
Farther left of Venus shines the orange giant star Aldebaran, the eye of Taurus the Bull. Aldebaran shines in front of another star cluster: the Hyades, bigger, looser, and dimmer than the Pleiades. The brightest stars of the Hyades form a V shape, with Aldebaran at the top left tip.
The Hyades star closest below Aldebaran is double. Can you resolve this pair with your naked eyes? If not, binoculars show its two components side by side: pinpoints of yellow-orange and blue-white.
It’s purely by chance that Aldebaran shines in front of the Hyades cluster. They actually have nothing to do with each other. Aldebaran is 65 light-years from Earth; the cluster is more than twice as far, at 150 light-years.
The distance difference between Venus and the Pleiades is much greater. Venus is our closest planetary neighbor; it is currently 5 light-minutes away. The Pleiades are about 420 light-years in the background.
Elsewhere, two star-and-planet pairs draw the eye to other parts of the evening sky.
Face southeast after dark and look high. There stand fire-colored Mars and, just to its right, blue-white Regulus, the brightest star of Leo, the Lion. Look far down to their lower left for another pair: Pale yellow Saturn with blue-white Spica to its left. Later this evening, the waning moon rises below these two.
Unseen stars, new worlds
These paired-up sky objects differ not just in distance but in age. Venus, Jupiter, Mars, and Saturn are by far the oldest. They formed 4.6 billion years ago along with the Earth, sun, and the rest of the solar system. That is a third of the way back to the Big Bang, which is now dated very accurately to 13.7 billion years ago.
The Pleiades all formed in a bunch 80 million years ago. We can tell this by how slightly even the brightest, fastest-burning members of the cluster have aged. They’re only about 2 percent as old as Venus in their foreground.
The Hyades are older, aged about 625 million years, as measured the same way. Aldebaran is harder to date, being all by itself, rather than part of a cluster with a common origin. Two billion years is a good estimate.
The very oldest stars are among the hardest to see. Some stars date from as much as 12 and 13 billion years ago, not long after the Big Bang when the universe was young. The stars from that era that still survive are mostly dim red dwarfs: modest little slow-burners that are only red- or orange-hot. Modest these dim stars may be, but they are by far the most numerous. Not one red dwarf is visible to the naked eye, but they account for 80 percent of all the stars in our part of space. While some of them are very old, others are young.
Moreover, these humble little suns are turning out to be rich with planets, in particular, small planets like Earth and not much larger, huddling close around their banked fires. Just last week astronomers of the European Southern Observatory, working with super-sensitive planet-hunting gear in the Andes, announced a remarkable conclusion. About 40 percent of all red dwarfs, they say, turn out to have a “super-Earth’’ orbiting in the star’s habitable zone, where temperatures would allow liquid water to exist on the surface.
Super-Earths are planets with 2 to 10 times Earth’s mass. We don’t have any in our solar system, but they’re common elsewhere. And by all indications, smaller planets with roughly one Earth mass - just beyond today’s detection limit, for the most part - are probably commoner still. Hundreds probably circle the thousand-plus red dwarfs within just a few dozen light-years of us.
So the bright stars lighting the sky may dazzle and charm, but the really interesting stuff may be going on around the little dim ones we mostly do not see.Alan M. MacRobert is a senior editor of Sky & Telescope magazine in Cambridge (SkyandTelescope.com). His Star Watch column appears the first Saturday of every month.