The sky, May 12-19: How old are the stars we can see?

Special to, May 12, 2024

Excerpts from weekly Sky&Telescope report.


■ The thickening crescent Moon shines just left of Pollux this evening. About twice as far to their right, Castor lies nearly on the same line.


■ After dark, use binoculars or a good finderscope to look 3° lower left of the thick crescent Moon. That’s about half the width of a typical binocular’s field of view. Can you make out the Beehive star cluster, M44, through the moonlight? The loose-scattered cluster is roughly ½° wide.

The Moon is 1.3 light-seconds from you. The Beehive is 600 light-years in its background.


■ The Moon shines to the right of Regulus at nightfall, as shown below. Notice that the Moon’s terminator is not yet perfectly straight. The Moon is exactly first quarter about 12 hours later for the time zones of the Americas, at 7:48 a.m. EDT Wednesday morning the 15th.

Moon passing Regulus, May 14-16, 2024

The Moon is exactly first quarter midway between Tuesday and Wednesday evenings for skywatchers in the Americas.


■ Now the evening Moon, about 12 hours past first quarter, shines closer over Regulus as shown above.


■ What is the oldest thing you have ever seen? For everyone in the world, it’s at least the Sun and other objects of the solar system, age 4.6 billion years. Everything on the Earth’s surface is much younger.

Next is Arcturus, very high in the southeast these evenings, which most people have surely seen whether they know it or not; it’s one of the brightest stars in the sky. It’s a Population II orange giant, age about 7 billion years, just passing through our region of the Milky Way.

Amateur telescope users have seen globular clusters. Most are older still, at least in part. For instance, white dwarfs in the familiar M4 in Scorpius have been dated at 12.7 ±0.7 billion years.

But what about individual stars that you can observe? Assigning dates to individual stars from the first eras after the Big Bang is still iffy; Astronomers have to work from the near-absence of heavy elements in their spectra. But a 6th-magnitude star in Boötes and a 7th-magnitude star in Libra, both in binocular range, await you on May and June nights. They probably date from about 12½ billion and at least 13 billion years ago, respectively. These will probably be the oldest things you have ever seen, or ever will. The Big Bang itself is well dated at 13.8 billion years.

One could pick nits. Pick a proton, any proton right in front of you, and it has very likely remained intact since the Big Bang’s first millionth of a second. And what a history it has been through since!


■ Tonight the dark limb of the waxing gibbous Moon occults Beta Virginis, magnitude 3.6, for telescope users across most of North America. The occultation happens after midnight in the Eastern and Central time zones, and late evening farther west. If you’re near the West Coast the Moon will miss the star.


■ The waxing gibbous Moon shines brightly in the south after dark. But not so brightly as to hide Corvus, the Crow, a little more than a fist directly under it. Cover the Moon with your hand to make Corvus easier. Its four main stars are all between magnitude 2.6 and 3.0. They’re within 8° of each other, smaller than your fist at arm’s length.


■ The Moon continues to illuminate the Virgo part of the sky. Early this evening look about 3° or 4° lower left of the Moon for Spica, Virgo’s brightest star. By midnight the Moon moves closer to Spica, which is now directly to its left.

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