This Week's Sky at a Glance, May 20 – 28

FRIDAY, MAY 20

■ With spring now in its final third, bright Vega shines nicely high in the east-northeast after dark. Its faint little constellation Lyra dangles from it toward the lower right.

■ Look 14° (almost a fist and a half at arm's length) upper left of Vega for Eltanin, the 2nd-magnitude nose of Draco the Dragon. Closer above and upper left of Eltanin are the three fainter stars forming the rest of Draco's stick-figure head. Draco always points his nose to Vega no matter how he's oriented. He seems curious about it.

The faintest star of Draco's head, opposite Eltanin, is Nu Draconis. It's a fine, equal-brightness double star for binoculars (separation 61 arcseconds, both magnitude 4.9). The pair is 99 light-years away. Both are hot, chemically peculiar type-Am stars somewhat larger, hotter, and more massive than the Sun.

SATURDAY, MAY 21

■ With summer still a month away (astronomically speaking), the last star of the Summer Triangle still doesn't rise above the eastern horizon until about 10 or 11 p.m. That's Altair, the Triangle's lower right corner. Watch for Altair to clear the horizon three or four fists at arm's length to Vega's lower right.

■ Before and during Sunday's dawn, Saturn hangs above the last-quarter Moon — by about the width of three fingers at arm's length for most of North America. How long into dawn can you keep Saturn, 1st magnitude, in definite sight?

SUNDAY, MAY 22

■ As spring moves into its final month Leo the Lion walks downward in the southwest, on his way to departing into the sunset in early summer. Right after dark, spot the brightest star fairly high in the west-southwest. That's Regulus, Leo's forefoot. The Sickle of Leo extends upper right from there; the rest of the Lion extends farther upper left from the Sickle.

■ Last-quarter Moon (exact at 2:43 p.m. EDT). The Moon rises around 2 a.m. tonight in dim Aquarius, with Saturn now glowing about a fist and a half to its upper right.

By Monday's early dawn the Moon and Saturn are higher in the southeast, forming a big triangle with Fomalhaut very low below them.

Far to the Moon's left by then are bright Jupiter and little Mars.

MONDAY, MAY 23

■ The Arch of Spring is sinking lower now and sets sooner. At the end of twilight it spans the west to northwest. Pollux and Castor form its roughly level top; brighter Procyon shines to their lower left; farther off to their lower right are Menkalinan and then, brightest of all, Capella.

■ As soon as it's dark, look very high due north for the Big Dipper floating upside down. If you have an open view low to the north, look there for the Dipper's opposite number: W-shaped Cassiopeia. The W is nearly upright. Midway between them are Polaris and the North Celestial Pole.

The farther north you live, the higher they all will be. If you're as far south as Tijuana or the Gulf Coast, most or all of Cas will be under your north horizon.

TUESDAY, MAY 24

■ Arcturus, high in the southeast, forms the pointy end of the long, narrow Kite asterism: the central part of Bootes, the Cowherd. The kite currently extends upper left from Arcturus, with its head bent upward. The kite is 23° long: about two fists at arm's length.

■ Look east in early dawn Wednesday morning, and there's the waning crescent Moon with Jupiter and Mars above it, as shown below. Binoculars help as dawn grows bright.

WEDNESDAY, MAY 25

■ Vega dominates the east-northeast after dark with Lyra hanging from it: a small, almost-equilateral triangle with Vega forming its top corner, and a larger parallelogram hanging to the lower right from the triangle's bottom.

The bottom two stars of that parallelogram, Beta and Gamma Lyrae, are the two brightest stars of the pattern after Vega. Gamma is the one farthest from Vega.

Most of the time these two are almost indistinguishable in brightness: Gamma is visual magnitude 3.25 and Beta is 3.4. But Beta is a famous eclipsing variable, one of the first discovered. If you take a look up at these two enough times, sooner or later you will catch Beta very obviously dimmer than Gamma, at its minimum brightness of mag 4.3. More often you're likely to catch it somewhere in between, when the difference is apparent but not so striking.

The components of Beta orbit each other every 13 days. The two stars are so close together relative to their sizes (both are B subgiants) that the gravity of one distorts the other into an ellipse. This adds to the system's variability throughout its orbit. So do a stream and disk of semi-opaque gas, as the larger, less massive star sheds material onto the other — by about one solar mass every 50,000 years. As a result of this transfer, we see the orbital period of the pair increasing by about 19 seconds per year. On astronomical timescales the Beta Lyrae system is in a fast, brief transition period. A quarter million years from now it will be something quite different.

THURSDAY, MAY 26

■ Can you see the big Coma Berenices star cluster? Does your light pollution really hide it, or do you just not know exactly where to look? It's 2/5 of the way from Denebola, the tail tip of Leo, to Alkaid, the end of the Big Dipper's handle: the tail tip of Ursa Major.

The cluster's brightest members form an inverted Y. The entire cluster is about 4° wide — a big, dim, irregular glow in a dark sky, roughly the size of a ping-pong ball at arm's length. It nearly fills a binocular's view.

FRIDAY, MAY 27

■ Bright Capella sets low in the northwest fairly soon after dark these evenings; how soon depends on your latitude. That leaves Vega and Arcturus as the brightest stars in the evening sky. Vega shines in the east-northeast. Arcturus is now very high toward the south.

A third of the way from Arcturus down to Vega look for semicircular Corona Borealis, with 2nd-magnitude Alphecca as its one moderately bright star.

Two thirds of the way from Arcturus to Vega is the dim Keystone of Hercules, now lying almost level. Use binoculars or a telescope to examine the Keystone's top edge. A third of the way from its left end to the right is 6th-magnitude M13, one of Hercules's two great globular star clusters. In binoculars it's a tiny glowing cotton ball. A 4- or 6-inch scope begins to resolve some of its speckliness. Located 22,000 light-years away high above the plane of the Milky Way, it consists of several hundred thousand stars in a swarm only about 140 light-years wide.

SATURDAY, MAY 28

■ Have you ever seen Alpha Centauri?! At declination –61° our brilliant, magnitude-zero neighbor star is permanently out of sight if you live north of latitude 29°. But if you're at the latitude of San Antonio, Orlando, or points south, Alpha Cen skims just above your true southern horizon for a little while late these evenings.

When to look for it? Just about when Alpha Librae, the lower-right of the two brightest stars of Libra, is due south over your landscape. At that time, drop your gaze down from there!

■ Bright Jupiter and fainter Mars are in conjunction, 0.6° apart, in early dawn Sunday morning May 29th as shown below. Look east-southeast, not very high. Binoculars help as dawn brightens.

Lastly: Make plans for the possible Tau Herculid meteor shower, which may put on a strong outburst on the night of May 30-31. Meteor researcher Joe Rao predicts that any meteor activity should peak around 5:00 UT May 31st (1 a.m. May 31st EDT; 10 p.m. May 30th PDT). This event will be either "all or nothing," says Bill Cooke of NASA’s Meteoroid Environment Office. Article by Don Machholz. Rao's research paper: Will Comet 73P/Schwassman-Wachmann 3 produce a meteor outburst in 2022?

 

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This Week's Planet Roundup

Mercury is hidden in the glare of the Sun.

Venus (magnitude –4.0) rises soon after the beginning of dawn. Look for it just left of due east. It's roughly two fists at arm's length lower left of Jupiter.

Venus will continue to rise near the beginning of dawn from now all the way through August.

Mars and Jupiter, very different at magnitudes +0.7 and –2.2 respectively, shine together before and during dawn in the east-southeast. Little orange Mars is slowly closing in on brilliant Jupiter from the upper right. You'll find them 4½° apart on the morning of May 21st, closing to 0.6° at their conjunction on May 29th.

Saturn, magnitude +0.8, glows in eastern Capricornus a good 35° or 40° upper right of Jupiter before dawn. The little star 2° to its lower right is Delta Capricorni, magnitude 2.8.

Uranus is buried in the eastern dawn.

Neptune, magnitude 7.9, is some 8° upper right of Jupiter just before dawn begins.

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All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions and graphics that also depend on longitude (mainly Moon positions) are for North America.

Eastern Daylight Time, EDT, is Universal Time minus 4 hours. (Universal Time is also called UT, UTC, GMT or Z time.)

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Want to become a better astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.

This is an outdoor nature hobby. For an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential magazine of astronomy.

Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas (in either the original or Jumbo Edition), which shows stars to magnitude 7.6.

Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5; nearly three times as many. The next up, once you know your way around, are the even larger Interstellarum atlas (stars to magnitude 9.5) or Uranometria 2000.0 (stars to magnitude 9.75). And be sure to read How to Use a Star Chart with a Telescope. It applies just as much to charts on your phone or tablet as to charts on paper.

You'll also want a good deep-sky guidebook. A beloved old classic is the three-volume Burnham's Celestial Handbook. An impressive more modern one is the big Night Sky Observer's Guide set (2+ volumes) by Kepple and Sanner.

Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically, meaning heavy and expensive. And as Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."

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Audio sky tour. Out under the evening sky with your
earbuds in place, listen to Kelly Beatty's monthly
podcast tour of the heavens above. It's free.

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"The dangers of not thinking clearly are much greater now than ever before. It's not that there's something new in our way of thinking, it's that credulous and confused thinking can be much more lethal in ways it was never before."
            — Carl Sagan, 1996

"Facts are stubborn things."
            — John Adams, 1770

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