Estimate Betelgeuse's brightness yourself
Use this photo to estimate Betelgeuse's magnitude using Aldebaran and, with reservations, Bellatrix. Magnitudes are in parentheses. When Bob King took this photo on December 22nd, Betelgeuse was much more like Bellatrix than Rigel.

Betelgeuse remains dim. The red supergiant Betelgeuse in Orion's shoulder has always been slightly variable, but for more than a month it's been in an unusually low dip. As of February 3rd it was still V magnitude +1.6 instead of its more typical +0.5. It looks obviously fainter than similarly-colored Aldebaran, magnitude +0.9, which it usually obviously outshines.

Go look! If you're familiar with Orion, he looks distinctly odd with his two shoulders now basically equal. The other shoulder is bluish-white Bellatrix, visual magnitude +1.6, but that's a poor comparison star because of its very different color. As we age our eye lenses yellow, so older people see the world through yellow filters. Thus your age affects how you judge the brightness of an orange-red star compared to a blue-white one. (For instance I'm 68, so I still see Bellatrix looking maybe 0.3 magnitude fainter than Betelgeuse even when their V magnitudes are the same right now).

In any case, Orion this winter wears an aspect you've never seen before and may never see again! Read Bob King's new The Latest on Betelgeuse, Plus a Bright Supernova and New Comet Iwamoto

And no, this does not mean Betelgeuse is about to go supernova. It's a massive star "close to the end" of its 10-million-year lifespan — but that's on an astronomical timescale! Expect to wait up to another 100,000 years or more for Betelgeuse to violently gulp down its core and blaze, for a few weeks, as bright as the full Moon.

Friday, Jan. 31

• More on Orion: he's high in the southeast right after dark, and he stands highest due south around 9 p.m. Orion is the brightest of the 88 constellations, but his showy main pattern is surprisingly small compared to some of his dimmer neighbors. The biggest of these is Eridanus the River to his right, enormous but hard to trace. Fainter Fornax the Furnace, to Eridanus's lower right, is almost as big as Orion! Even the main pattern of Lepus, the Hare cowering under the Hunter's feet, isn't much smaller than he is.

• Before moonlight returns to the early-morning sky, the supernova in a Virgo Cluster galaxy that Bob King discusses and charts is 12th magnitude, not too hard in a 6-inch scope through a moderately decent sky. And before the first light of dawn, Comet Iwamoto is a telescopically doable magnitude 9 or 10 near Vega.

Saturday, Feb. 1

• First-quarter Moon (exact at 8:42 p.m. EST). The Moon, half-lit, shines near the head of Cetus. In early evening look for 2.5-magnitude Alpha Ceti (Menkar), an orange giant, about 8° to the Moon's lower left. That's a little less than the width of your fist held at arm's length.

Farther to the Moon's right are the two or three brightest stars of Aries. Still farther to the Moon's upper left are the Pleiades and Aldebaran. Aldebaran is physically similar to Menkar but only a third as far away (65 light-years versus 220), which is why it looks so much brighter.

Sunday, Feb. 2

• The sky's biggest asterism (informal star pattern) is the Winter Hexagon, filling the sky toward the east and south these evenings. Start with brilliant Sirius at its bottom. Going clockwise from there, march right or upper right through Procyon, steeper upper right through Pollux and Castor, up to Menkalinan and then Capella on high, over and down a bit to Aldebaran as you face south (waving hi to the Moon tonight), then to Rigel in Orion's foot, and back to Sirius.

Betelgeuse shines inside the Hexagon, well off center.

Monday, Feb. 3

• The Moon shines near Aldebaran tonight. The Pleiades are farther on the Moon's opposite side (more or less the opposite side; it depends where you are and when you look).

• Look due east, not very high in early evening, for twinkly Regulus. Extending upper left from it is the Sickle of Leo, a backward question mark. "Leo announces spring," goes an old saying. Actually, Leo showing up in the evening announces the cold, messy back half of winter. Come spring, Leo will already be high.

Tuesday, Feb. 4

• The waxing gibbous Moon shines near the horn-tip stars of Taurus high over Orion this evening.

• After dark, look left of Orion for Gemini, headed up by Castor and Pollux at far left. The stick-figure Twins still lie on their sides.

Well below their legs is bright Procyon in little Canis Minor: the little dog whose top is barely visible in profile (in a dark sky; pardon the moonlight. Binoculars help.) The Little Dog is currently vertical, depending on your latitude. Procyon marks his rump.

Wednesday, Feb. 5

• The waxing gibbous Moon shines in a starry part of the heavens this evening. After dinnertime, look lower right of it for Betelgeuse and the rest of Orion, farther right or upper right of the Moon for Aldebaran, high overhead for brighter Capella, left or lower left of the Moon for Castor and Pollux, and farther down below it for Procyon.

• Algol should at its minimum brightness, magnitude 3.4 instead of its usual 2.1, for about two hours tonight centered on 12:21 a.m. EST; 9:21 p.m. PST. Algol takes several additional hours to fade and to rebrighten.

Thursday, Feb. 6

• Look left of the Moon this evening for Castor and Pollux. Lower right of the Moon is Procyon, the Little Dog Star. More than twice as far to the Moon's lower right shines Sirius, the big Dog Star.

Venus and Mercury in twilight, early February 2020
By late in the week, Mercury becomes higher and easier to spot in twilight, down to the lower right of Venus. The blue 10° scale is about the size of your fist held at arm's length.

Friday, Feb. 7

• Have you ever closely compared the colors of Betelgeuse and Aldebaran? Can you detect any difference in their colors at all? Normally, when Betelgeuse is the brighter of the two, I can't. But now that Betelgeuse is fainter, to me it looks distinctly redder.

Aldebaran, spectral type K III, is often called an "orange" giant, while Betelgeuse, spectral type M1-M2 Ia, is usually called a "red" supergiant. Their temperatures are indeed a bit different: 3,910 Kelvin and 3,590 Kelvin, respectively — and that's when Betelgeuse is normal. As it has dimmed this season its surface has also cooled slightly, by about 100 K.

Most of Betelgeuse's apparent color deepening this winter, however, is due to an illusion in the human eye: The colors of brighter objects appear, falsely, to be desaturated: tending paler (whiter) than they really are.

Saturday, Feb. 8

• Full Supermoon (exactly so at 2:33 a.m. Sunday morning EST). It's "super" because it's near perigee and therefore appears just a trace larger than average.

After dark, look for Castor and Pollux high above the Moon, Procyon and then brilliant Sirius way off to the Moon's right, and Regulus below it.

• Algol should be at minimum light for about two hours centered on 9:10 p.m. EST.

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 guide to astronomy.

Jumbo Pocket Sky Atlas cover
The Pocket Sky Atlas plots 30,796 stars to magnitude 7.6, and hundreds of telescopic galaxies, star clusters, and nebulae among them. Shown above is the Jumbo Edition for easier reading in the night. Sample chart.

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) and Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts with a telescope.

You'll also want a good deep-sky guidebook, such as Sue French's Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, or the bigger Night Sky Observer's Guide 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."

This Week's Planet Roundup

Mercury (magnitude –1.0) is having a good apparition low in the evening twilight. Look for it about 30 to 45 minutes after sundown, far to the lower right of Venus (about 25°). Mercury gets a little higher every evening.

Venus (magnitude –4.1, in Pisces) is the bright point shining in the southwest during and after twilight. Off to its upper right is the Great Square of Pegasus.

In a telescope Venus still appears fairly small (16 arcseconds) and gibbous (72% sunlit). But it will enlarge in size and wane in phase as it shines in the evening for the next four months.

Mars (magnitude +1.4, in the feet of Ophiuchus) glows in the southeast before and during early dawn. It's left or lower left of twinklier, Mars-colored Antares, which is about magnitude +1.1. Don't bother with a telescope; Mars is a very tiny 4.8 arcseconds in diameter.

Jupiter (magnitude –1.9, in Sagittarius) is very low in the sunrise glow. About 40 or 30 minutes before sunup, scan for it just above the southeast horizon well to the lower left of Mars. Nothing else in the vicinity is nearly as bright (airplanes excluded).

Saturn is barely emerging from the sunrise glow about 10° lower left of Jupiter. Try for it with binoculars about 20 or 30 minutes before sunrise. Good luck.

Uranus (magnitude 5.8, in southwestern Aries) awaits your binoculars or scope high in the southwest right after dark.

Neptune (magnitude 7.9, in eastern Aquarius) is still catchable just after dark, ever farther to the lower right of Venus. Finder charts for Uranus and Neptune (without Venus).

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 that also depend on longitude (mainly Moon positions) are for North America.

Eastern Standard Time (EST) is Universal Time (UT, UTC, GMT, or Z time) minus 5 hours.

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.


Image of Pencil BFB

Pencil BFB

January 31, 2020 at 9:28 pm

Saw Canis Major, Orion's Belt, Saiph, and Rigel 30 degrees above the western horizon at 1:00 AM on 30 January!

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