Nature brings us its own version of holiday lighting with the annual return of the luminous Geminid meteor shower. We also meet a binocular-bright star that may be experiencing Betelgeuse-like convulsions.

Geminid over Tucson
A brilliant Geminid tears across Orion over Tucson, Arizona, during the early morning hours of Dec. 14, 2017.
Eliot Herman

You can always count on the Geminids for a good show. Although the meteors can be seen from mid-November to Christmas Eve, Earth crosses the shower's core on the night of December 13–14. Under pristine skies, up to 120 meteors per hour might be seen during the early morning hours of Dec. 14th, when the radiant climbs highest in the sky. Unfortunately, observers will have to contend with the waning gibbous Moon this time around, which will mask all but the brighter meteors during the peak hours.

Before you consider snoozing instead of choosing to watch, NASA's Meteoroid Environment Office rates the Geminid meteor shower as the second-richest fireball producer behind the Perseids. And if you time it right, there are dark skies to be had — the Moon doesn't rise until around 9:30 p.m. local time, making for several hours of premium meteor watching.

Geminid meteor shower radiant diagram
The Geminid meteor shower can still be seen to good advantage before moonrise on Tuesday night, Dec. 13th. They'll appear to stream from a radiant near the star Castor in the constellation Gemini. Meteors headed toward the horizon will be mostly out of view, but those aiming upward upward will be well visible. The radiant reaches its peak altitude around 2 a.m. local time on Dec. 14th.
Sky & Telescope diagram

As soon as twilight ends, from about 6 to 7 p.m., keep an eye out for earthgrazers,  meteoroids that scratch the atmosphere at a shallow angle and produce long-lasting, often bright trails. Most earthgrazers will be seen moving from right to left as you face north or from left to right facing south. Because of perspective, these early Geminids enter the atmosphere nearly horizontally, that is, with paths more nearly parallel to the ground, which explains their greater length. Late at night, when the radiant stands nearly overhead, meteoroids bear down more directly from above.

The stars and Moon at the peak of the Geminid meteor shower
The radiant stands high in the southern sky during the peak viewing time, but the 70% waning Moon will brighten the sky.

Between 8 and 10 p.m., before the Moon becomes a problem, the radiant climbs from around 25° to a respectable 35°. That opens a fine observing window, especially for families who want to share the sight with their children before bedtime. Observers seeking the the "peak" experience, however diluted by moonlight, should watch from about 1 to 3 a.m. on the 14th. If you're clouded out, you'll still see the shower at half strength the following night, with a later moonrise to boot.

Phaethon dust
Astronomers hypothesize that solar heating causes cracks in the surface of the asteroid 3200 Phaethon, releasing the dust that feeds the Geminid meteor shower. Curiously, observations show that there's much more material in the Geminid stream that can be accounted for by the asteroid's current dust output, hinting that it may have been much more active in the past.

Material responsible for the shower originates with 3200 Phaethon, a 5.1-kilometer-wide asteroid that passes almost three times closer to the Sun than Mercury every 1.4 years. Around the time of perihelion, solar heating bakes and cracks its surface, releasing dust and rock fragments into space. Solar radiation pressure pushes the material back behind the asteroid along its orbital track. Every December, Earth hurtles through the Phaethonic exhaust, and the chaff strikes our atmosphere at 35 kilometers per second (79,000 mph). Heated by the impact, the particles glow brightly. The also compress and heat the air along their paths, ionizing air molecules and setting them aglow. The result is a meteor, still often called a "shooting star."

As physics does its job, you and I can relax on our reclining chairs and enjoy the performance. If you're an early evening watcher I suggest you face east. Those staying up late can face southwest or southeast. Keep in mind that you'll see the longest meteors 90° from the radiant, so rather than face the Gemini directly, look off to one side — except the side facing the bright Moon! For an hourly cloud forecast to help in planning your viewing session, visit the Astrospheric site

IMO magnitude chart
To determine your limiting magnitude during the Geminid meteor shower count the number of stars in Area 7 in Cepheus. See below for details.
Courtesy IMO

Would you like to make a contribution to Geminid science? The International Meteor Organization (IMO) is looking for dedicated observers to count shower members under bright moonlight. They request that participants watch for at least one hour and estimate their limiting magnitude every 30 minutes. Before you begin, read the helpful tips article then focus your attention on Area 7 in the constellation Cepheus. Count the number of stars in the triangle and record your meteor sightings. Share your report by going to the registration page and clicking on the blue Join Us Now button. Select the free user option and submit away!

Is RW Cephei Going "Betelgeuse" on Us?

Hypergiant illustration
This artist's impression depicts the hypergiant VY Canis Majoris with its vast convection cells and violent ejections. RW Cephei is a similar star more than 1,000 times the Sun's size that also flings gas into space. In both VY CMa and Betelgeuse (in 2019), ejected material cooled to form clouds of sooty material that partially obscured the stars.
NASA / ESA / R. Humphreys (University of Minnesota) and J. Olmsted (STScI)

RW Cephei, a red hypergiant and variable star, normally varies between magnitudes 6.0 and
7.3 and exhibits semi-regular fluctuations with no set period. Located about 2.5° southwest of better-known Delta (δ) Cephei, this circumpolar star is typically visible throughout its entire cycle in just a pair of binoculars. But in recent weeks its magnitude has plummeted, recalling Betelgeuse's Great Dimming that began in October 2019. You'll recall the star faded to about 60% of its normal brightness, reaching magnitude 1.6 (a historical minimum) by late January 2020.

RW Cep light curve
This V-magnitude light curve shows RW Cephei's variability and its recent fading below its usual minimum state.
Courtesy AAVSO

Recent magnitude estimates from the American Association of Variable Star Observers (AAVSO) put RW Cephei at V-magnitude 7.5. V magnitudes are determined by using a green filter that yields a brightness close to what the eye sees. Visually however, many variable star observers' estimates have put the star closer to 7.8 to 8.0. In a recent Astronomical Telegram, some astronomers are suggesting that the hypergiant is undergoing its own Great Dimming and possibly for the same reason: the star may have expelled a plasma bubble that cooled to form a debris cloud, temporarily obscuring its light.

RW Cep finder
RW Cephei is located near the small triangle of Delta (δ), Epsilon (ε) and Zeta (ζ) Cephei. The magnitude 6.6 star (labeled 66) is also shown on the AAVSO chart to aid in identifying the star field.

Use the map above to track down the star. This AAVSO chart will help you estimate its brightness. Many of us thrilled to see the changes in Betelgeuse three years ago. RW Cep may also be in tumult for the same reason and provide us another opportunity to witness stellar evolution right before our eyes.


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