Somethings going on in the sky. In Orion, the red supergiant star known as Betelgeuse is looking faint, dropping rapidly in brightness since October and is now at ~1.3 to 1.5 magnitude. Normally it burns brightly, as seen in the upper left in the photo below, but it has now dipped so low in magnitude it is not even in the top 20 brightest stars in the night sky.

Betelgeuse is a distinctly reddish, semiregular variable star whose apparent magnitude varies between +0.0 and +1.3, the widest range of any first-magnitude star. At near-infrared wavelengths, Betelgeuse is the brightest star in the night sky. In visible wavelengths, it is (was) the 9th brightest star in the night sky and 2nd-brightest in the constellation of Orion.

According to Wikipedia:

Due to its distinctive orange-red color, Betelgeuse is easy to spot with the naked eye in the night sky. It is one of three stars that make up the Winter Triangle asterism, and it marks the center of the Winter Hexagon. At the beginning of January of each year, it can be seen rising in the east just after sunset. Between mid-September to mid-March (best in mid-December), it is visible to virtually every inhabited region of the globe, except in Antarctica at latitudes south of 82°. In May (moderate northern latitudes) or June (southern latitudes), the red supergiant can be seen briefly on the western horizon after sunset, reappearing again a few months later on the eastern horizon before sunrise. In the intermediate period (June–July) it is invisible to the naked eye (visible only with a telescope in daylight), unless around midday (when the Sun is below horizon) on Antarctic regions between 70° and 80° south latitude.

Betelgeuse is a variable star whose visual magnitude ranges between 0.0 and +1.3. There are periods when it will surpass Rigel to become the sixth brightest star, and occasionally it will be even brighter than Capella. At its faintest Betelgeuse can fall behind Deneb and Beta Crucis, themselves both slightly variable, to be the 20th-brightest star.

It is also unimaginably huge. If placed where our sun is today, it would swallow Mercury, Venus, Earth and Mars and even Jupiter.

Currently, some astronomers have been speculating that the rapid dimming is a precursor to a supernova event. Some are saying no, that it’s just business as usual for a variable star. Looking at the data, it certainly is highly variable.

This much is clear: due to what it is, a red supergiant, Betelgeuse will eventually explode as a supernova. It is roughly 550-650 light-years away (parallax measurement is uncertain due to its size) and when it goes supernova, it will be spectacular.

The question is, is what we are seeing now just some behavior of a variable star, or the indications it is shrinking and about to explode?

According to the Astronomer’s Telegram, it may simply be periodic coincidence:

This appears to be the faintest the star has been measured since photoelectric observations have been carried out of the star. However, photoelectric photometry carried out during late-1926 / early-1927 by Joel Stebbins (1931: Pub. Washburn Obs., 15, 177) indicates that Betelgeuse declined to V’ ~+1.25 mag.

At its average maximum brightness light (V ~ 0.3 – 0.4 mag), Betelgeuse is the 6 – 7th brightest star. But by 2019 mid-December the star has slipped to the ~21st brightest star. The red supergiant is now closer in brightness to Bellatrix (V =+1.64 mag) than to Rigel (V =+0.13 mag). Wing three-band Near-IR and TiO photometry carried out at Wasatonic Observatory shows that Betelgeuse is also cooler with an inferred spectral-type near ~M3.5 Iab (Teff ~ 3,545 K from TiO-photometry).

This is about 150 K cooler than measured near maximum light. Analysis of the last 25-yrs of V-band and Wing TiO and Near-IR photometry shows a dominant ~425+/-10 day period as well as a long-term ~5.9+/-0.5 year period. The current faintness of Betelgeuse appears to arise from the coincidence of the star being near the minimum light of the ~5.9-yr light-cycle as well as near, the deeper than usual, minimum of the ~425-d period.

http://www.astronomerstelegram.org/?read=13365

Either way, it is fascinating.