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The Sky This Week, 2020 January 21 - 28

What's dimming Betelgeuse?
Orion as seen from the suburbs of Washington, DC, 2020 January 22
Orion as seen from the suburbs of Washington, DC, 2020 January 22
Imaged with a Canon EOS Rebel SL2 DSLR and an Omegon mechanical star tracker
from Alexandria, Virginia.
The right image is de-focused to show the colors and relative brightnesses of the
constellation's brighter stars. Note the brightness of Betelgeuse compared to that
of Bellatrix and Rigel.

The Moon starts the week off as a waning crescent in the pre-dawn sky and ends as a waxing crescent by the week’s end.  New Moon occurs on the 24th at 442 pm Eastern Standard Time.  Look for the Moon in the company of bright Venus after sunset on the evenings of the 27th and 28th.

As mentioned last week, the long winter nights are dominated by the stars of the Great Winter Circle and centered on the prominent outline of Orion.  This was the first constellation that I learned when I was very young, dominating the winter sky over my home in rural New England.  His three “Belt Stars” immediately caught my eyes as did the exceptional ruddy tint of Betelgeuse, whose contrast with the blue tints of Orion’s other bright stars fascinated me.  Since that time I have always regarded Betelgeuse as similar in brightness to Rigel, Orion’s other prominent star, and often I thought that Betelgeuse was slightly brighter.  Until recently.  When I viewed the Hunter in September from West Virginia all seemed normal, with Rigel and Betelgeuse sharing the mantle of the constellation’s brightest stars, but since then Betelgeuse has pulled something of a vanishing act.  When I looked at Orion from my suburban yard last night, I found Betelgeuse to be slightly dimmer than the star Bellatrix, which marks Orion’s opposite “shoulder”.  This means that Betelgeuse has faded by a full magnitude since September, and the star is now dimmer than at any time since reliable brightness measurements have been kept.

Many stars in the sky are known to vary in brightness.  Some, like Algol, the “Demon Star” in Perseus, were duly observed to vary by ancient Arabic astronomers.  Algol’s brightness changes are very predictable, and the star fades from second- to third-magnitude every 2.9 days.  The reason is simple: Algol is a binary star, and once every cycle the brighter star is partially obscured by the fainter companion.  Hundreds of these “eclipsing binaries” are scattered across the sky.

Another type of variable star also fades and brightens to a set rhythm.  These stars, known as Cepheid variables after their prototype Delta Cephei, go through regular pulsations with periods lasting from days to months.  Their brightness changes are caused by fluctuations of ionized helium in their outer layers.  They are intrinsically bright, and there is a well-documented correlation between their periods of oscillation and their luminosity.  Because they are so bright, we can use them to measure distances to many nearby galaxies.

Betelgeuse, on the other hand, is a “semiregular” variable star that displays two or possibly more variation modes, one of which is around 425 days and the other around 6 years.  It typically oscillates between magnitudes 0.5 to 1.3.  I have often seen it comparable to Rigel in brightness, but I have never seen it as dim as it is right now.  Explaining the mechanism of this variability is a challenge for astronomers because Betelgeuse is a very unusual star.  It is very old for a star, closing in rapidly (in cosmic terms) to its demise.  Internally it has depleted the hydrogen and other light elements in its core and now fuses these elements into heavier ones in onion-like layers surrounding a core of iron.  From a nuclear point of view, iron is inert, since it takes more energy to fuse it onto something heavier or split it into smaller nuclei than such a reaction would release.  This “shell-burning” phase has caused its outer layers to swell to a size comparable to the orbit of Mars around the Sun.  These layers are subject to upwelling of large convective cells of hot gas, and these cells may be the mechanism that cause the changes in the star’s brightness.  Most astronomers think that Betelgeuse will rebound from this deep minimum, but when that minimum actually will occur is anybody’s guess.  Watch the star, and watch this space.

Dazzling Venus continues to climb higher in the evening twilight sky.  She should be easy to pick out in the southwest shortly after sunset, and if you know where to look you should be able to spot her before the Sun goes down.  She is steadily climbing northward along the ecliptic, and by early February she will enter the northern hemisphere of the sky.

Ruddy mars may be seen in the gathering morning twilight, low in the southeastern sky.  He is pulling away from the bright star Antares in Scorpius as he skims along the southernmost reaches of the ecliptic.  This will be a good year for the red planet.  When he comes to opposition in the fall he will be well placed for northern observers and rival Jupiter in brightness.