Looking Up: Betelgeuse takes a dip!
Orion is certainly a favorite among the 88 constellations, probably second only to the star pattern of the Big Dipper (which is technically an “asterism” and part of a larger constellation, the Big Bear). One of Orion’s brightest stars, the red supergiant Betelgeuse, has faded lower than anyone remembers.
Betelgeuse is a slow-period variable star, so its brightness gradually changes, but it’s normally always considered “bright.” In only two months it has dropped from the 10th brightest star in the night sky to the 21st.
It’s interesting to note that Betelgeuse is also a star that is literally set to explode as a supernova, sometime between this very moment and about 100,000 years from now. Once it does, Betelgeuse will suddenly glow brighter than the moon, casting its red light all over the landscape. Massive stars like Betelgeuse end in a supernova blast when it runs short on nuclear fuel, and the dense core collapses on itself.
Despite the star’s unusual dip, astronomers are not necessarily expecting it to blow anytime soon.
While we may not stay up nights watching for Betelgeuse to ignite, you can easily see Orion any clear winter evening (in mid-January look southeast in early evening) and estimate the state of Betelgeuse’s brightness, comparing it with the other bright stars of Orion and nearby.
Betelgeuse typically varies from magnitude -0.2 (similar to Rigel) to approximately +1.3, a little brighter than Bellatrix, in Orion (+1.6).
Picture the main stars of Orion like a big rectangle, with the famous three-star “Belt” of Orion in the center.
Rigel is the very bright star in the corner to the lower right of the Belt. Bellatrix is to the upper right of the Belt. Betelgeuse is to the upper left. The star in the corner on the lower left of the Belt is known as Saiph and is about magnitude +2.
Astronomers started to witness the unusual dimming of Betelgeuse in December 2019. On Dec. 28, it was measured at only +1.5.
Betelgeuse changes brightness as its atmosphere expands and contracts. When the star is at its smallest and most hot, Betelgeuse would reach as far as the orbit of Mars. When it is at its largest and coolest, Betelgeuse would swell to the orbit of distant Jupiter. The star is so large, and so relatively near, that its disc has been (barely) resolved by professional observatories.
Betelgeuse is about 642.5 light-years from the Earth. It takes that many years for its light to reach our eyes.
Orion is the name of a mythological hunter.
While examining Orion, note the short line of stars marking Orion’s “Sword” which seems to dangle on one side of the Belt. In the center, you can see what appears to be a fuzzy star. This is the grand Great Nebula of Orion, or M42, well seen in binoculars and any telescope. It is a region where stars are being fashioned from the gas and dust of the nebula.
About stellar magnitude: Star brightnesses are classified on the magnitude scale. Each magnitude is 2.5 times brighter or dimmer than the next; the higher the number, the brighter it is. The faintest star you can typically see without optical aid on a dark night away from city and town lights is around +6.0. The brightest stars of the Big Dipper and the three stars of Orion’s “Belt” are around +2.0. The bright blue-white star Rigel, in Orion, is -0.3. The brightest star in the night sky, blue-white Sirius, is to the lower left of Orion and shines at -1.6.
To read more about Betelgeuse and its recent changes, see Bob King’s article at skyandtelescope.com, or Erika Carlson’s article at astronomy.com, among other sources.
Last quarter Moon is on Jan. 17.
Keep looking up!
Peter Becker is managing editor at The News Eagle in Hawley, Pennsylvania. Notes are welcome at firstname.lastname@example.org. Please mention in what newspaper or website you read this column.