Fans of Betelgeuse stand ready. The star we’ve watched fade to the faintest in recorded history may soon turn the corner. Astronomer Edward Guinan of Villanova University, who along with his team has been following the star’s ups and downs for the past 25 years, predicts that on February 21st, give or take a week, Betelgeuse will reach its lowest of lows and start to rebrighten.
Guinan bases his prediction on the star’s dominant pulsation period of 430 days which arrives on or about that date. You’ll recall that Betelgeuse physically changes size as it pulsates from bright to dim and bright again. A careful analysis of those pulsations reveals that it has more than one period or “heart beat.” It also varies over 6.06 years, 1,083 days and other intervals. Clearly, it’s a complex mess of a star with a lot going on.
Recent photos released by the European Southern Observatory (ESO) reveal amazing changes in the star since January 2019. Not only has the supergiant star faded but its shape has changed. A team led by Belgian astronomer Miguel Montargès suspects that Betelgeuse’s dramatic fading may be due either a cooling of the surface or dust ejected by the star in our direction.
A massive star like Betelgeuse forges more complex elements in its hot interior from simpler ones like hydrogen and helium. The new materials are lofted into its cool, outer atmosphere by powerful stellar winds where they join together to form a dusty haze enveloping the star. Astronomers have found water, silicon monoxide and aluminum oxide at Betelgeuse for example. Dust absorbs starlight, and if a particularly dense blob of dust were ejected in Earth’s direction, it would dim the star measurably. And if Betelgeuse arrived at the low point of its pulsation cycle the two would combine to make the star unusually faint.
Giant starspots, similar to sunspots but much larger, could also soak up some of the star’s light. Perhaps all three are at play.
If you’ve been making a habit of checking the star you’ve noticed that it’s very similar in brightness to its neighbor, Bellatrix, which shines at magnitude 1.6. My latest brightness estimate from 2 nights ago (Feb. 15, 8 p.m. CST) pegged it at 1.7. Guinen reports that the rate of the star’s decline has been slowing in the past few weeks. His most recent estimates of its brightness put the star around magnitude 1.60-1.62.
Astronomers everywhere have begun to focus their attention on Betelgeuse, bringing all manner of instruments to bear to dig all the information they can from its light. Guinan and a team other other scientists plan to observe the supergiant with NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA) in infrared light (invisible light that we can feel but not see emitted by warm objects ). Others will use the Hubble Space Telescope, radio telescopes and interferometry to chisel away at Betelgeuse. Interferometry is a technique astronomers use to recreate a much larger telescope by combining the light of multiple smaller telescopes — very handy for measuring things that look super-tiny (because they’re so far away) like a star’s size and shape.
Your efforts count, too! Amateur astronomers like you have contributed hundreds of recent visual, CCD, and photoelectric observations of the star to the American Association of Variable Star Observers (AAVSO).
Despite all the excitement about Betelgeuse “going supernova,” no one knows when that will happen but probably sometime in the next 100,000 years. Its current fluctuations, while exciting and revealing at the same time, don’t necessarily point to an imminent explosion. But when it does happen Betelgeuse’s last hurrah will be incredible because it will appear as a stellar pinpoint that will shine as brightly as the gibbous moon and be easily visible in full daylight.