Betelgeuse continues to fade, and astronomers are wondering what exactly that means. The star will go supernova at the end of its life, but that won't happen for tens of thousands of years or so. So what's causing the blackout?
Villanova University astronomers Edward Guinan and Richard Wasatonic were the first to report the recent darkening of Betelgeuse. Today, reports continue to come in about a further decrease in Betelgeuse's brightness. Astronomers also point out that while the star is still fading, its rate of darkening is slowing.
Betelgeuse is a red supergiant star in the constellation Orion. It left the main sequence about a million years ago and has been in the red supergiant stage for about 40,000 years.
It is the precursor to a Type II Supernova with a core collapse, which means that Betelgeuse will eventually burn up enough of its hydrogen for its core to collapse and explode like a supernova.
It is known as a semi-permanent variable star, which means that its brightness is variable. One of her cycles lasts for about 420 days, and the other for about five or six years. The third cycle is shorter; about 100 to 180 days. Although most fluctuations in brightness are predictable and follow cycles, this is the first time astronomers observe the current blackout.
Astronomers have been following Betelgeuse for a long time. Visual assessments of the star last for about 180 years, and more systematic measurements have been made by the American Observer Association since the 1920s.
About 40 years ago, astronomers at Villanova University began systematic photometric measurements of Betelgeuse's brightness. Photometric data over the past 25 years are the most complete, and according to these data, today, the star is dimmer than ever.
According to Guinan and Wasatonik's publication, Betelgeuse's temperature has dropped by 100 degrees since September 2019, and its brightness has dropped by almost 25 percent over the same time period.
According to all these measurements, the star's radius has grown by about 9 percent.
Betelgeuse magnitude in visible light, August 2018 to January 2020 (AAVSO – AAVSO Light Curve Generator 2)
In a way, we're lucky that Betelgeuse is so close, at least in astronomical terms. It is only 650 light-years away and is the only star besides our Sun on which we can see surface details. This helps astrophysicists understand what's going on there.
Like all stars, Betelgeuse generates heat in its core through fusion. Heat is transferred to its surface by convection. The streams that carry heat are called convection cells, which can be seen on the surface as dark spots.
As the star rotates, these spots rotate in and out of the field of view, contributing to the observed variability of Betelgeuse. Convection cells can be huge, especially on the surface of a giant star like Betelgeuse.
In 2013, scientists reported the presence of convective cells on the Sun that did not disappear for months. Could something like this be happening on Betelgeuse, contributing to the blackout?
This blackout episode may not be caused by the star itself, but by a cloud of gas and dust obscuring the light. Over time, Betelgeuse burns more fuel and loses mass.
As it loses mass, its gravitational hold on its outer edges weakens, and gas clouds leave the star into the surrounding regions. This could cause the current blackout episode.
Or could it be something else? We know a lot about the stars, but not everything. We also have never been able to observe other red supergiants in the way that we can Betelgeuse.
Astronomers know what's going to happen; they just don't know when.
Whatever the reason, we know what the final end of Betelgeuse looks like: a supernova explosion.
It is currently unknown if this blackout is related to the impending catastrophic death of this unstable star. As Ginan and Wasatonic say: “Betelgeuse's unusual behavior should be watched closely.”
When Betelgeuse eventually goes supernova, it will be the most breathtaking sight human has ever seen. Other supernovae, such as SN 185 and SN 1604, were much further away than Betelgeuse.
When Betelgeuse goes supernova, it will become the third brightest object in the sky, after the Sun and the Full Moon. By some estimates, it will shine even brighter than the moon.
This radiance will last for months, and the star will cast shadows on Earth even during the day. Betelgeuse will illuminate the sky like no other supernova. In about three years, the brightness will decline to its current level.
Six years after the supernova explosion, Betelgeuse will not be visible even in the night sky.
When exactly all this will happen, no one knows. And while this recent blackout is probably not directly related to a possible supernova explosion, Betelgeuse, astronomers don't know for sure.
This article was published by Universe Today.
Sources: Photo: (ALMA (ESO / NAOJ / NRAO) / E. O'Gorman / P. Kervella)
