The mystery surrounding the dramatic dimming of red supergiant Betelgeuse, observed over a period of several weeks back in winter 2019, has finally been resolved. In astronomy circles this event is now known as Betelgeuse’s Great Dimming.
A paper published in Nature by a team based at the VLT (Very Large Telescope) has concluded that one part of Betelgeuse underwent a temporary convective cooling process on part of its photosphere, allowing a previously ejected cloud of stellar plasma to condense into a kind of opaque nebulosity, obscuring some of the light from the star over a period of time.
Enormous stars like Betelgeuse have highly turbulent and dynamic surfaces and this event has helped us better understand the periodic mass loss that red supergiants experience in their last stages of evolution, as they literally ‘puff away’ vast swathes of their extended atmosphere into space.
It’s fair to say we’re still at the very earliest stages of truly understanding the complex processes controlling these dying stars.
I feel very privileged and humbled to have witnessed this exciting event (with others) from the surface of a tiny world over 550 light years away, and by extension 550 years after it actually happened.
Some interesting new information has emerged on Betelgeuse, the red supergiant that marks the left ‘armpit’ of Orion the Hunter.
1. It’s still burning Helium in its core so unlikely to go supernova until around 100,000 years.
2. It’s not as massive as previously thought. Earlier studies had shown its radius would extend to the orbit ofJupiter if placed in our solar system. This new data suggests its real radius is 60% of this.
3. It’s closer to Earth than previously measured, at 530 light years. This is 25% closer than we previously thought.
New data published in the Astrophysical Journal. Further reading here.
I can’t get over how much dimmer Betelgeuse in Orion appears at the moment. To my eye Aldebaran (a red giant) in Taurus now appears obviously brighter.
This was an image taken last winter.
Orion image from winter 2019
Many click bait astronomy articles have surfaced claiming the star might be dimming due to its impending collapse and rebound as a supernova, but Betelgeuse is a known variable star and similar changes in its brightness have been noted in the past, although recent changes appear to deviate from established patterns.
Unusual gravitational waves have also been detected in the vicinity of the red supergiant, adding to the sense of mystery. It’s important to note that these waves are merely in the vicinity, so could easily be generated by countless other sources behind Betelgeuse.
Still, it’s fun to speculate about the possibility of witnessing a relatively close supernova event in our lifetime. The last two major naked eye supernovas were recorded in 1572 and and 1054, in Taurus (the Crab nebula) and Cassiopeia (Tycho Brahe supernova). Both generated enough luminosity to be visible in daylight for several weeks, and shone as new ‘guest stars’ for around a year or so before fading.
Some work published by Dolan in 2016 estimated the impact of a Betelgeuse supernova on Earth and found it to be negligible. At 500 light years distance the residual energy of the vastly expanded shockwave would be exponentially diminished as it passed Earth. However the brightness predicted would be magnitude -12.4, making such an event more luminous than a full Moon!
For the mathematically motivated I enclose an extract from his calculations below.
The formation of a young protostar following the collapse of a previously inert dust cloud
We had a great turnout for March’s Urban Astronomy session last week at the Sea Cadet’s Hall in Inverness. The indoor presentation massively benefited from our new giant screen, expertly erected by Robbie (pictured below). Here’s a selection of slides from my presentation on naked eye observing and the life of giant stars.
Robbie putting the final touches to our new giant screen for indoor astronomy presentations and virtual sky guiding
– Naked eye and binocular observing
– Satellites: Iridium Flares and ISS
– Colour, temperature and mass of stars
– The Hertzsprung-Russell diagram
– Protostar formation from dark nebulae
– Main sequence burning and final fate of stars
– White dwarfs, supernovae, neutron stars and black holes.
As ever there were some superb questions during and after the talk. Stay tuned for upcoming events as myself and Caroline roll out the program.
In the simplest terms stars behave like black body radiators with colour linked to their surface temperatures.
The brightest stars in the night sky can be close – like Sirius – or giant stars very far away (eg. Betelgeuse, Rigel, Deneb).
The HR diagram. An elegant and reliable tool for describing the evolution of stars from main sequence burning into their final stages of life
The dog star ‘Sirius’ is now high and visible in winter skies looking South. Draw a line down and left from Orion’s belt and you can’t miss the brightest star in the night sky.
Sirius means ‘scorching’ and was considered a second Sun of sorts to many ancient cultures. Its incredible brightness is due to its close proximity. At only 9 light years away it’s the 5th closest star system to our Sun and a fairly typical hydrogen fusing main sequence star likely to live a long stable life of several billion years. This is in contrast to short lived giant stars like Rigel and Betelgeuse, which are very distant and appear bright due to their bloated sizes and massive energy output.
Procyon is sometimes mistaken for Sirius but it rises earlier, hence its name which means ‘before the dog’. The Arabs told a tale linking Procyon and Sirius as two sisters, who became separated by a great river (the Milky Way) while searching for their missing brother.