March 22, 2020 – What’s Going on With Betelgeuse?

Betelgeuse has been acting up lately.

As stars go, Betelgeuse is a beast!  It’s the bright, orange star defining the constellation Orion’s right (left as we look at it) shoulder.  It’s a huge, massive red giant star.  Huge, meaning if it were to replace our Sun it’s atmosphere would nearly reach Jupiter!  Massive, meaning nearly 20 times the mass of our Sun.  What does this mean?

Stars this massive don’t live long lives.  They usually live a hundred million years or less. Our Sun will make it to 9 billion years or longer.  Betelgeuse is around 10 million years old.

Betelgeuse was probably born as a blue giant star, like those in the Trapezium cluster inside Orion’s M42 nebula.  The Trapezium cluster is a tight collection of brilliant stars that is a pretty sight to see.  Betelgeuse is now furiously burning its hydrogen supply and it’s getting low.  Stars like this tend to expand and contract as the struggle between fusion and gravity gets intense.  Sir John Herschel noted its brightness variability in 1836 and indeed, Betelgeuse has brightness changes from magnitude 0 to 1.6.

The struggle will happen with our Sun too, but Betelgeuse will produce a spectacular finale while our Sun will just bloat, then shrink into a white dwarf.

Betelgeuse is fusing hydrogen into helium, just like the Sun.  Helium will fuse into carbon like the Sun, but this is the end of the road for the Sun.  Our Sun’s mass is insufficient to go beyond carbon or oxygen and once the helium is depleted, core collapse can’t get hot enough to fuse either.  No fusion, no outward pressure, no more star.  Our Sun will become a white dwarf in about 5 billion years.  Not so Betelgeuse.

Betelgeuse has plenty of mass and will fuse carbon, neon, oxygen, and silicon before hitting the wall.  It will also become less stable and more variable in brightness as this occurs.  As Betelgeuse fuses silicon its core will concentrate iron.  Once the iron mass in its core reaches about 1.5 times the mass of our Sun things will happen fast.  No matter the core’s mass, iron fusion uses more energy than it produces, so it just doesn’t happen.  The iron core collapses, and collapses, and collapses.  Even its own atoms can’t hold up the mass and get crushed.  Even the electrons cannot stop the crush, merge with protons and become neutrons.  What happens next depends on several factors but boils down to two possible results.  Neutron star or black hole.

Either way, Betelgeuse will go BOOM in a supernova!  It will be as bright as the Moon, visible in daylight.

Will it happen soon.  We don’t think so.  Its current variability has happened before, but maybe in a thousand years…?

What’s in the Sky?

March 28; dusk; west:  A waxing crescent Moon shares the evening sky with brilliant Venus, Aldebaran, and the Pleiades.