Betelgeuse is BIG (and variable)

Artist Conception of Betelgeuse

Betelgeuse is a bright star in the shoulder or armpit of the constellation Orion. It is easily visible in February and March from Vancouver around 10 pm, close to the southern Horizon.

Betelgeuse in Orion
Betelgeuse in the constellation Orion. Near the Southern Horizon from Coquitlam on Feb 18th, 2020

If you follow astronomy news at all you’ll have heard that Betelgeuse has recently been dimming. Betelgeuse is a variable star whose brightness is known to periodically rise and fall, but it’s recent dimming has been quite extraordinary. Especially exciting is speculation that Betelgeuse may explode as a brilliant supernova which would be visible even in daylight. Unfortunately the latest measurements have shown that Betelgeuse may be brightening again,

Less often reported is that Betelgeuse is BIG! – not too surprising given that it is classified as a red supergiant. So how big is it?

  • It is so big that its diameter is about 1300 times that of our sun.
  • It is so big that if placed at our Sun’s location, the outer edge of its photosphere would reach out to Jupiter.
  • It is so big that it was the first star to have its angular diameter measured. Stars are so far away that they appear as pinpoints of light whose angular diameter cannot be determined. But Betelgeuse was big enough that 100 years ago, in 1920, it was the prime candidate to submit to measurement of its angular diameter.
Betelgeuse Sun Size Comparison
Size Comparison of Betelgeuse and our Sun
Betelgeuse Overlay on Solar System
Betelgeuse overlaid on the Solar System
Image credit: ALMA (ESO/NAOJ/NRAO)/E. O’Gorman/P. Kervella

Michelson and Pease used the 2.5 m Hooker telescope at the Mount Wilson Observatory in California – the largest telescope in the world between 1917 and 1949 – but even it was not large enough to resolve the disk of Betelgeuse. Instead Michelson turned the telescope into an interferometer by attaching a framework with 4 six inch mirrors to the front of the telescope.

Michelson interferometer for measuring star diameters, attached to front of the 2.5m Hooker telescope.

The mirrors created 4 separate light paths of the star that combined, due to the wave nature of light, into an interference pattern.

Astronomical interferometers can produce higher resolution astronomical images than any other type of telescope – theoretically producing images with the angular resolution of a huge telescope with an aperture equal to the separation between the light paths. The separation between the mirrors of Michelson and Pease’s interferometer at Mount Wilson was about 20 feet (or 6 m) and this proved sufficient for them to measure the angular diameter of Betelgeuse as 0.047 arc-seconds.

Once you know the angular diameter, then if you also know the distance of the star, you can easily get its linear diameter in space. Michelson and Pease used several estimates of the distance to Betelgeuse available in 1920 to come up with a linear diameter of 386,000,000 km, somewhat smaller than more modern estimates. Using their 0.047 arc-seconds angular diameter with current estimates of the distance to Betelgeuse (about 724 light years) gives a linear diameter of 1.83 billion km — a truly colossal diameter, the equivalent of over 1,300 solar diameters!

Michelson and Pease published their results in the May 1921 edition of the Astrophysical Journal but a summary “Betelgeuse: How its Diameter was measured” appeared one month earlier in the April 1921 edition of the RASC Journal.

Betelgeuse is a peculiar star that is subjected to pulsation cycles that not only make is brightness vary but also make its size vary. Long-term monitoring by UC Berkeley’s Infrared Spatial Interferometer (ISI) on the top of Mt. Wilson show that Betelgeuse shrunk in diameter by more than 15% from 1993 to 2009. Recent images show that Betelgeuse has an asymmetric surface and appears to be shedding gas and dust at tremendous rates.

Asymmetric surface of Betelgeuse in Jan 2019
Astronomers used ESO’s Very Large Telescope to discovered a plume of gas ejected from Betelgeuse and a gigantic bubble that boils away on its surface. 
Image Credit: ALMA (ESO/NAOJ/NRAO)/E. O’Gorman/P. Kervella

Since Betelgeuse has a radius 1,300 times that of the Sun, it has a volume about 1.3 billion times larger than the Sun. But its mass is only about 8 – 20 times the Sun. This means the density of Betelgeuse is much, much lower than the Sun. The average density of the Sun is about 1.4 grams/cc – somewhat higher than the density of water. In contrast, the average density of Betelgeuse is just 12 billionths of a gram/cc. This is about a million times less dense than Earth’s atmosphere at sea level, or about the same as a vacuum found in an insulating Thermos bottle.