2021, February: Betelgeuse

Look for the bright rosy star Betelgeuse during February evenings.  It makes the shoulder of Orion the Hunter.

by Jeffrey L. Hunt

During February, nearly half of all the bright stars that are visible from northerly locations can be seen in the southern sky during early evening hours. (See our article about the Winter Stars.)

One of the most famous is Betelgeuse (“the armpit of the white-belted sheep”).  It is located at the upper left of the famous pattern Orion the Hunter.

The constellation is easily located.  Three stars that are of nearly equal brightness and equally-spaced are in the middle of two stars above the trio and two below.  Rosy Betelgeuse is above while sapphire Rigel is below.

Betelgeuse, like the other bright stars that shine during winter nights, is unusual.  It is nearly 100 times farther away than Proxima Centauri, the nearest star to the sun. It shines with the intensity of nearly 20,000 suns.

The star is nearing the end of its stellar life cycle.  According to stellar models, stars shine because they combine atomic nuclei into heavier elements in their cores at very high temperatures – millions of degrees. A small amount of the matter is converted into energy (E=mc²) that we see as sunshine and starlight.  In the sun and many stars, the primary fuel is the nucleus of the hydrogen atom, a single proton. Betelgeuse has consumed the hydrogen at its core.  To fire heavier atomic particles into heavier components, core temperatures increase.  This higher temperature pushes the outer visible layers of the star into space.  The star expands.  Betelgeuse’s diameter is nearly 700 times larger than our sun.  It is known as a red supergiant.

The light output of Betelgeuse is not consistent.  It is known as a variable star – its brightness changes.  Variable stars change brightness in rhythmic patterns that last days, weeks, or months.  Betelgeuse period is about 400 days.

Over a year ago Betelgeuse was a topic in the popular press.  The star’s brightness was dimming outside its normal range, leading to guesses that the star might be in a terminal state and that it might become a supernova, outshining a full moon. By mid-February 2020, the dimming was noticeable, even without a telescope.

Another competing hypothesis suggested that the star ejected a cloud of dust into space that blocked our view of the star creating an unusual dip in its brightness.

If you use Twitter, an account (@betelbot) tracks the changing brightness of Betelgeuse.  The bot aggregates observations of the star’s brightness and posts a summary of the observations each day. Watch the changing brightness of the star through the account as well as observations outside.  Happy viewing.

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