Betelgeuse is a red supergiant in Orion that's one of the most famous stars in the sky. It's an irregular variable that usually fluctuates between magnitudes +0.3 and +0.8, though on rare occasions its been known to peak at magnitude 0.0 and dim down to magnitude +1.2. The star currently hovers around magnitude +0.42 and marks the upper left-hand corner of the ancient hunter figure.

Although lettered alpha (α) Orionis, Betelgeuse is usually fainter than Rigel (β Orionis - mag. +0.13) and therefore the constellations second brightest star. The first person to record brightness fluctuations was Sir John Herschel in 1836 and on two occasions, in October 1837 and November 1839, he observed it to be brighter than Rigel. At best, Betelgeuse is comparable in brightness to Rigel and Capella (α Aur - mag. +0.08) but at the opposite end of the scale it appears more like Aldebaran (α Tau - mag(v). +0.75->+0.85).

The stars name is Arabic and is derived from "Ibt-al-Jauza", which means "the shoulder of the central one". It's often alternatively written as Betelgeuze or Betelgeux and pronounced in a number of ways; many people refer to it as "Beetlejuice", though "Beteljerze" is closer to the original Arabic. Eminent English 19th century astronomer William Lassell - who discovered Neptune's largest moon Triton, Saturn's moon Hyperion (co-discovered) and Uranus moons Ariel and Umbriel - described Betelgeuse majestically as "A most beautiful and brilliant gem; a rich topaz in hue and brilliancy different from any other star I have ever seen!".

Betelgeuse (Fred Espenak)

As with all M type stars the surface temperature of Betelgeuse is much cooler than the Sun; about 3,400 K compared to 5,800 K. The star is truly enormous in size with a diameter that could be as large as 1.67 billion kilometers (1.04 billion miles) or about 1200x that of the Sun. If placed at the centre of the Solar System it would easily contain the orbits of Mercury, Venus, Earth and Mars. However, Betelgeuse is much less denser that the Sun and therefore only between 8 to 20 times more massive. It's also likely that the star has used up its supply of hydrogen and is now fusing helium at its core. Eventually, it will explode as a supernova and that will be a spectacular event. The explosion will temporarily shine as bright as a crescent Moon, cast strong shadows on the ground and easily be visible in daylight. It could go bang tomorrow but we may have to wait another million years or so...only time will tell.

Hubble Space Telescope image of Betelgeuse revealing the huge ultraviolet atmosphere (Andrea Dupree (Harvard-Smithsonian CfA), Ronald Gilliland (STScI), NASA and ESA)

Betelgeuse is located 640 light-years distant and is the nearest of the vast red supergiants. As a result, it was the first star to be directly imaged by the Hubble Space Telescope in 1995; angular size is 0.05 arc seconds. The star is about 10,000x more luminous than the Sun. The chart below shows its position in Orion. Since located just north of the celestial equator it can be seen from every permanently inhabited location in the World. The best months are December, January and February. It appears overhead or near overhead from tropical regions.

Finder Chart for Betelgeuse (Alpha Orionis)

Finder Chart for Betelgeuse (Alpha Orionis) - pdf format

Betelgeuse Data Table

Bayeralpha (α) Ori
Flamsteed58 Ori
RA (J2000)05h 55m 10s
DEC (J2000)07d 24m 25s
Distance (ly)640
Apparent Mag.0.42(v)
Apparent Mag. Range0.0 -> 1.2
Absolute Mag.-5.85
Spectarl TypeM2Iab
Radius (Sol)950 -> 1200
Surface Temp (K).3,140 -> 3,641
Luminosity (Sol)7,500 -> 14,000
Age (million years)7.3
Other designationsHR 2601, HD 39801, HIP 27989

Sky Highlights - April 2017

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