Object: Betelgeuse (alpha Orionis)

Spectral class
: Orion
: 05 55.48 / +07.24
: 0.6
: M1-M2Ia-Ib
: Deep orange
Details sketch
Date / Time
Observing Location
Seeing / Transparency
Magnification / Field of View '
: 08/02/11 / 20.45
: Landgraaf
: 3 / 4
: Orion Optics UK 300mm
: 12mm Nagler Type 4
: 133 / 37
Observing report
Through the 12-inch dobson this bright, deep orange star looks simply stunning. Although about 70 degrees to the west there is the moon, almost at its first quarter, its bright light does not disturb the smashing view of Betelgeuse. With the naked eye I clearly see the orange colour of Betelgeuse, and in my 85mm refractor, the orange colour appears to me as very “deep” or intense. It looks much more orange than through the 12-inch telescope. However, I prefer the view through the 12-inch because of its light gathering power. Betelgeuse looks overwhelmingly bright, simply stunning. And on top of that, I love the diffraction spikes which are visible around the bright stars, which are produced in by the newtonians secondary spider.

At 133x, there are a handful of stars visible in the 37’ field of view of the 12mm Nagler. Some faint, others extremely faint, only visible with averted vision. It is a rather poor star field to observe. There are no double stars, coloured stars, or asterisms visible in the field of view. There is no background glow of unresolved stars or nebulosity.

betelgeuse finderchart

Image from Voyager by CapellaSoft

Betelgeuse, or alpha Orionis, is the eight brightest star in the night sky. Classified as a red supergiant (M21ab), together with Antares, it is one of the largest and most luminous stars known. Just have a look at the properties of this giant (some of them are however arbitrary):

Stacks Image 426

As you can see Betelgeuse radius is 1180 solar radii. On the sketch below you see the sun represented as one single pixel hovering above the gargantuan Betelgeuse. In fact, if Betelgeuse would be at the centre of our solar system, it would probably extend past the asteroid belt, even maybe engulf Jupiter and some space beyond. Incredible!

Stacks Image 432

In fact, the star is so big, that in 1920 it was the first star that had it’s angular diameter measured (0.047”) and it also was the first star from which the Hubble Space Telescope actually took the first conventional telescope image of the surface of another star. More recent studies produced images of Betelgeuse’s photosphere at different wavelenghts. Here you see 4 images that show the star and its surrounding atmosphere in different wavelenghts.

Stacks Image 438

However, Betelgeuse’s exact angular diameter remains a mystery. This is because it is a variable star, shows limb darkening and angular diameters that vary with wavelength. What makes things even more complicating is the fact that Betelgeuse has a complex assymetric envelope caused my severe mass loss involving huge plumes of gas being expelled from it’s surface. Some studies even speak of stellar companions, orbiting within the gaseous envelopes, which would contribute to the red giant’s eccentric behaviour.

About the images

Upper left:
This image shows the surface of Betelgeuse in UV light (Image credit: HST by NASA).

Upper right:
This wonderful image shows large nebulae around Betelgeuse (Image credit: ESO, VLT with the VISIR instrument in different Infrared wavelengths).

Lower left:
This image also shows large plumes of surrounding gas around Betelgeuse (Image credit: Pierre KervellaNaCo ESO VLT). The image was obtained at different wavelengths in the near infrared.

Lower right:
This is a radio image of Betelgeuse which shows the actual photosphere of the star (circle marked “optical disc), and the stars asymmetric atmosphere, which would expand beyond Jupiter’s orbit.(Image credit: NASA)

Betelgeuse article in Wikipedia
The brightest stars by Fred Schaaf