Betelgeuse or Alpha Orionis (Alp Ori) is the 2nd brightest naked eye star in the constellation Orion. With an apparent magnitude of 0.45v, Betelgeuse is the 9th brightest star in the entire sky (see: 50 Brightest Stars ). Its absolute magnitude is -5.14 and its distance is 428 light years. The Equinox J2000 equatorial coordinates are RA = 05h 55m 10.3s, Dec = +07° 24' 25".

Betelgeuse has a spectral type of B3V, a surface temperature of 3500° Kelvin and a luminosity 140,000 times the Sun. It has a mass of 18 solar masses and a diameter 1180 times the Sun.

The image above shows the uncropped view of Betelgeuse (North is up) through the Takahashi E-180 Astrograph.

Classified as a red supergiant, Betelgeuse is one of the largest and most luminous stars known. If it were at the center of our Solar System, its surface would engulf the inner Solar System (Mercury, Venus, Earth and Mars) and extend past the asteroid belt possibly to the orbit of Jupiter. However, with distance estimates in the last century that have ranged anywhere from 180 to 1,300 light years from Earth, calculating its diameter, luminosity and mass have proven difficult. Betelgeuse is currently thought to lie around 640 light years away, yielding a mean absolute magnitude of about -6.05.

In 1920, Alpha Orionis was the first star (after the Sun) to have its angular diameter measured. Since then, researchers have used a number of telescopes to measure this stellar giant, each with different technical parameters, often yielding conflicting results. Current estimates of the star's apparent diameter range from about 0.043 to 0.056 arcseconds. This is a moving target at best, as Betelgeuse appears to change shape periodically. Because of limb darkening, variability, and angular diameters that vary with wavelength, the star remains a perplexing mystery. To complicate matters further, Betelgeuse has a complex, asymmetric envelope caused by colossal mass loss involving huge plumes of gas being expelled from its surface. There is even evidence of stellar companions orbiting within this gaseous envelope, possibly contributing to the star's eccentric behavior.

Betelgeuse is believed to be only 10 million years old, but has evolved rapidly because of its high mass. It appears to be a runaway star from the Orion OB1 Association, which also includes the late type O and B stars in Orion's belt (Alnitak, Alnilam and Mintaka). Currently in a late stage of stellar evolution, Betelgeuse is expected to explode as a type II supernova, possibly within the next million years.

Distinctly reddish-tinted, it is a semiregular variable star whose apparent magnitude varies between 0.2 and 1.2, the widest range of any first magnitude star. The star marks the upper right vertex of the Winter Triangle, along with Sirius and Procyon.

Betelgeuse is easy to spot in the night sky, as it appears in proximity to the famous belt of Orion. In the Northern Hemisphere, it can be seen rising in the east just after sunset during January. By mid-March, the star is due south in the evening sky and visible to virtually every inhabited region of the globe. In large cities in the Southern Hemisphere (e.g., Sydney, Buenos Aires, and Cape Town) the star rises almost 49° above the horizon. Once May arrives, the red giant can be glimpsed but briefly on the western horizon just after the Sun sets.

The description above is based on the Betelgeuse entry in Wikipedia. For more information about Betelgeuse, see Stars (Jim Kaler).

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