Betelgeuse
A red supergiant variable star in Orion, nearing supernova.
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Betelgeuse is a red supergiant star located in the equatorial constellation Orion. Typically the tenth-brightest star in the night sky, it is the second brightest in its constellation after Rigel.
Its color is distinctly reddish, and it is a semiregular variable star with the widest brightness range of any first-magnitude star, fluctuating between apparent magnitudes +0.0 and +1.6, with a main period of about 400 days. At near-infrared wavelengths, Betelgeuse is the brightest star in the night sky. Its Bayer designation is Alpha Orionis, often abbreviated as Alpha Ori.
Main sequence
If placed at the center of the Solar System, Betelgeuse's surface would extend beyond the asteroid belt, engulfing Mercury, Venus, Earth, and Mars, given its radius of 640 to 764 times that of the Sun. Its mass is estimated at slightly under ten to a little over twenty solar masses. Measuring its distance has proven difficult, with current best estimates ranging from 400 to 600 light-years from the Sun—a wide uncertainty for a relatively nearby star. Its absolute magnitude is about −6.
After core hydrogen exhaustion
Less than 10 million years old, Betelgeuse evolved rapidly due to its large mass and is expected to end as a type II supernova, most likely within 100,000 years. When it detonates, it will shine as brightly as a half-Moon for over three months, but life on Earth will remain unharmed. Ejected from its birthplace in the Orion OB1 association (which includes the stars in Orion's Belt), this runaway star moves through the interstellar medium at a speed that creates a bow shock over four light-years wide.
In 1920, Betelgeuse became the first extrasolar star whose photosphere's angular size was measured. Subsequent studies have reported an angular diameter between 0.042 and 0.056 arcseconds, with variations attributed to non-sphericity, limb darkening, pulsations, and differing appearances at various wavelengths. The star is surrounded by a complex, asymmetric envelope roughly 250 times its size, resulting from mass loss. Among stars observed from Earth, only R Doradus and the Sun have larger angular diameters.
Starting in October 2019, Betelgeuse dimmed noticeably, and by mid-February 2020 its brightness had dropped by a factor of about three, from magnitude 0.5 to 1.7. It later returned to a more normal range, peaking at visual magnitude 0.0 and V-band magnitude 0.1 in April 2023.
Quick Facts
- Apparent magnitude range
- +0.0 to +1.6
- Main variability period
- ~400 days
- Radius
- 640–764 times the Sun
- Mass
- 10–20 times the Sun
- Distance
- 400–600 light-years
- Absolute magnitude
- −6
- Age
- less than 10 million years
Facts from the source article.
Lore & Background
Betelgeuse and its red coloration have been noted since antiquity; the classical astronomer Ptolemy described its color as 'more or less orange-tawny'. Three centuries before Ptolemy, Chinese astronomers observed Betelgeuse as yellow, which could suggest the star was in a yellow supergiant phase. Aboriginal groups in South Australia have shared oral tales of the variable brightness of Betelgeuse for an unknown period.
The variation in Betelgeuse's brightness was described in 1836 by Sir John Herschel in Outlines of Astronomy. From 1836 to 1840, he noticed significant changes in magnitude when Betelgeuse outshone Rigel in October 1837 and again in November 1839. A 10-year quiescent period followed; then in 1849, Herschel noted another short cycle of variability, which peaked in 1852. Later observers recorded unusually high maxima with an interval of years, but only small variations from 1957 to 1967.
The records of the American Association of Variable Star Observers show a maximum brightness of 0.2 in 1933 and 1942, and a minimum of 1.2, observed in 1927 and 1941. This variability in brightness may explain why Johann Bayer, with the publication of his Uranometria in 1603, designated the star alpha, as it probably rivaled the usually brighter Rigel. From Arctic latitudes, Betelgeuse's red colour and higher location in the sky than Rigel meant the Inuit regarded it as brighter, and one local name was Ulluriajjuaq ('large star').
Reader's Guide
Betelgeuse became the first extrasolar star whose photosphere's angular size was measured in 1920, and subsequent studies have reported an angular diameter ranging from 0.042 to 0.056 arcseconds; that range of determinations is ascribed to non-sphericity, limb darkening, pulsations and varying appearance at different wavelengths. It is also surrounded by a complex, asymmetric envelope, roughly 250 times the size of the star, caused by mass loss from the star itself. The Earth-observed angular diameter of Betelgeuse is exceeded only by those of R Doradus and the Sun.
Starting in October 2019, Betelgeuse began to dim noticeably, and by mid-February 2020 its brightness had dropped by a factor of approximately 3, from magnitude 0.5 to 1.7. It then returned to a more normal brightness range, reaching a peak of 0.0 visual and 0.1 V-band magnitude in April 2023. Infrared observations found no significant change in luminosity over the last 50 years, suggesting that the dimming was due to a change in extinction around the star rather than a more fundamental change. A study using the Hubble Space Telescope suggests that occluding dust was created by a surface mass ejection; this material was cast millions of miles from the star, and then cooled to form the dust that caused the dimming.
Betelgeuse is likely a binary star. The companion star, named Betelgeuse B or Siwarha, is not yet fully confirmed to be gravitationally bound to Betelgeuse, but likely is given other, indirect evidence for a companion. It would be much smaller and fainter than the red supergiant and is believed to orbit at a distance only a few times greater than the size of Betelgeuse.
More in Named Stars
Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Betelgeuse (CC BY-SA 4.0).
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