Rigel

Rigel at IC 2118.

Rigel (Beta Orionis / β Ori / 19 Orionis) is a star system of at least four stars that appear as a single blue-white point of light to the naked eye in the constellation of Orion. Although its Bayer designation "Beta" should correspond to the second brightest star in the constellation, its apparent magnitude of +0.18 actually places it as the brightest of the constellation, ahead of Betelgeuse (α Orionis). This system is located at a distance of approximately 860 light years (260 pc) from the Sun.

Name

Rigel is located on the left foot of the figure of the hunter Orion formed by the stars of the constellation. This is where her name comes from, from the Arabic Rijl jauza al-Yusra , "the left foot of the center", alluding to Orion. Another rarely used name for the star is Algebar or Elgebar, derived from the Arabic ar-Rijl al-Jabbār, "the foot of the giant".

In China it was known as 参?宿七, Shēn xiù Qī, “The Seventh of the Three Stars”. This curious name is due to the fact that, initially, the Asterism of the Three Stars consisted of just three, the famous Belt of Orion (see Chinese constellations). Four stars were later added to the asterism, but the name remained unchanged.

In Japan it was known as 源氏星, Genjiboshi, "The Star of the Genji Clan".

Physical characteristics

Farther from reliable parallax measurements, spectroscopic estimates place Rigel between 700 and 900 light-years—210 and 280 parsecs—from the Solar System. The best guess for the Hipparcos satellite is a distance of 860 light years.

The main component of the system is a bluish-white supergiant of spectral type B8Iab and with a surface temperature of 11,500 K. A very massive star —it has a mass of 18 solar masses—, it shines with a luminosity in the visible spectrum 50,000 times greater than that of the Sun; if ultraviolet light is considered, its luminosity increases up to 85,000 times that emitted by the Sun. Its radius is equal to 73 times the solar radius —equivalent to 0.34 astronomical units—, so if it were in the place of the Sun, it would extend almost to the orbit of Mercury. Very advanced in its evolution, it most likely contains an inert helium nucleus within it, although it could also be fusing helium into carbon and oxygen after having passed through the red giant phase. Stars of this mass end their lives. lives in a supernova explosion.

Comparison between the size of Rigel and the Sun.

Moving through a region with nebulosities and being so luminous, Rigel illuminates several nearby dust clouds, most notably IC 2118 (the "Witch Head Nebula"). Rigel also appears to be associated with the Orion Nebula, which, although located approximately in the same line of sight, is almost twice as far from Earth. Despite the difference in distance, the projection of Rigel's trajectory through space, considering its estimated age —about 10 million years— originally places it close to the nebula. Although Rigel is sometimes classified as a member of the Orion OB1 Stellar Association, she appears to be too close to us to be considered a real member of it, unless—like Betelgeuse (α Orionis)—she was expelled from her place. of birth.

Rigel is a variable star with small oscillations in its brightness, a rare phenomenon in supergiants. The range of variability oscillates between 0.3 and 0.03 magnitudes, equivalent to a variation between 3% and 30%, with a period of 25 days. It has been proposed that the variability is due to the existence of a fourth star in the system—see the stellar system section below—but it is generally considered that the true cause lies in pulsations on the surface of the parent star.

Star System

Rigel has been known to be a visual binary since at least 1831, when Friedrich Georg Wilhelm von Struve first measured the position of the secondary star. Although the latter is magnitude +6.7, its proximity to Rigel A—more than 500 times brighter—presents a challenge for telescopes smaller than 150 mm aperture. At the system's estimated distance, it is separated from the primary star by more than 2,500 astronomical units (au); consequently, no orbital motion has been detected, although they share their own motion.

Likewise, this stellar companion is a binary star, made up of two main sequence stars of type B9V (called Rigel B and Rigel C) whose orbital period is 63 years. In turn, the first of them is also a spectroscopic binary with a period of only 9.86 days. A fifth magnitude 15 star also appears to be associated with the system. Visually at 44 arcseconds—corresponding to a true separation of more than 11,500 au—it may be a K-type dwarf that takes about 250,000 years to complete one orbit around the bright inner system.