Star Facts: Rasalhague

Star Facts: Rasalhague

Rasalhague (Alpha Ophiuchi) is a binary star with a combined apparent visual magnitude of +2.07, making it the brightest star in the constellation Ophiuchus, and the 59th brightest star in the entire night sky overall. However, about 390,000 years ago, Rasalhague was at its point of closest approach to the Sun, at which time ancient observers would have seen it shining with an apparent visual magnitude of about +1.65. The star’s unusual name derives from the Arabic phrase meaning ‘Head of the Serpent-charmer’, with Rasalhague depicted on the forehead of an elderly man who is holding a huge serpent wrapped around his body, represented by the neighboring constellation of Serpens.

Quick Facts

• Constellation: Ophiuchus
• Coordinates: RA 17h 34m 56.06945s |Dec. +12° 33′ 36.1346″
• Distance to Earth: 48.6 light years
• Star Type: A5 III / K5–7 V
• Mass: (a Oph A) 2.40 sol|( a Oph B) 0.85 sol
• Radius: 2.6 sol
• Apparent Magnitude: +2.07 (Combined)
• Luminosity: 25.1 – 25.6 sol
• Surface Temperature: 7880K – 8050K
• Rotational Velocity: 240 km/sec
• Age: 0.77 billion years
• Other Designations: Ras Alhague, Rasalhague, 55 Ophiuchi, BD+12° 3252, FK5 656, HD 159561, HIP 86032, HR 6556, SAO 102932

Visibility

Rasalhague lies within the southern sky constellation of Ophiuchus, which is found between Scorpius and Sagittarius, but is only observable from latitudes of between +80° and -80°. The star itself is rather difficult to spot within the huge constellation, and finding it requires some star hopping. Fortunately, though, Ophiuchus contains many spectacular and famous objects that to some extent can be used to find Rasalhague, including seven Messier objects, M9, M10, M12, M14, M19, M62, and M107, all of which are globular clusters. While there are various ways to find Rasalhague within the constellation, one of the easiest ways is to locate the globular cluster M14, and then to draw a line almost due north for about 16 degrees to the first bright star, which is Rasalhague. Handily, there are no other bright stars on this line, although its path will pass just to the right of Beta Ophiuchi (Celbalrai), which is the fifth brightest star in the constellation.

Physical Properties

Binary System

Rasalhague is a binary system whose primary component, designated Alpha Ophiuchi A, has an estimated mass of about 2.4 times that of the Sun, while the secondary component, Alpha Ophiuchi B, is estimated to have only about 85% of the Sun’s mass. The mass of the secondary component suggests that this star is a type K5V to K7V star, which classification would place it on the main sequence. In contrast, the primary component, which has an A5 III classification, is an evolved white giant that has exhausted the hydrogen at its core, and is now about 25 times as bright as the Sun. Although Rasalhague had been known to have a companion for a long time, it was only in 2011 that investigators using adaptive optics were able to calculate the system’s orbital period accurately, based on the respective masses of the components. The companion is now known to orbit the primary star once every 8.62 years.

Rotational Velocity

Rasalhague is a very fast rotator, with its high rotational velocity of 240km/sec representing about 88.5 % of the speed at which the star would otherwise fly apart. This high spin rate has caused the star to assume a highly oblate shape, making its equatorial circumference 20% bigger than its polar circumference. This huge amount of distortion also raises the temperature along the star’s equatorial region by about 1,840K above that of the polar regions.

Somewhat surprisingly, the star’s spectrum shows an abnormally high level of singly-ionized calcium (Ca II), absorption. However, while this could possibly be a characteristic of the star’s composition, it is more likely to be the results of the stars’ light travelling through as-yet unidentified material between Earth and Rasalhague.

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