Kaus Borealis, known as Lambda Sagitarii, is the fifth brightest star in the constellation of Sagittarius, the celestial archer, and it is visible to the naked eye.
Key Facts & Summary
- Kaus Borealis is an orange giant star or subgiant, of spectral type K0 IV.
- It is located at around 78.2 light-years / 23.97 parsecs away from the Sun.
- Kaus Borealis has an apparent magnitude of +2.82 and an absolute magnitude of around 1.07.
- The star can sometimes be occulted by the Moon, and rarely by a planet since it is 2.1 degrees south of the ecliptic.
- Kaus Borealis was occulted by Mercury on the 5th of December 1865 and more recently by Venus on the 19th of November, 1984.
- Kaus Borealis has a mass of 2.6 times that of the Sun.
- The star is a lot bigger than our sun, with an estimated 11 solar radii.
- Though it is bigger, it is cooler, Kaus Borealis has average surface temperatures of around 4.770 K – it is around 1.000 K cooler than our sun.
- Kaus Borealis has a surface gravity of around 2.90 cgs and a rotational velocity faster than our sun, though still slow for a star, at an estimated speed of 3.81 km / 2.3 mi per second.
- The energy radiated by its expanded outer envelope is the cause of its orange-hue, which is typical for a K-type star.
- Kaus Borealis’s positing in the constellation marks the top of the celestial Archer’s bow.
- Kaus Borealis has a radial velocity of around -43.5 km / -27 mi per second.
- Though it is designated as alpha, Kaus Borealis is only the fifth brightest star in its constellation after Kaus Australis, the true brightest star, Nunki, Ascella, and Kaus Media.
- Kaus Borealis is also part of the Teapot asterism, a conspicuous asterism that dominates the constellation.
Though Kaus Borealis is designated as the alpha star, meaning the brightest of its constellation, that title belongs to Kaus Australis, the true brightest star of Sagittarius.
Designations tend to be given based on a star’s brightness, though this isn’t necessarily a rule of thumb. The name, Kaus Borealis, comes from the Arabic word qaws, meaning “bow” and the Latin “borealis” meaning “northern.” It was named as such since Kaus Borealis marks the northern tip of the celestial Archer’s bow.
Kaus Borealis isn’t associated with any known star group. Its age is currently unestimated. The star most likely formed out of a molecular cloud, or nebula, of gas and dust. Gravity pulled the swirling gas and dust together resulting in one of Sagittarius’s key stars, Kaus Borealis.
Distance, Size, and Mass
Kaus Borealis is located at around 78.2 light-years / 23.97 parsecs away from the Sun. It is one of the brightest stars in the night sky and the fifth brightest in its constellation.
The star has an estimated 2.6 solar masses, it has thus 260% of the sun’s mass. Kaus Borealis is a lot bigger than our sun, with an estimated 11 solar radii, or 1.100% the sun’s radius. It has an estimated angular diameter of around 4.24 milliarcseconds.
Kaus Borealis is an orange giant star or subgiant, of spectral type K0 IV. It has an apparent magnitude of +2.82 and an absolute magnitude of around 1.07, and it can sometimes be occulted by the Moon, and rarely by a planet since it is 2.1 degrees south of the ecliptic.
The star was occulted by Mercury on the 5th of December 1865 and more recently by Venus on the 19th of November, 1984. Kaus Borealis has an average surface temperature of around 4.770 K – it is exactly 1.008 Kelvins cooler than our sun.
Kaus Borealis has a surface gravity of around 2.90 cgs and a rotational velocity faster than our sun, though still slow for a star, at an estimated speed of 3.81 km / 2.3 mi per second while its radial velocity is at around -43.5 km / -27 mi per second.
The energy radiated by its expanded outer envelope is the cause of its orange-hue, which is typical for a K-type star. Kaus Borealis is at around 52 times brighter than our sun.
Kaus Borealis is located in the zodiacal constellation of Saggitarius, the celestial archer. This constellation is situated exactly towards the center of the Milky Way.
Together with Kaus Media and Kaus Australis, it marks the celestial Archer’s bow. The same stars together with Alnasl, Ascella, Nunki, Tau Sagittarii, and Phi Sagittarii form the Teapot asterism. This asterism is very bright and dominates the western half of Sagittarius.
Since the constellation of Saggitarius lies in the direction of the Milky Way’s center, it is located in the densest region of the galaxy. There are many interesting stars and deep-sky objects in Saggitarius, such as globular clusters, Messier objects, nebulae, and much more.
The month of August is the best time to try and observe the celestial objects in Sagittarius. Most objects are not visible year-round to northern observers and even farther north, they are completely invisible. When it comes to the mid-northern latitudes, they can only be seen above the southern horizon during the summer months.
Since Kaus Borealis is an orange dwarf, it will remain stable for long periods of time. Such stars are favorable for exoplanets that might reside in their habitable zone.
Did you know?
- The constellation of Sagittarius is one of the Greek constellations, first cataloged by the Greco-Roman astronomer Ptolemy, in the 2nd century C.E.
- Sagittarius is the 15th largest constellation in the sky and contains more Messier objects than any other constellation.
- In a sense, Sagittarius contains the center of the Milky Way, marked by the radio source / supermassive black hole, designated as Sagittarius A*.
- Among the stars of the Teapot asterism, of which Kaus Borealis is also part of, only Kaus Australis and Nunki were included in the list of the 58 navigational stars. Only the brightest and the most easily recognizable stars are included in this group.
- Kaus Borealis marks the top of the lid of the celestial Teapot asterism.
- Kaus Borealis is also part of a smaller asterism known as the Little Milk Dipper, formed by the stars in the Teapot asterism – Nunki, Phi, and Tau Sagittarii.
- The Chinese knew Kaus Borealis as the Second Star of Dipper – an asterism also known as one of the seven mansions of the Black Tortoise.
- The Egyptian astronomer Al Achsasi al Mouakket listed Kaus Borealis as “the keeper of the ostriches” in his 17th-century Calendarium. Its name was Rai al Naaim and later translated into Latin as Pastor Struthionum.
- The Arabian astronomer AI Tizini also called Kaus Borealis, Rai al Naaim in the 16th century. Borealis along with Polis was known in Arabic astronomy as the keepers of two groups of ostriches – two asterisms.