Asellus Tertius (κ Boötis) | Facts, Formation, Distance, Size & Mass

Asellus Tertius, designated as Kappa Boötis, is the ninth brightest star in the constellation of Boötes, the celestial herdsman. Kappa Boötis is also a double star.

Key Facts & Summary

The Kappa Boötis star system is comprised out of a spectroscopic binary system, designated as Kappa¹ Boötis, which consists of an F2 main-sequence star, and a secondary star which is much fainter and has only half the mass of the primary star, and Kappa² Boötis, which is a Delta Scuti variable star, and the brightest, biggest, and most massive out of all three stars.
Kappa¹ Boötis is located at around 161 light-years / 49.2 parsecs from our Solar System, while Kappa² Boötis is located at 154 light-years / 47.2 parsecs.
Kappa¹ Boötis has an apparent magnitude of +6.69, while Kappa² Boötis has an apparent magnitude that varies from +4.50 to +4.58 within a period of 1.08 hours.
Kappa¹ Boötis is an F2 main-sequence star that has around 1.40 solar masses, and around 1.43 solar radii.
Kappa² Boötis is an evolved A8IV subgiant star that has around 2.12 solar masses, and around 2.78 solar radii.
Kappa¹ Boötis is 3.801 times brighter than our Sun, while Kappa² Boötis is 28 times brighter.
The surface temperatures on Kappa¹ Boötis is at around 6,699 K, while on Kappa² Boötis, 7,760 K. Both stars are hotter than our Sun.
The surface gravity on Kappa¹ Boötis is at 4.32 cgs, while on Kappa² Boötis, at 3.66 cgs.
The rotational velocity of Kappa¹ Boötis is at around 38 km / 23.6 mi per second, while Kappa² Boötis is quite a fast-spinning star, having a rotational velocity of 128 km / 79.5 mi per second.
The radial velocity of these stars differs, which means that they may not share a common origin. Kappa¹ Boötis has a radial velocity of -22.09 km / -13.7 mi per second, while Kappa² Boötis is at -15.60 km / -9.69 mi per second.

κ Boötis

Kappa Boötis, also known as 17 Boötis, bears the traditional Latin name, Asellus Tertius, which only applies to the brightest star of this system, designated as Kappa² Boötis.

Asellus Tertius translates to “third donkey colt”. This name was approved by the IAU in late 2016.

Formation

The age of the primary star of the Kappa Boötis star system, Asellus Tertius or Kappa² Boötis, is unknown, however, the secondary star, Kappa¹ Boötis, is around 900 million years old. It is much younger than our Sun.

The stars of the Kappa Boötis star system formed from an interstellar medium of gas and dust. Gravity pulled the swirling gas and dust together and resulted in the creation of these stars when the right temperature was reached. These stars don’t appear to have a common origin, as their motion through space differs.

Distance, Size, and Mass

Kappa¹ Boötis is located at around 161 light-years / 49.2 parsecs, while Kappa² Boötis is located at around 154 light-years / 47.2 parsecs away from our Solar System.

These stars can be viewed even through small telescopes. The biggest star in the Kappa Boötis star system is Asellus Tertius.

Asellus Tertius has around 2.12 solar masses, or 212% of our Sun’s mass, and 2.78 solar radii, or 278% of our Sun’s radius. It is thus more than five times larger than our Sun.

The second-biggest star in the system is Kappa¹ Boötis, which has around 1.40 solar masses, or 140% of our Sun’s mass, and 1.43 solar radii, or 143% of our Sun’s radius. It is almost three times bigger than our Sun. The third star, the one that orbits Kappa¹ Boötis, has only half of its mass, and it is considerably fainter.

Other Characteristics

The primary star of the Kappa Boötis star system, Kappa² Boötis, also known as Asellus Tertius, is an evolved subgiant star of spectral type A8IV.

Asellus Tertius is also a Delta Scuti variable star, having brightness variations between magnitude +4.50 and +4.58, within a period of 1.08 hours. It is 28 times brighter than our Sun.

This star is also hotter than our Sun, having temperatures of around 7,760 K. It is thus 34% hotter than our Sun. Asellus Tertius is a fast-spinning star, having a rotational velocity of 128 km / 79.5 mi per second. The surface gravity on this star has been estimated at around 3.66 cgs.

The secondary star of the Kappa Boötis star system, Kappa¹ Boötis, is a main-sequence star of spectral type F2V. This star has an apparent magnitude of +6.69, much fainter than Kappa² Boötis.

Kappa¹ Boötis is hotter than our Sun, having surface temperatures of around 6,699 K. It is 15% hotter than our Sun, and 3.801 times brighter.

Kappa¹ Boötis has a surface gravity of 4.32 cgs, and a rotational velocity of around 38 km / 23.6 mi per second.

Star System

The Kappa Boötis star system is composed out of the subgiant star Asellus Tertius, and the two components of the spectroscopic binary system Kappa¹ Boötis.

The components are separated by an angular distance of 13.5 arcseconds. The spectroscopic binary system, Kappa¹ Boötis, has a radial velocity of -22.09 km / -13.7 mi per second, while Asellus Tertius has a radial velocity of -15.60 km / -9.69 mi per second. Thus, the stars don’t appear to have a common origin, however, the smaller star orbiting Kappa¹ Boötis might have formed out of the same interstellar medium as Kappa¹ Boötis.

Location

Kappa Boötis / Asellus Tertius is located in the constellation of Boötes, the celestial herdsman. It is the ninth brightest object in this constellation.

Asellus Tertius, along with Iota Boötis, Asellus Primus, and Lambda Boötis, is located in the far north of the Boötes constellation. They are collectively known as the Aulad al Dhi’bah – “the whelps of the hyenas”.

The constellation of Boötes is one of the first 48 Greek constellations, listed by the legendary astronomer Ptolemy, in his 2^nd century Almagest.

Boötes is now among the 88 modern constellations, being the 13^th largest in the sky, stretching for around 907 square degrees. There are many interesting stars and deep-sky objects in the constellation of Boötes, such as Arcturus, the fourth-brightest star in the night sky, Muphrid, a binary star system, Nekkar, a yellow giant star, Seginus, a Delta Scuti variable star, Alkalurops, a triple star system, Izar, another triple star system, Rho Boötis, an orange giant star, the globular cluster NGC 5466, the bright galaxies NGC 5248 and NGC 5676, the Seyfert galaxy NGC 5653, the Boötes Void, or the Hercules-Corona-Borealis Great Wall – the largest structure in the universe.

The constellation of Boötes is a part of the celestial sphere facing away from the plane of our Milky Way galaxy, and as such, it has few to almost zero globular clusters or nebulae. However, the constellation is prominent in many faint galaxies.

The best time to observe Asellus Tertius / Kappa Boötis, the other stars, and deep-sky objects in Boötes, is during the month of June when the constellation is the most prominent.

The Future

Kappa Boötis appears to be a stable star system, and it will continue to exist for many millions of years. The subgiant, Asellus Tertius, will go on to evolve on its stellar path and become a true giant star, many millions of years from now. At the end of its life, the star will probably become a white dwarf, since it isn’t massive enough to explode as a supernova.

Did you know?

Asellus Tertius is known to the Chinese as Tian Qiang yi – the First Star of Celestial Spear.
The Celestial Spear – known as Tian Quang – is a Chinese asterism formed by Asellus Tertius, Theta Boötis, and Iota Boötis. It is part of the Purple Forbidden enclosure, being among the three enclosures.
Asellus Tertius appears white in color, while Kappa¹ Boötis appears bluish.