Kaus Borealis (Lambda Sagittarii)

Kaus Borealis (Lambda Sagittarii) is a star in the constellation Sagittarius, magnitude 2.8.

The top of the Archer’s bow — once the keeper of the ostriches.

Medieval Arabic astronomers called it the keeper of the ostriches. Al Tizini named it Rāʽi al-Naʽāʼim, watching over two flocks of stars nearby: the Going Ostriches and the Returning Ostriches. Its modern name is plainer, joining the Arabic qaws, “bow”, to the Latin borealis, “northern” — the top of the Archer’s bow.

It is an orange giant of about 1.8 times the Sun’s mass, swollen to 11 times its width, with a surface near 4,770 K. It turns lazily: its equator moves at least 3.8 kilometres per second, about twice the Sun’s rate.

Lying just south of the ecliptic, it is sometimes covered by the Moon and, rarely, by a planet. Venus last passed in front of it on 19 November 1984; before that, Mercury did so in 1865.

Kaus Borealis (Lambda Sagittarii) is 78 light-years away: the light we see from it set off in 1948, 9 years before Sputnik became the first artificial satellite.

Did you know?

• Second star of the Dipper. Chinese astronomers counted it as the second star of Dǒu, the Dipper, the same group in which Ascella is the first.
• A close call with Mercury. On 17 December 2021 Mercury passed less than an arcminute from Kaus Borealis, as seen from Earth.

An orange star of magnitude 2.8 at the top of the Teapot’s lid in Sagittarius — the northern tip of the Archer’s bow. Compare its warm tint with blue-white Kaus Australis at the bottom of the bow.

What is a star?

When and where to see it

It is best placed in the evening sky around July, when it crosses the meridian at about 10 pm local time (around midnight in June). It can be seen from everywhere south of 65°N and never sets south of 65°S.

At its highest it stands 15° up from 50°N, 30° up from 35°N, 65° up from the equator, 80° up from 35°S.

Facts

Type

Star

Constellation

Sagittarius

Designations

Lambda Sagittarii, λ Sgr, 22 Sgr, HR 6913, HD 169916, HIP 90496

Magnitude

2.8

Position

RA 18h 28.0m, Dec −25° 25′ (J2000)

Spectral type

K1IIIb

Colour

orange (B−V 1.04)

Distance

78 light-years

Discovered

Known since antiquity

Open AstroPilot to see when Kaus Borealis (Lambda Sagittarii) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.