Nunki (Sigma Sagittarii)
Nunki (Sigma Sagittarii) is a star in the constellation Sagittarius, magnitude 2.1.
Two hot blue stars in the Teapot’s handle — and the nearest star expected to end as a supernova.
Nunki looks like one star, but it is two hot blue-white stars, each more than six times the Sun’s mass, swinging around each other every 135 days. Their surfaces are near 18,500 kelvin, more than three times hotter than the Sun’s, and their stretched orbit brings them closer together than Venus is to the Sun.
The pair is about 30 million years old, and its future is dramatic. In some 25 million years the larger star will swell into a red giant and pour gas onto its partner. The two should then merge into one star of over 10 solar masses — enough to explode. That makes Nunki the nearest star expected to end as a core-collapse supernova.
Its name is far older than the science. Nunki is Sumerian, the name of the holy city Eridu, “Divine Place of the Earth”. R. H. Allen popularised it for this star, though it probably first belonged to stars in the region of Vela.
Nunki (Sigma Sagittarii) is 230 light-years away: the light we see from it set off around 1795, 15 years after William Herschel discovered Uranus.
Did you know?
- Covered by Venus. Nunki lies just south of the ecliptic, where the Moon and planets travel. On 17 November 1981 Venus passed in front of it — the last time a planet hid it.
- Bayer’s low ranking. Johann Bayer’s star atlas Uranometria placed Nunki in the fourth magnitude class, though by modern measurement it is a second-magnitude star.
- The second returning ostrich. The Calendarium of Al Achsasi al Mouakket named it Thanih al Sadirah, the “second returning ostrich”.
A blue-white 2nd-magnitude star at the top outer corner of the Teapot’s handle in Sagittarius. It sits close to the ecliptic, so the Moon now and then passes over it — watch for it to wink out at the Moon’s edge.
When and where to see it
It is best placed in the evening sky around August, when it crosses the meridian at about 10 pm local time (around midnight in July). It can be seen from everywhere south of 64°N and never sets south of 64°S.
At its highest it stands 14° up from 50°N, 29° up from 35°N, 64° up from the equator, 81° up from 35°S.
Facts
- Type
- Star
- Constellation
- Sagittarius
- Designations
- Sigma Sagittarii, σ Sgr, 34 Sgr, HR 7121, HD 175191, HIP 92855
- Magnitude
- 2.1
- Position
- RA 18h 55.3m, Dec −26° 18′ (J2000)
- Spectral type
- B2.5V
- Colour
- blue-white (B−V -0.14)
- Distance
- 230 light-years
- Discovered
- Known since antiquity
Open AstroPilot to see when Nunki (Sigma Sagittarii) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.