Gomeisa (Beta Canis Minoris)
Gomeisa (Beta Canis Minoris) is a star in the constellation Canis Minor, magnitude 2.9.
Procyon’s little companion in the sky: a whirling blue-white star wrapped in a disc of its own gas.
Gomeisa spins like a top. Its equator races round at more than 200 kilometres a second, and it may turn once a day — the Sun takes almost a month. Flung out by that spin, a thin disc of hot gas circles its equator, reaching out to about three times the star’s radius and glowing with hydrogen light. That makes Gomeisa a Be star: a blue-white star of about three and a half solar masses, still burning hydrogen in its core.
What spun it up? Gomeisa probably has a hidden partner of under half the Sun’s mass, on an eccentric 170-day orbit. One idea is that it is the stripped core of a star that once poured gas — and spin — onto Gomeisa. If confirmed, the pair would join a very rare class of Be stars with hot subdwarf companions.
Its name is gentler: Arabic al-ghumaisa’, ‘the bleary-eyed woman’, short for a phrase meaning ‘the girdle of the bleary-eyed one’. The same short name was also used in Arabic for brilliant Procyon nearby.
Gomeisa (Beta Canis Minoris) is 160 light-years away: the light we see from it set off around 1865, 13 years before Edison lit his first practical light bulb — the start of light pollution.
Did you know?
- A heartbeat from space. Canada’s small MOST space telescope found Gomeisa varying by mere thousandths of a magnitude, in rhythms of a little over three cycles a day. That makes it a slowly pulsating B star.
- Variable on paper. Gomeisa was listed as a variable star in 1977, yet several later studies found no change in its brightness. Its clearest changes are in the hydrogen glow of its disc.
- The South River. In Chinese astronomy Gomeisa, Procyon and Epsilon Canis Minoris form Nán Hé, the South River. Gomeisa is its second star.
The second star of tiny Canis Minor, about four degrees north-west of brilliant Procyon — the two make up most of the Little Dog. Compare Gomeisa’s blue-white with Procyon’s soft yellow-white.
When and where to see it
It is best placed in the evening sky around February, when it crosses the meridian at about 10 pm local time (around midnight in January). It can be seen from everywhere north of 82°S and never sets north of 82°N.
At its highest it stands 48° up from 50°N, 63° up from 35°N, 82° up from the equator, 47° up from 35°S.
Facts
- Type
- Star
- Constellation
- Canis Minor
- Designations
- Beta Canis Minoris, β CMi, 3 CMi, HR 2845, HD 58715, HIP 36188
- Magnitude
- 2.9
- Position
- RA 7h 27.1m, Dec +8° 17′ (J2000)
- Spectral type
- B8Ve
- Colour
- white (B−V -0.09)
- Distance
- 160 light-years
- Discovered
- Known since antiquity
Open AstroPilot to see when Gomeisa (Beta Canis Minoris) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.