Alsephina (Delta Velorum)
Alsephina (Delta Velorum) is a star in the constellation Vela, magnitude 1.9.
The brightest eclipsing star known — two spinning, flattened suns that take turns to hide each other.
Every 45 days, one star slips in front of another. Alsephina is really two white suns, each about two and a half times the Sun’s mass, whose orbit we see almost edge-on. That makes it the brightest known eclipsing variable. A third, smaller star circles the pair once every 143 years.
Both stars spin so fast that they bulge at the equator. Their poles are hotter than their waists — about 10,100 kelvin against 9,700 for the larger star — so they would look brighter from above. We happen to see them side-on.
The name comes from the Arabic al-safīnah, “the ship”, once used for the whole of Argo Navis, the Argonauts’ vessel. By 1660 it had settled on this one star in Andreas Cellarius’s illustrated atlas Harmonia Macrocosmica. Precession will bring the south celestial pole close to it around AD 9000.
Alsephina (Delta Velorum) is 81 light-years away: the light we see from it set off in 1945, 12 years before Sputnik became the first artificial satellite.
Did you know?
- Spotted from Jupiter. The eclipses were announced in 2000. Observers on the ground and NASA’s Galileo spacecraft, then at Jupiter, caught the star’s variations independently.
- The False Cross. With Kappa Velorum and two stars of Carina it forms the False Cross, a pattern so like the Southern Cross that it has caused errors in navigation by the stars.
- A temple star. In China it was Tiān Shè sān, the Third Star of the Celestial Earth God’s Temple.
A white 2nd-magnitude star at one corner of the False Cross, a bigger, looser look-alike of the Southern Cross. To tell them apart, remember the true Cross is smaller and brighter, with a fifth star tucked inside and the dark Coalsack beside it.
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 south of 35°N and never sets south of 35°S.
At its highest it stands below the horizon from 50°N, 0° up from 35°N, 35° up from the equator, 70° up from 35°S.
Facts
- Type
- Star
- Constellation
- Vela
- Designations
- Delta Velorum, δ Vel, HR 3485, HD 74956, HIP 42913
- Magnitude
- 1.9
- Position
- RA 8h 44.7m, Dec −54° 43′ (J2000)
- Spectral type
- A2IV+A4V
- Distance
- 81 light-years
- Discovered
- Known since antiquity; found to be an eclipsing binary in 2000
Open AstroPilot to see when Alsephina (Delta Velorum) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.