Deneb Algedi (Delta Capricorni)
Deneb Algedi (Delta Capricorni) is a star in the constellation Capricornus, magnitude 2.8.
The Sea Goat’s tail — the nearest known eclipsing binary, blinking every 24½ hours.
Every 24½ hours, Deneb Algedi blinks. Two stars circle each other so tightly, and so nearly edge-on to us, that each passes in front of the other in turn. It is the nearest known eclipsing binary.
Its secrets came out in stages. Vesto Slipher found from its spectrum in 1906 that it was double; Clifford Crump worked out the orbit in 1921 from 69 measurements at Yerkes Observatory; and only in 1956 did Olin Eggen, at Lick Observatory, catch the eclipses.
The pair has a tangled past. The brighter star, about twice the Sun’s mass, is heavier than models predict, and its smaller partner is bloated — signs that gas once flowed between them. The name is Arabic for “the tail of the goat”: the fish-like tail of the Sea Goat.
Deneb Algedi (Delta Capricorni) is 39 light-years away: the light we see from it set off in 1987, 3 years before the Hubble Space Telescope was launched.
Did you know?
- A Behenian star. Medieval astrologers counted it among the fifteen Behenian stars, linking it with the gemstone chalcedony and the herb marjoram.
- Spinning in step. The brighter star turns at about 105 kilometres per second, in time with the orbit — unusually fast for a star with its peculiar, metal-rich spectrum.
- Hidden by the Moon. Lying just south of the ecliptic, it can be hidden by the Moon in an occultation; such events were observed in 1951, 1962 and 1988.
A white star of magnitude 2.8, the brightest in Capricornus, at the eastern end of the Sea Goat’s tail with Nashira just to its west. Every 24½ hours it dips by about a quarter of a magnitude in eclipse — a subtle change best caught by comparing it carefully with nearby stars.
When and where to see it
It is best placed in the evening sky around September, when it crosses the meridian at about 10 pm local time (around midnight in August). It can be seen from everywhere south of 74°N and never sets south of 74°S.
At its highest it stands 24° up from 50°N, 39° up from 35°N, 74° up from the equator, 71° up from 35°S.
Facts
- Type
- Star
- Constellation
- Capricornus
- Designations
- Delta Capricorni, δ Cap, 49 Cap, HR 8322, HD 207098, HIP 107556
- Magnitude
- 2.8
- Position
- RA 21h 47.0m, Dec −16° 8′ (J2000)
- Spectral type
- kA5hF0mF2III
- Colour
- yellow-white (B−V 0.29)
- Distance
- 39 light-years
- Discovered
- Known since antiquity; Olin J. Eggen discovered its eclipses in 1956
Open AstroPilot to see when Deneb Algedi (Delta Capricorni) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.