Star Facts: Deneb Algedi

Star Facts: Deneb Algedi
Image Credit: Akira Fujii/David Malin

Deneb Algedi (Delta Capricorni) is a binary system in the zodiac constellation Capricornus which has a combined apparent magnitude of +2.81, making it the brightest star in the constellation. Located about 39 light years away close to the ecliptic, Deneb Algedi is often occulted by the Moon, but only rarely by the planets.

Quick Facts

• Constellation: Capricornus
• Coordinates: RA: 21h 47m 02.44424s|Dec: -16° 07′ 38.2335?
• Distance: 38.70 light years
• Star Type: kA5hF0mF2III + A7m III
• Mass: d Cap A (2 sol) | d Cap B (0.73 sol)
• Radius: d Cap A (1.91 sol) | d Cap B (0.9 sol)
• Apparent Magnitude: +2.81 (Combined)
• Luminosity: d Cap A (8.5 sol)
• Surface Temperature: d Cap A (7,300K) | d Cap B (4,500K)
• Rotational Velocity: 105 km/sec
• Age: Undetermined
• Other Designations: Deneb Algedi (or Deneb Algiedi), Scheddi, d Cap, 49 Capricorni, ADS 15314, BD-16 5943, FK5 819, GCTP 5258.00, GJ 837, HD 207098, HIP 107556, HR 8322


Deneb Algedi is located within the constellation Capricornus, which for observers on Earth can be seen from latitudes of between +60° to -90°. In the Northern hemisphere, Capricornus can be seen from July to November, although it is best seen during early September at about 10 PM Local Time when the constellation is highest in the night sky.

Physical Properties

Like the famous double star Algol in Perseus, Delta Capricorni is a close eclipsing binary system in which the systems’ orbital plane is very close to our line of sight from Earth. The system has an orbital period of 1.022768 days, and as the secondary star eclipses the primary star, the system’s magnitude reduces by 0.24 magnitudes, while when the primary star eclipses the secondary the systems’ magnitude is reduced by 0.09 magnitudes.

Primary Star

The primary star in the system, designated Delta Capricorni A, is classified as an A7m III-type star, which indicates that it is an evolved giant that has exhausted the hydrogen fuel at its core. One other notable feature of the star is the fact that its rotational velocity of 105 km/sec, which is synchronous with the systems’ orbital period, is highly unusual for a star of this type.

Companion Stars

The secondary star in the system located one minute of arc away is of magnitude 15, and is either a G or K-type star with about 90% the Sun’s mass. There is also another 13th magnitude companion currently located about two minutes of arc away from the primary. Note that the distance between these companions and the primary star is increasing, which has raised the issue of whether the receding stars are related to the primary star at all.


The star’s traditional name, “Deneb Algedi”, derives from the Arabic phrase meaning “[the] tail of the goat”, as a reference to the fish-like tail of the sea-goat, Capricorn. In medieval times, the star was one of the fifteen “Behenian”, or fixed stars with a kabbalistic symbol, and was associated with the stone chalcedony, and the plant marjoram.

In China, Deneb Algedi forms part of an asterism known as “[The] Line of Ramparts”, along with 11 other stars located in the constellations Capricornus, Aquarius, and Pisces.

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