Rana (Delta Eridani)

Rana, Delta Eridani (δ Eri) is an orange subgiant star located 29.49 light-years away in the constellation Eridanus. With an apparent magnitude that varies from 3.51 to 3.56, it is the fifth brightest star in Eridanus, after Achernar, Cursa, Acamar, and Zaurak. Rana appears near Ran (Epsilon Eridani), one of the nearest visible stars to the Sun.

Star type

Rana is an evolved star of the spectral type K0 IV, indicating an orange subgiant. It has a mass of 1.33 solar masses and a radius 2.35 times that of the Sun. With a surface temperature of 5,027 K, it is 3.17 times more luminous than the Sun. The star is older than the Sun, with an estimated age of 6.194 billion years.

Rana has a projected rotational velocity of 0.7 km/s, indicating that it appears almost pole-on when seen from Earth.

The star was catalogued as a suspected RS Canum Venaticorum variable in 1983. RS Canum Venaticorum stars are close binary systems with active chromospheres and large stellar spots that cause the system’s brightness to vary.

However, more recent observations suggest that Delta Eridani does not show a high activity level typical for RS CVn variables. Additionally, interferometric observations with the VINCI instrument on the European Southern Observatory’s Very Large Telescope Interferometer (VLTI) did not detect a companion.

Rana (Delta Eridani), image credit: ESO/Digitized Sky Survey 2 (CC BY 4.0)

Name

The name Rana is the Latin word for “frog.” It was approved by the International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) on April 4, 2022.

The name appeared in Czech astronomer Antonín Bečvář’s Atlas Coeli Skalnate Pleso in 1951, but its origin is uncertain. It may have been transferred from the Latin name of Beta Ceti, Rana Secunda (“the second frog”). Beta Ceti was traditionally also known as Deneb Kaitos (“the tail of Cetus”) and now has the formal name Diphda. The bright Fomalhaut (Alpha Piscis Austrini) was known as Rana Prima, “the first frog.”

In Chinese astronomy, Delta Eridani was known as 天苑三 (Tiān Yuàn sān), the Third Star of Celestial Meadows. Celestial Meadows was an asterism formed by Rana with Zaurak (Gamma Eridani), Pi Eridani, Ran (Epsilon Eridani), Zibal (Zeta Eridani), Azha (Eta Eridani), Pi Ceti, Tau¹ Eridani, Angetenar (Tau² Eridani), Tau³ Eridani, Tau⁴ Eridani, Tau⁵ Eridani, Tau⁶ Eridani, Tau⁷ Eridani, Tau⁸ Eridani, and Tau⁹ Eridani. The asterism was part of the larger Hairy Head mansion, which represented the body of the White Tiger.

Location

Delta Eridani lies in the northern part of Eridanus, just east of the fainter Epsilon Eridani. Both Delta and Epsilon Eridani can be found using the stars of Lepus (the Hare), which appear just south of the bright constellation figure of Orion (the Hunter).

Rana appears along the imaginary line drawn from Arneb (Alpha Leporis) through Mu Leporis. It forms a flat triangle with Ran (Epsilon Eridani) and the brighter Zaurak (Gamma Eridani).

The location of Rana (Delta Eridani), image: Stellarium

Constellation

Rana is located in the constellation Eridanus. Representing the celestial River, Eridanus stretches farther from north to south than any other constellation. Its northern end appears near Rigel in Orion, while its southernmost end lies in the far southern sky.

Eridanus is one of the Greek constellations listed by the astronomer Claudius Ptolemy of Alexandria in his Almagest in the 2nd century CE. In Greek lore, the constellation is associated with the river into which Phaethon, the son of the Sun god Helios, fell after attempting to drive his father’s chariot. After first burning and then freezing the Earth, Phaethon was struck down by Zeus to prevent further disaster.

The constellation’s name was later taken as the Latin name for the Po, the longest river in Italy.

Eridanus is the sixth largest constellation in the sky, occupying an area of 1,138 square degrees of the predominantly southern sky. Its northernmost end lies on the celestial equator.

Eridanus constellation map by IAU and Sky&Telescope magazine (Roger Sinnott & Rick Fienberg) (CC BY 3.0)

The southern end of Eridanus is marked by Achernar (Alpha Eridani), the ninth brightest star in the sky. The fast-spinning blue-white main sequence star shines at magnitude 0.40 – 0.46 from a distance of 139 light-years.

Cursa (Beta Eridani), the second brightest star in Eridanus, appears near Rigel. The white giant shines at third magnitude from a distance of 90 light-years.

Other notable stars in Eridanus include Epsilon Eridani (Ran), the third nearest visible star to Earth (after Alpha Centauri and Sirius), the red giant Zaurak (Gamma Eridani), the orange giants Angetenar (Tau² Eridani), Sceptrum (53 Eridani A), Azha (Eta Eridani) and Beemim (Upsilon³ Eridani), the white subgiant Acamar (Theta¹ Eridani), the yellow giant Theemin (Upsilon² Eridani), and the F-type giant Beid (Omicron¹ Eridani). Eridanus is also home to the orange star Keid (Omicron² Eridani), the host star of the planet Vulcan in Star Trek lore, and to the Sun-like star 82 G. Eridani, which hosts a system of at least three planets.

Deep sky objects in the constellation include the planetary nebula NGC 1535 (the Cleopatra’s Eye Nebula), the reflection nebula IC 2118 (the Witch Head Nebula), the ring galaxy NGC 1291, and the grand design spiral galaxy NGC 1300. Eridanus also hosts the WMAP Cold Spot (the Eridanus Supervoid), a large region devoid of galaxies.

The best time of the year to see the stars and deep sky objects in Eridanus is during the month of December, when the constellation climbs higher above the horizon in the early evening. The entire constellation is visible from locations south of the latitude 32° N.

The 10 brightest stars in Eridanus are Achernar (Alpha Eri, mag. 0.40 – 0.46), Cursa (Beta Eri, mag. 2.796), Acamar (Theta¹ Eri, mag. 2.88), Zaurak (Gamma Eri, mag. 2.88 – 2.96), Rana (Delta Eri, mag. 3.51 – 3.56), Upsilon⁴ Eridani (mag. 3.55), Phi Eridani (mag. 3.55), Chi Eridani (3.70), Tau⁴ Eridani (mag. 3.57 – 3.72), and Ran (Epsilon Eri, mag. 3.736).

Rana – Delta Eridani