Acamar, otherwise known as Theta Eridani, is a binary system of stars located in the constellation of Eridanus, the River. It was thought for a period of time to symbolize the end of the celestial river until a brighter star named Achernar was discovered.
Key Facts & Summary
- The Theta Eridani system is composed out of two stars, θ¹ Eridani and θ² Eridani, and it is located at around 161 light-years / 49 parsecs away from the Sun.
- The primary star, θ¹ Eridani, is of spectral class A4 and has an apparent magnitude of + 3.2.
- The secondary star, θ² Eridani is of spectral class A1 and has an apparent magnitude of +4.3
- Thus, the Theta Eridani system has an apparent magnitude of 3.2 and an absolute magnitude of – 0.59. It is suspected that the star system is also variable.
- θ¹ Eridani is a blue-white subgiant with around 2.6 solar masses and a radius of around 16 times that of the Sun.
- θ² Eridani is also a subgiant, a little less massive than its companion but much more massive than our sun, at around 2.4 solar masses and its current radius is unknown.
- θ¹ Eridani is the brightest of the pair, being around 145 times more luminous than our Sun.
- θ¹ Eridani is also less hot than θ² Eridani, but almost twice as hot as our sun, with average surface temperatures estimated at around 8.200 K.
- θ² Eridani is the hottest star in the binary system, having surface temperatures of around 9.200 K, and it is also 36 times brighter than our Sun.
- The angular separation of the two stars is at around 8.3 arcseconds. It is unknown if the two stars are gravitationally bound to one another.
- θ¹ Eridani completes one rotation on its axis once every 569 days and has a rotational velocity of around 70 km / 43.4 mi per second. θ² Eridani is faster, spinning at 90 km / 55.9 mi per second.
The traditional name of Theta Eridani was Acamar, it is a derived word from the Arabic language which means “the end of the river,” referring to the star’s position which marked the end of the celestial river, the constellation Eridanus.
One of the first persons to describe Theta Eridani was the Greek astronomer Ptolemy. He described the star as a first magnitude star. Some believed that Ptolemy actually described Achernar, which today appear just above the horizon in Alexandria, where Ptolemy lived.
However, Achernar was not visible during Ptolemy’s life, it is now visible in Alexandria due to the precession of the equinoxes, and its declination in 100CE was -67, making it invisible even to the southern Egyptian town of Aswan.
Since most people were unaware of Achernar’s existence, a star brighter, and truly marking the celestial river’s end, they associate, for a period of time, Theta Eridani, as the star which marked the constellation’s end.
The current age of Theta Eridani is unknown. Whether the two stars formed at the same time and are gravitationally bounded is still uncertain.
However, Theta Eridani most likely formed from a molecular cloud of gas and dust. Gravity pulled the swirling gas and dust together and resulted in the ninth brightest star of the constellation of Eridanus, Theta Eridani.
Distance, Size, and Mass
The Hipparcos astronomy satellite measured the distance between Theta Eridani and our Sun. The binary system is approximately 161 light-years / 49 parsecs away from our Sun.
The primary star, θ¹ Eridani, is the biggest and most massive of the two. θ¹ Eridani has around 16 solar radii, and since a star’s radius is multiplied by 2 to estimate its diameter, it could have around 32 times the sun’s diameter. It’s also almost three times as massive as our sun, with an estimated 2.6 solar masses.
The secondary star, θ² Eridani, is also more than two times more massive than our sun, at an estimated 2.4 solar masses. Though its radius and thus its diameter is currently unknown, the second star is most likely several times bigger than our sun as well.
The Theta Eridani system has an apparent magnitude of 3.2 and an absolute magnitude of -0.59. It has a radial velocity of around 11.9 km / 7.3 mi per second.
The primary star, θ¹ Eridani, is the brightest of the system but it is cooler than θ² Eridani. θ¹ Eridani is blue-white subgiant, around 145 times brighter than our sun, and has average surface temperatures of around 8.200 K.
That is around 1.4 times hotter than our sun. The star has an apparent magnitude of +3.2 and a rotational velocity of 70 km / 43.4 mi per second. It completes one rotation on its axis once every 569 days.
The secondary star, θ² Eridani, is much fainter but at the same time hotter, and spins faster than θ¹ Eridani. θ² Eridani has an apparent magnitude of +4.3, and a rotational velocity of around 90 km / 55.9 mi per second.
The secondary star has surface temperatures of around 9.200 K. It is 1.59 times hotter than our sun. The star is also 36 times brighter than the sun.
Much of the Theta Eridani star system remains to be discovered. It is widely speculated that the stars may occult one another and that one or maybe both stars are variable in magnitude.
The two stars in the Theta Eridani system are separated by around 8.3 arcseconds. That is a huge distance, and some speculate that the stars aren’t even gravitationally bound to each other.
Acamar/Theta Eridani is located in the constellation of Eridanus, the celestial river. It is within the borders of the constellation.
The constellation of Eridanus is the sixth largest constellation in the night sky, and the best time to observe it and its components is in the month of December.
Both stars in the Theta Eridani system are not massive enough to end their lives in a supernova explosion. They seem stable, and they will continue to remain as such for millions of years.
One day, their hydrogen supplies in their cores will be exhausted, and the stars will begin to expand several times their current size. Much of these stars and their stellar system remain unknown, and as such future observations are needed to fully understand them.
Did you know?
- Since Ptolemy described Acamar as a first magnitude star, it might have been much brighter in the past then it is in the present.
- In Latin, the star’s name was translated from Arabic as Postrema Fluminis.
- The Chinese placed Acamar in the Celestial Orchard – an asterism formed by 11 stars. Acamar was named the Sixth Star of Celestial Orchard.
- In astrology, Acamar carries a note of one who is blessed with a direct line, to the source of all being.