Epsilon Reticuli, Latinized from ε Reticuli, is a

double star In observational astronomy, a double star or visual double is a pair of stars that appear close to each other as viewed from Earth, especially with the aid of optical telescopes. This occurs because the pair either forms a binary star (i.e. a bi ...

approximately 59

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s away in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

of Reticulum. The primary component is an orange subgiant, while the secondary is a

white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...

. The two stars share a common motion through space and hence most likely form a

binary star A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...

system. The brighter star should be easily visible without optical aid under dark skies in the southern hemisphere. In 2000, an extrasolar planet was confirmed to be orbiting the primary star in the system.

Star system

The primary component, Epsilon Reticuli A, is a subgiant star with a

stellar classification In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction grati ...

of K2IV, indicating that the fusing of

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

in its core is coming to an end and it is in the process of expanding to a

red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...

. With an estimated mass of about 1.5 times the

solar mass The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...

, it was probably an F0 star while in the

main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...

. It has a radius of 3.18 times the

solar radius Solar radius is a unit of distance used to express the size of stars in astronomy relative to the Sun. The solar radius is usually defined as the radius to the layer in the Sun's photosphere where the optical depth equals 2/3: :1\,R_ = 6.957\tim ...

, a

luminosity Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a st ...

of 6.2 the solar luminosity and an

effective temperature The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...

of 4,961 K. As is typical of stars with giant planets, it has a high

metallicity In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as a ...

, with an iron abundance 82% larger than the Sun's. The secondary star, Epsilon Reticuli B, is known as a

visual companion In observational astronomy, a double star or visual double is a pair of stars that appear close to each other as viewed from Earth, especially with the aid of optical telescopes. This occurs because the pair either forms a binary star (i.e. a bi ...

since 1930, and in 2006 was confirmed as a physical companion on the basis of its

common proper motion This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outsid ...

. It was noted that its

color indices In astronomy, the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature. The lower the color index, the more blue (or hotter) the object is. Conversely, the large ...

are incompatible with a main sequence object, but are consistent with a

. This was confirmed in 2007 by spectroscopic observations, that showed the

absorption Absorption may refer to: Chemistry and biology * Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which ...

spectrum typical of a

-rich white dwarf (spectral type DA). This star has a visual apparent magnitude of 12.5 and is located at a separation of 13

arcsecond A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The na ...

s, corresponding to a projected physical separation of 240 AU and an

orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...

of more than 2,700 years. It is estimated that Epsilon Reticuli B has a mass of and a radius of . Originally, when it was in the main sequence, it probably had a spectral type of A5 and a mass of , and spent 1.3 billion years on this phase. From a measured effective temperature of 15,310 K, it has a cooling age (time spent as a white dwarf) of 200 million years, corresponding to a total age of 1.5 billion years. This age is inconsistent with the primary estimated age of 2.8 billion years, which suggests a smaller mass for the white dwarf or a larger mass for the primary.

Planetary system

On December 11, 2000, a team of astronomers announced the discovery of a planet Epsilon Reticuli b. With a minimum mass of 1.17 that of

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

, the planet moves around Epsilon Reticuli with an average separation of 1.16 AU. The eccentricity of the planet is extremely low (at 0.06), and it completes an orbit every 418 days (or 1.13 years). Stability analysis shows that the planet's

Lagrangian point In celestial mechanics, the Lagrange points (; also Lagrangian points or libration points) are points of equilibrium for small-mass objects under the influence of two massive orbiting bodies. Mathematically, this involves the solution of th ...

s would be stable enough to host Earth-sized planets, though as yet no

trojan planet In astronomy, a co-orbital configuration is a configuration of two or more astronomical objects (such as asteroids, moons, or planets) orbiting at the same, or very similar, distance from their primary, i.e. they are in a 1:1 mean-motion resonan ...

s have been detected in this system.

References

External links

* {{DEFAULTSORT:Epsilon Reticuli K-type subgiants White dwarfs Binary stars Reticulum Reticuli, Epsilon Durchmusterung objects Gliese and GJ objects 027442 019921

1355 Year 1355 ( MCCCLV) was a common year starting on Thursday (link will display the full calendar) of the Julian calendar. Events * January 6 – Charles IV of Bohemia is crowned with the Iron Crown of Lombardy as King of Italy in Milan. ...

Planetary systems with one confirmed planet