18 Scorpii is a solitary

star A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth make ...

located at a distance of some from the Sun at the northern edge of the

Scorpius constellation Scorpius is a zodiac constellation located in the Southern celestial hemisphere, where it sits near the center of the Milky Way, between Libra to the west and Sagittarius to the east. Scorpius is an ancient constellation that pre-dates the Gre ...

. It has an apparent visual magnitude of 5.5, which is bright enough to be seen with the naked eye outside of urban areas. The star is drifting further away with a

radial velocity The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the rate of change of the distance or range between the two points. It is equivalent to the vector projection ...

of +11.6. 18 Scorpii has some physical properties in common with the Sun, a

G-type star A G-type main-sequence star (Spectral type: G-V), also often, and imprecisely called a yellow dwarf, or G star, is a main-sequence star (luminosity class V) of spectral type G. Such a star has about 0.9 to 1.1 solar masses and an effective tempe ...

. Cayrel de Strobel (1996) included it in her review of the stars most similar to the Sun, and Porto de Mello & da Silva (1997) identified it as a younger solar twin. Some scientists therefore believe the prospects for

life Life is a quality that distinguishes matter that has biological processes, such as Cell signaling, signaling and self-sustaining processes, from that which does not, and is defined by the capacity for Cell growth, growth, reaction to Stimu ...

in its vicinity are good.

Characteristics

18 Scorpii is a

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 ...

star of spectral and luminosity type G2 Va, with the luminosity class of 'V' indicating it is generating energy through the

nuclear fusion Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles (neutrons or protons). The difference in mass between the reactants and products is manifest ...

of hydrogen in its core region. Sousa ''et al.'' (2008) found its

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 ...

to be about 1.1 times that of the Sun, which means the abundance of elements other than hydrogen or helium is 10% greater. The radius of this star, as measured using

interferometry Interferometry is a technique which uses the '' interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber o ...

by Bazot ''et al.'' (2011), is 101% the

radius of the Sun 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_ = 6.9 ...

. When combined with the results of

asteroseismology Asteroseismology or astroseismology is the study of oscillations in stars. Stars have many resonant modes and frequencies, and the path of sound waves passing through a star depends on the speed of sound, which in turn depends on local temperatur ...

measurements, this allows the mass of the star to be estimated as 102% of the

Sun's 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 of ...

. This star is radiating 106% of the Sun's luminosity from its outer atmosphere at 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 5,433 K. It is this heat that gives the star the yellow-hued glow of a

. According to Lockwood (2002), it has a temporal photometric behavior very similar to the Sun. Its brightness variation over its entire activity cycle is 0.09%, about the same as the Sun's brightness variations during recent solar cycles. Using the technique of Zeeman-Doppler imaging, Petit et al. (2008) have detected its surface magnetic field, showing that its intensity and geometry are very similar to the large-scale solar magnetic field. The estimated period for the activity cycle of 18 Scorpii is about seven years, which is significantly shorter than the Sun's, and its overall chromospheric activity level is noticeably higher. Like the Sun, it has a hot corona with a temperature in the range of 1.5–2 MK and an

X-ray X-rays (or rarely, ''X-radiation'') are a form of high-energy electromagnetic radiation. In many languages, it is referred to as Röntgen radiation, after the German scientist Wilhelm Conrad Röntgen, who discovered it in 1895 and named it ' ...

luminosity of . Though 18 Scorpii is only slightly more metal-rich overall than the Sun, its lithium abundance is about three times as high; for this reason, Meléndez & Ramírez (2007) have suggested that 18 Scorpii be called a "quasi solar twin", reserving the term solar twin for stars (such as HIP 56948) that match the Sun, within the observational errors, for all parameters. 18 Scorpii is a young star at 2.9 Gyr (2.9 Billion years old). 18 Scorpii has not yet entered its stable burning stage. The Sun at 4.7 Gyr is at its most stable stage. Due to 18 Scorpii age, it is at the edge of range for a solar twin, and is more of a Solar analog. 18 Scorpii was thought to be 5.0 billion years old in the past, new measurements in 2013 found 18 Scorpii to be younger at 2.9 billion years old.NASA NEWS, Identification of oldest solar twin may help locate rocky exoplanets, August 29, 2013
/ref> 18 Scorpii was identified in September 2003 by astrobiologist Margaret Turnbull from the

University of Arizona The University of Arizona (Arizona, U of A, UArizona, or UA) is a public land-grant research university in Tucson, Arizona. Founded in 1885 by the 13th Arizona Territorial Legislature, it was the first university in the Arizona Territory. ...

Tucson , "(at the) base of the black ill , nicknames = "The Old Pueblo", "Optics Valley", "America's biggest small town" , image_map = , mapsize = 260px , map_caption = Interactive map ...

as one of the most promising nearby candidates for hosting life, based on her analysis of the HabCat list of stars. This is a solitary star, and does not display the level of excess infrared emission that would otherwise suggest the presence of unconsolidated circumstellar matter, such as a debris disk. In a paper published in April 2017, a candidate planet was found orbiting 18 Scorpii (HD 146233) with a period of , but a 2020 paper found the

signal to be a stellar activity cycle. The_life_cycle_of_a_Sun-like_star_(annotated)

References

External links

*
NASA article on 18 Scorpii18 Scorpii entry in the stellar databaseAstronomers Measure Sun-Like Brightness Changes of the Solar Twin, 18 Scorpii
* {{Stars of Scorpius G-type main-sequence stars Scorpii, 18 Scorpii, 18 Scorpius (constellation) BD-07 4242 Scorpii, 18 0616 146233 079672 6060