18 Scorpii is a solitary star located at a distance of some from the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

at the northern edge of the Scorpius constellation. It has an

apparent visual magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's lig ...

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

, 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 temp ...

. 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 Solar may refer to: Astronomy * Of or relating to the Sun ** Solar telescope, a special purpose telescope used to observe the Sun ** A device that utilizes solar energy (e.g. " solar panels") ** Solar calendar, a calendar whose dates indica ...

. 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 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 manife ...

of hydrogen in its core region. Sousa ''et al.'' (2008) found its metallicity 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 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_ = 6.957\t ...

. When combined with the results of asteroseismology 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 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 Corona (from the Latin for 'crown') most commonly refers to: * Stellar corona, the outer atmosphere of the Sun or another star * Corona (beer), a Mexican beer * Corona, informal term for the coronavirus SARS-CoV-2, which causes the COVID-19 di ...

with a temperature in the range of 1.5–2 MK and an

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

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

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 Margaret Carol "Maggie" Turnbull (born 1975) is an American astronomer and astrobiologist. She received her PhD in Astronomy from the University of Arizona in 2004. Turnbull is an authority on star systems which may have habitable planets, so ...

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. T ...

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 A debris disk (American English), or debris disc (Commonwealth English), is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris di ...

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

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