* Astronomy

A dim yellow star in the constellation Serpens Caput may be nearly identical to the Sun, say astronomers in South Carolina and Hawaii. The star could therefore host a life-bearing planet like Earth.
In 1997, astronomers in Brazil identified a solar twin in Scorpius named 18 Scorpii.

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18 Scorpii is located about 45.75 light-years from Earth. It lies at the northern edge of of Constellation Scorpius. The star can be found northwest of Zeta Ophiuchi, south of Yed Posterior (Epsilon Ophiuchi) and Yed Prior (Delta Ophiuchi), north of Graffias or Acrab (Beta1,2 Scorpii), and east of Zubeneschamali (Beta Librae).
18 Scorpii (HD 146233) is a yellow main sequence star that is marginally more massive, and more heavy-element enriched than the Sun. Its rotation period is 23 days and it appears to be slightly younger than the Sun (4.2 billion years compared with 4.54 billion years).


Solar twins are especially interesting to astronomers who search for extraterrestrial intelligence.

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Now Jeremy King and Simon Schuler, both of Clemson University, and Ann Boesgaard of the University of Hawaii have found a star that may be just as good a solar twin as 18 Scorpii.
Named HD 143436, the star shines at eighth magnitude, bright enough to see with binoculars. It is 140 light-years from Earth, and located in Serpens Caput, the western section of Serpens.


Position(2000): RA = 16 00 18.84 Dec = +00 08 13.2

King and his colleagues obtained high-resolution spectra of HD 143436 by using the Keck I Telescope in Hawaii. The star is spectral type G0, similar to the Sun's spectral type of G2. Its absolute magnitude is 4.87 ± 0.10. This is almost the same as the Sun's, which is 4.83. Thus, both stars emit nearly the same amount of light into space.
King's team finds HD 143436 to be just as hot as the Sun: its temperature is 5,768 ± 43 Kelvin, versus the Sun's 5,777 K. Furthermore, the star's mass is 1.01 ± 0.02 solar masses, and it has the same amount of iron as the Sun.
No one has yet discovered planets around HD 143436. But if HD 143436 has astronomers, they would see our Sun as an eighth-magnitude star in northern Eridanus.



King's team does note one difference: the light element lithium is 6 times more abundant on HD 143436 than on the Sun. However, 18 Scorpii's lithium level is also high - it is 4 times the Sun's. In addition, HD 143436 may have less oxygen than the Sun.
The astronomers estimate HD 143436's age to be 3.8 ± 2.9 billion years. Although this range overlaps the Sun's age of 4.6 billion years, it also means the star could be much younger. A young planet might not have had time to spawn intelligent life.
To reduce the uncertainties, King's team urges astronomers to study HD 143436 further, saying it "appears to hold great promise as a solar twin candidate." Until now, however, astronomers have neglected the star.
A Google search on "HD 143436" currently turns up nothing, and a search on the star's alternate name, HIP 78399, produces only one web page.
That's about to change. King and his colleagues will publish their work in the November issue of The Astronomical Journal.

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