Most of the stars visible to the naked eye in the night sky are much larger, more luminous and brilliant than the Sun. However, despite often labelled as an average star the Sun actually outshines most stars in the galaxy. It's believed up to 80% of all stars in the Milky Way are red dwarfs. This type of star is so dim that not one is bright enough to be seen from Earth with the naked eye. Even Proxima Centauri, the closest star to the Earth after the Sun, requires optical aid to be seen. When it comes to solar twins or stars that are incredibly similar to the Sun across all parameters, not many exist. One of the best examples is 18 Scorpii.
Astronomers classify the Sun as a yellow main sequence star of spectral type G (sub-class G2 V). Like other main sequence stars, G type stars are in the process of burning hydrogen into helium via nuclear fusion. They represent up to 4% of all the stars in the galaxy with of course much fewer fitting into the Sun's G2 sub-class. Naturally, Sun like stars are obvious targets in the search for life elsewhere. One star that fits the bill and is almost a carbon copy of the Sun is 18 Scorpii. In addition, it's a single star and visible to the naked eye, albeit faintly.
There have been many studies made of 18 Scorpii since the late 1990's. Cayrel de Strobel in 1996 included it in her review of the stars resembling the Sun. A detailed investigation made in 1997 by Gustavo Porto de Mello of the Federal University in Rio de Janeiro, Brazil and Licio da Silva of the National Observatory in Sao Paulo, Brazil determined that 18 Scorpii is a virtual carbon copy of the Sun. They concluded the mass, temperature, colour, surface gravity, rotation speed, surface activity and iron abundance of 18 Scorpii closely matches that of the Sun. There are small differences in luminosity and age with 18 Scorpii being a little bit older and more luminous.
More recent surveys support the original findings, although newer research suggests that 18 Scorpii has a lithium abundance about 3 times as high as the Sun, but overall the star is only slightly more metal-rich. In any case, 18 Scorpii is the closest star to us that's almost identical to the Sun. Consequently, it's a prime SETI target and a perfect example of a star with a Sun like evolution. It possibly contains a stable solar system of many planets, comets and asteroids not unlike our own. There may even be a small rocky body located at just the right distance for life to have evolved and even flourished. A dizzying thought!
18 Scorpii is 46 light-years distant and shines at magnitude +5.5, putting it at the fringes of naked eye visibility. It's located in northern Scorpio near the constellation border with Ophiuchus and about 18 degrees north and a little to the west of first magnitude red supergiant, Antares.
Perhaps the easiest way to locate 18 Scorpii is to first focus on Yed Prior (δ Oph - mag. +2.73) and Yed Posterior (ε Oph - mag. +3.23). These two stars are separated by just over a degree with Yed Prior the brighter of the two. Imagine a line connecting them and extend it southwards towards ζ Ophiuchi (mag. +2.54). Just south of the halfway mark is υ Ophiuchi (mag. +4.6). Then change direction and proceed 3 degrees directly west to arrive at 18 Scorpii.
Finder Chart for 18 Scorpii - pdf format (credit:- freestarcharts)
18 Scorpii / Sun - Data Table
|RA (J2000)||16h 15m 37s||---|
|DEC (J2000)||-08d 22m 06s||---|
|Spectral type||G2 Va||G2 V|
|Age (billion years)||4.7 ± 0.6||4.57|