Introduction

Most of the stars visible to the naked eye in the night sky are much larger, luminous and more brilliant than our own Sun. However, despite being often labelled as a very ordinary average star, the Sun actually outshines most of the other stars in the galaxy. This is because astronomers believe that up 80% of the stars in our galaxy are in fact red dwarfs; stars so dim that not one is bright enough to be visible from Earth with the naked eye, even though the next closest star to the Earth after the Sun, Proxima Centauri is itself a red dwarf. But when it comes to solar twins, stars that are incredibly similar to our Sun across all parameters, there are few and one of the best examples is 18 Scorpii.

Solar classification

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 and represent up to 4% of all the stars in our 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. It is also a single star just like the Sun and what's more, it is visible to the naked eye, albeit faintly.

Solar twin

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 and a detailed investigation 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.

18 Scorpii (ESO Digitized Survey)

They concluded that 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 the luminosity and age with 18 Scorpii being a little bit older and more luminous.

More recent surveys support the original findings although newer research has suggested 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 is an almost identical copy of the Sun. Consequently, it is a prime SETI target and a perfect example of a star that has had a Sun like evolution over billions of years and possibly contains a stable solar system of many planet, 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.

Location

18 Scorpii is 46 light-years from Earth and shines at mag. 5.5 putting it on the fringes of naked eye visibility. It is located at about the most northern position possible in the constellation of Scorpio near the border with Ophichus and about 18 degrees north and a little to the west of first mag. Antares.

18 Scorpii Star Chart

18 Scorpii Star Chart - pdf format

Name 18 Scorpii Sun
HD 146233 ---
HIP 79672 ---
Constellation Scorpio ---
RA (J2000) 16h 15m 37s ---
Dec (J2000) -08d 22m 06s ---
Visual Mag. +5.50 -26.74
Distance (light-years) 45.7 0.0000158
Spectral type G2 Va G2 V
Absolute Mag. +4.77 +4.83
Metallicity (Solar) 1.04 1.00
Mass (Solar) 1.01 1.00
Radius (Solar) 1.02 1.00
Luminosity (Solar) 1.08 1.00
Temperature (K) 5800 5778
Rotation (days) 23 25
Age (billion years) 4.7 ± 0.6 4.57

Perhaps the easiest way to locate 18 Scorpii is to focus on easy to identify stars δ (delta) Oph (Yed Prior) and ε (epsilon) Oph (Yed Posterior) in Ophichus. These two stars lie on the western side of Ophichus and are separated by just over 1 degree with Yed Prior the brighter of the two.

Then imagine a line connecting Yed Prior (mag. 2.73) with Yed Posterior (mag. 3.23) and extend this line southwards towards mag. 2.54 ζ (zeta) Oph. Just south of the halfway mark along this line is star υ (upsilon) Oph (mag. 4.6). Once here, change direction and proceed 3 more degrees directly west to arrive at 18 Scorpii.

Conclusion

18 Scorpii is a faint but naked eye single star that is a double of our Sun. There is a real possibility of a similar system of planets, asteroids, comets and other bodies around 18 Scorpii like our own Sun along with the potential of alien life.

Just maybe, right now there is an extra-terrestrial looking through a telescope on a planet or moon somewhere around 18 Scorpii at a similar faint yellow star in his/her night sky and wondering what if....

For these reasons alone, 18 Scorpii is an amazing star.


Sky Highlights - May 2017

Mercury
Mercury reaches greatest elongation west on May 17, 2017

Meteor Shower
Eta Aquariids meteor shower peaks on May 5th and 6th, 2017

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for May 2017

Northern Hemisphere
Evening
West:- Mars (mag. +1.6)
South:- Jupiter (mag. -2.4)
Midnight
Southwest:- Jupiter
Southeast:- Saturn (mag. +0.2)
Morning
South:- Saturn
East:- Venus (mag. -4.7)

Southern Hemisphere
Evening
West:- Mars
North:- Jupiter
Midnight
Northwest:- Jupiter
North:- Saturn
Morning
Northwest:- Saturn
East:- Venus, Mercury (mag. +2.5 to -0.3), Neptune (mag. +7.9)

Deep Sky
Naked eye / binoculars:-
Melotte 111 - Mel 111 - The Coma Star Cluster (Open Cluster)
Messier 44 - M44 - The Praesepe (Open Cluster)

Telescopes:-
Messier 67 - M67 - Open Cluster
Messier 51 - M51 - The Whirlpool Galaxy (Spiral Galaxy)
Messier 97 - M97 - The Owl Nebula (Planetary Nebula)
Messier 101 - M101 - The Pinwheel Galaxy (Spiral Galaxy)
Messier 65 – M65 – Spiral Galaxy
Messier 66 - M66 - Intermediate Spiral Galaxy
Messier 95 - M95 - Barred Spiral Galaxy
Messier 96 - M96 - Intermediate Spiral Galaxy
NGC 4244 - Spiral Galaxy
NGC 4565 - Needle Galaxy - Spiral Galaxy

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