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Two Suns in the Sky

I am always curious what it would look like on alien worlds. I have a background in physics and solid modeling (Lightwave 3D).

Centauri A and Centauri B in the sky from a hypothetical planet orbiting around Centauri B

Centauri A and Centauri B in the sky from a hypothetical planet orbiting around Centauri B

Two Suns

On a particular summer day, such as in early July, the sun will set at roughly 9:10 PM and will rise the next day at about 6:05 AM. This occurs in states like Michigan, Minnesota, Wisconsin, and also in any other states at the same latitude.

You always hear about weather reports where the rain will end and the sun will come out later in the day. You will even see the small graphics on television weather broadcasts showing the clouds with the sun peaking out from behind.

However, what would it be like if the weather reports showed the graphic, not with one sun but with two. Yes, the rain will end and the two suns will shine in the afternoon. The first one will set at 9:10 PM while the second one will set at 9:45 PM. They both will rise in the morning at 6:05 AM and 6:40 AM respectively.

We don’t see this because our sun is not part of a binary system. Interestingly enough, slightly more than half of the stars like the sun, a G-type main sequence star, have binary companions. What would it be like to see two stars shining in the sky?

Alpha Centauri

One possibility of what it would be like can be found only 4.35 light years away where two sun like stars exist. They are Alpha Centauri A and Alpha Centauri B, where Centauri A is 1.1 times the mass of the sun and Centauri B is 0.93 times the mass of the sun. Despite only being slightly more massive than the sun Centauri A is over 1.5 times more luminous than the sun while Centauri B is only half as luminous as the sun.

The stars are far enough apart so that life, such as human life, can exist on any planet orbiting in the habitable zone of either star without the other star harming it. The two stars orbit either other in elliptical orbits with a period of nearly 80 years. The maximum distance between the two stars is 35.6 AU with the minimum distance at 11.2 AU. One AU is equal to about 93 million miles (150 million kilometers) which is the distance of the earth from the sun.

Now, when we look up in the sky, why do we only see one star, our sun? It is estimated that more than 85 percent of all stars are in a binary or even triple star system. More than half of sun like stars have a binary companion. So, where is ours?

What Happened to Our Twin?

There are two theories concerning what happened to the sun’s companion. One involves a small red or brown dwarf star called Nemesis and the other one suggests that our companion is the same size as the sun but is now much further away and is given the designation HD 186302.

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Nemesis got its name because of all the mass extinction events that have occurred in earth’s history. Like many binary stars, their orbits are elliptical. If Nemesis exists and is traveling in an elliptical orbit around the sun then it is speculated that its closest approach would bring it through the Oort cloud (roughly the orbit of Pluto). The disruption in the cloud could dislodge a multitude of comets that could then devastate life on earth. What makes this theory plausible is that mass extinction events have occurred on earth roughly every 27 million years, which would be the orbital period of Nemesis bringing if from the distance of the Oort cloud out to about 1.5 light years from earth. Most of the binary stars that exist, like Alpha Centauri A and B, are in tight close orbits that is a consequence of their age as they eventually settled down, as their orbital periods became more constant. As the sun aged, it is believed that Nemesis broke away. This is probably due to the great distance between both stars. However, some cosmologists think that Nemesis is still in orbit with the sun but since it is possibly only two times as massive as the planet Jupiter, it would be hard to detect at more than a light year away.

The second theory points to the possibility that the sun was ejected from the stellar nursery that it was born in, leaving its stellar companion behind. Star HD 186302 is incredibly close to the size and luminosity of our sun. It is also the same age. Even its chemical makeup is similar. What is surprising is its distance. It is 184 light years away.

Centauri A and Centauri B with A on the left and B on the right.  The planet would be orbiting B.  This view shows A at its closet approach of 11.2 AU.

Centauri A and Centauri B with A on the left and B on the right. The planet would be orbiting B. This view shows A at its closet approach of 11.2 AU.

Maybe Someone is Looking Back at Us

So, seeing two stars in the sky would be interesting. Maybe somewhere in countless binary systems throughout our galaxy and even beyond, intelligent life is looking up at a sky filled with two stars and maybe even three stars in a trinary system. As they look up might they wonder what it would be like if only one star filled their sky like what we see when we look up at our sun.

  • Alpha Centauri
    A Candidate for Terrestrial Planets And Intelligent Life


BRENDA ARLEDGE from Washington Court House on July 30, 2021:

I wonder if they really see 2...i think it would be awesome, but maybe a bit scary.

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