Astro Bob: What would the sun look like from Alpha Centauri?

We're stuck orbiting one star, but if we could see the sun from say, a planet orbiting inside the Alpha Centauri system, we'd be pleasantly surprised.

Proxima Centauri planet M. Kormesser ESO.jpg
This artist’s impression shows a view of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the solar system. The double star Alpha Centauri also appears in the image to the upper-right of Proxima itself. (ESO / M. Kornmesser)

Looking back from a distance, whether that be time or physical separation, can provide us with a fresh perspective on who we are and how far we've come. In our daily lives, Earth appears flat and limitless under the shielding blue sky, but seen from afar it's a little ball suspended in the incomprehensible vastness of outer space. Just looking at the photo below makes us all feel a little closer.

Earth and moon from Mars MRO.jpg
Earth and the moon photographed at Mars on October 3, 2007 by NASA's Mars Reconnaissance Orbiter. At the time it was taken, Earth was 88 million miles (142 million km) from Mars. (NASA)

Every one of us will spend our entire life going around a single star, the sun. We celebrate each orbit with a birthday. Birthdays humanize a maddeningly repetitive cosmic cycle. That's our job though, right?. To soften the edges of existence and create meaning in life through love, curiosity and purpose.


Earth Voyager 1 Feb 1990 full res.jpg
On Feb. 14, 1990, NASA’s Voyager 1 took this photo of Earth at a distance of 3.7 billion miles (6 billion km) from the sun. The image inspired Carl Sagan to write: "Look again at that dot. That's here. That's home. That's us." (NASA)

In the modern era, space missions like Apollo and Voyager have provided fresh perspectives on how we see Earth in a cosmic setting. We can do the same for the sun by taking an imaginary trip to next nearest star system, Alpha Centauri. From here we can look back at our own solar system from a new vantage point.

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Alpha Centauri and the Southern Cross are just now returning to view in the dawn sky seen from the southern hemisphere. (Stellarium)

Alpha Centauri is a triple star in Centaurus the centaur , a constellation best visible from the southern hemisphere. The brightest stars, Alpha Centauri A and B, appear as a single star to the naked eye, but a small telescope will split them into a brilliant, close pair. Both are similar to the sun in size and located 4.37 light years or about 26,000,000,000,000 (trillion) miles away. Alpha Centauri C, better known as Proxima Centauri, is a smidge closer than the bright binary at 4.24 light years.

Alpha Centauri Sun and Proxima sizes RJ Hall CC BY SA 3.0.jpg
This illustration compares the relative sizes of the sun, Alpha Centauri A and B and Proxima Centauri. RJ Hall / CC BY-SA 3.0

Proxima is a much fainter, smaller and less massive star called a red dwarf about one-seventh the diameter of the sun. While Alpha Centauri AB is the third brightest nighttime star after Sirius and Canopus, Proxima shines meekly at magnitude 11.1 and requires a small telescope to see. But it has something its brighter, bigger siblings don't have — planets. If we're going to look back at the sun it would be nice to do it from the surface of a planet just as we look up into the sky from Earth's surface.


Proxima Centauri b ESO M. Kornmesser.jpg
Artist's conception of Proxima Centauri b as a rocky-like exoplanet, with Proxima Centauri and the Alpha Centauri binary system in the background. Proxima b is the closest exoplanet to the solar system. (ESO / M. Kornmesser)

The innermost of the Proxima's two planets, called Proxima Centauri b, is a rocky sphere just 1.17 times the mass of Earth that orbits just 4.7 million miles (7.5 million km) from its host sun. While that's close, remember that Proxima is far cooler and fainter than our sun, so the planet lies within the "Goldilocks Zone," where liquid water could exist on its surface. Not that we have any evidence of that yet, just that it's possible.

Standing on the surface of Proxima b the sky would appear very similar to the one on Earth. That's because the majority of stars that make up the constellations are so far away that they shift only a little when we travel 4.24 light years away, the same way the moon appears in the same spot in the sky from one end of my home town to another. Four light years is a drop in the bucket when it comes to celestial distances.

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From the Alpha Centauri star system the sun would be a brilliant star in the constellation Cassiopeia. Instead of a W it would look like a big zigzag. (Stellarium with additions by the author)

Where might we find the sun from Proxima, or for that matter from anywhere in the Alpha Centauri system? As you scanned the sky and your gaze landed on the familiar W of Cassiopeia you'd instantly notice a brilliant star below and to its left. Yep, that's it — the sun! At magnitude 0.4 it would appear as bright as Betelgeuse in Orion or Procyon in the Little Dog (Canis Minor). Located where it is, the sun would stretch Cassiopeia's shape into a striking zigzag that inhabitants of the planet would find as eye-catching as Orion's Belt. The next time you go out at night, look high in the northern sky for the W and imagine the sun right next to it.


Sirius from Alpha Centauri.jpg
From Alpha Centauri, Orion would keep his shape but add Sirius right next to Betelgeuse! The reason Sirius shifts so much and not the other stars is because it's close to both the sun and Alpha Centauri. Even moving 4 light years results in a noticeable displacement. (Illustration: Bob King)

Even in a large telescope there'd be no hint of our solar system's planets, just a brilliant, starry point in the blackness of space. Only specialized instruments would be able to detect the sun's slight wobble caused by the gravitational tugs of the planets, especially Jupiter and Saturn. Otherwise the sun would appear identical to a bright star viewed from Earth.

It makes me smile knowing that the light of our lives holds a prominent place in Proxima b's sky. But if we power up our rocket ship and travel farther and farther from the sun, it becomes fainter and fainter. From a couple hundred light years it merges into the smoky band of the Milky Way as one of the more than 250 billion suns in our galaxy.

How quickly we blend into the cosmos and disappear! Yet as small and seemingly insignificant as our planet appears, there's no place like home.

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at .

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