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Where is Proxima Centauri?

You probably know by now that a planet has been discovered around the very nearest star. Guy Ottewell shows you where in space that star is.

Stars within 12 light-years of the sun, via Guy Ottewell's Astronomical Companion. Click to view larger, Guy says:

Stars within 12 light-years of the sun. The lines on the grid are 4 light-years apart. Diagram via Guy Ottewell’s Astronomical Companion. Click to view larger; Guy Ottewell says, “It’s quite vivid when it fills the screen.”

Editor’s Note: Proxima Centauri is the nearest star, and on August 24, 2016 astronomers announced it likely has a planet. This star is the nearest of a triple star system, which we on Earth see with the eye alone as the single star Alpha Centauri, visible from Earth’s Southern Hemisphere. They are about four light-years away.

The diagram above shows you where Alpha Centauri is, with respect to other nearby stars. It’s like that in the Nearest Stars section of the Astronomical Companion, but from a viewpoint that has moved closer and to a different angle so as to get a better look at Alpha Centauri. Included are stars within 12 light-years from the sun. The glows of light representing the stars are millions of times larger than the stars themselves, which would be microscopically small on this scale.

The grid serves to show the equatorial plane, and also the scale, the lines being 4 light-years apart. The slightly thicker line is the vernal equinox direction (the Earth-sun direction at March 20).

Imaginary stalks from the plane to the stars show how far north or south they are. I’ve cropped the picture so that some of the stars are off the top or bottom, but they are obscure stars you may not have heard of, with designations such as Lalande 21185, Luyten 726-8, DX Cancri. Most stars, including most of those near to us, are smaller than our sun – red dwarfs.

The exceptions near to us are Sirius, Procyon, and Alpha Centauri.

Alpha Centauri is the third-brightest star (that is, as seen from our place in space, and not counting the sun). Like the first- and second-brightest – Sirius and Canopus – it is a southern star. Indeed it’s much farther south than either of those, which is why it has no traditional name in our culture (except a rather faux-traditional one, Rigilkent).

The space diagram shows it at a steep southward angle from the sun. This angle (its declination -61°) means it doesn’t peep above the horizon till you go down to the latitude of northern Florida; to see it properly you might go south of Earth’s equator.

Then you would see in your telescope that it is a double star – one of the widest and easiest to “split.” Here is part of my diagram of the pair with which I used to fill a space in Astronomical Calendar 2016:

The double star of Alpha Centauri, via Astronomical Calendar 2016 by Guy Ottewell.

The double star of Alpha Centauri. Astronomers call them Alpha Centauri A and B. Image via Astronomical Calendar 2016 by Guy Ottewell.

Again, the symbols for the stars are vastly larger than the bodies of the stars would be. It isn’t really that the B star revolves around the A one: they both revolve around their common center of gravity. You can see that this year, 2016, is the year when B appears closest to A, though in the true (untilted) orbit it will reach periastron in 2035. The blue lines are one second of arc apart – that’s the apparent size of a tennis ball ten miles away.

Alpha Centauri A is a star much like the sun, slightly larger and of about the same 4.6-billion-year age or slightly older; B is slightly smaller and cooler. In their elliptical orbits around their common center of gravity, they range from about 11 astronomical units (sun-Earth distances) apart when closest, to 36 when farthest apart – in other words, from something like the sun-Saturn to something like the sun-Pluto distance.

And the distance of this star system from us is only 4.4 light-years: nearer than all other stars. Except for one, discovered in 1915 (by Robert Jones in South Africa).

It is one of those numerous dwarfs whose surfaces are reddish, meaning cooler and dimmer. Only about 1/7 as wide as the sun, and at a magnitude (brightness) of 11, it is about 100 times too dim to be seen with the naked eye. And it is more than 2 degrees away from the Alpha Centauri pair; on observatory photographs, there are thousands of background stars in between. Yet studies of it found that it is only 4.25 light-years away from us. Hence it is dubbed Proxima Centauri, “the nearest [star, of the constellation] Centaurus.”

The nearest stars not only have the largest parallax (apparent angular shift as we go around the sun) but are liable to have large proper motion (travel across the starry background from year to year). Proxima is found to be still coming gradually toward us (to be nearest, at only about 3 light-years, about 27,000 years into the future. And it is probably, though not quite certainly, gravitationally bound to the Alpha Centauri pair 0.2 light-year away from it, in an enormous, slow orbit of something like 500,000 years. So it can be called Alpha Centauri C.

Yes, these are humiliating numbers, and I hesitate to crush you further with the reminder that a light-year is nearly 6,000,000,000,000 miles, and the distance across the Milky Way galaxy is something like 30,000 times greater than the distance to these our nearest neighbors in it.

Since the breakthrough in 1992, thousands of exoplanets have been discovered. The detection methods are calculations based on measurements of extremely small fluctuations in stars’ positions, brightness, or spectral lines. which may show either that a star is moving in a slightly wavy course around a common center of gravity with something else, or that something else is passing in front of it and interrupting its light. At first the discoveries were mostly like the giant planets of our solar system, though often much closer to their stars. The hope was to find something Earth-like in size, and within its star’s “habitable zone,” that is, not too close in or far out for water to exist in the liquid state. Life may have to be water-based. That has begun to happen.

It may seem surprising that the discovery at our nearest star has come only after thousands more remote. Part of the reason was caution: there was a false excitement in 2012 with Alpha Centauri Bb, a claimed planet around one of the great pair. Now comes Proxima b, announced this month.

Caution still prevails: it’s officially a “candidate” exoplanet.

But it’s essentially certain, and is very exciting. It’s Earth-sized; and it’s within the habitable zone. And if the very nearest star has an Earthoid, there are probably millions of others in the galaxy.

This particular Earthoid isn’t necessarily able to harbor life, even microbial. The planet is much closer in to its star than Mercury is to the sun, therefore is tidally locked, one side always baked by the star and the other always facing frigid space.

And don’t forget that even if there is a Proximan with a telephone, and one day you receive a call from her asking, “What is your name, how many legs do you have, and how many sexes are there in your world?” it will be more than four years before she receives your reply and more than eight before you know what she thinks of it.

Bottom line: Diagram and explanation showing the location in space of the Alpha Centauri system, whose nearest member – Proxima Centauri – now appears to have a planet.

Guy Ottewell

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