2015-05-28

Close Neighbors: Alpha Centauri at minimum separation

Alpha Centauri is the nearest star to the unaided eye, and the third brightest of the sky. A small telescope reveals it to be a beautiful double system: two stars similar to our Sun revolve each other in the span of a human lifetime. Because of their relative proximity, angular separation and position angle of the two components change notably in the matter of years, making this system particularly interesting to observe. This year, in November, the angular separation reaches a minimum of only four arcseconds.

"Selfie" with the three brightest stars: Sirius, Canopus and Alpha Centauri. Although the latter also has proper names, it is best known with its iconic Bayer designation. Image: 8mm fisheye lens, Canon EOS 600d camera.

With a declination of -61°, Alpha Centauri is invisible from mid-northern latitudes. On the southern hemisphere it is best seen in (southern) winter skies. It is circumpolar south of about 29° S. Northerners living south of 29° N (e.g. southernmost US continental states and Hawaii, Canary Islands) might get a glimpse of it when it scarcely rises above the southern horizon in summer (it culminates around midnight end of April). The above image I took last year from southern Chile (47°S), where Alpha Centauri passes almost overhead.

The double nature of Alpha Centauri is noticeable only in a telescope. In spring 2015, I was still able to separate the two companions (dubbed Alpha Centauri A and B, respectively) using a small 3” refracting telescope at magnifications of 70 and higher.

Alpha Centauri is centered right in the band of the Milky Way. The wide angle image was done with a 35mm lens and a Canon DSLR camera in Chile. At Alpha Centauri's position, North is roughly top left and West is top right. The inset is used with permission of Jared Males, University of Arizona. It was gained with the Visible Wavelength Science Camera (VisAO) of the 6.5m-Clay telescope at Las Campanas Observatory, Chile, at 690nm and at the diffraction limit of the named telescope. The angular resolution reached 22 milliarcsecons. Note that Beta Centauri, the second brightest star of the Centaurus constellation, has no physical connection to Alpha. Credit: Jan Hattenbach (Wide-angle image), Jared Males (A-B inset)
This image is a combination of a wide angle shot I took on May 18, 2015 using a 35mm lens and a Canon EOS 450Da camera, and a very special image I can show here thanks to Jared Males, astronomer of the University of Arizona. Jared has a much bigger scope at his disposal than I can ever dream of: the 6.5 meter Clay Telescope at LasCampanas Observatory in Chile. On the night of May 3, 2015, he took this image of Alpha Centauri A and B during a very unusual observing session. You might want to read his blog here, it is highly recommended.

Jared allowed me to use his image, so I mounted it into mine. The position angle of Alpha Centauri A and B corresponds to the orientation of my wide angle shot. Don’t even think of seeing or photographing the two companions with such a clear separation unless you have a diffraction-limited 6m-telescope at hand!

It might not be obvious by looking at this image, but the current close position of the two companions is unfavourable for ongoing attempts to find planets around the nearest stars – and to confirm those that might have already been found. In 2012, a team of European astronomers using ESO’s facilities in Chile announced the discovery of an earth-sized planet in orbit around Alpha Centauri B, but this claim has not been confirmed ever since by other teams on the ground or by the Hubble Space Telescope. The HST, however, might have glimpsed signs of another planet, also at Alpha Centauri B, as was announced earlier this year. Because of the closeness of Alpha Centauri A, all measurements suffer from contamination of the brighter component. (A has 150% of the Sun’s luminosity, and is therefore brighter than B, which shines only half as bright as our star. That’s why it’s a bit easier to search for planets around B.)
True and apparent orbits of Alpha Centauri B. Nothe this graphic is turned compared to the photograph above: North is down, East is right. Credit: SiriusB [CC BY-SA 3.0] via Wikimedia Commons.

But not for long: Alpha Centauri A and B revolve each other in 79.9 years, so they will separate again. Because the apparent trajectory of B around A seen from Earth is different to the true trajectory due to our particular viewing angle, this year’s minimum angular separation is not synonymous to the true minimum separation of the two stars (aka periapsis or periastron). Periapsis will be reached again in May 2035, when both stars will approach each other to 11.2 astronomical units (AU), about the distance in which Saturn revolves around the Sun. 

The above image shows both the apparent and the true orbit of B around A. It also shows that the current minimal separation is not the closest possible: Shortly after periapsis, in November 2037, the angular separation will shrink to just 1.71”, while in reality the two stars will be already receding from each other again. It will be much harder then to separate the two in smaller telescopes. Due to their excentric orbit, A and B recede from each other as far as 35.6 AU (roughly the distance between the Sun and Pluto). The next apoapsis will occur in 2075.

Interestingly, the graphic shown is only temporarily, as the apparent orbit changes. As each orbit goes by, its shape (its apparent eccentricity) continues to narrow, caused by the true proper motion of the system. Alpha Centauri is approaching our Solar System with about 25 kilometers per second, but it will not hit us. Closest approach will be around 29,240 AD at 2.97 light-years.

Alpha Centauri's third component, Alpha Centauri C or Proxima Centauri, is much more difficult to find. It is shwn here in a magnified crop of an image taken with a 70mm telephoto lens. Also in this image: the iconic constellation Crux, the Southern Cross. Credit: Jan Hattenbach (Wide-angle image & Proxima Centauri inset), Jared Males (A-B inset)
Alpha Centauri, however, is (most probably) a triple system. The third component (sometimes called Alpha Centauri C, but much more commonly known by it’s name Proxima Centauri) is much more difficult to see. It’s a small red dwarf with only 11mag, and thus invisible unless in a medium sized telescope. Separated from A and B by more than 2° (about four lunar disks), it is very difficult to identify among the many stars in the Milky Way. It is, however, easy to photograph with a moderate telephoto lens. The second inset in the image below shows Proxima Centauri. I took the corresponding image last year using a 70mm lens; the inset is magnified for convenience

Proxima is 4.243 light years from Earth, a bit closer than A and B (hence the name ‘Proxima’, which means ‘closest’ or ‘next to’ in Latin).

Further reading: I highly recommend Andrew James' website for further detailed information and useful graphics on Alpha Centauri.

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