Something to do on a Moonlight Night

Finding and Observing Proxima Centauri - (V 645 Centauri)

by Doug Brown

With a magnitude of 10.7, Proxima Centauri can be glimpsed even on a bright Moon lit night using a reasonable pair of binoculars or a small telescope. This faint star is exceptional for one reason, it is the nearest star to the Solar System. At about 4.3 light years, Proxima Centauri is marginally closer than the easily identifiable and bright star Alpha Centauri. In space, both stars are separated by about one sixth of a light year. The question as to whether Proxima Centauri is gravitationally linked to the famous double star system of Alpha Centauri is not yet fully resolved.

Only after being interested in astronomy for several years did I get around to looking for Proxima Centauri. When I did however, I found it more difficult than I had thought owing to the problem of accessing a detailed enough star chart that clearly and unambiguously showed the exact position of the star. I tried several different charts but even so, trying to find Proxima Centauri proved to be much harder than I had anticipated. To make matters worse, I was unsure whether I had actually observed Proxima Centauri or just some other nearby look-alike reddish/orange star. I could not claim a positive sighting. This uncertainty was compounded by the star's faint magnitude and its location in a relatively rich star field. For other observers, the two charts supplied with this article should remove any of that uncertainty.

Proxima is a well known variable flare star and has been described by Robert Burnham in his Celestial Handbook as possibly one of the most active stars known of its type. Flare stars are usually dim red dwarfs which for some reason, increase and decrease their brightness over a matter of minutes. These changes in brightness appear to occur at random sometimes hours, days or months between each event. It is currently believed that such changes in brightness are caused by flares on the star's surface similar to the solar flares that the Sun produces from time to time.

Compared to the Sun, red dwarfs are relatively dim objects so when a major surface flare occurs, these flares add substantially to the overall brightness level of the star. With much brighter stars, a surface flares contribution to the luminosity of a star is usually lost amid the star's overall brilliance. As Proxima Centauri is intrinsically around 13,000 times less luminescent than the Sun, any increase in brightness over a short period can often be readily observed.

According to Burnham*, Proxima Centauri has a diameter around 64,000 kilometres or about five times the size of the earth, hence it is a very small star when compared to the Sun. Its a bit like comparing a pea with a large rock melon. To put this into some context, it must be remembered that the Sun itself is considered a dwarf star on the stellar scale.

Using the Charts

The two charts shown here differ in both their scale and limiting stellar magnitudes. Chart A (21K) shows Alpha and Beta Centauri (pointers to the Southern Cross) and Alpha Circinus. The latter is a bright naked eye star about four degrees from Alpha Centauri in the direction of the South Celestial Pole. Also, Alpha Circinus forms an easily identifiable right angle triangle with the other two stars. The cut off magnitude of Chart A is close to 10. Note the asterism shaped like a broken cross marked on the chart. Chart B (17K) is a more detailed map of the immediate region of Proxima Centauri with a limiting magnitude of about 12. It should provide enough background stars for the observer to zero in on Proxima Centauri.

To locate Proxima Centauri, I suggest that you first find the cross shaped asterism using Chart A. This should not be too difficult. Now with the help of Chart B, in the direction of Alpha Circinus look for a reddish/orange star about half a degree from the asterism (the diameter of the Moon). Depending on the instrument you are using, Proxima Centauri and the asterism might be in the same field of view. If not, you will need to move your instrument a small amount in a direction towards Alpha Circinus.

Good luck.

* (Editor's note) Burnham refers to the three volume work of Robert Burnham Jr published c1978. The three volumes examine the objects (notable features, double stars and deep-sky) in each constellation in great detail. Burnham also included information about specific objects (white dwarfs, nova etc.). It is a well prepared work. Even though it is now somewhat dated it is still a useful work for any astronomer.

ABOUT THE AUTHOR

Doug Brown has been an ASWA member for 17 years, originally attracted to the Society after attending a public viewing night in 1983. He built his own 20 cm Newtonian, which he has since converted for photographic use, as a prototype of a larger scope he still intends to build. Doug most enjoys photographing some of the lesser known objects and abides by the motto that "unphotographed photons are wasted photons". He has yet to succumb to the temptations offered by CCDs, automatic guiders and imaging software.

Doug remains a valuable member of the Society, having served on Council, and actively participates in viewing nights, Astro-Camps, etc. He is of the strong view that the future of ASWA will remain bright as long as the membership stays focussed on the real reason for ASWA's existence. To him it is providing an opportunity for like-minded people to come together to develop and share their love of astronomy.