Wayfarers All

     “And you, you will come too, young brother; for the days pass, and never return, and the South still waits for you. Take the adventure, heed the call, now ere the irrevocable moment passes!” (The Wind in the Willows)

     And with Sirius, that Grand Finale, our tour is ended. From the early spring nights we spent struggling mightily to catch a glimpse of the impossible DX Cancri, to the Blazing Glories of the winter firmament, to end with the brightest luminary of them all. “Having seen all, we move on” (Dante, The Divine Comedy). But not quite. There are still unseen stars to the south, forever hidden beneath the implacable curvature of the Earth, never to grace the skies of Howard County, Maryland. To “see all”, we must go south. Far south.

     The situation is not as bad as it could have been. We have missed out on observing just four star systems, not visible from our vantage point at latitude 39 degrees north. Among these are yet two more red dwarfs: 7th magnitude Lacaille 9352 in the constellation Piscis Austrinus (brightest red dwarf in the sky), and 13th magnitude GL 1061 in Horologium (at 11.991 light years (ly), the furthest object to make it onto our list). Otherwise, these two are quite similar to the 16 red dwarfs we have already observed from our home turf. But the other two… Now, these are worth a journey.

     ALPHA CENTAURI: The Alpha Centauri system, also known as Rigel Kentaurus, is the third brightest star in the Earth’s sky (after Sirius and Canopus). It is composed of three stars, Alpha Centauri A and B, and Proxima Centauri. The A and B components are a close binary (they would both fit within the orbit of Pluto), and cannot be separated with the naked eye. Besides being our closest neighbors (other than Proxima), these two members also happen to be the most similar to our Sun of anything found in the local neighborhood. Alpha Centauri A is one tenth more massive and one fourth larger than the Sun, and shines with the radiance of one and one half Suns. Alpha Centauri B is the smaller of the pair, having about 90 percent of the Sun’s mass and one half of its luminosity. They orbit about a common center of gravity once each 79.9 years, separated by at most 36.6 Astronomical Units (AU) and at their nearest by only 11.2 AU (about the distance between the Sun and Saturn). 

     In October 2012, after many years of searching using a variety of means, a single, roughly Earth-sized planet was finally detected circling Alpha Centauri B by the HARPS (High Accuracy Radial velocity Planet Searcher) instrument at the European Southern Observatory’s La Silla Observatory in Chile. This undoubtedly superheated world (now called Alpha Centauri Bb) orbits its star at a distance of only 3.6 million miles, far closer than our own Mercury is from the Sun. A single orbit would take only 3.2 Earth days. One can easily imagine a world entirely covered by molten rock, with no solid surface anywhere. Although by no means a desirable piece of real estate, its very existence does make the prospect more likely that there might possibly be more favorably-placed planets in the system yet to be discovered.

     The third (and dimmest) member of the system, Proxima Centauri, is a distant 0.2 ly from the other two, mostly in our direction, making it (at 4.2 ly) the closest of all stars to the Sun. It was discovered in 1915 by South African astronomer Robert Innes. Proxima Centauri is a nondescript red dwarf of 0.12 solar mass and 0.0017 solar luminosity. It appears in our sky as an 11th magnitude star at a considerable distance (10^m of right ascension and almost 2º of declination) from the other two components of the Alpha Centauri system. Once again, we have to marvel that so many of the stars in our immediate vicinity, even the closest of them all, cannot be seen with the naked eye.

     EPSILON INDI: The very last star on this journey, Epsilon Indi is also one of the strangest. A main sequence star of stellar class K, it boasts three quarters solar mass and 22 percent of its luminosity. Its surface temperature is a balmy 4,630º K. Although it is only 1.3 billion years old, its metallicity is somewhat less than the Sun’s.

     These traits by themselves would not call for any particular interest in Epsilon Indi. And in fact, other than its long-noted high proper motion (ninth highest of any star), none was paid to it until 2003, when the star was determined to not be alone. This time, the as yet unseen companion (actually, as it turned out, companions) was not a suspected planet, but a new type of star – a brown dwarf. In Epsilon Indi’s case, it turned out that we were dealing with another triple star system – Epsilon Indi A, and a binary pair of brown dwarfs (Epsilon Indi Ba/Bb). The two sub-components have a mass of approximately 50 and 30 Jupiters respectively, and are separated from each other by 2.1 AU, circling each other once every 15 years. The pair itself is a distant 1,500 AU from Epsilon Indi A.

     But it now seems they are not the only objects making up the Epsilon Indi system. Measurements of the primary star’s motion have strongly indicated that there is still a third, as yet undetected, member between 5 and 20 times the mass of Jupiter, and orbiting the system’s A star at a distance of somewhere between 10 and 20 AU, and inclined to the star’s equator by 20º.

     But despite such an abundance of exotic companions, no evidence has been found to point toward the existence of any planets or other rocky, dusty, or icy material in the system. Epsilon Indi appears to be solely a gathering of stellar oddities.