The Nautical Almanac # 38 --- Rigil Kentaurus (The Alpha Centauri Star System)

The 2016 meeting of the International Astronomical Union (IAU) was as fractious as the 2006 meeting. In 2006, a majority of the small minority of members who were present and thus able to vote decided to demote Pluto from Planet to Dwarf Planet (the discovery of other "Dwarf Planets" since seems to have confirmed their decision, but since Dwarf Planets are still Planets maybe the terminology, not the status, needs to be changed). 

In 2016, they did it again, changing the name of the very well-known star Alpha Centauri to Rigil Kentaurus ("The Foot of The Centaur" in Astro-Arabic). Alpha Centauri is a triple star system (like Beta Centauri) made up of what should be Alpha Centauri Alpha, Alpha Centauri Beta, and Alpha Centauri Gamma. 

Alpha Centauri Alpha (left), Alpha Centauri Beta (right) and Proxima Centauri (in the red circle)

However, the IAU did not bother to rename Alpha Centauri Beta to Rigil Kentaurus Beta. Since the third star in the system was never named Alpha Centauri Gamma to begin with, but Proxima Centauri, it's easier to think of Rigil Kentaurus as the name of the star system and to remember the stars as Alpha Centauri A and B and Proxima Centauri. Rigil Kentaurus ("Rigil Kent" for short) is the 38th numbered entry into the Nautical Almanac. 

Why all this hoopla? Well, because the Rigil Kentaurus star system is the Sun's closest neighbor. In older books, Alpha Centauri (Alpha) was listed as the closest star, at a relative walk in the park distance of 4.5 light years away. When Alpha Centauri Beta, its orbital companion, was discovered, Beta was found to be 4.3 light years away. When more powerful telescopes found Proxima Centauri it was found to be only 4.25 light years away from Earth. (For comparison, the Sun's own Oort Cloud is one light year away, making Proxima even closer to the exurbs of our stellar neighborhood.)

Alpha Centauri Alpha and its exoplanet

Alpha Centauri Beta and its exoplanets

Proxima Centauri

The world of Proxima Centauri b

This would not be all that intriguing but for the fact that Alpha Centauri Alpha is a yellow star amazingly like our own Sun, and that it has exoplanets in orbit about it, some in the "Goldilocks" or "Habitable" Zone. And so does Alpha Centauri Beta, a tangerine-colored star. The two Alpha Centauris are about as far from each other as Neptune is from the Sun --- nearly 3 trillion miles, but not beyond the reach of spacecraft. That means that if intelligent life exists on the exoplanets of one it could be in contact with intelligent life on the exoplanets of the other, possibly forming a United Federation of Planets --- or a Romulan Empire. And though Proxima is further away and in distant orbit around A and B, it too has an exoplanet in the Habitable Zone. So scientists study the Rigil Kent system on a constant basis. The Voyager spacecraft were launched in the direction of Alpha Centauri complete with "Greetings from the children of planet Earth." One day we may get greetings back --- or hungry alien arthropods may come to eat us. Who knows? 

"Greetings, Earthlings!"

Eating Earthlings.

Alpha Centauri Alpha is 1.1 times the mass of the Sun, with 1.2 times its radius, and 1.5 times its luminosity. It is a Zero Magnitude star, the third brightest in our night skies (it can only be seen from the Southern Hemisphere). It burns at 10,000 degrees F., just like our own Sun, and is 4.4 billion years old, just slightly younger than our Sun. It is a yellow Main Sequence star, and very stable. It appears to be a quite livable star for Earth purposes --- in fact, astronomers have long been fascinated that two such similar stars are so close together --- and Alpha Centauri Alpha has been known since antiquity. 

Alpha Centauri Beta was not known to science until more recently, when finer instruments discerned its presence. Alpha Centauri Beta, also a Main Sequence star, is 0.9 times the mass of our Sun, 0.8 times the radius, and is about half as luminous as our Sun. It is about 6.5 billion years old, two billion years older than our Sun, and is a yellowish star with a pinkish tinge, burning at about 9500 degrees F. Indistinguishable from Alpha Centauri Alpha from Earth, it shares the Zero Magnitude designation with its companion star. 

Just as Castor and Pollux seem to be heavenly twins from our vantage point on Earth and the Rigil Kentaurus system and the Beta Centauri system were called "The Brothers" by the Chinese, it is entirely possible that from a far distant planet circling another star, our Sun and Alpha Centauri are "The Friends." 

Yellow stars like the Sun (sometimes called "Yellow Dwarfs") are the second most common type of star in the Milky Way, exceeded only in numbers by "Red Dwarfs." Proxima Centauri is a Main Sequence Red Dwarf, measuring an 11 on the magnitude scale (invisible to the eye). It is 600 times less bright than the Sun, meaning that its exoplanet is a shadowy place. Its mass is only 1/12 that of our Sun's mass, it's radius is only 0.15 that of the Sun --- about the size of Saturn --- and it burns at about 5000 degrees Fahrenheit. Proxima is about five billion years old, and it is expected to have a long lifespan of possibly trillions of years (Red Dwarfs burn fuel even more slowly than Yellow Dwarfs). Thus, it will have an incredibly long life to allow its exoplanets to develop life, even though many scientists expect the presently-known Earth-sized exoplanet of Proxima Centauri b to be at best Mars-like. Time will tell.   

The Voyager Plaque. An uproar ensued over whether a short line representing the vagina should be inscribed on the female figure.

Music, greetings, news broadcasts, and Chuck Berry (!) all exist on the Voyager gold record, If anybody is out there listening our first radio transmissions are an expanding bubble now 100 light years across.

For Bud Jiho