Rigel-Kentaurus, The Star of the Southern Sky
What is the thing that makes southern sky is so special? For me, i simply answer, because southern sky have one unique star, that is: Alpha Centauri (α Cen / α Centauri/ Rigil Kentaurus). For simplicity, i will call it, just by Rig-K, that is because i like it that way. In particular, the name Rigil Kentaurus derived from the Arabic phrase Al Rijl al Kentaurus, meaning “foot of the centaur,” (of course because it is placed on the foot of the Centaurus constellation) but is most often referred to by its Bayer designation Alpha Centauri, the official name for astronomy.
Why Rig-K so special? First, a simple as this: Rig-K is the brightest star in the southern constellation of Centaurus. Yet Rig-K is a system of three stars (let’s say a trinary system), one of which is the fourth brightest star in the night sky. Rig-K is easily found in a far “pointer” toward the west of Southern Cross. The components of trinary system are too close to be resolved by naked eye and usualy perceived as a single source of light.
The second reason is, Rig-K is the closest star system to us at, let’s say 4.5 light-years distant (about 42 million million km). So, there must be many-many researches can be conducted on this stars.
The largest member, α Cen-A, resemblance our sun (with spectral type of G2V, so it is safe to say that this is a twin of the sun), but slightly larger & brighter. The second member, α Cen-B, smaller & dimmer, with spectral type K1V with orangish-yellow-white light. Two of this as close as 11.2 AU (1.7 billion km, around the distance between Sun to Saturn), with elliptical orbit; and the farther distance about 36 AU (6 billion km; around Sun to Pluto); with one period of 80 years. From this data, one can derive that the sum of the two masses is around two times of the Sun.
The third member, known as α Cen-C/Proxima Centauri, is about 0.2 light year (around 2 million million km or 400 times the distance between Sun – Neptune) from the other two; which is around 1/20 distance of Rig-K to the Sun. Because of the distance is too far compared to the other two, there is dubious wether this is the member of system, but current trend still believed it is the member of the system; that is why this third member known as the Proxima Centauri. Proxima Centauri is a dim red dwarf with a spectral type of M5 – much fainter, cooler, and smaller than the Sun. Proxima is so faint that astronomers did not discover it until 1915. But i will not dwell on the Proxima’s problem, i will go further for the third reason.
The third reason, the ultimate reason & the most compelling reason. α Cen-A is a special place, because it may offer life conditions similar to our solar system. Remember that α Cen-A is resemblance our Sun? That is a good clue, that we might looking in there for possibility of habitable planet outside of our system. And because it is near, we can have more & more detail understanding of our ‘twin’ neighbour.
So, that is the reason for the Rig-K, now, why did i say, the ultimate reason? Well, in a melodramatic manner, the human endeavor in astronomy is looking for ‘another earth’; that is why we always hear the jargon of campaign, ‘extra solar planet’, ‘habitable planet’, or something like that. I am not good on this subject of study, but something that i can share is: A star must pass five tests before we can call it a promising place for terrestrial life. Most stars would fail easily. In the case of α Cen-A passes all five tests, α Cen-B passes either all but one, and only Proxima Centauri out. So here are the tests, compared with our Sun (which is the place of our life, of course).
Test number one, maturity and stability, which means it has to be on the main sequence. Main-sequence stars fuse hydrogen into helium at their cores, generating light and heat. Because hydrogen is so abundant in stars, most of them stay on the main sequence a long time, giving life a chance to evolve. The Sun and all three components of Rig-K pass this test.
Test number two, The Goldilock’s Problem. If the star is too hot, it will ceased to exist quickly, if the star is too cold, it will not have enough power to sustain possibility of life. It has to be just right.
The hotter stars (compared to Sun), (know as the spectral type of O, B, A and early F) burn out fast, die quickly. On the other, the cooler, (M and late K), produce energy which insufficient to sustain life, because they may not permit the existance of liquid water on their planets. Yellow G-type stars (Sun and Sun alike) can give rise to life. Late cool F and early hot K may be fine. So, the α Cen-A pass, α Cen-B can be yes or can be no. But Proxima is out.
Test number three, the system must show stability. Meaning, the brightness of the star(s) should not vary very much, or the possible life would not survive the changing between bright & dim, which causing ‘frozen’ & ‘fried’ effect on planet. α Cen-A & α Cen-B in itself is a good star, as they pass test number two, but α Cen-A & α Cen-Bis a binary, so if there is a planet with supporting life in one of the star, there is variability of the other pair affecting the planet, the brightness of the other increases as the stars approach and decreases as the stars recede. Fortunately, the variation is too small to matter, and α Cen-A & α Cen-B pass this test. How about Proxima? Red dwarfs known as a flare star, which likely to bursting out itself up to two or triple times its brightness in the order of minutes. So, this harsh variability is make the possibility is nearly zero. Proxima’s out.
Test number four, is the stars’ ages. The Sun is about 4.6 billion years old, so on Earth life had enough time to develop. A star must be old enough to give life a chance to evolve. α Cen-A & α Cen-B are even older than the Sun, they have an age of 5 to 6 billion years, therefore they pass the test. Proxima, may be only a billion years or so old, then Proxima fails this test.
Five, the last test. Do the stars have enough heavy elements (Astronomers call the elements heavier than helium as “metals”) – such as carbon, nitrogen, oxygen and iron – that biological life needs ? Like most stars, the Sun is primarily hydrogen and helium, but 2 percent of the Sun’s weight is metals. Although 2 percent may not sound a lot, it is enough to build rocky planets and to give rise to us. And again, α Cen-A & α Cen-B pass this test. They are metal-rich stars.
So, two of the three pass the test.
Now, α Cen-A pass the test, α Cen-B nearly pass all; but this is the ultimate question: Is there planet in Rig-K? At least one warm, rocky, with plenty of water, with habitable possibility planet, just like Earth? We do not know yet. If Rig-K is ‘close’ in a sense of astronomical scale, why we cannot find one? (at least yet). Because, no matter the star pass the test, but without any planet, we have nothing.
For a binary system, planets which orbit one of the star must not too far away from the particular star, or the orbit become unstable. If the distance exceeds a threshold value, the other star will strongly disturbs the orbit of the planet. For the system of α Cen-A & α Cen-B, the threshold value is 11 AU, with the limit of stable planetary orbit is about 2 AU. For the case of our Solar System, we see that we have Mercury (0.4 AU), Venus (0.7 AU), Earth (1 AU) and Mars (1.5 AU). Therefore, the system might have one or two planets in the habitable zone (Earth-like). Once the ability to observe the system is improved, which we can derived the higher & higher resolution, hopefully we can find the answer, wether we are alone in the universe or not, or , at least that we have some other place to go from Earth. If one ask, how far is 1 AU? 1 AU (Astronomical Unit) is approximately 150 million km, which is quiet a short distance in an astronomical sense.
So, if humanity looking for another place, or even another intelligent life, Rig-K is the place to go. It is the best candidate. Oh, how unique & special Rig-K, the star of the southern sky.