Discovery and further study of Alpha Centauri – Our Closest Neighbor Part III

Alpha Centauri was historically thought off as a single star. But with advancements in science and technology, more was revealed about this far off world, which also is the closest to us. What are the two stars like? Are there any other objects in the system? Read on to find out the answer. Also, learn about how small changes in one variable can make huge differences in the way a star operates.

Part 1: The brightest star in the day sky

Part 2: Alpha Centauri – As observed in nature and culture

Part 4: The Proxima Puzzle, Stellar Proximities, and Earthly Purview

When one became two and went for a spin

On December 21, 1689, Jean Richaud observed a comet pass through the sky from Puducherry, India. To his surprise, Alpha Centauri appeared to be split by the comet. This led to the eventual discovery of it being a binary star system. In honor of his discovery, one of the official identifiers for Alpha Centaury is RHD 1. In July 1709, a French Astronomer, Louis Feuillée made independent observations regarding the binary nature of Alpha Centauri. According to him, the binary system consisted of the much bigger Alpha Cen A and a smaller Alpha Cen B, which were separated by a distance comparable to the size of Alpha Cen B. He was observing from Concepción, Chile.

Abbe Nicolas Loius de La Caille became the first astronomer in 1752 to measure the distance between the stars Alpha Cen A and Alpha Cen B. Observing from Cape of Good Hope, he measured the distance to be 20.5 arcsec in the sky. In 1761, Nevil Maskelyne, observing from Saint Helena calculated that the two stars are separated by 15.6 arcsec. This showed that there was a change in the angle between the stars, indicating a movement, which eventually led to the study on orbits of the Alpha Centauri system. On January 12, 1855, edition of the Monthly Notices of the Royal Astronomical Society, estimation of Alpha Cen B’s orbit was published for the first time. It was computed by Eyre Powell, working from an observatory at Chennai, India. While Powell calculated the duration of one orbit at 75.3 years, another astronomer John Russell Hind, came up with the orbit period of 80.94 years.

A better duration period of Alpha Centauri’s orbit was provided by William Finsen in February 1926. The duration was calculated as 80.089 years. Presently, the time taken to complete a single orbit, in the Alpha Centauri system that is accepted, was provided by Dimitri Pourbaix. 79.91 years is what is widely considered as Alpha Centauri’s orbit duration and it was calculated in 2002. How far is Alpha Centauri from the Sun? The first person to provide a clear answer to this question was Thomas Henderson. Publishing his results in the April 1842 edition of Memoirs of the Royal Astronomical Society, he pegged the distance at 3.573 light years. The modern estimate is 4.366 light years.

Inner workings of Alpha Centauri

In order to determine the lifetime of a star, we need to know two things, the mass of the star (indicating the amount of fuel at its disposal) and how bright it shines (which tells us how fast it uses the fuel). Based on this we can determine that generally large stars have shorter burn cycles (since they need to burn the fuel at a higher rate) and smaller stars tend to last longer. And so, how do things shape up for our neighboring star family? Having a similar mass as the Sun, Alpha Cen AB are expected to have a lifetime of around 10 billion years. The kind of nuclear fusion processes, as well as placement of different zones in Alpha Cen A and Cen B, are very similar to that of the Sun. Proxima Centauri, however, is in for the long haul. A size equal to 1/10th of the Alpha Cen A and Cen B means that, at the current rate, Proxima can last for more than 2 trillion years!

Alpha Cen A is present in the middle of its own Maunder minimum, characterized by a lack of spots or flares related activity on its surface. Asteroseismic studies performed on Alpha Cen A and Cen B have revealed that the star system is between 6-6.5 billion years old, a good 1.5 billion years older than our Sun. In order to understand the composition of these stars, it is necessary to study with more focus. We might assume that carrying masses similar to our Sun, Alpha Cen A, and Cen B also behave similarly. You’d be surprised to know that with just little change in mass, these stars exhibit widely different levels of brightness, for example.

Alpha Cen A is only slightly heavier than the Sun, but this results in a 50% increase in brightness. And in such an environment, a planet similar to earth, orbiting at a similar distance (1 AU) will be too hot to have liquid water. It is exactly the opposite for Alpha Cen B. A 50% reduction in output as compared to the Sun, although being ever so lighter (in mass), at 1 AU, an Earth-like planet would be a frozen world.

Our search of exoplanets shows that the stars can be expected to have a planetary system, provided they are composed of heavy metals. Captain William Jacob was the first to build a hypothesis about the possibility of planets orbiting Alpha Cen AB in 1856. As you read, planets are being discovered in the Alpha Centauri system, with Alpha Cen Bb being the first among them. Planet formation in such systems is the same as in single stars, except it depends how close the stars themselves are to each other. In binary systems, a planet can orbit around a single star or around the system. In Alpha Centauri’s case, we can expect planets to be orbiting either Cen A or Cen B or in orbit in the region between Alpha Cen AB and Proxima. Scientists believe that the Alpha Centauri system may have been born out of the same region in the Milky Way galaxy.

Notes:

  • Beech, M. (2015).Alpha Centauri. 1st ed. Springer International Publishing. [Minor planet (12343) Martin Beech has been named in recognition of his contributions to meteor physics.]

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  • The nomenclature used properly should be (α – Alpha, ß – Beta). It must be noted that the use of actual words than Greek letters is aimed to make reading easier.
  • Arcsec is 1/3600 of a degree of an angle
  • Alpha Cen Bb is located so close to Cen B that they may be tidally locked, meaning one side of the will always be in extremely hot day mode and the other in complete icy darkness.

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