The Proxima Puzzle, Stellar Proximities, and Earthly Purview – Our Closest Neighbor Part IV

Long after Alpha Centauri was discovered and studied, small bits of information began to emerge regarding another star lurking in the vicinity. Was this a star? If so, is it part of Alpha Centauri and thus make it a three-star system? Or is it just the most surprising co-incidence that it hangs out nearby, yet is separate? Proxima has been mystifying ever since. Btw, it is the actual nearest star to us.    

Part 1: The brightest star in the day sky

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

Part 3: Discovery and further study of Alpha Centauri

Proxima Centauri, the ‘other’ star

The closest star to the Sun is also not easily visible in the night sky. Being a faint and undersized star, Proxima Centauri lacks any distinctive make-up which could help us to identify it better. The presence of a dynamic mix of stars in the background doesn’t help either. Robert Innes holds the distinction of discovering the star through painstaking observation that span over many days. Using a blink comparator and multiple photos, he observed a yet unknown star, in the vicinity of Alpha Centauri.

The distance of Proxima Centauri was a tough nut to crack. Joan Voûte, based on his observation deduced that Proxima Centauri was 1,444 AU further away from Alpha Centauri. He published his results in the June 7, 1917, issue of the Monthly Notices of the Royal Astronomical Society. Robert Innes, concluded his own calculations to determine the distance of the star he had discovered. He concluded that it was indeed 18, 151 AU closer to the Sun than Alpha Centauri. This was based on the observations between 1816 and 1917. In fact, it was him who suggested that the star be named‘Proxima Centaurus’ (indicating the close distance and the neighborhood it was a part of). However, Centauri is used more often these days. Also, the current measure of the distance of this star from the Sun is pegged at about 11,000 AU closer than Alpha Centauri.

Proxima, an unsolved mystery

Proxima Centauri is placed precariously at the bottom of the main sequence of stars. Even a little mass is lost, then it would become a brown dwarf and not a star anymore. Its luminosity is very low, at only 0.17% as that of the Sun. Proxima is of the same age as Alpha Centauri, yet will outlive the binary system. For the most part of its life, it is expected to be stable at its core but seems to disagree at its surface. Being a ‘flare star’, it exhibits a sudden increase in brightness followed by a slow path to normalcy. This is a typical characteristic of the majority of red dwarfs and can last from a few seconds to several minutes long. Proxima’s flare activity was first discovered by Harlow Shapley in 1951.

Not all is clear about Proxima though since it’s still unknown as to how can this star, as well as other red dwarfs, can exhibit any magnetic field. When a star is less than 40% of the mass of the Sun, the radiative layer that is necessary to generate a magnetic effect, is absent. The mystery deepens when successive simulations of such stars with only convective zone show that they need to have a stable magnetic field at the surface, although that is clearly not what we observe. The flares are a cyclical process indicating a dynamic magnetic aspect of Proxima.

The search is on to discover planets orbiting in the habitability zone of Proxima.  In 2016, astronomers discovered a nearly Earth-sized planet in this region. A paradox with respect to Proxima is whether the red dwarf gravitationally bound to Alpha Cen AB, forming a Tri-Star system or is it just by random chance that it is close to their vicinity. The fact that it lies on the gray area separating a probable Alpha Cen ‘C’ or otherwise, adds to the confusion., As of now, we can only take solace knowing Proxima is gravitationally bound to Alpha Centauri, but only ever so slightly. It is subject to extensive observations and calculations that are yet to reach a conclusion.

Shrinking interstellar distances

When you read that a star is 11,000 AU closer than another, yet more than 4 light years away, it is challenging to fathom such large distances. Martin Beech, the author of Alpha Centauri explains it in a very interesting manner. Here it goes:

  • Let’s say the Sun is the size of a grain of sand (arbitrarily fixed at 1 mm across)
  • Earth is a 109 times smaller than that sand grain and hence 0.01 mm wide.
  • The diameter of earth’s orbit (taken as a circle) will be 21.5 cm
  • Alpha Centauri is, then located 29 km away from us, on the same scale
  • Both Alpha Cen A and Cen B are about the size of our Sun, hence 1 mm across
  • At their closest contact (Periastron) they are 1.2 m apart and at the farthest (Apastron) 3.8 m apart.
  • Proxima Centauri will then be 1.2 km away from Alpha Cen AB system. It will be 0.1 mm across, being 1/10th the size of the sand grain.

How close can they get?

By measuring the velocity of a star and its relative position to Solar system, scientists are able to discern who among them will be closest to us and for how long. Since the past 50,000 years, Alpha Centauri has been our nearest stellar neighbor. The distance is getting closer, with the closest point that will be reached 23,000 years hence.

32,000 years from now, Alpha Centauri will be dethroned as our closest star (system) when Ross 248 will take that spot. But this is not for long. 10,000 years from that point Gliese 445 will become the nearest star to Sun and will hold that place for about 8,000 years. Alpha Centauri will regain the top spot and retain it for another 30,000 years. After this point, Ross 128 will be our nearest star (outside of the Sun, of course)

Ross 248, Gliese 445 and Ross 128 are all red dwarfs, similar to Proxima Centauri. Gliese 710, a red-brown dwarf star will pass just outside of the Oort cloud, making it the first close encounter of the solar system with another star. It will not gravitationally disturb any planets at that distance, although it is expected to push a considerable number of comets towards the inner Solar system. All of this will occur only a few million years into the future.

Conclusion

The future is uncertain yet full of possibilities. We do know for certain, how our Sun or Alpha Centauri will behave over the next few million years. Whether interstellar travel will enable us to visit our star neighbors, we don’t know yet. Will sufficient technology exist in the future, if yes, then how far are we from taking our first flight towards the stars? What are the possibilities of finding Earth-like planets that might support our species? Can we discover and observe more stars, planets and the related that’ll help us understand the universe better than we know now? Sure, technological advances have pushed the boundaries of possibilities since man first started to wonder what lay beyond the skies. It will definitely redefine the limits of our knowledge of the cosmos. But there’s one factor that is constant in all of our existence. Martin Beech, the author of Alpha Centauri thinks the main force behind any advancement of our species will be human grit, imagination, and social evolution.

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.
  • A blink comparator is used to place multiple star images (captured on photographic plates), which aids in the detection of any movement of a star in relation to its background. Stars that are closer to the observer tend to have a larger shift, called proper motion. Closer the star, larger the proper motion.
  • When a star is less than 0.08 times the mass of Sun, they end up being ‘Brown Dwarfs’, with low surface temperatures. Some are found at distances closer than that of Proxima Centauri. For comparison, they are about the size of Jupiter.
  • The first known brown dwarf was discovered by Maria Teresa Ruiz, a Chilean astronomer.
  • It is now understood that flares occur due to a sudden release of an enormous amount of magnetic energy. It may also be a product of collisions with an asteroid and comet-like objects.
  • The first exoplanet was discovered in 1995, with its announcement made in the Nature journal (Nov ‘95). Working from the Geneva Observatory, Michel Mayor and Didier Queloz discovered the exoplanet 51 Peg b, orbiting its home star 51 Pegasi.
  • The first Earth-like exoplanet discovered around a red dwarf is Kepler-186f, located in the habitability zone, orbiting the star Kepler-186. It was announced in April 2014.

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