Thursday, April 11, 2013

Nearest Stars: Past, Present and Future

At present, the nearest known star is Proxima Centauri which lies at a distance of 4.24 light years from the Sun. Proxima Centauri is a red dwarf star and it is way to faint to be visible with unaided eyes from Earth. Nevertheless, Proxima Centauri is likely to be part of a triple star system with Alpha Centauri - a pair of sun-like stars located slightly further away at 4.37 light years. Although Alpha Centauri is a binary star system, it appears to the unaided eyes as a single star with a combined visual magnitude of -0.27, making it the 3rd brightest star in the night sky after Sirius and Canopus. Alpha Centauri and Proxima Centauri have not always been the nearest stars to the Sun. Over the course of time, a number of stars have been predicted to have come or will come much closer to the Sun.

Figure 1: Distances of the nearest stars from 20,000 years ago until 80,000 years in the future. Credit: Matthews, R. A. J. (1994)

Alpha Centauri and Proxima Centauri are predicted to come as close as 3 light years from the Sun about 27,000 years from now, before receding away. They will continue to remain as the nearest stars until about 33,000 years from now. Beyond that, a red dwarf star named Ross 248 will become the closest star to the Sun between 33,000 to 42,000 years from now.

Looking further into the future, a star named Gliese 710 is particularly interesting since it is travelling nearly head-on towards our Sun. Even though this star is currently about 64 light years away, it is estimated to have a high probability of approaching within a mere one light year from the Sun at approximately 1.4 million years from now. As a result, Gliese 710 is likely to perturb the Oort cloud and send an influx of comets into the inner solar system. However, the increase in impact rates in the inner solar system due to the influx of comets is expected to be very small.

About 7.3 million years ago, a triple star system named Algol passed within 9.8 light years of the Sun. Although it may not seem like a very close approach, Algol has a combined mass that is 5.8 times the mass of the Sun. In addition, the combined luminosity of Algol is a whopping 100 times the luminosity of the Sun. At closest approach, the gravity from Algol might have been sufficient to perturb the Oort cloud. An observer on Earth at that time would have seen Algol shining as a brilliant star with a visual magnitude of about -2.8. That is over 3 times brighter than Sirius appears at present. Currently, Algol is about 93 light years away.

Figure 2: An image of Sirius - the brightest star in the present night sky. Credit: Greg Parker, New Forest Observatory

Zeta Leporis is another star which came relatively close to the Sun in the past. Bobylev & Vadim V. (2010) estimated a closest approach of 4.16 light years from the Sun about 861,000 years ago while García-Sánchez et al. (2001) estimated a closest approach of 5.34 light years from the Sun about one million years ago. Zeta Leporis is 14 times more luminous than our Sun and if it had came as close as 4.16 light years, it would have appeared as a very bright star in the night sky with a visual magnitude of about -2.5.

On 11 March 2013, the discovery of a binary brown dwarf system located at a mere 6.5 light years away was published in a paper by Kevin Luhman, an associate professor of astronomy and astrophysics at Pennsylvania State University. The last time a stellar /substellar object was found to be this close to the Sun occurred nearly a hundred years ago in 1916 when Barnard’s star was discovered at 6.0 light years from the Sun. This binary brown dwarf system is designated WISE 1049-5319 since it was detected by NASA’s Wide-field Infrared Survey Explorer (WISE). Such a discovery shows there are still objects left to be found even within those few light years of space that define the Sun’s neighbourhood. The discoverer of WISE 1049-5319 said: “There are billions of infrared points of light across the sky, and the mystery is which one - if any of them - could be a star that is very close to our solar system.”

- Matthews, R. A. J. (1994), “The Close Approach of Stars in the Solar Neighbourhood”, Quarterly Journal of the Royal Astronomical Society Volume 35: 1-9
- Bobylev, Vadim V. (2010), “Searching for Stars Closely Encountering with the Solar System”. Astronomy Letters Volume 36 (3): 220-226
- García-Sánchez, J.; Weissman, P. R.; Preston, R. A.; Jones, D. L.; Lestrade, J.-F.; Latham, D. W.; Stefanik, R. P.; Paredes, J. M. (2001), “Stellar Encounters with the Solar System”, Astronomy and Astrophysics Volume 379 (2): 634-659
- Garcia-Sanchez, J.; Preston, R. A.; Jones, D. L.; Lestrade, J.-F.; Weissman, P. R.; Latham, D. W. (1997), “A Search for Stars Passing Close to the Sun”, The First Results of Hipparcos and Tycho, 23rd meeting of the IAU, Joint Discussion 14
- K. L. Luhman (2013), “Discovery of a Binary Brown Dwarf at 2 Parsecs from the Sun”, arXiv:1303.2401 [astro-ph.GA]