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Sigma Orionis, much more than a star

A detailed study on this multiple star system led by Spanish astrophysicists has allowed them to determine with unprecedented accuracy the physical parameters of the most massive stars composing it

Alicante, 27 January 2015

About three million years ago, hundreds of stars were formed from a dense cloud of dust and gas in the constellation of Orion (The Hunter). The star that attracted most of the mass was Sigma Orionis (Sigma Ori), today the fourth brighter star in Belt of Orion and the one illuminating the famous Horsehead Nebula. While sigma Orionis, formed around a lot of stars of different masses, brown dwarfs and isolated planets (objects similar to Jupiter's mass, but free-floating in the star cluster). Smaller objects in Orion's belt have 10,000 times less mass than sigma Orionis.

Knowing how often low-mass stars, brown dwarfs and isolated planets are born and evolve, involves knowing first what happens to their blue high-mass stellar neighbours. To this end, an international team of astronomers led by Spanish researchers Sergio Simón-Díaz, from the astrophysical research institute of the Canary Islands (IAC) / University of La Laguna (ULL), José Antonio Caballero, from the Centre for Astrobiology (CAB, CSIC INTA) and Javier Lorenzo, from the University of Alicante, has studied in detail sigma Orionis multiple star, which continues to amaze the beholder. The results of this study have been published today in The Astrophysical Journal. The research work have been developed by fourteen scientists, four of whom belongs to the University of Alicante’s Department of Physics, Systems Engineering and Signal Theory, apart from Javier Lorenzo, Ignacio Negueruela, Ricardo Dorda and Amparo Marco.



Stellar banquet

Paraphrasing Dr. Eldon Tyrell in Ridley Scott's Blade Runner (1982), <<the light that shines with an intensity a thousand times greater, lasts a thousand times less>>. This is exactly what happens in the Universe: stars that are very bright, with large masses and high temperatures and they die in supernova explosions when they have a few million years. By contrast, the least massive stars have a quiet and long life, with a duration comparable to the age of the universe, which allows the development of life on the planets orbiting them. 

Even after eons, the unmistakable stamp that massive stars leave can be seen in virtually everything: our own chemical composition, spatial distribution of stars and nebulae that occur at the end of their lives, the design of spiral arms in galaxies or, interestingly, the number of low-mass stars. This effect is because low-mass stars and brown dwarfs (intermediate objects between the smallest stars and larger planets) are just the 'leftovers of the banquet` of massive stars , as explained by Sergio Simón-Díaz, first author of the article. 

Sigma Orionis is a very hot three million years old star. Its temperature reaches about 30,000 K - five times hotter than the Sun. This high temperature causes the star to have a bluish colour, in contrast to the less massive stars, which have reddish colours. José Antonio Caballero recalls that researchers showed that sigma Orionis is actually a multiple star consisting of six blue stars instead of five as previously thought before: two of them are very closed massive stars circling each other with an orbital period of about 143 days. A third somewhat less massive star orbits about 100 astronomical units (100 times the average distance between the Sun and Earth), with a much longer period of about 157 years. Finally, the cluster is completed with three other stars slightly cooler and less massive, accompanied by numerous stellar remnants. 

Now, these researchers together with 11 other partners in Spain, Germany, Chile, USA, Belgium and Hungary, have been observing in detail the central trio of stars (sigma Ori Aa, sigma Ori Ab y sigma Ori B) and measuring all their physical parameters with unprecedented accuracy. The closest pair period of about 143 days could now be determined with an error of only 11 minutes, as Simón-Díaz pointed, making it possible to schedule specific observations in certain phases, for example, space telescopes the X-ray in the periastron, that is, the point at which the two central stars have lower separation.


Devouring stars

The study has also allowed them to accurately determine the masses of the three stars with different methods. In all, the mass of the trio exceeds 40 solar masses,as stated by Simon-Diaz. These findings, together with interferometric observations in progress, are an excellent input for theoretical models that attempt to explain the structure and fate of these ‘devouring stars’. 

Finally, Caballero added that they have also measured the number of high-energy photons emitted by the trio as a whole. These photons from Sigma Orionis Aa, Ab and B are the 'comb mane' of the Horsehead Nebula and announce the start of a new feast of high-mass stars in the region. In a few million years, when sigma Orionis Aa (and perhaps Ab) explodes as a supernova and cleans the neighboring region, there will still be a lot cooler and smaller stars plus a few large, massive and very hot stars, that will be found at that time immersed in clouds near the Horsehead Nebula and the life cycle of stars will continue!


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