An artist's representation of binary stars
An artist's representation of binary stars | Wikimedia commons
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Bengaluru: Call it the astronomical equivalent of twins, born of the same womb at the same time.

Binary stars — systems consisting of two gravitationally-bound stars that appear as a single pinpoint from Earth — are born bound to each other, new research suggests, shedding light on a mystery that has baffled scientists for years.

An international team of researchers, comprising scientists from the RIKEN Cluster for Pioneering Research in Japan, Chalmers University of Technology in Sweden, and University of Virginia in the US, along with other collaborators, has observed a molecular cloud that is on its way to form a binary star system.

While scientists have made radio observations before, information about the formation of star systems is so scarce that each new observation can be a potential gold mine.

The molecular cloud will eventually collapse under its own weight, and two protostars will start to form among the gas and dust.

The researchers, whose study was published in Nature Astronomy, used the Atacama Large Millimeter/Submillimeter Array (ALMA) telescope array in northern Chile.

The telescopic array observed at high spatial resolution a star-forming region called IRAS07299-1651, which is located about 5,500 light years away and full of gas and dust.

It finally offers evidence for the theory of multiple star formation, which states that the cores of multiple-star systems emanate from the same disc of material, as opposed to a more massive star capturing the other when two star systems collide or merge.


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Binary stars and their formation

The two stars observed by the researchers are separated by a large distance, about six times the distance between Neptune and the sun. They are orbiting each other over a period of 600 years and have a combined mass of 18 times the sun’s.

Most massive stars — those that are over 10 times the mass of the sun — tend to have an orbiting companion star.

Scientists have known for a while that the more massive a star, the larger its probability of being in a binary system.

But how a binary system forms has been an area of mystery: Are companion stars born together from a spiralling disc of material, or do they eventually encounter each other in a cluster of stars (stars don’t normally split into two)?

“This is an exciting finding because we have long been perplexed by the question of whether stars form into binaries during the initial collapse of the star-forming cloud or whether they are created during later stages,” researcher Yichen Zhang of the RIKEN Cluster for Pioneering Research was quoted as saying in a press release that accompanied the study.

“Our observations clearly show that the division into binary stars takes place early on, while they are still in their infancy,” he added.

‘Seeing through the dust’

The formation of binary star systems had been hard to deduce so far because the gas discs that surround young stars prevent and distort signals and light.

It is hard for scientists to visually discern what is happening in the thick of all the haze.

“We need to see through the dust which blocks the ultraviolet and optical light,” researcher Abhijeet Borkar, an astrophysicist at the Astronomical Institute of the Academy of Sciences, Czech Republic, who has earlier worked with ALMA, told ThePrint.

“The dust is also super bright in infrared. So we need sub-millimeter wavelengths to see through, and still have good sensitivity and resolution,” he said. “ALMA can additionally observe carbon monoxide, carbon sulphide, and even hydrogen cyanide molecules.”

The properties of these molecules are fixed for specific temperatures and pressures, so the environmental conditions around the target star are easier to know. They also act as identifiers for additional ingredients in the mix. For example, carbon monoxide is a ‘molecular tracer’ and tightly tied to hydrogen. So, the reading can be used to determine the amount of molecular hydrogen in the star system.

The researchers’ findings also add value to one of the theories about the mechanism of formation of protostars: The second, much less massive, star is actually feeding on material that is surrounding its companion, leading to a gradual fragmentation of the disc of material that is originally around the primary star. Over time, the second star also grows massive enough for both the stars to have similar characteristics.

“The reason this is amazing is that we are literally seeing stars being born, how they’re born and what method they are born from,” said Borkar.

“It’s a big step in understanding how stars form, especially in binaries. This is only the third or fourth time we have observed it happening right now in-situ,” he added.


Also read: Uranus remains mysterious but now we know why it spins on its side


 

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