Indian astronomers identify new category of stars in transition

Six Indian astronomers from Bengaluru have identified a new category of stars that fills a gap in the stellar transition from childhood to youth. Astronomers so far have not had a complete picture of this evolution cycle, primarily because of lack of adequate detecting methods.

Stars that have not yet lit up are called pre-main sequence stars, and those that shine brightly are main sequence stars. After analysing 15 months of data on 225 pre-main sequence stars and 2,167 main sequence stars, PhD student Suman Bhattacharyya and his team at CHRIST (Deemed to be University), Bengaluru, developed a predictive model to find transition phases between these two categories.

So far, they have identified 98 such transition phase stars. The team's research was published in the Monthly Notices of the Royal Astronomical Society journal in August.

"Star formation of massive stars proceeds through well-studied, definitive stages before reaching the youth. Our own Sun is now passing through this youth stage," Bhattacharyya says. Stars pass through infancy (known as protostar), then the so-called child stage (known as pre-main sequence or PMS) and gradually move to the youth stage, which is when hydrogen burns at their cores to produce helium and thus releases energy.

Is there an intermediate phase between childhood and youth in the stellar life cycle? The question had been haunting astronomers. Research wasn't producing evidence that stars might possess such an unseen phase.

Bhattacharyya - who "fell in love" with astronomy after attending a lecture series by astrophysicist Somak Raychaudhury at Presidency University, Kolkata - started working with some of his professors and fellow students on the subject during his MPhil when his supervisor, Blesson Mathew, tasked him to search for those rare stars in transition. He used a machine learning algorithm that he had developed for an unrelated and yet behaviourally connected sector: market crash prediction.

"The randomness of the market is similar to the randomness of the star-formation process from the dust cloud," Bhattacharyya says. 

During the protostar and PMS phases, a star is usually surrounded by thick discs of dust and gas. This makes it difficult to observe such stars using ground-based facilities, says Gourav Banerjee, co-author of the paper. However, an understanding of the transition from PMS to main sequence phase is vital for addressing the astrophysical aspects connected to disc dynamics and the evolution of a star's rotation rate. Few studies exist in scientific literature that examine and attempt to better understand stars in the transition phase.  

After the protostar phase of the stellar evolution cycle, when the star gathers all of its mass, it starts contracting and temperatures rise up to millions of degrees Celsius, causing hydrogen atoms to fuse with each other and turn into helium. This contraction stage of the stellar evolution process is the PMS phase.

In the main sequence phase, a star starts producing energy from fusion. It was theoretically predicted that the evolution from PMS to the main sequence must pass through an intermediate phase because this is not a sudden jump. Contraction due to gravity stops at some point and then stars slowly evolve towards the fusion process. This phase has been defined as the transition phase by the Bengaluru team. 

This discovery will help astronomers fill a gap in stellar evolution. The study of the properties of these may reveal some unknown facts about stellar physics in the near future.