Carrying out the first-ever comprehensive analysis of blue stragglers, Indian researchers found that half of the blue stragglers in their sample are formed through mass transfer from a close binary companion star.

What are Blue Stragglers?

• A blue straggler is a main-sequence star in an open or globular cluster that is more luminous and bluer than stars at the main sequence turnoff point for the cluster.
• The most likely explanation is that blue stragglers are the result of stars that come too close to another star or similar mass object and collide.
• The newly-formed star has thus a higher mass and occupies a position on the HR diagram which would be populated by genuinely young stars.
• One-third of them are likely formed through collisions of 2 stars, and the remaining are formed through interactions of more than 2 stars.

How are they formed?

• A bunch of stars born at the same time from the same cloud form a star cluster.
• As time passes, each star evolves differently depending on its mass.
• The most massive and bright stars evolve and move off the main sequence creating a bend in their track, known as the turnoff.
• Stars above this bend or brighter and hotter stars are not expected in a cluster, as they leave the main sequence to become red giants.
• But in 1953, Allan Sandage found that some stars seem to be hotter than the turnoff of the parent cluster.

Behind the nomenclature

• Initially, these blue stars still straggling above the turnoff were not part of these clusters.
• However, later studies confirmed that these stars are indeed cluster members, and they were termed “Blue Stragglers”.
• The only probable way these stars can still be present in these clusters is if they have somehow acquired extra mass along the way while on the main sequence.
• Confirming the mechanisms of the mass gain required a study using a large sample of blue-straggler stars and estimates of the mass they have gained.

What have Indian researchers found?

• Research showed that these stars are primarily present in the older and massive star clusters. And due to their large mass, they are segregated towards the centre of the clusters.
• The researchers compared the mass of the blue stragglers to the mass of the turnoff stars (which are the most massive ‘normal’ stars in the cluster) and predicted the formation mechanisms.
• The study will help improve understanding of these stellar systems to uncover exciting results in studies of large stellar populations, including galaxies.
• Following these findings, the researchers are conducting detailed analyses of individual blue stragglers in the catalog to obtain their stellar properties.

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