BLUE STRAGGLER

Description

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Context

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, one third are likely formed through collisions of 2 stars, and the remaining are formed through interactions of more than 2 stars.

About

Blue stragglers are a class of star observed in old, dense stellar systems such as globular clusters.
They stand out because old stellar populations are expected to be devoid of blue (high-mass) stars which possess very short lifespans.
The blue stragglers in an old stellar population must therefore have formed long after the system as a whole.
They are bigger and bluer than the rest of the stars.

Globular cluster

A globular cluster is a spherical collection of stars.
Globular clusters are very tightly bound by gravity, giving them their spherical shapes and high concentrations of stars toward their centers.
Globular clusters are found in nearly all galaxies.
In spiral galaxies like the Milky Way, they are mostly found in the outer, spheroidal part of the galaxy—the galactic halo.
They are the largest and most massive type of star cluster, tending to be older, denser, and composed of fewer heavy elements than open clusters.
The Milky Way has over 150 known globulars and possibly many more undiscovered.
The origin of globular clusters and their role in galactic evolution remain unclear.

Origin of Blue Stragglers

A clue to their origin is that they are only found in dense stellar systems, where distances between stars are extremely small (a fraction of a light year).
In these dense environments (for example the cores of globular clusters), collisions between stars are relatively common, and it has long been believed that blue stragglers are the result of the merger of two old, red stars.
This merger produces a star with a greater mass (hence bluer in colour), and severely disrupts the two stars involved, mixing hydrogen into the stellar core and giving the star a new lease on life.
This model for the formation of blue stragglers was confirmed through observations of the nearby globular cluster, 47 Tucanae, taken with the Hubble Space Telescope.
These observations showed that one of the blue stragglers (BSS 19) has a mass almost twice that of the normal stars in the cluster, and an extraordinarily high rotation rate at 75 times faster than that of the Sun.
Both the high mass and the rapid rotation of this star bears witness to its origin in a stellar merger event.