ISRO Missions and Discoveries

Why are blue straggler stars different from the norm?


From UPSC perspective, the following things are important :

Prelims level: Blue Straggler Stars

Mains level: Not Much

Researchers from the Indian Institute of Astrophysics, Bengaluru have studied the eccentricities of blue straggler stars.

What are Blue Straggler Stars?

  • 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.
  • Blue stragglers were first discovered by Allan Sandage in 1953 while performing photometry of the stars in the globular cluster M3.

What did the Indian researchers study?

  • Eccentricity is the deviation of a planets’ or stars’ orbit from circularity — the higher the eccentricity, the greater the elliptical orbit.
  • For this, the researchers also made use of the observations by the UVIT instrument (Ultra Violet Imaging Telescope) of ASTROSAT, India’s first science observatory in space.

(a) Stellar ageing of stars

  • To know what blue stragglers are, it is necessary to understand how stars are classified and their evolution, studied.
  • Our Sun, for example, is what is called a main sequence star, and, given its mass and age, it is expected that once it has converted all its hydrogen into helium, its core will get denser, while outer layers expand.
  • So, it will bloat into a red giant.
  • After this phase, its fuel spent, it will shrink, becoming a smaller, cooling star called a white dwarf star at the end of its life.

(b) Sequencing of stars

  • To study the behaviour of the star, you could plot a graph of the colour of a star, which is an indication of its surface temperature, against its magnitude, which is related to the total energy given off by it.
  • If you do this for all the stars in a globular cluster, a large number of stars are seen to find a place within a band known as the main sequence.
  • Our Sun is a main sequence star, too, and the expectation is that all main sequence stars follow a pattern of evolution pretty much like our Sun’s fate, which was described earlier.
  • There are a few stars that, just at the stage of their lives, when they are expected to start expanding in size and cooling down, do just the opposite.
  • They grow brighter and hotter and blue in colour, thus standing out from the cooler red stars in their vicinity in the colour-magnitude diagram.
  • Since they lag behind their peers in the evolution, they are called stragglers, more specifically, blue stragglers, because of their hot, blue colour.

Outcome of the research: Reasons for Blue Stragglers behaviour

  • The puzzle of why a blue straggler is more massive, and energetic than expected may be resolved in several ways.
  • One that these do not belong to the family of stars in the cluster, and hence are not expected to have the group properties.
  • Second, the straggler draws matter from the giant companion and grows more massive, hot and blue, and the red giant ends up as a normal or smaller white dwarf.
  • The third possibility is that the straggler draws matter from a companion star, but that there is a third star that facilitates this process.


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