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UV-Bright Stars Spotted in Globular Structure NGC 2808

  • 22 Jan 2021
  • 8 min read

Why in News

Recently, astronomers have spotted rare hot Ultra Violet (UV)-bright stars in the massive intriguing globular cluster in the Milky Way Galaxy called NGC 2808.

  • India's first multi-wavelength space satellite AstroSat helped astronomers in this.

Key Points

  • Data:
    • Scientists combined data of Ultraviolet Imaging Telescope (on board AstroSat) with observations made using other space missions such as the Hubble Space Telescope and the Gaia telescope along with ground-based optical observations.
      • Hubble Space Telescope: The HST or Hubble (NASA) is a space telescope that was launched into Low Earth orbit in 1990 and remains in operation. It is one of the largest and most versatile space telescopes till date.
      • Gaia is a space observatory of the European Space Agency, launched in 2013 and expected to operate until 2022. The spacecraft is designed for astrometry: measuring the positions, distances and motions of stars with unprecedented precision.
  • Findings:
    • About 34 UV-bright stars were found to be members of the globular cluster (NGC 2808). One of the UV-bright stars was found to be about 3000 times brighter than the Sun with a surface temperature of about 1,00,000 K.
    • Hot UV-bright stars have been distinguished from the relatively cooler red giant and main-sequence stars.
    • Most of the stars were found to have evolved from a solar stage called the horizontal branch stars with hardly any outer envelope. Thus, they were bound to skip the last major phase of life called the asymptotic giant phase (it is one of the last major phases in the life of stars) and directly become dead remnants or white dwarfs.
      • The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red giant branch in stars.
  • Significance:
    • Properties of Stars: The findings will help in determining properties of these stars such as their surface temperatures, luminosities and radii.
    • Evolution of Stars: These present excellent laboratories where astronomers can understand how stars evolve through various phases between their birth and death.
      • Death of star: It is not clear how these stars end their lives as not many of them are detected in these fast-evolving phases, making their study crucial.
    • UV radiations: UV-bright stars are speculated to be the reason for the ultraviolet radiation coming from old stellar systems.
  • About NGC 2808:
    • NGC 2808 is a globular cluster in the constellation Carina. The cluster belongs to the Milky Way, and is one of our home galaxy's most massive clusters, millions of stars. It is estimated to be 12.5-billion years old.
    • It is said to have at least five generations of stars.

Stellar Evolution

  • Nebula:
    • A nebula is a cloud of gas (mostly hydrogen and helium) and dust in space.
    • Nebulae are the birthplaces of stars.
  • Main Sequence Stars:
    • Main sequence stars are stars that are fusing hydrogen atoms to form helium atoms in their cores.
    • Most of the stars in the universe i.e. about 90% of them are main sequence stars. The sun is a main sequence star.
    • Towards the end of its life, a star like the Sun swells up into a red giant, before losing its outer layers as a planetary nebula and finally shrinking to become a white dwarf.
  • Red Dwarf:
    • The faintest (less than 1/1000th the brightness of the Sun) main sequence stars are called the red dwarfs.
    • Proxima Centauri, the nearest star to the Sun, is a red dwarf.
  • Red Giant:
    • Red giants have diameters between 10 and 100 times that of the Sun.
    • They are very bright, although their surface temperature is lower than that of the Sun.
    • A red giant is formed during the later stages of the evolution as it runs out of hydrogen fuel at its centre.
    • A very large red giant is often called Red Supergiant.
  • Planetary Nebula:
    • Planetary nebula is an outer layer of gas and dust that are lost when the star changes from a red giant to a white dwarf.
  • White Dwarf:
    • A white dwarf is a very small, hot star, the last stage in the life cycle of a star.
    • White dwarfs are the remains of normal stars, whose nuclear energy supplies have been used up.
    • White dwarf consists of degenerate matter with a very high density due to gravitational effects.
  • Nova:
    • Novae occur on the surface of a white dwarf in a binary system.
    • If the two stars of the system are sufficiently near to one another, material (hydrogen) can be pulled from the companion star’s surface onto the white dwarf.
    • When enough material builds up on the surface of the white dwarf, it triggers a nuclear fusion on a white dwarf which causes a sudden brightening of the star.
  • Supernova:
    • A supernova is the explosive death of a star and often results in the star obtaining the brightness of 100 million suns for a short time.
    • The extremely luminous burst of radiation expels much or all of a star’s material at a great velocity, driving a shock wave into the surrounding interstellar medium.
    • These shock waves trigger condensation which is a nebula paving the way for the birth of a new star.
  • A neutron star is the collapsed core of a massive supergiant star.


  • It is a multi-wavelength astronomy mission on an IRS-class (Indian Remote Sensing-Class) satellite in a 650-km, near-equatorial orbit.
  • Launch: It was launched by the Indian launch vehicle PSLV from Satish Dhawan Space Centre, Sriharikota in 2015 by ISRO.
  • It is the first dedicated Indian astronomy mission aimed at studying celestial sources in X-ray, optical and UV spectral bands simultaneously with its five unique X-ray and ultraviolet telescopes working in tandem.
  • One of the unique features of AstroSat mission is that it enables the simultaneous multi-wavelength observations of various astronomical objects with a single satellite.
  • The Ground Command and Control Centre for ASTROSAT is located at ISRO Telemetry, Tracking and Command Network (ISTRAC), Bangalore, India.
  • This has put India in an exclusive club of countries which have multi wavelength space observatories.
  • The minimum life of the AstroSat mission was expected to be 5 years.


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