Context: The LIGO Scientific and VIRGO Collaborations (LSC) have detected an unusual object whose mass falls in between that of a typical black hole and a neutron star. 


  • The LIGO and VIRGO detectors observed gravitational wave signals emerging from the coalescing of binary black holes in 2015.
  • Since then they have detected mergers of pairs of black holes, pairs of neutron stars and black hole-neutron star duo. 

About VIRGO and LIGO

  • The Virgo interferometer is a large interferometer designed to detect gravitational waves predicted by Einstein's general theory of relativity. 
  • The instrument's two arms are three kilometers long and located in Santo Stefano a Macerata, near the city of Pisa, Italy
  • Virgo is part of a scientific collaboration of laboratories from six countries: Italy and France, the Netherlands, Poland, Hungary and Spain. 
  • Other interferometers similar to Virgo have the same goal of detecting gravitational waves, including the two LIGO (Laser Interferometric Gravitational-wave Observatory) interferometers in the United States (at the Hanford Site in Washington State and in Livingston, Louisiana).

About the observations:

  • The waves coming from the signal indicate that the primary object in this merger had a mass of about 23.2 times that of the Sun and the smaller, secondary object had a mass of about 2.6 times the solar mass. 
  • The two objects combined to form a large black hole of mass 25.6 times the Sun’s mass, having lost 0.2 solar masses.
  • The two black holes that merged were locked in the disk surrounding a quasar, a supermassive black hole that shoots out blasts of energy.
  • Later, an observatory called the Zwicky Transient Facility (ZTF) in California, USA detected a blast of light. 

The puzzle:

  • Visible black hole collision: The flash of light made scientists wonder whether they had spotted the rare visible black hole collision. Black holes aren't supposed to make flashes of light.
  • The mass ratio:  It was 1:9 for the pair. This is the largest disparity in masses that has been observed till now between members of the combining pair of objects. 
  • Identity of second mass: With the mass of 23.2 solar masses, the primary object qualifies the criteria for a black hole but the calculated mass of the secondary is too light to be a black hole and too heavy to be a neutron star.
    • It becomes very difficult to detect any signatures of neutron star ‘tides’ which could have given us insights about the star. 
    • Scientists cannot confirm or rule out whether there is a ‘mass gap’ between the maximum mass of the neutron star and the minimum mass of a black hole.

The implications

  • Black holes can be visualized: We can actually visualize black holes by observing the surrounding matter they light up. It's how the Event Horizon Telescope snapped the now-famous image of a supermassive black hole, last year. 
    • That image is not exactly the black hole itself, but rather the glowing gas and dust bordering its event horizon.

Black holes:

  • A black hole is a place in space where gravity pulls so much that even light can not get out. 
  • The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying. 
  • Because no light can get out, people can't see black holes.
  • Space telescopes with special tools can help find black holes.

How Do Black Holes Form?

  • Stellar black holes are made when the center of a very big star falls in upon itself, or collapses. When this happens, it causes a supernova. 
  • A supernova is an exploding star that blasts part of the star into space.

Stellar evolution: It is a description of the way that stars change with time.

The life of a star: 

  • Stars are born out of the gravitational collapse of cool, dense molecular clouds.
  • Death of star: Once the hydrogen in the core has all been burned to helium, energy generation stops and the core begins to contract.This causes the star to expand enormously and increase in luminosity – the star becomes a red giant.
  • Once the helium has all been converted, the inert carbon core begins to contract and increase in temperature.
  • What happens next depends on the mass of the star
    • Stars less than 8 solar masses
      • The carbon core continues to contract until it is supported by electron degeneracy pressure. Eventually, the outer layers of the star are ejected completely and ionised by the white dwarf to form a planetary nebula.
  • Stars greater than 8 solar masses
    • The contracting core will reach the temperature for carbon ignition, and begin to burn to neon. 
    • If the core has a mass less than about 3 times that of our Sun, the collapse of the core may be halted by the pressure of neutrons. In this case, the core becomes a neutron star. 
    • If the core has a mass greater than about 3 solar masses, even neutron pressure is not sufficient to withstand gravity, and it will collapse further into a stellar black hole.

The Hertzsprung-Russell diagram (HR diagram) plots the absolute magnitudes of stars against their spectral type (or alternatively, stellar luminosity versus effective temperature). 

  • As a star evolves, it moves to specific regions in the HR diagram, following a characteristic path that depends on the star’s mass and chemical composition.


Image Source: NASA