Gravitational Wave Observations

Sagittarius A*: Black Hole at the Centre of our Galaxy imaged


From UPSC perspective, the following things are important :

Prelims level: Event Horizon Telescope (EHT), Black Hole, Saggitarious A*

Mains level: Read the attached story

Scientists from the Event Horizon Telescope (EHT) facility revealed the first image of the black hole at the centre of our galaxy i.e. the Milky Way.

The Milky Way is a spiral galaxy that contains at least 100 billion stars. Viewed from above or below it resembles a spinning pinwheel, with our sun situated on one of the spiral arms and Sagittarius A* located at the centre.

What is Sagittarius A*?


  • Pronounced Sagittarius ‘A’ star, it refers to the believed location of the supermassive black hole in the centre of our galaxy.
  • About 50 years ago, astronomers identified an area within the constellation of Sagittarius that was the strongest region of radio emission – thus making it the likely centre of the Milky Way.
  • It possesses 4 million times the mass of our sun and is located about 26,000 light-years—the distance light travels in a year, 5.9 trillion miles (9.5 trillion km)—from Earth.

What is an event horizon?

  • Black holes are extraordinarily dense objects with gravity so strong that not even light can escape, making viewing them extremely challenging.
  • A black hole’s event horizon is the point of no return beyond which anything—stars, planets, gas, dust and all forms of electromagnetic radiation—gets dragged into oblivion.
  • The closer someone came to a black hole, the greater the speed they would need to escape that massive gravity.
  • The event horizon is the threshold around the black hole where the escape velocity surpasses the speed of light.

What are the recent observations?

  • The image of Sagittarius A* (SgrA*) gave support to the idea that the compact object at the centre of our galaxy is indeed a black hole, strengthening Einstein’s general theory of relativity.
  • The image was obtained using the EHT’s global network of observatories working collectively to observe radio sources associated with black holes.
  • It showed a ring of light —super-heated disrupted matter and radiation circling at tremendous speed at the edge of the event horizon—around a region of darkness representing the actual black hole.
  • This is called the black hole’s shadow or silhouette.

How did Einstein’s theory found its proof here?

  • According to Einstein’s theory, nothing can travel faster through space than the speed of light.
  • This means a black hole’s event horizon is essentially the point from which nothing can return.
  • The name refers to the impossibility of witnessing any event taking place inside that border, the horizon beyond which one cannot see.

About EHT Facility

  • EHT project is a large telescope array consisting of a global network of radio telescopes.
  • It combines data from several very-long-baseline interferometry (VLBI) stations around Earth, which form a combined array.
  • It provides an angular resolution sufficient to observe objects the size of a supermassive black hole’s event horizon.
  • In 2019, the eHT facility made history by releasing the first-ever image of a black hole, M87* — the black hole at the centre of a galaxy Messier 87, which is a supergiant elliptic galaxy.


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