Written by Gautham Sabhahit

Over a century ago, Albert Einstein formulated his general theory of relativity (GR, also called Einstein’s theory of gravitation) describing the motion of objects in space and time under the influence of nearby matter and radiation. The theory formulated ‘gravitational force’ as a geometric property of space-time itself – the energy and momentum from matter bend the fabric of space-time which then tells objects in its vicinity how to move. The bending of space-time itself can give rise to interesting phenomena. Two such effects we explore below are gravitational reddening (Fig 1), where light from objects in near-vicinity of strong gravitational fields appears redder (or ‘red-shifted’) and orbital precession (Fig 2), an effect where close orbits around massive objects ever slightly precess which is already well-tested for the orbit of Mercury around our Sun.

General relativity has stood the test of time with multiple experiments in weak gravitational fields, including the famous Eddington eclipse test in 1919, independently verifying the theory. Comparatively, GR has been tested in conditions of strong space-time curvature in only a few instances – one of which involves studying the orbits of stars and gas around the supermassive black hole (SMBH) at the centre of our own galaxy called Sagittarius A*. The highly relativistic environment surrounding Sag A* offers a test-bed for GR in presence of strong gravitational fields, allowing us to test effects such as orbital precession and gravitational red-shift. 

Since the early 1990s, different groups have studied the orbits of the stars in the vicinity of our Galactic SMBH. One star in particular, S0-2, was observed for almost a decade in the near-infrared and was reported in 2002 to have a highly eccentric orbit. The star is deep in the gravitational well of the black hole, and light itself struggles to escape the strong gravitational pull of the black hole. GR predicts the light from this star to be severely red-shifted (appearing redder) when it is closest to the black hole, i.e., at the pericenter. The gravitational redshift of S0-2 observed at pericenter agreed more consistently with GR than Newtonian theory of gravity. These observations a century after Einstein formulated his theory confirming its validity even in strong gravitational fields, shows how right Einstein was all along. 

The star SO-2 has been observed to perform two full orbits around the black hole since observations began in 1995, with a period of just 16 years! In 2002 and 2018, S0-2 was observed at the pericenter zooming past at almost 3% the speed of light. Multiple other stellar objects have been identified ever since, some with even tighter orbits than S0-2. Being so close to Sag A*, the orbit of S0-2 precesses, meaning the star does not return to its ‘original’ position after a full orbit. The recent peri-central approach in 2018 and the orbit was observed by a multi-telescope instrument called GRAVITY which is capable of accurately measuring the position of S0-2. Once again, the observed precession of the star failed to match the orbit as predicted by Newtonian gravity, but closely matched the predictions of GR confirming its applicability even in strong g-fields.

Sag A* and the star S0-2 made headlines a couple of years ago when Prof. Reinhard Genzel and Andrea Ghez were awarded the Nobel Prize in Physics ‘for the discovery of a supermassive compact object at the centre of our galaxy’. A few months ago, the Event Horizon Telescope (EHT) released the first-ever image of the accretion disk around Sag A*. If you want to know more about the first photo of the central black hole in our galaxy, do have a look at the recent Astronote by our astronomer Marc Sarzi! (Link)

Additional resources:

Video on YOUTUBE showing the motion of stars around Sgr A*

Detection of gravitational redshift paper

Paper that reported the results on precession


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