Tag Archives: Sagittarius A*

Einstein’s General Relativity passes the test at the centre of our Galaxy

Measurements of a star passing close to the supermassive black hole at the centre of the Milky Way confirms the predictions of Einstein’s theory of general relativity in a high gravity environment.

An artist visualization of the star S0–2 as it passes by the supermassive black hole at the Galactic centre. As the star gets closer to the supermassive black hole, light it emits experiences a gravitational redshift that is predicted by Einstein's General Relativity. By observing this redshift, we can test Einstein's theory of gravity (Nicole R. Fuller, National Science Foundation)

An artist visualization of the star S0–2 as it passes by the supermassive black hole at the Galactic centre. As the star gets closer to the supermassive black hole, light it emits experiences a gravitational redshift that is predicted by Einstein’s General Relativity. By observing this redshift, we can test Einstein’s theory of gravity (Nicole R. Fuller, National Science Foundation)

A detailed study of a star orbiting the supermassive black hole at the centre of our Galaxy, reveals that Einstein’s theory of general relativity is accurate in its description of the behaviour of light struggling to escape the gravity around this massive space-time event.

The analysis — conducted by Tuan Do, Andrea Ghez and colleagues — involved detecting the gravitational redshift in the light emitted by a star closely orbiting the supermassive black hole known as Sagittarius A*. The redshift was measured as the star reached the closest point in its orbit — which has a duration of 16 years — to the black hole.

Lasers from the two Keck Telescopes propagated in the direction of the Galactic centre. Each laser creates an artificial star that can be used to correct for the blurring due to the Earth’s atmosphere. (Ethan Tweedie) 

Lasers from the two Keck Telescopes propagated in the direction of the Galactic centre. Each laser creates an artificial star that can be used to correct for the blurring due to the Earth’s atmosphere. (Ethan Tweedie)

The team found that the star experienced gravitational redshift — which occurs when light is stretched to longer wavelengths and towards the red ‘end’ of the electromagnetic spectrum by the effect of gravity — as it gets closer to the black hole,  conforming to Einstein’s theory of general relativity and its predictions regarding gravity.

At the same time, the results defy predictions made by the Newtonian theory, which has no explanation for gravitational redshift.

Ghez says: “(The findings are) a transformational change in our understanding about not only the existence of supermassive black holes but the physics and astrophysics of black holes.”

The major difference between general relativity and the Newtonian calculation of gravity is, that whereas Newton envisioned gravity as a force acting between physical objects, Einstein’s theory saw gravity as a geometric phenomenon.

The presence of mass ‘curves’ space it occupies. Physical objects, including light, must then follow this curvature. As John Wheeler infamously put it: “matter tells space how to curve, space tells matter how to move.”

Testing relativity in regions of high gravity

Image of the orbits of stars around the supermassive black hole at the centre of our galaxy. Highlighted is the orbit of the star S0–2. This is the first star that has enough measurements to test Einstein’s General Relativity around a supermassive black hole. [Credit: Keck/UCLA Galactic Center Group]

Image of the orbits of stars around the supermassive black hole at the centre of our galaxy. Highlighted is the orbit of the star S0–2. This is the first star that has enough measurements to test Einstein’s General Relativity around a supermassive black hole. [Credit: Keck/UCLA Galactic Center Group]

The new research resembles an analysis conducted last year by the GRAVITY collaboration, except in this new expanded analysis, the team report novel spectra data.

Although general relativity has been thoroughly tested in relatively weak gravitational fields — such as those on Earth and in the Solar System—before last year, it had not been tested around a black hole as big as the one at the centre of the Milky Way.

Observations of the stars rapidly orbiting Sagittarius A *provide a method for general relativity to be evaluated in an extreme gravitational environment.

Do explains why these kind of tests are important:

“We need to test GR in extreme environments because that’s where we think the theory might break down.”

“If we can see which predictions from general relativity have deviations, that gives us clues as to how to build a better model of gravity.”

A figure showing the challenges the Ghez team had in processing decades of image data and spectroscopy input to follow the star S0–2. (Zina Deretsky, National Science Foundation)

A figure showing the challenges the Ghez team had in processing decades of image data and spectroscopy input to follow the star S0–2. (Zina Deretsky, National Science Foundation)

To obtain their results, the team analyzed new observations of the star S0–2 as it made its closest approach to the enormous black hole in 2018. They then combined this data with measurements Ghez and her team have made over the last 24 years.

The team has many avenues of investigations available to them from here, Tuan tells me.

He continues: “Two of them I’m excited about are testing space-time around the black hole by looking at the orbit of the star S0–2.”

“GR predicts that the orbit should precess, or rotate, meaning that it won’t come back where it started.”

The team should also be able to start using more stars other than S0–2 for these tests as the time baseline of observations increase and technology improves

Do concludes: “ These measurements open a new era of GR tests at the Galactic centre so it’s very exciting.”


This research appears in the 26 July 2019 issue of Science.

Why our galaxy’s black hole has a small appetite

As we know by now, most galaxies, including our own, have a supermassive black hole at their center. However, the one in our galaxy, Sagittarius A* (pronounced Sagittarius A-star) has a surprisingly small appetite, something which has puzzled astronomers for years.

This composite image combines observations using infrared light and X-ray light that see through the obscuring dust and reveal the intense activity near the galactic core.
IMAGE: X-RAY: NASA/CXC/UMASS/D. WANG ET AL.; OPTICAL: NASA/ESA/STSCI/D.WANG ET AL.; IR: NASA/JPL-CALTECH/SSC/S.STOLOVY Via MIT News.

Sagittarius A* is 4 million times as massive as the sun, but it is devouring much less material than it would be expected for a black hole of its size. By studying 3 million seconds of observations taken by the Chandra X-Ray Observatory, a group of researchers from the MIT and the University of Massachusetts at Amherst believe they have found the answer.

“The black hole doesn’t have a chance to do its meat-grinder thing and turn that matter into energy,” says Joey Neilsen, who contributed to the research as a postdoc at MIT’s Kavli Institute for Astrophysics and Space Research. “All of that stuff basically escapes before the black hole can destroy it.”

The group’s work focused on iron and X-rays: iron can be found on the surface of stars, as well as in the gas surrounding a black hole; the iron is much hotter on the black hole. To determine where the majority of X-rays were coming from, the researchers calculated the temperature of the iron, given its X-ray emission lines. They calculated that there’s plenty of hot iron, suggesting much of the X-ray activity arose not from a cluster of stars, but from gas surrounding the black hole. So if there’s all this hot material around it, why doesn’t the black hole consume more?

The likeliest, and so far, only reasonable explanation for such finicky eating is was that the gaseous material ejects itself in the form of hot wind, before the black hole can consume it.

“We think most of the energy, or a lot of it, is going toward pushing the gas away from the black hole and not letting it fall in,” says Mike Nowak, a research scientist at MIT Kavli. “Now we have a better understanding of what parts are active, and what aren’t.”

Now the question remains if this is something specific for Sagittarius A*, or if it is something typical for most supermassive black holes. This, however, is a question all but impossible to answer.

“However, the degree of inactivity — a factor of a million or so — is remarkable, in part because we have so much better and higher-resolution data in this case,” notes Genzel, who was not part of the research team. “The beautiful high-resolution Chandra spectroscopy presented in this paper will, for the foreseeable future, never be available in other galactic nuclei.”

Black hole at the center of our galaxy bursts out

As far as black holes go, Sagittarius A*, the supermassive black hole at the center of the Milky Way is pretty boring. It emits about the same energy as the sun, despite being 4 million times more massive.

Sagittarius A*

The region studied by Chandra. The bright point in the center is the burst caused by Sagittarius A*

Astronomers have observed that about once a day, the black hole awakes, emitting a brief burst of light before settling back into its slumber. It’s unclear what causes these flares,and astrophysicists have long tried to figure out the cause, in an attempt to better understand mature black holes like Sagittarius A*.

Recently, a team including researchers from NASA, MIT, the University of Amsterdam and the University of Michigan have used NASA’s Chandra X-Ray Observatory to detect the brightest flare ever observed from Sagittarius A*. The flare was recorded from 26,000 light years away, is 150 times brighter than the black hole’s normal X-ray luminosity, and could be an important piece of the puzzle.

“We’re learning what black holes do when they’re old,” says Joey Neilsen, a postdoc at MIT’s Kavli Institute for Astrophysics and Space Research. “They’re no young whippersnappers like quasars, but they’re still active, and how they’re active is an interesting question.”

The results were published in The Astrophysical Journal.

Picky black holes

Black holes are the nastiest places, devouring everything and anything around them. Astronomers detect them by observing the light energy given away when a black hole swallows a nearby object. As black holes age, the team explained, they behave somewhat curiously; you’d expect an older, bigger black hole to suck up more and more matter, but that’s not really how things are.

“Everyone has this picture of black holes as vacuum sweepers, that they suck up absolutely everything,” says Frederick K. Baganoff, a research scientist at MIT Kavli. “But in this really low-accretion-rate state, they’re really finicky eaters, and for some reason they actually blow away most of the mass available for them to consume.”

To detect such faint signals, the team used NASA’s Chandra X-Ray Observatory, a giant space-based telescope, and their attempt paid off.

“Suddenly, for whatever reason, Sagittarius A* is eating a lot more,” says Michael Nowak, a research scientist at MIT Kavli. “One theory is that every so often, an asteroid gets close to the black hole, the black hole stretches and rips it to pieces, and eats the material and turns it into radiation, so you see these big flares.”

The team has reserved a whole month on the Chandra telescope to study Sagittarius A*, in an attempt to see just how often these flare-ups are.