Q&A: Seeing a ‘Cosmic Monster’

Radio astronomers have revealed the first image of a giant black hole at the center of our galaxy.

Known as Sagittarius A* or Sgr A*, it is located about 27,000 light-years from the sun and is about 6.5 billion times more massive than the sun.

The Milky Way and the location of the central black hole as seen from the Atacama Large Millimeter/submillimeter Array. Image: ESO/José Francisco Salgado (josefrancisco.org), EHT Collaboration

The image shows a bright ring formed as light bends in the intense gravity surrounding a black hole. The research, conducted by the Event Horizon Telescope (EHT) collaboration, was published in six articles in the Astrophysical Journal Letters and used a network of telescopes from Antarctica to Greenland to capture the image.

We asked cosmologist and astrophysicist associate professor Christian Reichardt of the School of Physics to explain why what we see for the first time is so important.

what exactly have scientists found?

Astronomers have captured a picture of the supermassive black hole lurking at the center of the Milky Way, called Sagittarius A* (as you might guess from the name, it’s part of the constellation Sagittarius, also known as The Archer).

However, this was not easy, because despite their mass, black holes are small and as the name implies – dark.

What the astronomers have actually seen is a ring of gas and matter orbiting the black hole. This gas is heated to blistering temperatures as it falls in — the bright spots in the image reach a trillion degrees.

All of this matter will eventually fall onto the black hole, making it much more massive.

Scientists knew that stars orbit something invisible, compact and massive at the core of the Milky Way. Image: ESO/B. Tafreshi (twanight.org)

But taking the picture was a challenge.

First, you have to see through all the other things between us and the center of the Milky Way.

Second, it is small. Astronomers had to use a telescope with radio sources over Earth to image the ring, using a technique called very long baseline interferometry to combine data from telescopes scattered around the planet, from the South Pole to Greenland.

Finally, the hot gas is swirled around the black hole at very high speeds. Astronomers had to stare at the black hole for 10 nights to see it, but the gas moved on timescales from minutes to hours.

Astronomers have effectively addressed this by taking thousands of short exposures and then combining them with the removed orbital motion.

Why is it important?

This is the first time we have seen the black hole at the center of our galaxy. And only the second time we’ve directly imaged a black hole (in 2019, the same telescope captured a picture of a much larger black hole in a nearby galaxy called M87*).

We already had a pretty good idea that a huge black hole, about four million times more massive than the Sun, was lurking in the center of the Milky Way.

Comparison of the dimensions of two black holes: M87* and Sagittarius A*. Photo: EHT Collaboration (Courtesy: Lia Medeiros, xkcd)

You may recall that the 2020 Nobel Prize was co-awarded to Andrea Ghez and Reinhard Genzel for the “for the discovery of a supermassive compact object at the center of our galaxy”.

But the wording was carefully chosen to avoid saying black hole, because we hadn’t seen it. Now we have.

With these new observations, we can also test whether gravity works as we think it should in the extreme conditions near a black hole. And it seems.

The image of the spinning gas and matter falling into the black hole matches the predictions of Einstein’s general theory of relativity. It seems that general relativity works to predict how space-time will be distorted near a black hole.

What does it tell us about our galaxy?

This image confirms that the Milky Way is home to a supermassive black hole. This puts the Milky Way in good company, as we believe these black holes live at the center of most galaxies.

It confirms that our black hole is the mass we would have expected from looking at the orbits of stars near it, and that there is no non-black hole explanation for the previous observations.

An artist’s impression of the anatomy of a black hole. Graphic: ESO

Why are black holes important?

Black holes are cool and are among the most extreme objects in the galaxy. They provide unique windows into the strong gravity regime and test our understanding of how gravity works.

Black holes are also believed to play a vital role in how galaxies grow and create stars.

We believe that essentially all galaxies have a supermassive black hole like Sagittarius A* at their center. The impact of matter on this black hole can release phenomenal amounts of energy, enough to actually change the dynamics of gas lumps across the galaxy and control how quickly stars can form.

Banner: Event Horizon Telescope Collaboration

Leave a Comment