Sagittarius A* – Now We Have A Photo Of The Milky Way’s Black Hole

Last week, astronomers revealed the first ever photo taken of the black hole at the center of our galaxy, Sagittarius A*, which is abbreviated Sgr A*. This photo is a sensation for both technical and scientific reasons and it marks a milestone in the understanding of the galaxy we live in.

In this week’s blog post, we will explore what information this photo brings us and how this information compares to what astronomers previously have inferred from indirect measurements of our galaxy’s black hole (read my earlier post about the center of the Milky Way here).

This is the first image of Sgr A*, the supermassive black hole at the center of our galaxy. It’s the first direct visual evidence of the presence of this black hole. Credit: EHT Collaboration.

A Doughnut On The Moon

Let’s start with investigating how spectacular the photo itself is.

The photo above is not a single photo. Instead, it’s an average of several different photos that the Event Horizon Telescope, EHT, took during 10 days in April 2017. The EHT is a so-called array of several telescopes located at different locations all over the world (North America, South America, Hawaii, Europe and Antarctica – see the full list here). By pointing all these telescopes towards the same target, in this case the Milky Way’s black hole, then it is possible to obtain a resolution that corresponds to having one telescope the size of the Earth. Pretty amazing!

All these telescopes are radio telescopes. This means they are able to detect radiation from space that have longer wavelength than what we can see with our human eyes, and in the case of EHT it means they collect photons that are 1.3 mm long. Radio waves can be very long, several kilometers, so one millimeter is in the lower end of the radio spectrum.

It’s generally very tricky to observe photons in the mm-region from Earth because this exact range (around 1mm) is what our atmosphere prefers to absorb. The atmosphere protects us from radiation, sure okay, but it also blocks out, or at least dims down, a lot of the radiation that contain valuable information about space.

Fun fact and totally unrelated to black holes: The APEX telescope is located in northern Chile and is among the telescopes used in the EHT array, and I spent almost a month at APEX in 2010 doing service observations. I will write about that some other time.

With this impressive Earth-covering array in place, the EHT can resolve incredibly small objects. The photo of Sgr A* corresponds to photographing a doughnut on the surface of the Moon. In case you forgot, the Moon is located almost 400,000 km away from us, and if you want a doughnut for scale, I think it’s best you go get one at the bakery. You know – for science.

Comparing To The Black Hole M87*

Back in April 2017, the EHT collaboration photographed Sgr A*. This is the black hole located 27,000 light years away from us in the very center of our galaxy. It has a mass of 4 million suns that are squeezed into a small sphere only 17 times bigger than the Sun.

The scientific collaboration also photographed other black holes, including M87*, which is a black hole at the center of the Messier 87 galaxy located 53 million light-years away from us. A photo of the black hole M87* was revealed in April 2019, only two years after the data was collected. The Sgr A* photo was revealed five years later.

The reason for it taking so much longer to obtain the photo of “our” black hole is that it is 1,000 times less massive than M87* and the swirling disk around the black hole therefore changes on a much shorter time scale making it difficult to get a proper image. M87* changes during hours and there is hence enough time to capture a decent observation.

Our galaxy’s black hole (right) compared to the M87* black hole located in a galaxy 53 million light years away from us. Both photos are from the EHT collaboration, but the Sgr A* photo took more than twice the time of M87* to make because of it’s small size and rapid motion. Credit: EHT collaboration (acknowledgment: Lia Medeiros, xkcd).

Measuring Matter In (Or At Least Very Near) the black hole

Prior to this photo, astronomers had all their information about Sgr A* from measurements based on the motion of surrounding stars. By observing how surrounding stars would orbit the black hole, that is how fast they were moving and in which direction, astronomers could estimate the size and mass of the black hole.

Illustrative view of how stars orbit a black hole. While it is not possible to see what is actually in this seemingly empty region, the motion of orbiting stars reveal what mass and size the black hole has. Credit: Astronomicca.

In 2019, astronomers estimated that the black hole had a mass of 4.154 million suns. The radius of the black hole was estimated in 2018 to be 50 micro arc-seconds on the sky, which was in good correspondence with the 52 micro arc-seconds predicted by relativity theory. These angles are so small that they make up an almost infinitely small part of your thumb if you stretch your arm towards the sky.

Do these black hole measurements change with this new direct image of Sgr A*?

Well, they are based on the same photos (those famous 10 days back in April 2017 where EHT made the observations), so in that sense there is not much new in terms of data. Instead, what this photo shows us is that the black hole indeed “looks” like what we saw indirectly from the data and that the black hole indeed looks and behaves like a circular object. We see this not from the black hole itself, but from the shadow it casts from the surrounding hot disk of gas.

Black Hole Accretion Disk

The orange/red ring around the black hole is a very hot (millions of degrees) disk with gas that swirls around the center. This gas is slowly being eaten by the black hole. Even though black holes are all-eating by nature, they are still confined by the laws of the universe. Specifically, the conservation of angular momentum (conservation of “rotation”) prevents any matter from falling into the black hole in one go.

Instead, the surrounding gas orbits the black hole. Each atom slowly loses momentum as it bumps into other gas particles (friction). The energy from the momentum loss is transformed into heat and when the temperature gets high enough, the disk starts to emit light. That light is what we see in the photo. As previously mentioned, atoms lose momentum due to friction in the swirling disk. This slowly transports the gas closer to the black hole’s horizon, and at some point the gas will be so close, that it falls into the black hole and is forever gone from our view. Once something enters a black hole, it never leaves.

The photo shows this shiny accretion disk beautifully and together with the shape and shadow of the black hole itself, I think it makes up for a spectacular astronomical and technical achievement.


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