Sagittarius A* and the Event Horizon Telescope

FAS Astronomers Blog, Volume 30, Number 7.

Some 26,000 light years away, between the constellations Sagittarius and Scorpius, is the center of the Milky Way galaxy. Our galactic center is extremely dense with stars packed in much tighter than here in the galactic outskirts. The editors of EarthSky note that it would outshine a full moon if dust didn’t obscure our view of it. Lurking there is a strange object known as a supermassive black hole. You can find more about black holes in a previous article.

Back in 1974, Bruce Balick and Robert L. Brown first identified that a bright radio source at the Milky Way center was in fact a black hole. In 1982, Brown gave it the name Sagittarius A* (“Sagittarius A star” or “Sag A star”) because of its location near the radio object Sagittarius A. Today astronomers think there are supermassive black holes at the center of most galaxies.

Because of the dust obscuring our view of the galactic center, we can’t see this black hole directly. Until just recently, the best we could do was to observe the motion of stars at the center of the Milky Way. Over a twenty five year period, Reinhard Genzel (Max Plank Institute for Extraterrestrial Physics) and Andrea Ghez (UCLA Galactic Center Group) measured the rapid movement of these stars and demonstrated that the Milky Way center is home to a supermassive black hole around four million times the mass of the Sun. In 2020, Genzel and Ghez shared half of the Nobel Prize in Physics for their discovery.

Even if the galactic center wasn’t obscured by dust, we still couldn’t “see” this black hole directly. The idea is that a black hole is black because light falls into it and its gravity is too intense for light to escape. However, we could, in theory, go beyond what Genzel and Ghez did and see the material circling around a black hole just as it’s about to fall through the event horizon and disappear forever. Until recently no one had done this.

Then along came the Event Horizon Telescope (EHT). This telescope is different than both the standard backyard telescopes most of us are familiar with and the telescopes NASA has sent into space to capture light across the electromagnetic spectrum (see the Great Observatories). The EHT captures radio emissions. Radio waves pass through the dust between us and the galactic center giving us a view that isn’t possible with other telescopes.

In addition, the black holes targeted by the EHT are extremely small and at a great distance. So, a single radio telescope such as Arecibo or Green Bank just isn’t large enough. The answer is to create a virtual telescope from several single radio telescopes or groups of telescopes using something called “Very Long Baseline Interferometry.” Without going into the details, the idea is that all the telescopes take images at exactly the same time. These images are then combined and processed in a way that results in a single image as if the virtual telescope has a dish the size of the Earth.

The final images are also modified so that the radio waves appear as if they were visible to the naked eye. This is all quite complicated, and it takes years to produce one final image. Sky & Telescope has a short video showing how this works. The folks at the National Radio Astronomy Observatory (NRAO) have an infographic with additional information.

The EHT took the first “pictures” of two black holes utilizing eight participating observatories in April 2017. These images were not a couple of quick snap shots. The telescope captured radio waves over a period of days collecting several thousand trillion bytes of information.

In April 2019, after two years of processing, the final EHT image of the black hole at the center of galaxy M87 (M87*) was unveiled. The image showed the bright event horizon around the black hole and the shadow of the black hole in the center. The EHT results were published in a series of papers in the April 2019 issue of The Astrophysical Journal Letters.

Sagittarius A* (Sag A*) was the next. After five years of processing, the EHT collaboration produced the first image of Sagittarius A* in May of this year (2022). It showed a similar bright event horizon and shadow as seen with M87*. The process took longer because of the more rapid rotation of material around Sag A* as compared to M87*. The EHT results were published in another series of papers in the May 2022 issue of Astrophysical Journal Letters.

Note that M87* is around 1,500 times larger than Sag A*, but it is some 2,000 times more distant. Therefore, the two black holes have approximately the same apparent size. Also, keep watching, the folks running the EHT collaboration are now looking to put together a video. Although it could take a few years before it’s published.

In March 2024, the Event Horizon Telescope published updated images of M87* and Sagittarius A*, this time in polarized light showing magnetic fields surrounding the black holes.