In the center of our galaxy, astronomers say there is a monster black hole. They call it Sagittarius A* (said aloud: Sagittarius A “star”), and it has a mass some 4 million times greater than our sun. All that mass is compressed into a single point, a singularity, whose gravity is so strong, stars orbit around it.
Surrounding the black hole is what’s called the “event horizon,” which is the boundary beyond which nothing can escape the black hole’s gravity, not even light. We’ve never directly seen a black hole or its event horizon.
Tomorrow, that might change.
At 9 am Eastern on Wednesday, April 10, an international collaboration of scientists called the Event Horizon Telescope is releasing the results of an attempt to photograph Sagittarius A*.
You can watch them reveal the possible image of this black hole and the center of the galaxy right here in this live stream:
What will they reveal? We really don’t know.
Usually, before a big scientific announcement, journalists are given a look at the findings under the condition they’re not allowed to publish anything about them before an embargo date. But that didn’t happen for this announcement.
But if the scientists have been successful, if they really have produced some images, here’s what we may see: A dark circle representing the black hole, its event horizon, and a larger dark region where light can conceivably escape from but is unlikely to. And all of that could be surrounded by a bright halo of material being mashed to smithereens by the intense gravity. It’ll be rad. (It’s also possible the image will be blurry, imperfect, or something unexpected.) It’s possible they will release images of the black hole at the center of the Messier 87 galaxy, too.
How to take a photograph of a black hole
There’s a reason why we’ve never seen a picture of the black hole at the center of our galaxy: despite its huge mass, Sagittarius A* is actually very small and is surrounded by bright gas and swirling material.
Taking a picture of the shadow cast by Sagittarius A*’s event horizon is like “taking a picture of a DVD on the surface of the moon” from the surface of the Earth, Dimitrios Psaltis, an astrophysicist at the University of Arizona and one of the lead scientists on the effort, once told me.
In science speak, the shadow cast by Sagittarius A* is expected to be around 50 microarcseconds wide when viewed from the Earth. An arcsecond is 1/3600 of a degree. And there are 1 million microarcseconds in an arcsecond. Again: The shadow cast by Sagittarius A* is tiny. Compared to the full moon, the shadow cast by Sagittarius A* is 37.2 million times smaller.
To take a picture of something that small, you need a huge telescope, one the size of the Earth.
In April 2017, eight radio telescopes located in Antarctica, Greenland, South America, North America, Hawaii, and Europe all pointed their dishes to the black hole in the center of our galaxy. They were looking out for the narrow band of radiation that’s expected to be emitted from the bright ring of material around the black hole. And in the middle of the bright ring, they hoped to see the silhouette of the black hole itself.
Each telescope ultimately captured an enormous amount of data that needed to be combined to reveal the image of the center of the galaxy. As the National Science Foundation explains, these eight telescopes were turned into a virtual giant parabolic dish.
Think about a simple mirror telescope. In it, the curved surface of a mirror reflects light back to a central point, where an image is brought into focus. The Event Horizon Telescope does a similar thing. “When the EHT sites are synchronized, their recordings can later be perfectly aligned in the same way that the mirror aligns the optical light,” the National Science Foundation explains in a video.
But the task of this synchronization is really hard. Part of the reason why this announcement has been two years in the making is because the data files were too big to be transferred by digital means. The hard drives had to be flown from the observatories to get processed. And the dataset in Antarctica was inaccessible for months due to harsh winter conditions.
Another reason is that the scientists need to account for the rotation of the Earth. The Earth rotates, meaning the individual observatories making up the Event Horizon Telescope are moving too, introducing a type of blur into the data. It took extremely precise atomic clocks at each of the observatory sites to ensure all the data would line up and the resulting image would be clear.