The Event Horizon telescope has captured another image of a black hole. This time, the telescope has captured the massive black hole at the center of our own galaxy, the Milky Way.
The newly recorded black hole is called Sagittarius A*, or Sgr A*, and is located about 26,000 light-years from Earth. Astronomers from the European Southern Observatory and the US National Science Foundation estimate that this makes the black hole appear the same size from Earth as a donut on the moon. However, the mass of Sagittarius A* is about four million times that of the Sun.
Sagittarius A*.
Source: EHT Collaboration
Scientists have suspected for years that there is a huge black hole at the center of the Milky Way. However, black holes cannot be seen directly because they do not emit light. The Event Horizon telescope was therefore used again to capture the black hole. In 2019, the EHT also captured the black hole M87, which is located 55 million light-years from Earth.
The telescope does not actually capture the black hole itself. Instead, the image shows a sort of silhouette of the black hole, surrounded by an orange ‘ring’ of light. The black hole is surrounded by particles that are swallowed by the black hole. These particles are heated as they race toward the black hole, causing them to glow. In effect, the image shows a ‘shadow’ of Sgr A* surrounded by the light being swallowed by the black hole.
The location of Sgr A*. Source: EHT Collaboration
Capturing the black hole was not easy because of its distance from Earth. A single telescope would have to be the size of Earth to achieve this. Instead, scientists came up with an alternative. The Event Horizon telescope is actually made up of an array of radio telescopes spread across five different continents. These telescopes work together to observe the same object and thus function as one large telescope.
The telescope observed Sagittarius A* over several nights in April 2017, collecting data for ‘many hours at a time’. According to scientists, this is comparable to using a long exposure time on a camera. All images were then combined into an ‘average’ of all collected images.
This turned out to be more difficult than with the previously shown M87 black hole. Sgr A* is closer to Earth, but also smaller and less active. In addition, the ring of light encircling the black hole showed strange flaring, which allowed the appearance of Sagittarius A* to vary from minute to minute. “A bit like trying to get a clear picture of a puppy chasing its tail quickly,” writes EHT researcher Chi-kwan Chan.
A total of 300 researchers from 80 different institutes worked together to deploy the Event Horizon Telescope. The scientists worked for five years to combine and analyze the data from the telescope using supercomputers. That required a total of 100 million ‘CPU hours’. The astronomers shared six papers on their findings, which have been published in The Astrophysical Journal Letters.
Sagittarius A* compared to M87, which was already photographed in 2019. Source: EHT Collaboration