Last year, researchers first "heard" black holes by detecting gravitational waves from two such objects colliding. Now they want to see the black hole with their own eyes, or at least its silhouette.
Next month, astronomers are set to use radio telescopes around the globe to create the equivalent of a single "planetary" instrument that will allow them to capture images of black holes by illuminating the huge cloud of gas and stellar matter that orbits them. Their target is a supermassive black hole in the center of the Milky Way known as Sagittarius A * (Sgr A *), as well as an even more massive object in the nearby galaxy M87.
Earlier observations using the Event Horizon Telescope (EHT) gave some very intriguing results, but faceless spots remained in the place where the black holes should have been in the photo. For the first time this year, EHT will receive support from laboratories in Chile and Antarctica, and this additional power will help it improve image resolution. Astronomers hope to see black holes collect the floating gas around them into dense structures and spew long streams of stellar matter. They also hope to map out the focus and shape of the event horizon and test whether Albert Einstein's general theory of relativity works under such extreme conditions.
The EHT will only be able to capture the target once a year, provided the weather is good and the position is in which both black holes are clearly visible in observatories around the globe. This year, the team will observe the sky for 5 nights from a 10-day working "window" from April 5-14. Data intensive work will then begin, which may take a year or more, depending on the results of the surveys. EHT director Shen Dolman of the MIT Observatory in Westford jokes that this is "a pleasure put on hold and rewarded squared."
Visualizing black holes is challenging, not only because their intense gravity captures even photons of light. The main problem is that these objects are surprisingly small: Sgr A * has a mass of four million Suns (!), But its event horizon is only 24 million kilometers across, which is only 17 times wider than the Sun. To see something so small (by cosmic standards) at a distance of 26,000 light-years from us, it takes a telescope of truly global power.
In the optical wavelength range, the black hole is hidden from us by a veil of dust and gas that darkens the heart of the galaxy. Radio waves will pass through it much more easily, but even they are still hampered by ionized gas clouds. The best telescopes that are sensitive to the shortest (millimeter length) radio waves have only been developed in the past few decades. In the early 2010s, Dolman and others at EHT began testing the idea with such equipment in Hawaii, California, and Arizona. They later expanded the array to include the famous Large Millimeter Telescope from Mexico. The result was an acceptable image of a black hole from M87, but scientists still have not been able to understand exactly how black holes twist and heat gas clouds.
However, in order to see the event horizon itself, the EHT must become even more powerful. Over the years, it has grown from a poorly funded adventure to a project of international importance, supported by 30 major scientific institutions in 12 countries. Next month, the Italian Atacama Large Millimeter / submillimeter Array (ALMA) telescope in Chile will be connected to it, which will increase the EHT sensitivity by several orders of magnitude.
You can learn more about the strategy and plans of astronomers in a review article on the Science Journal portal.
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