On isolated mountaintops across the planet, scientists await word that tonight is the night. The complex coordination between dozens of telescopes on the ground and in space is complete, the weather is clear, tech issues have been addressed—the metaphorical stars are aligned. It is time to look at the supermassive black hole at the heart of our Milky Way galaxy.
This “scheduling Sudoku,” as the astronomers call it, happens each day of an observing campaign by the Event Horizon Telescope (EHT) collaboration, and they will soon have a new player to factor in; NASA’s James Webb Space Telescope will be joining the effort. During Webb’s first slate of observations, astronomers will use its infrared imaging power to address some of the unique and persistent challenges presented by the Milky Way’s black hole, named Sagittarius A* (Sgr A*; the asterisk is pronounced as “star”).
In 2017, EHT used the combined imaging power of eight radio telescope facilities across the planet to capture the historic first view of the region immediately surrounding a supermassive black hole, in the galaxy M87. Sgr A* is closer but dimmer than M87’s black hole, and unique flickering flares in the material surrounding it alter the pattern of light on an hourly basis, presenting challenges for astronomers.
“Our galaxy’s supermassive black hole is the only one known to have this kind of flaring, and while that has made capturing an image of the region very difficult, it also makes Sagittarius A* even more scientifically interesting,” said astronomer Farhad Yusef-Zadeh, a professor at Northwestern University and principal investigator on the Webb program to observe Sgr A*.
The flares are due to the temporary but intense acceleration of particles around the black hole to much higher energies, with corresponding light emission. A huge advantage to observing Sgr A* with Webb is the capability of capturing data in two infrared wavelengths (F210M and F480M) simultaneously and continuously, from the telescope’s location beyond the Moon. Webb will have an uninterrupted view, observing cycles of flaring and calm that the EHT team can use for reference with their own data, resulting in a cleaner image.
The source or mechanism that causes Sgr A*’s flares is highly debated. Answers as to how Sgr A*’s flares begin, peak, and dissipate could have far-reaching implications for the future study of black holes, as well as particle and plasma physics, and even flares from the Sun.
Black holes, predicted by Albert Einstein as part of his general theory of relativity, are in a sense the opposite of what their name implies—rather than an empty hole in space, black holes are the most dense, tightly packed regions of matter known. A black hole’s gravitational field is so strong that it warps the fabric of space around itself, and any material that gets too close is bound there forever, along with any light the material emits. This is why black holes appear “black.” Any light detected by telescopes is not actually from the black hole itself, but the area surrounding it. Scientists call the ultimate inner edge of that light the event horizon, which is where the EHT collaboration gets its name.
The EHT image of M87 was the first direct visual proof that Einstein’s black hole prediction was correct. Black holes continue to be a proving ground for Einstein’s theory, and scientists hope carefully scheduled multi-wavelength observations of Sgr A* by EHT, Webb, X-ray, and other observatories will narrow the margin of error on general relativity calculations, or perhaps point to new realms of physics we don’t currently understand.