By Karín Menéndez-Delmestre
To observe black holes, you need to be aware of what's going on around them
Has the reader ever wondered how we observed black holes? If, as is supposed, not even light can escape its gravitational force, then how do we build a real image of these objects? To see black holes, we need to keep an eye on what happens around them.
Let's start with the basics. There are so-called primordial black holes (those that formed shortly after the Big Bang), but most of them correspond to the "corpse" of a very massive star – at least about thirty times the mass of the Sun – that collapses on itself. The star implodes and generates a very high density in a very compact region, forming a gigantic gravitational field – this is the black hole.
If on Earth only particles with a speed that exceeds 11.2 km / s are able to leave the planet, in a black hole the force of gravity is so great that even light (with a speed of 300 thousand km / s) cannot escape. Due to this great attraction force, the gas particles that approach the black hole (those that go beyond the so-called “event horizon”) are “swallowed”. The reader must remember the impact on the media that had the announcement from the first image of a black hole in 2019 – it was precisely the event horizon, that bright edge, that the power of several ground-based telescopes together managed to capture.
The light that forms the captured images of black holes is the light emitted by the gas that orbits in the vicinity of these holes. This gas, heated by friction, forms a thin disc around the black hole, reaching high temperatures and shining at different energies, from the coldest in the infrared, through the ultraviolet and optical, to the hottest, in X-rays. . And voilà: this is the potential for images of black holes, depending on the telescope used.
With a mass equivalent to hundreds of thousands, up to tens of billions of solar masses, supermassive black holes have the ability to significantly alter the orbits of nearby stars. Therefore, even if they are not actively “swallowing” gas, we can detect its presence by observing its effect on nearby objects. That is how we learned that our Milky Way is home to a supermassive black hole right in its center. Although it is dormant, that is, it is not “swallowing” large amounts of gas, it was possible to detect it by monitoring its impact on the orbits of neighboring stars, over more than a decade, with the largest telescopes in the world (in Hawaii and in Chile).
Researchers (including myself) are committed to identifying and studying supermassive black holes in other galaxies, as we now know that every massive galaxy is home to a black hole in its bowels. Looking at distant galaxies, we can see how they are the scene of a complex dance between the growth of the star mass and the growth of a supermassive black hole. Outbreaks of star formation alternate with periods when strong winds generated by the supermassive black hole momentarily "shut down" star formation. If we want to understand how our home, the Milky Way, was formed, we need to understand the details of this dance.
The images of black holes allow us to confirm many of the hypotheses about the nature of these objects, but, beyond the event horizon, we can only rely on theoretical models. To truly understand what happens in a black hole, we need to bring observations and theory together – the progress of knowledge depends on both the one and the other. In 2020, we look forward to the launch of the James Webb space telescope. With a 6.5m folding mirror, it will be the largest telescope ever launched into space and will allow us to discover thousands of supermassive black holes. I am already preparing my proposal to obtain data.
Karín Menéndez-Delmestre is an astronomer, professor at the Valongo Observatory at the Federal University of Rio de Janeiro
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