How do astronomers detect black holes if they cannot be observed with any kind of instrument?

One of the signs that a black hole exists in the universe is dictated by the x-rays that it emits outside of its reach, although from within its location, black holes usually don’t emit anything, especially not light. The detection of x-rays in space is how Cygnus-X1 (see the graphic below), the first black hole discovered by humans, was “seen” by the instruments installed inside a suborbital Aerobe rocket.

However, you should keep in mind that x-ray satellites weren’t invented in the time of Cygnus-X1’s discovery, nor were there space observatories for experts to take a closer look at the black hole. The exact location of the source for the emitted x-rays could not be determined, and after more than five years, astronomers have detected a visible star whose motions were being influenced by an invisible companion nearby. Along with the star’s varying motion or movement, the intensity of the x-rays near the heavenly body showed rapid fluctuations, which basically meant that the invisible companion is extremely large and powerful, with having a calculated or theorized mass that is at least ten times that of the Sun and other smaller stars in the universe.

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Since the object that is shown to be exerting a surprisingly tremendous gravitational force on the nearby heavenly body is not itself a star, and since its mass is well above the size of a neutron star, which is about 3.2 solar masses, experts say that it must be a black hole. This theory was created in accordance with the General Theory of Relativity that predicts the existence of black holes in space.

What is Cygnus-X1?

We know that the Cygnus-X1 is a black hole, but we don’t exactly know the details surrounding this mysterious black hole. Cygnus-X1 is widely accepted by many scientists to be a black hole, including Kip Thorne and later, Stephen Hawking, after losing the debate to Thorne while claiming that the x-ray source is not a black hole. It got its name from the Cygnus constellation, where it is located, and the “X-1” is supposed to represent its x-ray source.

It was discovered in 1964, specifically during a sub-orbital space flight that can detect x-rays from a distance. Although it is more than 50 years old, the discovered x-ray is currently one of the strongest x-ray sources captured on Earth. Because of the mystery surrounding the x-ray source, Cygnus-X1 is still the most studied object in space until today. Because black holes cannot be easily seen using advanced telescopes, scientists often rely on x-ray sources to identify and locate black holes. Because of Cygnus-X1’s discovery, astronomers now have another way to detect black holes in space.

Research suggests that Cygnus-X1 is approximately five million years old, and it is theorized to have been formed from a progenitor star that has 40 million solar masses or more. That specific star is supposed to explode as a supernova, but because the heavenly bodies surrounding it or near to it are still intact, the star may have just imploded or collapsed in itself and created a black hole.

In terms of what is currently going on in the case of Cygnus-X1, the visible blue supergiant variable star near the black hole, designated as HDE 226868, is continuously losing portions of its gaseous mass to the Cyg-X1. This is evident by the detection of a vast stream of hot gas that does not flow directly into the black hole. The stream instead spirals around the black hole and gradually flows its way inward and towards the black hole’s boundary of no return called event horizon. It is the star’s superhot gas that emits flicking that x-rays that signal as its last cry or groan, and it is also this radiation that indicated and showed the presence of Cygnus-X1 black hole on the sub-orbital spaceship in 1964.

Because scientists have found out that they will be able to detect other black holes using x-ray sources, many such cosmic irregularities have been detected in space since then. In fact, there are enough reasons to believe that all galaxies may have a supermassive black hole at their center, although most black holes are still not detected until today. Thankfully, there are still no black holes detected in the Milky Way galaxy that can affect our solar system.

Additional reading:

Black hole (Wikipedia)
Cygnus X-1 (Wikipedia)

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