Could Saitama escape from a black hole

#Cygnus X-1 - Astrodicticum Simplex

This is the transcription of an episode of my Star Stories podcast. The episode is also available as an MP3 download and YouTube video. You can also find the whole podcast at Spotify.

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Star Stories Episode 406: Cygnus X-1

The constellation Swan can be observed very well in the northern hemisphere, especially in summer. With a little imagination, you can actually see a flying swan in the cross-shaped arrangement of the brightest stars in this region of the sky. Its tail is formed by Deneb, one of the brightest stars in all of the sky; the also hard to miss star Albireo forms the neck of the bird. Very close to the heavenly gooseneck is the object that we are talking about today and of which you cannot see anything with the naked eye. Until 1964, nothing was known of its existence. It was only seen when we were able to shoot X-ray eyes into the sky and a lot more time passed before we understood what it was all about. But then it turned out to be extremely extraordinary and answered a long open question in the history of astronomy in a rather revolutionary way.

In 1964, two rockets flew into the sky from New Mexico. Not into space, only once to the limit of the atmosphere and then back again. On board were instruments that were able to measure X-rays. They are not only found in hospitals but also everywhere else in the universe. There are plenty of cosmic phenomena that are capable of generating high-energy X-ray light. But we cannot see it from the ground because it is blocked by the earth's atmosphere. During this first search for objects in space that produce X-rays, eight were found. One of these sources was in the constellation Swan and was named Cygnus X-1 “Cygnus” because the constellation is officially called Swan in Latin. “X” because X-rays come from there, so “X-Rays” in English. And “1” because it was the first known object of its kind in this constellation.

Cygnus X-1 was extremely bright in the X-ray light. With normal telescopes, however, nothing could be seen in the sky. Neither with radio telescopes. At least not then. But what was discovered with these first attempts: It is worthwhile to look at the sky in X-rays. In 1970 NASA therefore sent “Uhuru” into space, a satellite that had searched the entire sky for X-ray sources for more than two years. He found almost 300 objects and also researched Cygnus X-1 in more detail. With the new and better data, it was also found that the Cygnus X-1 is flickering a little. The intensity of the X-rays changes a little, several times per second. That was interesting. Because that means that the processes that take place there take place in a relatively small space. The larger an object, the slower it can change. Put simply, it takes time for information to get from one end of the object to the other. Nothing can move faster than light. In order to generate bright X-rays, some high-energy processes must take place in the Cygnus X-1. However, they cannot change over such a large area as quickly as desired, in order to change the intensity of the radiation several times per second. That only works if the whole thing is no bigger than 100,000 kilometers. Which is quite large - but, for example, significantly smaller than a star like our sun, which is 1.4 million kilometers in diameter.

Cygnus X-1 can definitely not be a normal star. But what else it could be was unknown. Mainly because the position could not be measured as precisely either, which was not possible with the X-ray telescopes. In 1971, however, it was still possible to measure radio emissions that seemed to come precisely from the direction of Cygnus X-1. With the data from the radio telescope, the position could be determined more precisely. And exactly where they came from was a star called HDE 226868. A giant star, a so-called blue-white supergiant that is about 30 times larger than our sun, about 30 times as much mass as it and a temperature of more than 30,000 degrees. It shines 200,000 times brighter than the sun - but it doesn't glow in X-rays. Such a giant star is impressive - but not able to generate the intense X-ray radiation that comes from Cygnus X-1.

The breakthrough came in 1971. Astronomer Louise Webster and her colleague Paul Murdin of the Royal Greenwich Observatory in England, and independently the Canadian astronomer Tom Bolton, both made the same discovery. The star HDE 226868 wobbles. It wobbles very quickly. It comes a little closer to the earth, then it moves away again a little, then it comes again, etc. It wobbles back and forth with a period of only 5.6 days. Which is interesting in itself, but becomes even more interesting when you take a closer look at the radiation from Cygnus X-1. This not only shows changes within seconds, but also changes its brightness in the course of around five and a half days. Exactly in the same rhythm as the star itself. That only allowed one conclusion: the star is not alone, but orbits something. At a very short distance, so that it only takes him five and a half days to complete one round. That something is Cygnus X-1, the source of X-rays. And because you now knew how long the star needs for one round, you could also calculate the strength of the gravitational force that Cygnus X-1 exerts on the star. Or the mass that Cygnus X-1 must have: Almost 16 times as much mass as our sun.

And now it's getting exciting. On the one hand, you now knew that you had something with a lot of mass. On the other hand, it has to take up a very small space. It can't be a star, that was clear beforehand. So it has to be something that is smaller than a star but has more mass than a typical star. One candidate for this are neutron stars, which I have often spoken of in star stories. These are the extremely compact remains that are left over when a large star at the end of its life can no longer do nuclear fusion and collapses. Neutron stars are definitely small, only a few tens of kilometers at most. But they cannot have more than at most three times the mass of the sun. There are not many options left: the only thing that Cygnus X-1 can still be is a black hole!

That such a thing could exist, at least in theory, had been clear for a long time in the 1970s. Albert Einstein's general theory of relativity predicts that mass can be compressed so much that in the end nothing and no light can permanently escape from its vicinity. At the time, nobody knew whether such extreme objects could also exist in reality. Cygnus X-1 was the first concrete object that had a realistic chance of being a real black hole. Two scientists who worked particularly intensively on black holes at the time even made a bet about it. Kip Thorne and Stephen Hawking: In 1974, Hawking bet that Cygnus X-1 is not a black hole; Thorne bet against it. Hawking was actually convinced that there were black holes, but if he was wrong, he wanted to at least win the bet as a consolation, as he has often said.

Over time, the evidence has become increasingly clear that Cygnus X-1 is indeed a black hole. But let's look again first, where this X-ray radiation actually comes from. To do this we have to go to the giant star HDE 226868 again. It is really big. It has gotten enormously hot over time; an enormous amount of energy and radiation is produced inside it. It pushes outwards, causing the star to inflate. At some point it has become so big that it can no longer hold on to the outer layers of its matter due to its gravitational force. And what's next door? The black hole. The matter repelled by the star can be captured by the black hole, then swirls around the hole extremely quickly in a disk and is heated to a few million degrees in the process. Gas that hot does a lot of interesting things, but among other things it emits very bright X-rays. So it is not the hole itself that shines, but its surroundings.

This was viewed in detail in 2001 with the Hubble space telescope. And saw no X-rays, which Hubble can't. But ultraviolet radiation. Which is also generated by the hot gas. An analysis of this light has shown that “chunks” are repeatedly released from the hot disk full of gas. They move closer and closer to the hole on a spiral path, getting faster and darker. Until the light suddenly disappears at some point. As I said: You didn't see it DIRECTLY - but if you analyze the light precisely enough, you can find out how something has to move there in order to produce the exact changes in brightness observed. And Hubble saw exactly what you should see when gas repeatedly disappears from the disk in the black hole.

Today we know with a probability bordering on certainty that Cygnus X-1 is a black hole. At some point it was a big star. Really big, with more than 40 times the mass of the sun and part of a binary star system with the also not small HDE 226868. A few million years ago the star ran out of fuel for nuclear fusion. It collapsed and due to its high mass with such force that a black hole was created. The companion star was more long-lived and is still there today. The black hole is hardly bigger than an asteroid, almost 50 kilometers in size. Or, to be precise, this is the diameter of the event horizon, i.e. the limit beyond which the gravitational force becomes so strong that no more light can escape. The disk of gas around the hole has a diameter of some 10,000 kilometers. At a distance of 14 million kilometers there is the star HDE 226868. It keeps supplying the disk and Cygnus X-1 can continue to shine brightly in the X-ray light.

The distance of the whole system to the earth is only a little more than 6000 light years. What makes Cygnus X-1 one of the closest black holes to us. But of course there is no need to worry, this is more than enough distance for it to pose any danger to Earth. Today we discovered tons of other black holes; we even took a picture of a black hole. But Cygnus X-1 was the first one we found. In 1990 Stephen Hawking was also convinced and admitted to having lost the bet with Kip Thorne.

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