Are supernovae actually possible or not

Suspicious weakness of light in Orion's shoulder : Are we facing a supernova in the cosmic neighborhood?

If you look up to the sky after dusk in the coming months, you could witness a spectacular spectacle: the explosion of the star "Betelgeuse" in the constellation Orion. Betelgeuse is currently playing a bit crazy, which could be a sign of its imminent end.

Scientists around the world are fascinated. "That would be a sensation, and for me personally, a childhood dream would come true," says Dieter Breitschwerdt, the director of the Center for Astronomy and Astrophysics at the TU Berlin.

During the past 1000 years, humans could only observe the death of such a star with the naked eye twice - in 1572 and 1604. But since the sky observers of that time could not see the two exploding stars beforehand with the means at their disposal, they interpreted that bright one Lights up as "Nova Stella", ie as a new star. That is why today the explosive death of a star is still referred to as a supernova.

What we see was a long time ago

Supernova researchers have a hard time. According to Breitschwerdt, it starts with the observation: “So far, no supernova precursor star has been observed shortly before its explosion.” A few stars explode every second in space. However, almost all of these stellar disasters occur in distant galaxies. That is why they are only discovered afterwards, and despite their brightness, at best with telescopes. With Betelgeuse, however, it could soon hit a star in our own Milky Way, which, according to cosmic standards, also shines right on our doorstep. It is "only" about 640 light years away from us.

Even the word “soon” has to be categorized cosmically: It can happen today - or in a hundred thousand years. And if Betelgeuse's supernova actually lit up in the night sky today, it would have happened 640 years ago. The news of the star explosion would have been traveling through space since the late Middle Ages to reach us today.

Astrophysicists have long included Betelgeuse on their list of stellar death row inmates. One of the clearest signs of its approaching end is its reddish color, which can already be seen with the naked eye. From it you can read the temperature of its outer gas layers, which emit this reddish light. They are only about 3500 degrees. This is much cooler than, for example, the surface of the sun, whose yellowish-white light shows temperatures of almost 6000 degrees.

A volume of a billion suns

Despite its comparatively cool and therefore dark surface, we see Betelgeuse as one of the brightest stars in the sky. From the abundance of light arriving at the earth and its distance, it can be calculated that it emits around 100,000 times more light than the sun. This great luminosity despite the cool surface only allows one conclusion: Betelgeuse is very large. The point of light in the sky is actually a huge star ball, with 20 times greater mass than the sun - but significantly lower density: our sun would find space in it around a billion times. Betelgeuse is thus probably one of the largest stars in the Milky Way, a prime example of a type that astronomers refer to as the “red supergiant”.

Like all stars, Betelgeuse is heated up by the radiation that is released when atomic nuclei fuse in its dense, hot center. And as in all stars, in Betelgeuse initially only hydrogen atomic nuclei fused to helium atomic nuclei.

In a massive star, however, this fusion reactor is running at full speed and consuming a lot of hydrogen. Even though Betelgeuse has not been lit for 10 million years, it already draws its energy from at least one additional source: helium atomic nuclei have long since merged in it. This creates carbon and oxygen.

The ignition of the helium fusion in Betelgeuse's hot belly increased the radiation pressure inside him. And that in turn inflated its outer layers of gas into the red supergiant we see today. Its current brief description is: hot core, cool, extremely voluminous shell, which nevertheless emits a lot of light overall.

Energy crisis when the cores no longer fuse

As energy replenishment for this high level of radiation, Betelgeuse will ignite new atomic nucleus fusions with increasing age, in which increasingly larger atomic nuclei are built up: sodium, neon, magnesium, oxygen, etc. However, nobody knows which of these fusion processes Betelgeuse has already arrived at is - or arrived 640 years ago.

But if the so-called “silicon burning” has already started there, the end is not far off. Iron will be produced from silicon in complex nuclear reactions. However, iron is no longer another fuel for a star's fusion reactor. As soon as Betelgeuse begins to fill up with iron, the energy production of his star power plant ceases. This also reduces the central gas and radiation pressure, which then have nothing to counter the weight of the star's shell. And then everything goes very quickly. The outer masses of gas begin to fall inward.

There they hit the dense core of iron, are thrown back and rush out again. The chaos of collapsing and thrown back gas masses transforms the star's gravitational collapse into an explosion of highly heated stellar matter. The hot gas masses that are diverging from one another will radiate as much light for weeks as many billions of normal stars put together. In the sky of the earth, Betelgeuse's supernova can be seen for weeks as a glistening, shining “star” that will light up the nights like a full moon.

Neutron star or black hole

However, the inner dense core of the former star will survive the explosion and be further compressed and condensed by its own gravity. Its final state will depend on the mass remaining to it. At less than about two solar masses, its collapse will end with atomic nuclei and electrons merging to form neutrons. The resulting “neutron star” will only be ten to twenty kilometers in size.

Its matter will be so tightly packed that even a heaped teaspoon of it would contain as much mass as in the Great Pyramid of Cheops. However, if the collapsing former center of Betelgeuse should contain more than two solar masses, its collapse will not end with an unimaginably dense neutron star. In this case there would be no holding back against its further collapse into a black hole.

But it's not that far yet. Betelgeuse is still alive and shining. A small message from the astronomer Edward Guinan from the University of Villanova in Pennsylvania (USA) in "Astronomer’s Telegram" on December 8, 2019 could soon turn out to be his anticipated obituary. She transformed the star old man into a global media star: Betelgeuse has become significantly darker. As measurements of its spectrum show, its surface temperature has also dropped by over 150 degrees.

Dark harbinger?

Even inexperienced observers of the sky will see it at first glance: the constellation Orion no longer looks like it has been in memory. Until recently, Betelgeuse, the left shoulder star from our point of view, was the seventh brightest of all stars in the sky. But now it doesn't even shine in the top twenty anymore. When asked, Edward Guinan stated that Betelgeuse's weakness of light not only continued, but that its brightness had even decreased further. But that the decrease in brightness heralds its supernova is anything but certain.

The majority of astronomers even disagree, Breitschwerdt for example: "I think that the most plausible explanation for the decrease in brightness is an instability in energy generation, which is quite common with supergiants." As a result, Betelgeuse pulsates and thereby periodically changes its brightness. Edward Guinan also tends to this interpretation, but also points to another possible explanation: "Perhaps the unstable star has thrown away a larger gas cloud that has cooled and now temporarily weakens the starlight."

In fact, astronomers have been observing more or less regular fluctuations in the brightness of Betelgeuse for a long time. But it has never been as dark as it is now. In any case, Guinan also says that he currently “often looks up to the sky to make sure that a much brighter star is not shining in his place”.

The supernova as the (original) origin of life

It would be death, but also obstetrics. Because, according to Guinan, supernovae are of great importance for the cosmos and its history: "New stars and new planets are born from the gas that is ejected into space during a supernova."

The explosion cloud of every supernova contains all the chemical richness of the substances that the star had previously built up in its hot belly during its nuclear mergers. In addition, most of the other missing atomic nuclei, which the star could not build, but which today also belong to the periodic table of the elements, are formed in the heat of the explosion of a supernova. It was only through countless star deaths like this that a boring hydrogen universe gradually became a chemical paradise during its 13.7 billion year history. Only in him could life come into being - at least once here on earth, perhaps elsewhere too.

If at some point in the coming weeks, months or years we actually do experience the Betelgeuse supernova, we should not just marvel at the celestial spectacle. We should also remember that we are stardust ourselves and that the stuff of stars in our brains has become aware of where it comes from.

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