What is a star explosion

Star explosions as element suppliers of the universe

Massive stars have already reached the end of their development after tens of millions of years and, in a huge explosion, throw almost all of their matter out into space - material for new celestial bodies.

If a star has about ten times as much mass as the sun, it first burns - like all other stars - hydrogen to helium inside. When the hydrogen is used up, helium is burned to carbon, and finally carbon to oxygen. So it goes on via silicon and magnesium to the element iron. “These burning phases, which take place at the end of the star's life, take place in a few days or even just hours. The silicon burning itself takes less than a day and then forms the iron core inside the star. This iron core cannot become arbitrarily large - it becomes unstable as soon as its mass has reached a certain limit mass and collapses to form a neutron star, ”explains Hans-Thomas Janka from the Max Planck Institute for Astrophysics in Garching.

The Crab Nebula

A neutron star has more mass than the sun, but is only about twenty kilometers in size - so the matter in it is extremely densely packed. Only the iron core becomes a neutron star, the remaining ninety percent of the star's mass flies outwards in a huge explosion, a supernova. Not only are the substances created in the star distributed in space, from carbon to iron: in the dramatic moments of the explosion, heavy elements such as gold, silver, titanium, uranium and so on are also formed at extremely high energies. All chemical elements that are heavier than iron can only be formed directly in such explosions.

Remains of such catastrophes are so important to the astronomers of the US X-ray satellite Chandra that they celebrated the “first light” of the satellite with the X-ray source Cassiopeia A, some 11,000 light years away. This supernova should have been observed on Earth around 320 years ago - but apparently nobody noticed the star explosion at the end of the 17th century, because there are no records. The explosion presumably took place behind a thick haze of dust, so that not much light reached the earth.

Supernova remnant Cassiopeia A

With the help of the Chandra satellite, scientists documented the finely structured gas envelope around the supernova remnant, which is around ten light years in diameter. The chemical composition of the cloud could be derived from the recorded X-ray spectra. The outer areas of the nebula are surprisingly rich in iron. Surprising because iron must have formed deep inside a star. On the other hand, the astronomers found silicon-rich gas, which is produced in the upper layers of the star, still near the central star. Apparently the iron shot out of the star very quickly during the explosion and overtook the original outer shell of the star.

The explosion threw huge amounts of matter into the surrounding space. While the hot gas initially flows into space at a few thousand kilometers per second, in a few hundred thousand years it will be sufficiently cooled to become part of new stars and planets. If there hadn't been countless supernovae before the formation of the domestic solar system four and a half billion years ago, our life would be unthinkable.

Despite their important role in the universe, astronomers still do not know exactly how the explosions happen. Because by the time you see the bright supernova, the actual explosion is over. The researchers only have to look at the glowing remains, regrets Hans-Thomas Janka: “In order to draw conclusions from this information about the processes that took place during the explosion and possibly also in the last few seconds before the explosion, we need theoretical models . So with these theoretical models we have to calculate back, so to speak, from the observed properties of the explosion to what happened in the deepest interior of the exploded star. "

Supernova 1987A

In the interior of the star, neutrinos seem to play the dominant role. They contain 99 percent of the energy released when the star collapses. Only one percent goes into the explosion, of which only one percent goes into visible light. So astronomers only see the tip of the iceberg - and are still puzzling today what exactly is driving the explosion.

Neutrinos could provide an answer because they really carry information from the center of the explosion. However, the elementary particles can only be detected with a great deal of effort, since they pass through matter practically undisturbed. Gravitational waves - tiny perturbations in space and time which, according to theory, occur when a neutron star is formed - could also provide the researchers with important information. So far, however, they have never been directly detected.

So the supernova researchers are in a tight spot: The light can be seen almost through space, but contains hardly any information about the star explosion itself. Neutrinos and gravitational waves come from the center of the explosion, but are with the distant objects which the astronomers have to do every night to no longer prove on earth. Hans-Thomas Janka and his colleagues all have a dream: a supernova in the middle of our Milky Way.