How compressed space

Science in dialogue

What is the temperature in space, what is it in the sun? And how does the big temperature difference come about?

The temperature differences in space are extremely large. They range from near absolute zero to several billion Kelvin inside very massive stars or near black holes. The different temperatures are due to the different physical processes that take place in the cosmos and during which heat is either generated or can be lost again.

Temperature as a physical term describes the state of motion of atoms and molecules. The faster the particles in a body move, the higher its temperature. This is a disordered, i.e. H. statistically random movement. The lowest possible temperature would be reached when the particles are at rest. This so-called absolute zero point (zero Kelvin) is minus 273.15 degrees Celsius. According to the third law of thermodynamics, it can never be reached exactly. Because a law of quantum physics (Heisenberg's uncertainty principle) says that it is fundamentally impossible to exactly measure the location and the speed of a particle at the same time. But that is precisely what would be possible if a particle no longer had any kinetic energy at all.

Now the universe is not a homogeneous body, but a strongly structured entity, with galaxies, stars, planets, moons and the space in between. The different bodies in space have different chemical compositions and different levels of mass density. In some places there is a vacuum with only about one particle per square centimeter (or less) or mass densities of 10 to 27 kilograms per cubic meter. Massive stars or even neutron stars have densities of 10 to the power of 17 kilograms per cubic meter.

Temperatures in interstellar space vary widely: they range from near absolute zero in molecular clouds (i.e. in areas where stars are formed) to several hundred million Kelvin in remnants of supernova explosions. The temperatures essentially depend on the physical processes that are responsible for heating and cooling. Very efficient heating occurs in the form of shock waves in supersonic currents (cf. the heating of the shuttle when it re-enters the atmosphere), which are very common in interstellar space, while cooling is usually achieved through radiation from atoms and molecules.

In stars, e.g. B. in the sun, high temperatures arise mostly due to nuclear fusion processes and gigantic gravitational forces in older, already developed objects. Temperatures of up to several million Kelvin prevail in the sun: about 5,700 Kelvin on the surface. In the corona - this is where the diffuse light comes from. B. sees a total solar eclipse - the temperature is around 2 to 5 million Kelvin. Inside the sun - where energy is generated by nuclear fusion - there are temperatures of around 15 million Kelvin.

These high temperatures inside the sun, but also in other stars, are a result of gravitation. In a gas ball (nothing else is a star) the particles are compressed by their own gravity until the pressure inside can withstand the gravitational pressure.

This also happens in our sun, which consists mainly of hydrogen and helium. Due to the large mass - and the associated high force of gravity - the particles inside the sun are increasingly compressed. The temperature rises and the hydrogen atoms have enough energy to fuse to form helium atoms. Since these are lighter than hydrogen atoms, the “missing” mass is converted into huge amounts of thermal energy according to the formula E = mc² found by Albert Einstein. The result is an increase in temperature.

The different temperatures that one encounters in the cosmos therefore reflect the different physical processes in which either heat is generated or can be lost again, e.g. B. Cooling by radiation. But even in the places in space that are far away from warmth-giving stars, it cannot be colder than about three Kelvin. This is how “warm” the so-called background radiation is, which is considered a remnant of the Big Bang and permeates the entire universe.

Prof. Dr. Dieter Breitschwerdt, Director of the Center for Astronomy and Astrophysics at the TU Berlin.