Like the white dwarf stars

Star evolution - the mass decides

How bright a star shines, how long it exists and what is left of it later, ultimately only depends on one parameter: its mass.

Stars consist of about three quarters of hydrogen, a quarter of helium and a very small proportion of heavier elements such as carbon, oxygen or gold. This also applies to our sun. Inside, hydrogen fuses to form helium at around 15 million degrees Celsius. Four hydrogen nuclei each form a helium nucleus - however, the end product, the helium nucleus, is slightly lighter than the starting material, the four hydrogen nuclei. This tiny difference in mass is converted directly into heat and light - according to Einstein's famous formula: \ (E = mc ^ 2 \) (energy is equal to mass times the speed of light squared).

The hydrogen burning

Every second, 600 million tons of hydrogen melt into 594 million tons of helium in the center of the sun. The sun converts six million tons of matter into pure energy every second and becomes six million tons lighter every second. Shouldn't she have used up her supply long ago? By no means, over the nearly five billion years of its existence, the sun has not even lost a thousandth of its mass.

Star field in the constellation Sagittarius

The sun's energy output may seem wasteful, but there is enough fuel: According to theoretical models, the sun is only just middle-aged. It will shine for another five billion years. Only then is most of the hydrogen inside used up. Once the “hydrogen burn” is over, the end of a star is not far away. It is true that helium fuses to form heavier elements, which in turn form even heavier elements. But these processes are no longer very effective - the star quickly runs out of fuel.

How long the hydrogen burning lasts depends on the mass. Because a star with twice the mass of the sun does not shine twice as brightly, but around eight times as brightly as the sun. The amount of energy emitted by a star increases approximately with the third power of the star's mass \ (M \), so the luminosity is proportional to \ (M ^ 3 \). How strong this relationship is depends in turn on the star's mass. A star with three times the solar mass is therefore almost 27 times (three times three times three) as bright as the sun.

The evolution of the stars depends strongly on the initial mass. Stars with more than five to eight solar masses (some stars have almost a hundred solar masses) explode as a supernova at the end of their life. In these gigantic explosions, the outer layers of the star are thrown into space, and the core collapses into a neutron star or black hole. There are also supernovae in stars with lower mass - but only in certain binary star systems.

Explosive ending

Stars the size of the Sun, on the other hand, burn quite unspectacularly - and for quite a long time. A star with ten times the mass of the sun shines around 4,000 times brighter than the sun, but its lifespan is relatively short: it is over after less than fifty million years. The sun, on the other hand, is a little over ten billion years old. A star with only a tenth of the solar mass shines with less than a thousandth of the sun's luminosity - and that for well over a hundred billion years. All stars in the universe with less than about three quarters of the solar mass must therefore still exist in any case, since their lifespan exceeds the world age assumed today of about 13.7 billion years.

From the weight class of the sun or just above it, a number of stars have already ended their core fire and filled space with beautiful nebulae. In about five billion years, our sun will also swell, far beyond the Earth's orbit. The surface cools down and the sun no longer shines yellow-white, but reddish - it has become a red giant. The remaining fuel remains inside burn faster and faster and the sun finally gets into an unstable phase, flickers and pulsates for a while. The radiation from within drives more and more matter from the thin outer layers into the planetary system and further into space. This so-called stellar wind causes the sun to lose almost half of its mass.

In the remainder of the sun, the energy source will eventually dry up, and without the radiation pressure from within, the star will shrink to a white dwarf. This small, very hot object is about the size of the earth, but contains a good half the mass of the sun. The white dwarf cools down over billions of years. For the first few thousand years, it still stimulates the previously released gas to glow: the sun will be a white dwarf with a beautiful fog for some time - and in the middle of it the earth, which will then, however, have long since become uninhabitable.

Lots of open questions

That’s the theory. But nobody knows how well the stellar evolution is really understood. The basic idea may be correct and the evolution of the stars is certainly better understood than the formation of the stars - but many, many details are still unclear:

A Hertzsprung-Russell diagram

  • What role does the rotation of a star play?
  • Some stars pulsate, which means that they regularly inflate and shrink a little. How does this phenomenon affect the further evolution of the star? Do stars have a pulsating phase several times in their life?
  • When and how does convection occur? During convection, bubbles of hot material rise, cool down and sink back into the star's interior - like in boiling water on the stove top. The energy is transported inside stars by radiation and convection. The exact details are still largely unclear.
  • Star wind causes further problems - because such wind blows parts of its outer layers out into space. So the star is losing mass.
  • Magnetic fields certainly play a major role in the formation and development of stars (the sun has a strong and very complex magnetic field, which can be easily recognized by the sunspots, among other things).
  • And how do all these phenomena depend on the mass of the stars?

As long as all these things are not understood, information about the age of the stars can only be made very imperfectly. The age of globular clusters is a frequently mentioned lower limit for the age of the universe - although it is often forgotten that the information on the age of globular clusters fluctuates between 11 and 15 billion years. If the formation and evolution of stars were better understood, the age of the stars could also be estimated much better.