Cosmic minions burst the astronomical worldview
They are too small and cold to be stars, but too warm and massive for planets: brown dwarfs. Since their discovery in 1995, they have kept puzzling astronomers. For just in recent years, the researchers repeatedly came across "fast stars" that refute all common definitions and assumptions. The question: "What is a planet?" Has since been more open than ever.
Just over 15 years ago, everything seemed simple: stars were massive, hot, self-luminous celestial bodies. Planets, on the other hand, were the mere companions of these suns, smaller objects that passively reflect the starlight. And the dependencies also seemed to be clear: there were stars in a double pack or individually, but planets always revolved around a central star.
But then came the year 1995 and with it the discovery of the first brown dwarf, a mysterious object that was neither star nor planet. Its mass was not enough to get the hydrogen fusion - and thus the starlight - into its interior. The little thing just glowed darkly. Meanwhile, the astronomers know hundreds of such brown dwarfs, almost weekly new ones are discovered - and almost every find raises further questions.
Astronomers are encountering ever smaller, colder and lighter objects, making the demarcation of the brown dwarfs against planets almost impossible. And then there are the mini-systems: dust disks and planets that revolve around a planet-sized dwarf like a miniature version of the solar system.
Brown dwarfs and star formation
Whether a brightly shining star arises or "only" a brown dwarf, a "prevented" star, decides at birth.
Stars do not form individually in empty space, but in huge clouds of gas and dust. Starweights of this kind are found, for example, in the well-known Orion Nebula or in the dust columns of the Horsehead Nebula. Due to its own gravity, sometimes also by external pressure waves, areas of these clouds collapse and become hotter and denser. This collapse initially emits energy in the form of radiation. Compressed by their own mass, however, lumps of hydrogen gas and dust are formed, which are so dense that the radiation can no longer escape. The interior of these lumps heats up more and more, the atoms move closer and closer together.
Fusion: the star engine starts
Then it suddenly happens: The temperature inside the lump increases to more than five million Kelvin. In this infernal furnace of heat and pressure, even the strong repulsion between the atomic nuclei must be small: they begin to merge, releasing vast amounts of energy. The nuclear fusion is ignited - a star is born.
The fusion of hydrogen nuclei into helium forms the fuel that will keep the newly created celestial body glowing for billions of years. At the core of our sun, this stellar fusion reactor converts the vast amount of 564 million tons of hydrogen into helium every second. A good four million tons per second become radiation and make our central star the yellow glowing dwarf star that will remain there for billions of years.
If the mass is not enough
So far so good. But the whole thing can go wrong too. For then, if the gaseous and dust lumps in the stars do not contain enough matter. Only from just under eight percent of the sun's mass, which corresponds to about 75 times the planet Jupiter, the mass of the protostar lump is sufficient. Only then does it generate enough heat and pressure to ignite the hydrogen fusion.
But what happens if the mass is insufficient? Astronomers developed their first theories in the 1970s. They postulated that a particular type of celestial body, a "black" or "brown dwarf" would have to emerge if a cluster of matter in the formation of stars only aggregates between 13 and almost 75 Jupiter masses.
Pressure and temperature in its interior are then no longer sufficient for hydrogen fusion, but for another type of nuclear fusion, deuterium fusion. The "heavy hydrogen" consisting of one proton and one neutron combines with another proton to form a helium-3 nucleus. Radiation energy is also generated, but much less than with hydrogen fusion.
Brown dwarf as infrared light-Funzel
The young brown dwarf thus formed does not shine yellowish or whitish like our sun, but dark red-brown. A large part of its radiation is not emitted as visible light or more energy-rich waves, but as infrared light, as heat radiation. Its surface temperature is below 1,800 Kelvin, which is considered the minimum stellar temperature.
And even this rather funzelige lighting phase is not long lasting. Because its fuel Deuterium is very rare in the gas clouds of the starweed, of course, its proportion measured on "normal" hydrogen at only 0.015 percent. Accordingly, the deuterium supply in the young celestial body is exhausted, and after only a few million years the deuterium fire is extinguished. From now on, the brown dwarf slowly cools down, becoming darker and less radiation-intensive. So far the theory. But you could not prove this thought building for more than 20 years
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