Are brown dwarfs cold or hot

Secrets of brown dwarfs

Astronomers want to learn more about planetary gas giants from stars that could not ignite the nuclear reaction inside

In the beginning there is the fog. In order for a new star to form, an imbalance must have developed in an interstellar gas cloud of sufficient density. The cloud agglomerates under the influence of gravity to form a protostar until the temperatures inside are high enough to ignite the nuclear fusion. Since the overall angular momentum is retained, but the cloud draws its arms like a figure skater, it turns faster and faster.

However, it can happen that there is simply not enough raw material available. Either because the gas cloud itself was too small. Or because a giant star in the vicinity blew away too much fuel. The protostar may also have arisen as part of a multiple system and its brothers have secured a larger share of the primary supply.

Exactly how much nuclear fuel is required depends on its composition - just as alcohol burns more easily than diesel. However, the heavier the raw material, the easier it is to ignite stars. If the composition corresponds to our sun, then one reckons with about 75 Jupiter masses (a thousand Jupiter masses correspond approximately to one solar mass). In the young universe, however, the starting conditions were still 90 Jupiter's masses.

If the protostar is lighter, fusion reactions can still occur: from 65 Jupiter masses, for example, lithium fusion begins, and from 13 Jupiter masses, deuterium nuclei fuse with protons to form helium. But because usually neither lithium nor heavy hydrogen (deuterium) are among the main components of a baby star, it only glows very weakly, it just glows.

Brown dwarfs usually reach about the size of Jupiter. The lighter stars are bigger than the heavier ones because the gravity is weaker here. The mini-stars age relatively quickly. They begin to cool down shortly after the fusion reactions have ignited. The heaviest known representatives have a surface temperature of 2900 Kelvin. The fate of a brown dwarf is relatively unspectacular: it cools down over time until, after a few billion years, it wanders through space as an ice ball.

For astronomers, such an ice-cold gas ball is still very exciting - it should have many properties in common with planetary gas giants that orbit a sun. The researchers suspect, for example, that just like planets, clouds form in their atmosphere. Unfortunately, the brown dwarfs are difficult to observe because of their low temperatures. Instead of visible light, they only emit infrared radiation. But at least they might be easier to discover and explore from a distance than planets in other solar systems.

So it is fitting that astronomers, with the help of the Spitzer Space Telescope, have now succeeded for the first time in precisely measuring the distance to a number of these objects. The researchers publish their results in the journal Science. Knowing the distance is a prerequisite for finding out more about a celestial object. Because how much brightness arrives on earth from a star, of course, depends not only on its luminosity but also on the distance.

In this way, the researchers were able to determine the luminosity, temperature and mass of some brown dwarfs. In fact, it confirmed the assumption that the objects fill the gap between large gas planets and stars. Surprisingly, however, it was also found that the spectrum of a brown dwarf does not necessarily correspond to its temperature. This is exactly the hoped-for indication of cloud formation in the atmosphere; it can also be a result of different surface gravity or vertical currents in the atmosphere.

WISEP J1828 + 2650, one of the brown dwarfs whose surface temperature was previously attested, seems to be 250 degrees hotter after all. The object WD 0806−661B is currently the hottest candidate for the price of the coldest brown dwarf. With six to ten masses of Jupiter, it should be around 60 to 100 degrees Celsius on the two billion year old star. Ross 458C, which is about seven times as heavy as Jupiter, is also one of the smallest brown dwarfs.

The author's eBook "The New Biography of the Universe", which describes the development and structure of our universe in detail, is available on Amazon (Mobi, DRM-free), iTunes (for iPad) and Beam eBooks (ePub, PDF, DRM- free) available.

(Matthias Matting)

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