Hunting brown dwarfs

INR --- | --- BD 2

What are brown dwarfs?

First

Stars form within giant clouds of gas within the disk of the Galaxy, such as the gaseous nebula M16, spectacularly pictured by the Hubble Space Telescope. Next

Individual stars form around small density perturbations within the molecular cloud:
more material -- more gravity -- attracts more material
as the gas collapses, it releases potential energy which is transformed to heat within the core of the forming protostar. Next

If the central temperature of the protostar climbs above a threshold value (about 3 million degrees Celsius), nuclear reactions can get underway
higher temperatures -- faster motion -- higher energy particles -- break into nucleus
hydrogen burns to form helium - a reaction currently powering the Sun Next

The more massive the gas cloud, the more massive the star, ,br> the more energy released in formation, the more energy produced due to nuclear reactions
the higher the energy production, the higher the luminosity and the higher the surface temperature
hence stars settle into an equilibrium state, where there is a well-defined relation between temperature and luminosity. Next

Stars in the galactic disk have a chemical make-up very similar to the elemental abundances in the Sun, but their spectroscopic appearance changes drastically depending on their temperature. Hence, the well known sequence of spectral types

O B A F G K M plus R N S on the giant branch
Spectral type depends on the stellar mass: high mass stars are profiligate with their fuel supply, and have brief, but spectacular, careers; M dwarfs live forever.
Stellar lifetimes range from several million years for stars with 100 solar masses
through 10 billion years for the Sun to several trillion years (10¹²) for the lowest mass stars, M dwarfs
Next

Collapsing `stars' with total masses below a certain critical level, about one tenth the mass of the Sun, never become hot enough to trigger nuclear reactions. These `failed stars' are known as 'brown dwarfs'. Next

Unlike stars, brown dwarfs have no central energy source to maintain their luminosity; the only energy available is the heat stored during the collapse of the parent gas cloud. As that energy is radiated and lost, the brown dwarf gradually cools and fades into oblivion. The rate of cooling is mass dependent
a brown dwarf just below the hydrogen-burning limit can take 10 billion years to reach 10^-6L(sun)
a brown dwarf of 0.01 M(sun) (10 Jupiter masses) takes only 10⁷ years to reach the same limit. Next

Very low-mass stars, brown dwarfs and gas-giant planets are almost identical in size. This is due to a property known as degeneracy: the atoms and molecules are as closely packed as possible, so `less stuff' doesn't mean smaller.

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INR --- | --- BD 2

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