In astronomy, a frozen star, besides a disused term for a black hole, is a type of hypothetical star that, according to the astronomers Fred Adams and Gregory P. Laughlin, may appear in the future of the Universe when the metallicity of the interstellar medium is several times the solar value.
Due to opacity effects, as metallicity of the interstellar gas increases both the maximum and minimum masses a star can have will decrease. For the latter case, it's expected that an object with a mass of 0.04 solar masses (40 times the mass of Jupiter), that currently would become a brown dwarf unable to fuse hydrogen, could do so ending in the main sequence with a surface temperature of 0 °C (273 K, thus frozen), much cooler than the dimmest red dwarfs of today, and ice clouds forming in its atmosphere. The luminosity of these objects would be more than a thousand times lower than the faintest stars currently existing and their lifetimes would also be sensibly longer.
In astronomy, metallicity is used to describe the abundance of elements present in an object that are heavier than hydrogen or helium. Most of the physical matter in the Universe is in the form of hydrogen and helium, so astronomers use the word "metals" as a convenient short term for "all elements except hydrogen and helium". This usage is distinct from the usual physical definition of a solid metal. For example, stars and nebulae with relatively high abundances of carbon, nitrogen, oxygen, and neon are called "metal-rich" in astrophysical terms, even though those elements are non-metals in chemistry.
The presence of heavier elements hails from stellar nucleosynthesis, the theory that the majority of elements heavier than hydrogen and helium in the Universe ("metals", hereafter) are formed in the cores of stars as they evolve. Over time, stellar winds and supernovae deposit the metals into the surrounding environment, enriching the interstellar medium and providing recycling materials for the birth of new stars. It follows that older generations of stars, which formed in the metal-poor early Universe, generally have lower metallicities than those of younger generations, which formed in a more metal-rich Universe.
Observed changes in the chemical abundances of different types of stars, based on the spectral peculiarities that were later attributed to metallicity, led astronomer Walter Baade in 1944 to propose the existence of two different populations of stars.
These became commonly known as Population I (metal-rich) and Population II (metal-poor) stars. A third stellar population was introduced in 1978, known as Population III stars. These extremely metal-poor stars were theorised to have been the "first-born" stars created in the Universe.Metallicity distribution function
The Metallicity distribution function is an important concept in stellar and galactic evolution. It is a curve of what proportion of stars have a particular metallicity ([Fe/H], the relative abundance of iron and hydrogen) of a population of stars such as in a cluster or galaxy.
MDFs are used to test different theories of galactic evolution. Much of the iron in a star will have come from earlier type Ia supernovae. Other [alpha] metals can be produced in core collapse supernovae.
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