Colorful Dwarfs
Most of the stars in our Milky Way galaxy are small, faint cosmic embers known as red dwarfs. In fact, about 70 percent of the stars in the galaxy are red dwarfs, including our closest stellar neighbor, Proxima Centauri. The very name, "dwarf," hints at smallness, and introduces a question long associated with these stars: How small can a star be and still be a star?
The words astronomers use to describe stars evoke the fairy tales of childhood: white dwarfs, red giants, red dwarfs, and blue giants, among others.
The characteristic that distinguishes a giant star from a dwarf star is size, which depends on the star's mass and its stage in life. The characteristic that distinguishes a red star from a blue star is temperature. When an iron bar is first removed from a blacksmith's forge, it appears white hot. As it cools, its color changes from bright orange to dull red, then black.
You now know something about red dwarfs just by reading the name: They have relatively low surface temperatures and mass.
In astronomical jargon, most red dwarfs are classified as class "M" dwarfs. The M comes from a system that classifies stars based on surface temperature. The surfaces of M dwarfs are around 4,500 degrees Fahrenheit (2,500 C) -- less than half the surface temperature of the Sun.
M dwarfs recently lost their title as the coolest stars, though, with the discovery of L dwarfs, a class of cool, dim objects that barely qualify as stars.
Mass determines nearly everything about a star: its temperature, its color, and how rapidly its interior evolves. The amount of energy flowing through the star's surface determines its temperature and color.
A star emits more or less energy depending on the pressure in its core. The more mass, the higher the pressure. Because their masses are so low, the core pressure in red-dwarf stars is low, so the stars are cool, red, and last a long, long time -- perhaps a trillion years or longer, compared to about 10 billion years for a star like the Sun.
Just how little mass is required to instigate the nuclear fusion process motivates much contemporary research. If there's too little mass, the central pressure never climbs high enough to trigger fusion. But add a little mass and fusion begins -- and a star is born. The "star/not-a-star" mass region seems to lie between seven and eight percent of the mass of the Sun.
Even before the recent discovery of L dwarfs, we knew of a few objects that clearly fell below the star/not-a-star mass limit. The act of formation provides enough heat for the youngest of these objects to produce visible light, just like an iron bar recently removed from the forge. We see them in the process of cooling. Dull red is brownish, hence the name "brown dwarf."
Red dwarfs are a stopover on a continuing journey of discovery. Astronomers used to think they were the coolest stars, but no more. They continue to have value, though, since binary stars containing M dwarfs are the hunting grounds for brown dwarfs. Whatever processes form stars, they seem to favor low masses, since there are more M dwarfs than any other kind of star. Are there more L dwarfs than M dwarfs? Sky surveys toil to answer this question.
This document was last modified: October 19 2009.
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