Our original write-up:
For many years astronomers were faced with questions that offered no ready answers: What are objects like that are intermediate in mass between stars and planets? Are they prevalent in the Universe and could they be a source of dark matter? Thanks partly to observations from Keck's first decade, astronomers can now provide those answers.
Stars form as gas collapses and heats up within interstellar clouds; brown dwarfs are thought to form in the same way, but with masses too low to generate sufficient heat to ignite hydrogen fusion. Without a central energy source, brown dwarfs continue to cool forever, much like embers plucked from a roaring fire.
Just like embers, brown dwarfs become redder as they cool, so the best prospect for discovering them is with surveys conducted at infrared wavelengths, beyond the red extreme of the visible spectrum. One survey in particular, the Two Micron All Sky Survey (2MASS), scanned the entire night sky at these wavelengths between 1997 and 2001.
We selected brown dwarf suspects by hunting for objects detected by 2MASS but invisible (or at least very dim) on optical images of the same patch of sky. In 1997 we began observing these candidates at the Keck Observatory to obtain their spectra, a kind of fingerprint that splits the light into its component "colors" to reveal the object's temperature and chemical makeup. With these spectra we were able to verify over two hundred previously unidentified brown dwarfs in the vicinity of the Sun. Because of the distinctive new range of temperatures and chemical makeup of these objects, we were forced to revise the century-old spectral type sequence familiar to all astronomers.
Figures 1 and 2 show artist's renditions comparing these objects with the Sun. All are plotted to the same scale. On the far left is the limb of the Sun. To its right is shown a very low mass star (a so-called "late-M dwarf"), a couple of brown dwarfs (a hotter "L dwarf" and a cooler "T dwarf"), and the planet Jupiter. These objects have masses ranging from 1050 times that of Jupiter (for the Sun) through 75, 65, 30, and 1 Jupiter mass for the late-M dwarf, L dwarf, T dwarf, and Jupiter, respectively. The colors of the brown dwarfs are chosen to match an age of 1 billion years. Despite the range in mass, all four of the low-mass objects are approximately the same size, ten times smaller than the diameter of the Sun.
Figure 1 shows how these objects might appear to the human eye: the M and L dwarfs are red, while the T dwarf is dimly magenta, due to lack of light -- actually absorptions by sodium and potassium atoms -- in the green portion of the spectrum. Figure 2 shows the same objects as they might appear to eyes sensitive to near-infrared light. Here the M and L dwarfs are slightly orange or red compared to the Sun, but the T dwarf is distinctly blue due to a lack of light in the "green" and "red" caused by absorption from methane. Methane is also abundant in the atmosphere of Jupiter and this, along with the clouds and bands of other complex molecules, gives it alternating patches of pink and blue.
Not only have our Keck observations enabled us to learn more about the makeup of these objects, they have also allowed us to determine the answer to the second question. Figure 3 shows the makeup of a typical slice of the Milky Way - specifically the sample of nearby stars within 8 pc of the Sun and visible from the northern hemisphere - with stars on the left and brown dwarfs on the right. The stars are composed of 4 blue A stars, 1 green-tinted F star, 5 yellow G stars (one of which is the Sun itself), 22 orange K stars, 87 red M stars, and 9 small white dwarfs. The brown dwarfs on the right are composed of a few red M and redder L dwarfs along with lots of magenta T dwarfs (or cooler objects), most of which are so cool that even Keck cannot detect them. Despite the fact that there are at least as many brown dwarfs as stars, the stars are responsible for most of the mass, as the scale shows. Brown dwarfs can be ruled out as making any significant contribution to the dark matter questions which continue to trouble cosmological investigations.
(Artist's renditions by Dr. Robert Hurt of the Infrared Processing and Analysis Center.)
Final text with images incorporated