Fainter and Smaller than the Red Dwarf stars are the so-called Brown Dwarfs.
As the name implies, a brown color is not part of the spectral classification
system, so these objects do not even make their own light. This means Brown
Dwarfs are not undergoing thermonuclear fusion of Hydrogen into Helium with
the release of gamma radiation. Therefore, by definition, Brown Dwarfs are not
even stars. One more time, but certainly not the last, it is the mass of the
star that determines its fate. Like all other stars, a small cloud of dust and
gas collapses under some external stimulus. In the case of the Brown Dwarf,
the mass of this collapsing cloud is so small (less than 0.08 Suns) that gravitational
pressure is never sufficient to raise the kinetic energy of Hydrogen nuclei
to 7 million K. Thermonuclear fusion never begins. The star is never born. It
is a star "wannabe." The entire lifecycle of the Brown Dwarf is found
below and to the right, compared to the Sun's lifecycle below and to the left.
These objects never reach the main sequence, never ignite hydrogen into helium,
never make their own light, and therefore do not glow in the sky in a manner
such that we can find them with a conventional telescope.
Brown Dwarfs are seen with an Infrared Telescope
Brown Dwarf stars were theoretically predicted to exist, but
had not been photographed until recently, due to the fact that they do not produce
visible light on their own, owing to the fact that they are not massive enough
to sustain thermonuclear reactions in their core. In this photograph taken by
the Hubble Space Telescope, the left image shows what the center of the Orion
Nebula looks like in visible light, while the right image is taken with an infrared
camera. This camera sees objects which are not glowing visibly, but glow with
infrared radiation. The same principle is applied in night vision goggles, where
the goggles sense heat instead of visible light. Use of infrared cameras are
among fire fighters who enter a smoke-filled house and can find people among
the cloudy interior because of infrared energy emitted by people. Infrared goggles
are used by Navy Seal teams when doing covert operations in the dark, and are
shown in several memorable movies like "Patriot Games," "Cliff
Hanger," and "Navy Seals." The predatory alien in "The Predator"
used infrared vision and could not find Arnold when he covered himself with
cold mud from the river bank.
The infrared camera of the HST could find the hot-glowing brown
dwarf stars in the middle of the dusty nebula of Orion. They are glowing in
the right image, but not because they are making visible light, but because
their pressure from gravity is making the interior particles glow at infrared
wavelengths to which the HST camera is attenuated. The numerous points of light
in the right image are brown dwarf stars.
Brown Dwarfs were first visibly discovered October 27, 1994 by
the Palomar telescope in California. The first image is seen below and to the
left. To the right is an HST image of the same Brown Dwarf at much higher resolution.
The name of the object is Gliese 229B. This tiny Brown Dwarf shines dimly next
to its vastly brighter M1 dwarf companion. Yea, that's right ... this is a picture
of a Brown Dwarf-Red Dwarf binary system! Isn't that just the coolest thing
... a picture of both star types that are the subject of this page. There are
so many cool pictures out there for you to find! Okay, back to the Brown Dwarf.
Gliese 229B has a surface temperature of only about 1000 K, and its infrared
spectrum contains absorption bands of methane, which are NEVER found in stars.
significance of the cool temperature of Gliese 229B is seen in the image to
your left. Gliese 229A is a Red Dwarf whose diameter is quite a bit smaller
than our Sun, and whose spectrum indicates a surface temperature of 3800 K.
The M class Red Dwarf stars extend down into the L spectral class where there
is some mixing of stellar types. These dim and even smaller Red Dwarfs have
a surface temperature of 2000 K, which is the lowest limit of the main sequence.
Below that temperature, stars are not stars anymore because their interiors
cannot sustain thermonuclear fusion. We have reached that realm of the Brown
Dwarf objects. What is cool to me is that Gliese 229B is an object 1000 K cooler
than the lowest main sequence star. These Brown Dwarfs, as seen in the image
to your left are very small, and with masses near 0.01 Suns. At this extremely
low range, questions abound as to whether to classify them as "stars"
or large planets.
What are Planets?
By definition, a planet is an object independently orbiting a
star. The mass of 0.08 Suns defines the bottom of the main sequence. This mass
is 80 times that of Jupiter. When we descend below this mass level, we are in
the realm of the Brown Dwarfs, and further descent brings us to objects like
Jupiter. The movie "2010 - Space Odyssey" tells a story of Jupiter
suddenly becoming a star. While Jupiter is similar to the Sun in composition,
elemental ratios, and gaseous nature, it lacks the necessary mass to generate
internal core temperatures required to ignite thermonuclear fusion. At a mass
of 13 Jupiters - just under 0.013 Suns - even deuterium burning must halt, and
therefore the object is perhaps better termed a "planet."
the left is the HR Diagram showing the "lifecycle" of these Jupiter-like
planets. It is difficult to determine whether an object is a star or planet
... a smallest of the Brown Dwarfs or something else. Stars and Brown Dwarfs
are expected to form from dusty gases in interstellar space, while planets form
from the leftover dust and gas after the star has fully formed. Is is possible
to say that Brown Dwarfs as small as Jupiter might form independently from interstellar
gas without a companion star? Are there Jupiters scattered throughout the Milky
Way that we may never find? As for now, we know of some 50+ extrasolar planets.
These are objects that are orbiting companion stars in such a way that the host
star is moved back and forth by the orbital motion of the planet. The nearly
50+ objects are all less massive than the lower-limit Brown Dwarf of 0.013 Suns.
There are the subject of intense research and we know so little about them at
present, so I choose to leave the subject open-ended here. Cool, huh? Planets
orbiting other stars!
your left is a sampling of some of the extrasolar planets discovered to date.
The method of search and discover is to look at a particular star over long
periods of time and try to determine if the star is moving back and forth ever
so slightly. This indicates an object in orbit around the star. By watching
the movement and measuring the timing, astronomers can determine the mass of
the planet and its distance from the parent star. As you can see, most of the
discovered planets are Jupiters in mass, but orbiting much closer to their stars
than our Jupiter does. This close proximity of a gas planet to a star has posed
some problems for planetary formation theories. To learn more specifics about
extrasolar planets, go to the Extrasolar
Planet Encylopedia, or click directly to the Extrasolar
Planet Catalogue. The list keeps growing almost daily, and the most prolific
finder of these planets is Geoff Marcy, who discovered planets 70
Virginis (Jan, 1996), and 51
Pegasi (Sep, 1996). These were two of the first planets to be found orbiting
stars other than our own Sun. The connection just shown is to his article in
the "Astrophysical Journal," and is quite complex, but I have put
the link in here to show you what professional astronomers do, and also to show
you why we are avoiding the complicated math in this course.
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