Astro Bob blog: Not quite planet, not quite starA look at the netherworlds of brown dwarfs, often called failed stars.
By: Bob King, Duluth News Tribune
Not quite planet, not quite star
The Seven Dwarfs in the famous "Heigh ho" sequence in Walt Disney's Snow White and the Seven Dwarfs. animated by Shamus Culhane. Credit: Walt Disney/RKO Pictures
"Heigh-ho, heigh-ho, it's home from work we go!" Who doesn't recall
the seven Disney dwarfs breaking into song after digging all day in
the mines in Snow White and the Seven Dwarfs. My favorites were Happy
and Dopey, which probably says a lot about my personality, but my wife
would cast me as Grumpy in a second. Dwarfs pop up in astronomy, too.
The other day we looked at red dwarfs, those small stars that populate the
galaxy in the billions. There are more of these humble stellar fires than all the
giant and supergiant stars thrown together.
Recall that red dwarfs contain a lot more matter than Jupiter but are
only a couple times its size. They've enough kindling in the form of
hydrogen gas and sufficient pressure in their cores for nuclear fusion
to occur. Once a star begins fusing hydrogen, it releases prodigious
amounts of energy, literally becoming a sphere of fiery gas. The only
thing keeping it from blowing apart is the counteracting pressure of
This comparison diagram gives you a good idea of how brown dwarfs stack up against the sun and planets. Brown dwarfs range between about 13 and 80 times the mass of Jupiter. Below that we enter the realm of planets while above it are the true stars. Credit: Jon Lomberg/Gemini Observatory
Planets never get hot enough to "burn" hydrogen and therefore don't
radiate light of their own. Some may be the right size, but
the matter in their interiors is simply not dense (or hot) enough for
fusion to take hold. Planets shine by "borrowed" light from their host star which
they reflect back into space.
Red dwarfs are true stars, but their brown dwarf cousins are not.
Brown dwarfs fall somewhere between giant planets like Jupiter and red
dwarfs in size and temperature. A typical one might pack 45-80
Jupiter's masses of mostly hydrogen into a sphere about Jupiter's
size. The least massive of them contain only 13 Jupiter's worth of
material. One of the most amazing traits of all brown dwarfs whatever their
mass is they're all about Jupiter's diameter or some 11 times the size of Earth.
The red object at center is the dim brown dwarf 2MASS 0415–0935. It emits almost no visible light at all and was photographed in the infrared. It has a surface temperature of just 770 degrees Fahrenheit, only a few hundred degrees hotter than your oven. Credit: U.S. Naval Observatory
A hefty brown dwarf briefly burns deuterium (a form
of hydrogen) in its youth, but that internal fire soon peters out, and the star
cools down like an unplugged iron, fading from dull red to black.
The heat they do retain comes from the release of the remaining thermal
energy in their cores. Typical brown dwarf surface temperatures are around
1300 degrees or about the same as an electric stove coil set on high.
Perfect for heating up water for tea.
If that sounds cold for a star, we still have a ways to go before hitting bottom.
The coolest brown dwarfs hardly budge the thermometer at just 300+ degrees.
Astronomers have detected methane gas and steamy water vapor in their atmospheres.
These substances couldn't exist on our blisteringly hot sun, but we do find them
on planets. One thing is clear: brown dwarfs are the bridge between true stars and
the giant gas planets like Jupiter and Saturn.
The first brown dwarf was discovered in 1995 and now hundreds are known.
Most give off little visible light -- they're just too
cool -- but they do radiate heat energy or infrared light. A
telescope able to see in infrared will have a much easier task
finding brown dwarfs because they appear brighter than in a typical
telescope. It's similar to how firefighters use infrared detectors to pinpoint heat
sources that would otherwise be invisible to the eye.
This artist's concept shows simulated data predicting the hundreds of failed stars, or brown dwarfs, that NASA's Wide-field Infrared Survey Explorer (WISE) is expected to add to the population of known stars in our solar neighborhood. The green pyramid represents the relatively tiny volume surveyed by NASA's Spitzer Space Telescope. Credit: AMNH/UCB/NASA/JPL-Caltech
Recently, the Spitzer Space Telescope, an orbiting observatory optimized
for infrared observing, detected 14 of the coldest brown dwarfs ever
discovered. They glow so feebly not a single one is visible in even the largest ordinary
telescope. Their temperatures range from 350 to 620 degrees, which
makes you wonder where we draw the line between planets and stars.
Most brown dwarfs float freely in space and are thought to form like
other stars, from dense clouds of dust and gas that collapsed under the
force of gravity -- with one difference. They started out with far less material.
Others formed as companions around larger stars like the pair of brown dwarfs
that orbits the red dwarf star Epsilon Indi.
Spitzer looked at a very tiny slice of sky to spot chilly dwarfs
hundreds of light years away. Based upon that small sample,
scientists believe there are at least a 100 within 25 light years
of the sun. That's why they expect the WISE (Wide-field Infrared
Survey Explorer) telescope, which can survey chunks of the sky 40
times larger than Spitzer, to ferret out a bevy of browns in the
months to come. These should be close enough for astronomers to employ
the Hubble and other large, ground-based telescopes to tease out a
few more of their dark secrets.