The Starbase One Astronomy & Space Collection/STARBASE_ONE.ISO/a96/disk12/brdwarfs.txt

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A Galaxy Dweller's Guide to Planets, Stars and Dwarfs


"Twinkle little star, how I wonder what you are . . ."

Today, you might just as easily find astronomers humming this nursery
rhyme as well as children.  Rapid advances in telescope technology --
adaptive optics, space observatories, interferometry, image processing
techniques -- are allowing astronomers to see ever fainter and smaller
companions to normal stars.  As telescopic capabilities sharpen,
conventional definitions for planets and stars may seem to be getting
blurry. In the search for other planetary systems, astronomers are
turning up objects that straddle the dim twilight zone between planets
and stars, and others that seem to contradict conventional wisdom, such
as a planetary system accompanying a burned-out compacted star called a
neutron star.


Stars

Stars are large gaseous bodies that generate energy through nuclear
fusion processes at their cores -- where temperatures and pressures are
high enough for hydrogen nuclei to collide and fuse into helium nuclei,
converting matter to energy in the process.  Stars are born out of
clouds of hydrogen, that collapse under gravity to form dense knots of
gas.  This collapse continues until enough pressure builds up to heat
the gas and trigger nuclear fusion.  The energy released by this
"fusion-engine" halts the collapse, and the star is in equilibrium.

A star's brightness, temperature, color  and lifetime are all
determined by its initial mass.  Our Sun is a typical middle-aged star
halfway through its ten billion-year life.  Stars can be 100 times more
massive than our Sun, or less that 1/10 its mass.  A Hubble Space
Telescope search for dim stars suggests that most stars in the galaxy
are about 1/5 the mass of our Sun.

Following a fiery birth, stars lead tranquil lives as inhabitants of
the galaxy.  Late in a star's life, fireworks can begin anew as changes
in the core heat the stars further, eject its outer layers, and cause
it to pulsate.  All stars eventually burn out. Most collapse to white
dwarf stars -- dim planet-sized objects that are extraordinarily dense
because they retain most of their initial mass.  Extremely massive
stars undergo catastrophic core collapse and explode as supernovae --
the most energetic events in the universe.  Black holes and neutron
stars -- ultra dense stellar remnants with intense gravitational fields
-- can be created in supernova blasts.

At least half of the stars in the galaxy have companion stars.  These
binary star systems can undergo complicated evolutionary changes as one
star ages more rapidly than the companion and dies out.  If the two
stars are close enough together, gas will flow between them and this
can trigger nova outbursts. Supernovae and novae are key forces  in a
grand cycle of stellar rebirth and renewal.  Heavier elements cooked up
in the fusion furnaces of stars are ejected back into space, serving as
raw material for building new generations of stars and planets.


Planets

Though the universe contains billions upon billions of stars, until
recently only nine planets were known -- those of our solar system.
The Solar System provides a fundamental model for what we might expect
to find around other stars, but it's difficult to form generalities
from just one example.  It may turn out that nature is more varied and
imaginative when it comes to building and distributing planets
throughout the Galaxy.

In it simplest definition, a planet is a nonluminous body that orbits a
star, and is typically a small fraction of the parent star's mass.
Planets form out of a disk of dust and gas that encircles a newborn
star.  These embryonic disks have been observed around young stars,
both in infrared and visible light.  The planets' orbits in our solar
system trace out the skeleton of just such a disk that encircled the
newborn Sun.

Planets agglomerate from the collision of dust particles in the disk,
and then snowball in size to solid bodies that continue gobbling up
debris like cosmic Pac-Men.  In the case of our solar system this led
to eight major bodies, thousands to tens of thousands of miles across.
(The ninth planet, Pluto, is probably a survivor of an early subclass
of solar system inhabitants called icy dwarfs).  A planet's mass and
composition are determined by where it formed in the disk.  In the case
of our solar system the more massive planets are found far from the
Sun, though not too far where material didn't have time to agglomerate
(because orbital periods were so slow that chances for collisions were
minimal).

Unlike asteroids which are cold chunks of solar system debris, a
planet must  be massive enough to have at least once had a molten core
that differentiated the planet's interior.  This is a process where
heavier elements sank to the center and lighter elements float to the
surface.  According to this idea, planets should have dense
rocky/metallic cores.  Depending how far they formed from their parent
star, they may retain a dense mantle of primordial hydrogen and
helium.  In the case of our solar system this establishes two families
of planets: the inner rocky or terrestrial planets such as Earth and
Mars, which have solid surfaces, and the outer gas giant planets
Jupiter and Saturn that are mostly gaseous and liquid.  Massive planet
like Jupiter are still gravitationally contracting and shine in
infrared light.

Ironically, the first bonafide planetary system ever detected beyond
our Sun exists around a neutron star - a collapsed stellar core left
over from the star's self-detonation as a supernova. Resembling our
inner solar system in terms of size and distribution, these three
planets  orbiting the crushed star probably formed after the star
exploded.  Apparently a disk must have formed after the stellar death,
from which the planets agglomerated.  Other suspected extrasolar
planets also seem to defy conventional wisdom. An object orbiting the
star 51 Pegasus may have the mass of Jupiter, but is 20 times closer to
the star than Earth is from the Sun.


Brown Dwarfs

Brown dwarfs are the galaxy's underachievers.  They never quite made it
as stars. Like stars, brown dwarfs collapse out of a cloud of
hydrogen.  Like a planet they are too small to shine by nuclear fusion,
and radiate energy only through gravitational contraction.  (More
massive brown dwarfs might have initiated fusion, but could not sustain
it.)  Their predicted masses range from several times the mass of
Jupiter to a few percent the mass of our Sun.  Spectroscopically, the
cool dwarfs may resemble gas giant planets in terms of chemical
composition.


A Color-Guide to Dwarfs

The different type of so-called "dwarfs" in the Galaxy would even
befuddle the storybook character, Snow White:

White dwarfs  -- Burned-out stars that no longer shine through nuclear
fusion, and have collapsed to Earth-sized objects.  Ironically, their
surface temperature rises as they collapse and so the star is
white-hot.

Yellow dwarfs -- Normal stars with our Sun's temperature and mass.

Red dwarfs  -- Stars that are small, cooler and hence, dimmer than our
Sun.  The cooler a star the redder it is, just as a dying ember fades
from yellow-orange to cherry-red.

Brown dwarfs -- Substellar objects that have formed like a star, but
are not massive enough to sustain nuclear fusion processes.

Black dwarfs -- White dwarfs that cool to nearly absolute zero.  The
universe isn't old enough yet for black dwarfs to exist.