|What makes a planet different than a star? Has
anyone been out of our galaxy before? If so,
when and where did they go?
|Question Date: 2002-12-03|
First, let's talk about stars and planets. Stars
are balls of gas that are so massive that the
pressure inside of them causes a nuclear reaction.
That makes them bright and hot (like the sun,
sometimes even hotter).
Planets can vary a bit,
and I don't know the exact definition. Planets
like the earth and Mars are rocky with a metallic
center -- in other words, they're solid. But
planets like Jupiter and Saturn are mostly gas,
like a star. The difference is that they aren't
big enough to cause the nuclear reactions, so they
aren't so hot and don't emit much light (certainly
none visible to the eye, what we see is the
reflection of the sun).
What's interesting is
that there are things called "brown dwarfs" that
are bigger than planets like Jupiter and seem
almost to be stars but aren't quite.
galaxy: no human being has been past the Earth's
moon. So we're stuck in one corner of our solar
system (our sun and it's planets). The solar
system is just a tiny part of the galaxy (which
has billions of stars). And there are many
galaxies in the known universe. The universe is a
very big place!
A planet is usually defined as an object (which
might be solid, liquid, gas, or a combination of
all three) which is too small to produce nuclear
fusion (which is the process by which the sun
burns) and which orbits a star. The Earth is an
example of a planet and orbits the sun, which is a
A star is usually defined as a body
of gas which is large enough and dense enough that
the heat and crushing pressure at its center
produces nuclear fusion. This is a fancy way of
saying that it glows or burns, like our sun. Some
stars are known to have planets orbiting around
them (our sun is one example!), but not all stars
A solar system is a star
with its planets. Our solar system consists of the
sun, the Earth and eight other planets. There are
smaller objects in the solar system, like the
comets, asteroids, and each planet's moons.
A galaxy is a collection of stars (some of
which have planets) which clump together, held to
each other by their combined gravity. The galaxy
we live in is called The Milky Way. There are 200
billion stars in it, and they aren't very closely
packed together, so it is quite a big place. No
one from the Earth has ever left the galaxy - it
is just too far to go. Even if you traveled at
the speed of light, it would take about ten
thousand years to get there. The farthest that any
object launched from Earth has traveled is eight
billion miles, which is to say it is just on the
outer edge of our solar system. The farthest any
person has ever traveled is to the Moon - just
240,000 miles away.
Good places to learn
more about these things
A star is basically a giant ball of very high
temperature gas...it is made up of > 99.9 % of
Hydrogen(H) and Helium( He). The critical part
of a star is that the temperature at its center is
high enough to enable H to slam into another H and
make He and also generate a LOT of LIGHT ENERGY.
A planet on the other hand comes in two
flavors: There are the Giant planets that are
MORE or LESS the same COMPOSITION as a star (H
plus He); however, because they are smaller than a
star, a planet never reaches an internal
Temperature high enough to ignite the
thermonuclear reaction of H+H= He plus ENERGY.
Hence planets don't give off light by internal
generation. (They do reflect light of course)
These are called gas giant planets and JUPITER
is a good example. There is another type of
planet called a TERRESTRIAL planet which is made
up of ROCK and Metals. The Earth or Mars is an
example. In the case of Earth, it is 30 % Fe metal
and 70 % rock.
A star is generally defined by its ability to
generate light via nuclear (fusion) reactions that
convert hydrogen into helium. Planets are formed
from the collection of gas and dust that surrounds
a star. They do not have the necessary mass, when
sufficiently compressed by its own gravity, to
support the hydrogen-to-helium fusion
reactions. On a side note, there are "in
between" sized objects that aren't quite massive
enough to become full blown stars, but can support
other types of fusion (protons into deuterium).
These are called brown dwarfs and they can
emit some light early in their life. They are not
quite stars, but also not really
Nobody has ventured out of our
galaxy yet (at least from Earth!). Actually,
nobody has been further away than the moon!
The formation of planets and stars begins with one
large event. Scientists believe the process
begins when a large cloud of dust and gas
collapses. The material that collapses to the
center forms the star and the rest of the material
is left to rotate as a cloud around the star.
Eventually, after tens of thousands of years, the
dust in the cloud clumps together, forming
planetesimals. These bodies collide and become
larger bodies known as planets. Stars are
sources of energy; they emit light and heat
created through nuclear fusion of hydrogen and
helium in their cores. In contrast, planets have
no renewable source of energy and cool off slowly
through time. There are observational
differences between stars and planets, too. When
you look up into the night sky, stars are the
bodies that appear to twinkle and have fixed
locations relative to each other. Planets, on the
other hand, don't twinkle and they move relative
to the stars.
No, no one has ever been out of
our galaxy. Only unmanned spacecraft have been to
the outer planets of our solar system, far from
out of our galaxy. NASA currently has 3
interstellar probes in space, and you can read
about two of them at voyager
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