|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 star.
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 have planets.
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
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 place 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
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 planets.
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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