|What makes a planet different from a star?
Stars generate energy in their very centers by
nuclear reactions. The main nuclear reaction in
the Sun is that two Hydrogen atoms slam together
at such energy so that the protons fuse and a new
element called Helium forms. When two atoms of
Hydrogen come together to form one Helium , the
mass of the two hydrogens is a little bit larger
than the mass of the Helium. That difference is
converted into energy by the formula:
energy= mass difference X speed of light
Planets on the other hand DO NOT generate
heat by nuclear fusion. For the most part
planets are simply cooling off from their initial
state. 4.5 billion years ago the earth was a big
globe of molten stuff!, and it has cooled off in
the last 4.5 billion years.
A star gets power from nuclear reactions
occurring at its core.Planets don't have nuclear
reactions powering them. That is the
primary difference. They also form in
different ways, with stars being formed of mostly
hydrogen and light elements, while planets are
formed of heavier elements. Many of the heavier
elements in planets originally were formed by the
nuclear reactions at star cores. So literally you
are made of dead star parts. It's pretty cool.
Stars are massive enough that the hydrogen that
makes them up is under enough pressure in the
center that it can fuse to become helium. This
provides the energy that makes the star shine.
Planets don't have enough mass to create such
stupendous pressures and temperatures that are
needed for nuclear fusion.
In general, stars are much more massive than
planets and at some point in their "lives" are
capable of undergoing nuclear fusion (which is how
stars generate their energy). Planets, on
the other hand, do not undergo nuclear fusion,
and usually orbit stars.
A couple things make planets different than
stars. First is the way they move in the sky.
Ancient Greek astronomers referred to planets as
asteres planetai, or wandering stars,
because their position in the sky moves over time.
In 2006, the International Astronomical Union, a
collection of astronomers whose job it is to name
and classify stars and planets, declared that a
planet in our solar system must orbit the sun,
must be round in shape, and must have a much
larger mass than anything else in its orbit.
The makeup of planets is also different than
stars. Planets form from an already existing star
when matter (solid or gas) from the star begins to
condense around ice or rock cores to form a
rock/ice planet (for example, the earth) or gas
around a central core (for example,
Jupiter).Stars are also classified as having
high enough temperatures to perform nuclear
reactions in its core that burn hydrogen. As
it turns out, to have these high temperatures, a
star needs a mass of at least 75 times the size of
Jupiter. Therefore, a star is also much larger
than any of the planets in our solar system.
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