Each star in the
sky is an enormous glowing ball of gas. Our sun is a
medium-sized star.
tars can live for billions of years. A star is born when
an enormous cloud of hydrogen gas collapses until it is
hot enough to burn nuclear fuel (producing tremendous
amounts heat and radiation). As the nuclear fuel runs out
(in about 5 billion years), the star expands and the core
contracts, becoming a giant star which eventually explodes
and turns into a dim, cool object (a black dwarf, neutron
star, or black hole, depending on its initial mass). The
largest stars have the shortest life span (still billions
of years); more massive stars burn hotter and faster than
their smaller counterparts (like the Sun).
The composition of stars is studied
using spectroscopy in which their visible light (the
spectrum) is studied.
GROUPS OF
STARS
In the universe, most stars occur in groups of at least
two stars. Two stars that are locked in elliptical orbit
around their center of mass (their barycenter) are called
a binary star system. About half of all stars are in a
binary star system.
There are larger groups of stars,
called clusters. These are relatively unorganized
collections of stars. An open cluster is a loose
collection of up to about 1,000 stars. Examples of open
clusters include the Pleiades and Hyades.
Huge, organized collections of stars are called galaxies.
Our solar system is located in the Milky Way Galaxy, a
spiral galaxy. For more in-depth information on galaxies,
click here.
All groups of stars are held together by gravitational
forces.
GLOBULAR CLUSTER
A globular star cluster is a spherical group of up
to a million stars held together by gravity. These remote
objects lie mostly around the central bulge of spiral
galaxies
WHY ARE STARS HOT AND BRIGHT?
Nuclear Fusion and Nucleosynthesis
Stars are giant nuclear reactors. In the center of stars,
atoms are taken apart by tremendous atomic collisions that
alter the atomic structure and release an enormous amount
of energy. This makes stars hot and bright. In most stars,
the primary reaction converts hydrogen atoms into helium
atoms, releasing an enormous amount of energy. This
reaction is called nuclear fusion because it fused the
nuclei (center) of atoms together, forming a new nucleus.
The process of forming a new nucleus (and element) is
nucleosynthesis.
For more information on nuclear fusion in stars, click
here.
WHAT IS THE
CLOSEST STAR?
The closest star to us is the sun! Other than that, the
closest star is Proxima Centauri, aka Alpha Centauri C
(the dimmest star in the Alpha Centauri system). Proxima
Centauri is 4.3 light-years from the Sun. It has an
absolute magnitude of 15.5.
WHY DO STARS TWINKLE?
The scientific name for the twinkling of stars is stellar
scintillation (or astronomical scintillation). Stars
twinkle when we see them from the Earth's surface because
we are viewing them through thick layers of turbulent
(moving) air in the Earth's atmosphere.
Stars (except for the Sun) appear as
tiny dots in the sky; as their light travels through the
many layers of the Earth's atmosphere, the light of the
star is bent (refracted) many times and in random
directions (light is bent when it hits a change in density
- like a pocket of cold air or hot air). This random
refraction results in the star winking out (it looks as
though the star moves a bit, and our eye interprets this
as twinkling).
Stars closer to the horizon appear to
twinkle more than stars that are overhead - this is
because the light of stars near the horizon has to travel
through more air than stars overhead and subject to more
refraction. Also, planets do not usually twinkle - they
are big enough that this effect is not noticeable (except
when the air is extremely turbulent).
Stars would not appear to twinkle if
we viewed them from outer space (or from a planet/moon
that didn't have an atmosphere).
STELLAR WIND
Stellar wind is ionized gas that is ejected from the
surface of a star (including the Sun). Older (evolved)
stars give off stronger stellar winds than younger stars.
