Special Stars
[Key Words] [General] [Binary
Stars] [Variable Stars]
General
A star is not just a star. In the previous section, you
have learned that as star goes through many stages in its life,
it becomes a different type of star. These are not the only types
of stars. This section will explain about more types of stars,
such as binary
stars and variable stars.
Binary Stars
A binary star is actually two or more stars. The gravity
produced by each star keeps them together. Centrifugal
force keeps these two stars from crashing in to one another.
There are different types of binary stars. Sometimes binary stars
have equal
masses, but usually the stars have different masses.
An example of this would be a binary star consisting of a white
dwarf and a red giant. Other binary stars are composed of more than two stars.
An example of this is two pairs of binary stars revolving around
each other. These types of binary stars usually have highly
elliptical orbits.
Variable Stars
Variable
stars are stars where the total light we see from earth
changes. This can happen because of one of two reasons. Either an
eclipse
is taking place in a binary star system between two stars (known
as an extrinsic variable), or the star is actually
giving off more or less light (known as an intrinsic
variable). Observations of the magnitude of variable stars
over time yield a graph called a light
curve.
Extrinsic Variable:
In a binary star system, two stars revolve around each other.
Sometimes, earth is right along their plane of orbit. When this
is the case, one star will eclipse another star, preventing it's
light form getting to earth. Although the total output of light
from the star is the same, when this happens, the binary star
looks dimmer because of the eclipse.
Intrinsic Variable:
These variable stars actually change in brightness. There are
three different types of these, cepheids,
long-period variables, and novae.
Cepheids
are the most numerous and well known variable stars.
Cepheids are supergiants that pulsate, which changes surface
temperature and spectral type. This happens over a period of a
few days to a few months. There is a numerical relationship
between the period of pulsation and the star's absolute
magnitude. By finding out the star's absolute magnitude by its
pulsation rate and comparing it to its apparent magnitude, an
astronomer can then determine how far away the star is. This is
why cepheids are know as astronomical yardsticks.
Long-period
variables are red supergiants with very
long periods of pulsation and extreme magnitudes.
Novae
are explosions on binary stars. A nova occurs usually between a
red giant and a white dwarf. About every 50 years, matter is
transferred from the larger star to the smaller star. A nuclear
chain reaction begins to take place on the surface of the smaller
star. The reaction ceases, and material gets blown off of the
surface of the star. The star glows brightly, and days or weeks
later, after the star fades, the process begins again.
Previous Topic: Life
Cycle of a Star Next Topic: Spectral
Types
[Key Words] [General] [Binary
Stars] [Variable Stars]
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