Stars
A diagram showing the various parts of a
star.
The Parts of a Star
In order to understand what we see when we look at
stars similar to our
Sun in the X-ray
region of the EM spectrum, we need to understand "the parts of
stars". A star during most of its life is called a "main-sequence
star". It consists of a central core, convective and radiative zones, the
photosphere, the chromosphere, and the
corona. The core
of the star is where all the
nuclear
fusion reactions occur to power the star. The convective and radiative
zones allow the energy produced in the nuclear reactions to move outward. In
the convective zone, energy is transported by the hot gases swirling around,
just like hot air moving heat from a hair dryer to your hair. In the
radiative zone, energy is transported by
radiation,
like heat from a light bulb. Next, there is the
part of the star that we see in
visible light, the
photosphere. We often refer to the photosphere as the "surface of the
star", although it is not a real surface like the surface of the Earth.
Just outside the photosphere is the chromosphere (a thin layer which appears
red to us when we can see it because of all the hydrogen found there). This
part of the stellar atmosphere can only be seen when the main
disk (or photosphere)
of the star is blocked off, like during an
eclipse, or if we
look at the Sun using a special filter which only lets in the red light of
hydrogen. Finally, the outermost part of the stellar "atmosphere"
is the corona. It is a very hot (over a million degrees!) thin gas.
X-rays from "normal" stars
Just like for the Sun, what we see in X-rays when we look at other main
sequence stars are not the surfaces, but the coronae. Many of the fine
details we can see in our Sun are lost when we observe stars much farther
away, but we can still study how the X-ray emissions vary from star to star.
In this way, scientists have been able to learn a great deal about the
relationship between the X-ray emissions and other properties of the stars,
such as the mass, the
age, the rotation period, and the chemical composition.
While our Sun is a fairly bright X-ray source because it is so close to us,
it is not easy to detect other main sequence stars in X-rays. In fact,
it was not until the
Einstein satellite
was launched in 1978 that astronomers had a sensitive enough detector to open
up the field of X-ray observations of the nearby stars.
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