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Structure of a normal star
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 detector sensitive enough to open up the field of X-ray observations of the nearby stars.

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

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All material on this site has been created and updated between 1997-2012.

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