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Remember the line from the children’s saying that goes: “Starlight, star bright, first star I
see tonight”? Looking at the light from stars is exactly how astronomers can learn about
them. But, stars do not just off so-called visible light, the type we can see with the
human eye. Rather, astronomers need to study stars in many different types of radiation
to get a complete picture of what is really happening of them.

Dr. Nancy Brickhouse studies stars with the Chandra X-ray Observatory and other
telescopes and instruments. She explains some of the basics of what stars are.

A star is a ball of hot gas and in the center it's very very dense and in the center a star is
producing nuclear fusion reactions, and that's basically what makes the star hot. As you
go out into the atmosphere of the star, eventually you reach what we call the photosphere
which is the surface of the star. The visible light color of the star is actually coming from
the photosphere.

But, even though that might sound straightforward, there’s really nothing simple about
stars. To begin with, there are many different kinds of stars: huge red stars called red
giants, small hot white stars called white dwarfs, small cool brown stars called brown
dwarfs, and more. Dr. Brickhouse explains how astronomers can keep track and X-rays
are used to help categorize these very different objects.

Stars have a temperature that's related to the color of the star so the blue stars are hotter
than the yellow and red stars, and so we have a classification scheme that's going from
hot to cool and the sun is considered to be a cool star. And this is based on their visible
light properties. The color is basically related to the color of the photosphere. When we
look at these stars in x-rays we're looking above that surface, so we're looking higher up
in the atmosphere of the star, where the gas is really not as dense, but it's much hotter.
And it turns out that both hot stars and cool stars make x-rays but the processes are
different. So when we look at cool stars we're looking at processes that are more like the

But where does our Sun fit into the scheme of things? What does Chandra and its X-ray
data tell us about our nearest star?

We can look at the sun with other satellites and there have been x-ray missions for just
studying the sun, but it would wipe out Chandra's detectors if we used Chandra to look at
the sun, but we can study other stars and it turns out that 10 to a hundred thousand times
more energy is produced in x-rays than what the sun produces, so by the standards of
other stars, the sun is kind of a wimpy x-ray producer.

So, astronomers can’t use Chandra to study our Sun directly. But, what kind of insight
do astronomers actually get by studying other Sun-like stars using Chandra. It turns out
to be a very valuable thing to do.

One of the reasons to look at other stars is to get some understanding of what the
processes are that cause the x-ray emission and what kind of diversity there might be
because it will help our models if we just have one system to look at even though we can
look at it in a lot of detail we don't necessarily understand the possibilities that would
help us constrain our models.

Studying all kinds of stars – including those like our Sun and those that are very different
– remains an important area of research in astronomy and astrophysics. The starlight we
wish upon in that rhyme turns out to be much more complicated and informative than we
may have ever expected. Researchers using Chandra are helping to address some of
questions whose answers may only be found in their X-ray light.

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