| In the Universe
Universe there are more than ten thousand billion billion
stars. (picture 1) In our galaxy,
the Milky
Way, one counts more than one hundred billion of them.
These stars are small and large Suns. To be able to understand
the Sun, one must initially be interested in stars. Some
of them are similar to the Sun in its younger days, others
look like the Sun at the end of its life: a star which
will have ejected its external layers to become a planetary
nebula. (picture 2) Astronomers
specialize in the study of a quite precise phase of
the star life or in the understanding of a well-defined
star type. This way they obtain a complete vision of
stars and their evolution.
Many of the stars do not live alone: they form groups
(picture 3) or live in pairs.
(video 1) Within such binary
systems, each star turns around the other one. From
this rotation, the astronomers manage to determine the
mass of both stars.
Some of the stars lose a part of their mass. This ejected
material is either attracted by the companion of the
star (picture 4) whenever
there is one, or is spread in the interstellar environment.
Many stars pulsate;
they are called variable stars. (video
2) The study of these pulsations provides information
about the star's interior.
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