During the course of a star's lifetime, it goes through different phases as the nuclear fusion reactions inside it change.
A star can become a type of giant star called a red giant when it has converted all the hydrogen fuel in its core into helium and begins burning up the hydrogen in its outer layers to resist gravity's inward pressure. As this happens, the star swells greatly in size and it glows more brightly. Due to subsequent changes in the fusion reactions, the star may move in and out of the red giant state, becoming different types of giant star.
Image: A comparison of the relative sizes and masses of different star types (credit: NASA, ESA and A. Field/STScI)
Dr Chris Lintott gives a tour of the stars that make up Orion, The Hunter, a constellation easily seen during the winter months in the northern hemisphere. This includes the instantly recognisable Orion's Belt - a diagonal line of three stars under which hangs a sword - a vertical collection of stars and nebulae.
As the Sun ages, it will gradually become a red giant as its hydrogen fuel begins to run out. Its surface will expand to approximately 100 times its current size as its core shrinks, and the inner Solar System will be engulfed.
Howard Stableford of Tomorrow's World takes to a hot air balloon to explain how a star explodes in a supernova and leaves behind a neutron star or a black hole.
Professor Brian Cox explains that we can predict what will happen to the Sun by looking at other stars in our galaxy. He shows how the Solar System will change as the Sun becomes a red giant.
A giant star is a star with substantially larger radius and luminosity than a main-sequence star of the same surface temperature. Typically, giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun. Stars still more luminous than giants are referred to as supergiants and hypergiants. A hot, luminous main-sequence star may also be referred to as a giant. Apart from this, because of their large radii and luminosities, giant stars lie above the main sequence (luminosity class V in the Yerkes spectral classification) on the Hertzsprung–Russell diagram and correspond to luminosity classes II or III.
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