Page 23 - Curriculum Visions Dynamic Book

 These are hot blue stars deep inside an elliptical (oval) galaxy seen in ultraviolet light. This picture shows 8,000 blue stars in galaxy M32, some 2.5 million light-years away in the constellation Andromeda.
The ultraviolet light comes from extremely hot, helium-burning stars at a late stage in their lives. Unlike the Sun, which burns hydrogen into helium, these old stars exhausted their central hydrogen long ago and now burn helium into heavier elements.
Stars are born when enough interstellar matter comes together for a large gravitational field to develop.
As gravity pulls the matter (which is mostly hydrogen) together, it heats up the gas so much that nuclear reactions take place. That generates more heat, and the star begins to shine. This is the birth of a star.
As hydrogen burns, it is converted to helium. During this conversion four hydrogen nuclei change into one helium nucleus. But not all of the nuclear energy that bound the hydrogen atoms is required to form a helium nucleus, and so some is released as radiation. Among the forms of radiation released are heat and light, which make a star shine brightly.
Mature stars
Not all stars have the same life. Stars that are smaller than the Sun burn hydrogen much more slowly than a star that is bigger than the Sun. As a result, small stars have a longer life than their large, hydrogen-eating cousins.
A star about half the size of the Sun cannot generate the massive burning that occurs in a big star. Because of this it burns its fuel more slowly and lasts for a long time, often billions of years, as our Sun demonstrates. On the other hand, a star 50 times the size of the Sun can process hydrogen much more quickly and spectacularly. As a result, although it contains more hydrogen than a small star, it uses it up faster and lasts for only three million years.
 This is the “polar-ring” galaxy NGC 4650A, located about 130 million light-years away. The bright bluish clumps, which are especially prominent in the outer parts of the ring, are regions containing shining young stars.
gravitational field The region surrounding a body in which that body’s gravitational force can be felt.
Small stars can last as long as galaxies. Big, bright blue stars, on the other hand, with lifetimes ten thousand times shorter than small stars, come and go very fast during the lifetime of a galaxy, being born among the clouds of gas and dust in the galaxies. You can see this happening in many pictures in this book.
gravity
bodies.
The force of attraction between
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