A dense, glowing ball of hydrogen, helium, and trace quantities of heavier elements that shines with energy released from a hydrogen thermonuclear fusion reaction in the center, or core. Our Sun is an example of a star. Stars can appear red, orange, yellow, blue, or white, according to their masses and effective temperatures. Stars are born from clouds of gas and dust, live for millions or billions of years, depending on their masses, and die sometimes spectacular deaths. Stars usually exist in very large collections called galaxies.