Star, large incandescent ball of gases held together by its own gravity. The sun is a fairly normal star in its composition, parameters, and color. It is believed that stars originate as condensations out of interstellar matter. In certain circumstances a protostar will form, slowly contracting under its own gravity, part of the energy from this contraction being radiated, the remainder heating up the core; this stage may last several million years. At last the core becomes hot enough for thermonuclear reactions to be sustained and stops contracting. Eventually the star as a whole ceases contracting and radiates entirely by the thermonuclear conversion of hydrogen into helium; it is then said to be on the main sequence. When all the hydrogen in the core has been converted into helium, the now purely helium core begins to contract while the outer layers continue to “burn” hydrogen; this contraction heats up the core and forces the outer layers outward, so that the star as a whole expands for some 100–200 million years until it becomes a red giant star. Although the outer layers are comparatively cool, the core has become far hotter than before, and thermonuclear conversions of helium into carbon begin. The star contracts once more (though some expand still further to become super-giants) and ends its life as a white dwarf star. It is thought that more massive stars become neutron stars, whose matter is so dense that its protons and electrons are packed together to form neutrons; were the sun to become a neutron star, it would have a radius of less than 12.5 mi (20 km). Finally, when the star can no longer radiate through thermonuclear or gravitational means, it ceases to shine. At this stage some stars may undergo ultimate gravitational collapse to form black holes.