In contrast to the planets, the stars seem to be motionless, or fixed to their spots on the celestial sphere. But it only looks that way because they are so far away from us―distances that are measured in light-years. In reality the fixed stars also move in the sky, but they do so at such a slow pace that their motion (called “proper” motion) becomes visually apparent only over thousands of years and otherwise can be proven to exist only through precision measurements. The fixed stars shine with their own light. They are suns like our Sun, that is, spheres of incandescent gases (called plasma by scientists). In the interior of these spheres, energy is produced through thermonuclear reactions. The number of suns in the universe is vast beyond guessing. The unaided eye can detect about 4,000 stars on a very clear night; a telescope reveals many millions in the region of the Milky Way alone. The stars are so far removed from Earth that their distance is difficult to measure. Distances up to about 70 light-years can be established with relative accuracy by using a method similar to triangulation, which is used by surveyors. Greater stellar distances are determined indirectly. Because a star (like any other luminous body) appears fainter the farther away it is, scientists try to calculate a star's magnitude theoretically by applying methods based on physics. They then compare the results with the actual magnitude observed. However, this method is less accurate, and the distances given by different astronomers vary considerably, diverging sometimes as much as 100 percent, especially for very remote stars. Many stars do not always exhibit the same brightness. Instead they change their magnitude in cycles anywhere from several hours to several years in length, but most commonly with so-called periods of up to 100 days. In many of these variable stars the magnitudinal fluctuations occur with considerable regularity, whereas in others they follow no clearly recognizable pattern. There are two reasons for the fluctuations in brightness. Either the physical characteristics of the star change―it may be growing larger or smaller or its surface temperature may rise and drop―or the star is concealed by another star that stands in our direct line of vision―as the Sun is hidden by the Moon in a solar eclipse―and blocks the light of the star behind it from our view for a certain period. One of the most fascinating areas of research in modern astronomy is the study of the life history of stars. Stars are formed from huge interstellar gas clouds in whose interior gases and particles of matter gradually concentrate. A star is born when, at the core of this concentrated mass, energy is first produced through the conversion of hydrogen, the most prevalent element in space, into helium. The star then goes on to spend the main part of its life in a stable state, shining with a steady light. But at some point all the hydrogen is used up and the star enters its next life stage. It tries to find alternate sources of energy by transforming the helium it has produced. But this is accomplished only with difficulty. The interior parts of the star contract while the outer ones expand. The star swells up in size and becomes a red giant. Eventually, when all sources of energy are exhausted, the star's life comes to an end. The star goes through a final, cataclysmic reaction and then either dies in a spectacular supernova explosion or simply ceases to emit light. The outer layers go hurling into space as gas nebulas, and what remains is either a star corpse whose glow gradually fades away―a white dwarf―or a neutron star. Conditions in such a neutron star are beyond anything imaginable: The mass of the entire Sun can be compressed into a ball 6 to 12 miles in diameter, and one cubic centimeter of matter weighs 10 million to 1 billion tons. The life span of a star depends largely on its initial mass. Heavy stars with more than five times the mass of our Sun live a relatively short time, about 100 to 200 million years, whereas stars like our Sun have a life span of 7 to 10 billion years.