Introduction

Stars come in many different shapes, sizes, and colors, each of which signifies a different type and life cycle. All stars are born in the same manner - they begin as nebulae, clouds of dust, hydrogen, and helium. These clouds slowly condense as they age, forming small hot areas of gas called protostars. The temperature of these protostars increases until nuclear fusion begins in the cores. At that point, a young star is born. The stars' sizes and eventual fates are determined by the heat, size, shape, and location of the original protostars. Stars formed in this manner generally begin their lives in clusters, but then are influenced by other objects around them and slowly drift apart. Mature stars are classified and their origins revealed by their masses, sizes, temperatures, colors, and magnitudes.

Blue Stars

Blue stars (such as Rigel and Regulus) are extremely hot, with average temperatures of 20 000-45 000 ° F. These stars are formed from very hot, large, and dense cores. It takes them about 100 million years, a relatively short time, to use up all their nuclear fuel. At this point, a blue star slowly expands to become a blue supergiant (Rigel) or an even larger blue-white supergiant. After millions of years, they cool down to form red supergiants, such as Betelguese.

In death, these supergiants will follow one of two major courses of action, depending on their mass. Those of great mass tend to be unstable and will explode, forming a supernova during which brightness is dramatically increased and the outer layers of the star thrown off into space. At the same time, the core of the star implodes, resulting in a dense remnant called a neutron star. These neutron stars rotate and send out pulses of radio waves, thereby earning the name pulsar. The most massive supergiants follow the same course with one exception - the neutron star they form is so dense that nothing, not even light, can escape its gravitational pull. These dense star remnants are called black holes.

Yellow Stars

Yellow stars actually come in three colors that correspond to their average temperatures and position in their life cycle: yellow-white (9000-11 000 ° F), orange (6500-9000 ° F), and red (5500-6500 ° F). Examples of these stars are the sun, Aldebaran, and Antares, respectively. These stars have relatively long life spans of about 10000 million years. When fusion eventually stops in the cores of these stars, expansion takes place and larger, cooler, more luminous red giants such as Aldebaran (actually orange-colored) are formed. These giants will simply extend their outer layers to form planetary nebulae, such as the Ring Nebula in Lyra. At the center of such a nebula will remain a small white dwarf, which is very dense and contains most of the original mass of the star. White dwarves do not generate their own heat, but instead radiate what heat they have left over a period of thousands of millions of years until they cool into red dwarves and eventually cold, dead black dwarves. (Note: under certain circumstances, white dwarves can become neutron stars; see Supersoft X-Ray Sources.)

The Hertzprung-Russell Diagram

The Hertzprung-Russell (H-R) diagram, named after the two astronomers who developed it in the early 1900's, is the plot of stars' temperature against their brightness. The diagram reveals a pattern of stars that follow a curved band from the upper left to the bottom right corner of the diagram. This band is called the main sequence and the stars that make it up are called dwarves (Types of dwarves were described above). The exception to this rule is the location of the white dwarves, which are small, faint, and very dense due to their origins in planetary nebulae. Giants and supergiants, which are brighter than other stars of the same temperature, are located in the upper right corner of the graph. Note: Black dwarves and black holes are not plotted on the diagram because these star types had not been discovered when Hertzprung and Russell developed their diagram.

The Structure of Stars in Their Prime

Stars at various stages of their life cycle have very different structures; however, stars in the main sequence have a general structure that is roughly the same as, and is exemplified by, our sun (shown in the diagram). The sun, like all stars, is made up of hydrogen and helium gases, which undergo nuclear fusion in the hot, fiery core. The core, at a temperature of 15 000 000 ° C, fuels the sun's shine and is made entirely of gases. From the core, energy moves out toward the surface of the star through the convection zone, an area in which gas rises and sinks in a manner similar to that of gas bubbles rising in boiling water. The sun's surface, its outer layer of gases, is much cooler than the rest of the sun at 5500 ° C. The chromosphere is a thin and spiky layer of gas outside the surface of the sun. During eclipses, the chromosphere can be seen as a reddish glow. The corona is an even thinner layer of gas that surrounds the chromosphere. It is much hotter than the sun's surface at 2 000 000 ° C, possibly because of the energy from the sun's magnetism. The solar wind, a steady stream of ions and electrons form the sun, is actually an extension of the corona. Large loops of gas that extend from the surface of the sun are called prominences and can stand up to 30 000 km high. Prominences are generally short-lived and fall to the surface again after only a few hours, but they can be held in position by the magnetic field for as long as several months. Sunspots, phenomena related to prominences, are cooler areas on the sun's surface where the magnetic field is especially strong. Sunspots usually appear according to a regular cycle that rotates on an 11-year basis. The most recent peak of the cycle occurred in 1991. When the sunspot cycle is at its peak, solar flares with temperatures of up to 4 000 000 ° C can erupt and last anywhere from ten minutes to an hour. Solar flares can speed up the solar wind and cause a magnetic storm on earth. Solar flares are also responsible for auroras, or the northern and southern lights.