Star

In this section we will examine four stars in detail. At the high end of the mass scale is Alnilam, the central star in Orion's belt, whose radius is 50 times that of our Sun. Next comes Regulus, the brightest star in the constellation Leo (the Lion). Regulus has a radius five times our Sun and a lifetime of 300,000,000 years. Next is the Sun, with a lifetime of 10,000,000,000 years, and finally Proxima Centauri, the nearest star beyond the Sun, but so tiny, at one tenth the radius of the Sun, and faint that it is invisible to the unaided eye. These four stars are representative of the different properties and life cycles that stars can have.

Luminosity. Although Regulus is only 4.5 times more massive than the Sun, its luminosity, or rate of energy output, is 200 times greater. Stable stars obey a mass-luminosity relation, which can be expressed as an equation of the form L = 3.5M, where L is the luminosity in solar units, and M is the mass in solar units. Since luminosity is related to fuel consumption rate, more massive stars have to burn their fuel much more rapidly than less massive ones to remain in hydrostatic equilibrium.

Lifetime. The mass-luminosity relation spells trouble for Regulus. Although Regulus has nearly five times as much fuel as the Sun does, it fuses it into helium 200 times faster. We therefore expect that Alnilam will live as a healthy star only about 0.025 (5/200) times as long as the Sun. By the same argument, tiny Proxima Centauri should live for an enormously long time. Long after the Sun, Regulus, and Alnilam have gone out, Proxima will still be glowing.

Energy transport. Energy flows from hot regions to cool regions. If you let a cup of hot chocolate sit for a while, it gradually gets cold as its heat dissipates into the Figure 4. The light curve of Algol, one of the most famous variable stars in the sky. Illustration by Hans & Cassidy. Courtesy of Gale Group. surroundings. Therefore, energy flows from a star's intensely hot core outward to its surface, and it does so in two ways. One is called radiation, which is the normal flow of electromagnetic radiation through a medium such as a star's gas. The other is called convection, and occurs when large, hot bubbles of gas rise, deposit their heat into a cooler, higher layer of the star, and then sink back down where they are reheated to begin the cycle anew. Convection is the phenomenon that builds cumulus clouds into towering thunderstorms on a hot summer day. Massive stars like Alnilam and Regulus have convective cores and radiative envelopes (envelope is the term used to describe the layers outside the core). Less massive stars like the Sun have radiative interiors with a convective zone just below their surface. Proxima Centauri is convective throughout. The type of transport mechanism a star uses at any point in its interior is determined by the local temperature structure, which in turn is governed by the star's mass.

Surface Temperature. When we speak of a star's surface, we usually mean the photosphere, which is the thin layer from which the star emits most of the visible light that reaches our eye. The photosphere is not a surface as we usually think of it, since it is thousands of times less dense than air. Below the photosphere, however, there is still enough stellar material between a ray of light and empty space that the light cannot escape. Above the photosphere, light can escape without interacting with any of the star's matter, and this defines the boundary between the star's interior and its atmosphere. More massive stars are hotter than less massive ones, because their gravity is stronger and their gas pressure (which is related to temperature) has to be higher to counteract this strong gravity. Regulus's photosphere is about 12,000K (21,092°F; 11,700°C), and at this temperature it blazes with a brilliant, white light. Proxima Centauri, if you could see it, would be a dull red, with a photosphere of only about 3,000K (4,892°F; 2,700°C).

Atmosphere. The photosphere is the innermost layer of the star's atmosphere. The Sun's photosphere is only 300 mi (500 km) thick-minuscule when compared with its radius of almost 210,000 mi (350,000 km). We might expect the temperature to keep dropping as we move outward though the atmosphere, but this is not the case. In the Sun, the temperature rises sharply a few thousand kilometers above the photosphere. This region, which in the Sun is about 10,000K (17,492°F; 9,700°C), is called the chromosphere. Further out, the temperature rises even further, culminating in a corona of perhaps 2,000,000K (2,000,000°C). Finally, beyond the corona, the temperature drops off and we have reached empty space and the "end" of the star. The existence of chromospheres and coronae baffled the scientists who discovered them and no one yet fully understands the nature of these regions.

Circumstellar environment. Most stars lose mass in a stellar wind. In stars like the Sun, the wind is an insignificant portion of the total mass, but many stars have enhanced winds that carry off an important part of their mass. Because mass is the property that governs a star's evolution, mass loss can play an important role in altering the star's evolution. The star Betelgeuse, for example, is an enormous, cool, red star that may end its life in a catastrophic supernova explosion—unless its strong wind carries off enough to prevent it. Additionally, stellar winds are a contributor to the replenishment and enrichment of the interstellar medium, the thin gas between the stars. During its life, therefore, a star contributes to the evolution of the galaxy it belongs to, as well as to future generations of stars.