Stars That One Might Observe

Stars that one might observe with the naked eye have a life of their own. Many of the visible stars such as the sun are as old as the universe itself and their formation and lifespan gives clues to the processes of creation and destruction that have fueled the creation of the universe. Stars have a definite structure, defining characteristics, a life cycle, and a place in the universe.

Everyone knows that stars emit light and are, therefore, visible to the naked eye and to astronomers who use technology to observe the photons. A casual observer might even think the stars "twinkle," but this effect is a result of Earth's air movements that change the straight path of the stars' rays. Stars are considered light-emitting or luminous bodies. Their luminosity is "the rate at which a star emits energy" and is measured in watts compared relatively with the luminosity of the sun (Green, 2005, p. 2). Another facet of visibility of stars is their color spectrum. There are six bands that make up the electromagnetic spectrum and stars are capable of emitting all six, but not all of them do. The six bands are radio waves, infrared rays, visible light, ultraviolet rays, X rays, and gamma rays (Green, 2005, p. 3). A star's color is affected by the energy produced by it and measuring the emission lines in the star's electromagnetic spectrum is one way for scientists to determine the temperature and energy of a star.

Stars have many variants such as age, size, mass, and luminosity, but one thing they have in common is that "about 75% of all stars are members of a binary system, a pair of closely spaced stars that orbit each other" (Green, 2005, p. 1). Stars are also grouped together in galaxies such as the Milky Way which hosts more than 100 billion stars. "Three-dimensional computer models of star formation predict that the spinning clouds of dust may break up into two or three blobs; if true, this would explain why the majority of the stars in the Milky Way are paired or in groups of multiple stars" (, p.1). Clearly, the structure and formation of individual stars mimics the structure of other important elements of the universe.

What exactly is a star? "A star is a huge, shining ball in space that produces a tremendous amount of light and other forms of energy. The sun is a star, and it supplies Earth with light and heat energy...The sun and most other stars are made of gas and hot, gaslike substance known as plasma (Green, 2005, p. 1). The characteristics that define stars fall into five categories that are all related and interdependent. These categories are brightness, color, surface temperature, size, and mass. "These characteristics are related to one another in a complex way. Color depends on surface temperature, and brightness depends on surface temperature and size. Mass affects the rate at which a star of a given size produces energy and so affects surface temperature" (Green, 2005, p. 2).

The creation of stars and their life cycle depends largely on their mass. There are three types of stars: high-mass, intermediate-mass, and low-mass. The middle group is the most widely studied and understood (and contains well-known stars like the sun). The evolution of the intermediate-mass stars is a good example of the process by which stars are born, live, and die. This star begins as a swirling cloud of gas that takes 100,000 years to collapse into a protostar. Hydrogen fusion begins in the protostar and causes the creation of a T-Tauri star which is a variable brightness star. This new star contracts for 10 million years until the core energy is balanced with gravity. "The star has begun the longest part of its life as a producer of energy from hydrogen fusion, the main-sequence phase...The amount of time a star spends there depends on its mass" but is likely to be billions of years (Green, 2005, p. 6). When the balance between the production of fusion energy and gravity shifts, compression occurs and the star enters the "red giant" phase where it expands greatly and appears red in color. Another 100 million years brings another change and the star enters horizontal branch phase in which it grows smaller and burns steadily. The star endures another expansion and greater weakening in the next "asymptotic giant phase" and eventually becomes a "white dwarf" resembling a planet. The last phase for an intermediate mass star is the "black dwarf" phase in which it becomes too faint to even see (Green, 2005, p. 7).

The life cycle of a star is greatly affected by its mass. "More massive stars have greater central temperatures and densities and so exhaust their nuclear fuel more rapidly (in spite of the fact that they have more of it) than do lower mass stars" (Lochner, 2004, p. 3). Simply put, "the larger a star is, the shorter its life. Of course all things are relative -- stars live for billions of years" (, p.2).