What is it about the stars that has intrigued humankind for thousands of years? Most people who look up towards the sky in search of stars feel no particular compulsion to learn about stellar life cycles or a star's absolute magnitude or spectral class. Their interest is not in the mathematical calculations and scientific data that fascinate astronomers. They look skyward for the same reason that poets and songwriters have done so for centuries -- to absorb the silent beauty of a starry night. Throughout history, stargazing has been motivated by more than admiration for celestial objects, however. As early as 3,000 B.C., the ancient Babylonians studied the night sky and identified various constellations. Other early civilizations created star maps and tracked the position of the stars for navigation and timekeeping purposes. Their efforts were succeeded by advancements in science and technology that enabled modern society to understand more fully those twinkling lights in the sky.
Although Galileo used a telescope in the early seventeenth century to make important discoveries about our solar system, his instrument had limitations -- chiefly, distortion of the image. The first practical reflecting telescope was invented by Isaac Newton around 1670. By the close of the 1600s, there were a number of telescopic observatories. During the next two centuries, hundreds of stars were observed and catalogued. In 1838, Friedrich Bessel computed the distance of a star for the first time. Further studies and innovations helped astronomers to make detailed observations of the spectra, or white light, of stars, which was key to determining a star's chemical composition. In 1849, the first photos of stars were taken at an observatory in Boston.
In recent times, astronomers have learned a great deal about stars. From observation of stars at different stages, astronomers have theorized that stars have existed for hundreds of millions or billions of years. Stars are formed from dust and gas in space. They are born in regions of space called nebulae, in which gravity prompts interstellar matter to contract, generating heat. The result is the creation of a protostar. If a protostar's temperature rises high enough, nuclear fusion reactions at its core will transform it into a true star. During this stage, a star is classified as a main-sequence star.
A main-sequence star is essentially a huge ball of glowing gas with a lifespan of about 10 billion years. It is fueled by hydrogen, and when this starts to run out, further gravitational contractions occur and the star collapses. It then undergoes elemental changes that convert it into a red giant or a red supergiant if it has enough mass. After around 100 million years, supergiants collapse due to gravitational forces. It is a spectacular event. The collapse itself takes under a second and is followed by an enormous explosion called a supernova -- so bright it can outshine the parent galaxy. The remnants, or debris, are flung throughout space, becoming a nebula in which new stars will be born.
Everyone can readily identify the star that is indispensable to life on Earth -- the Sun. Due to its proximity to Earth, it bathes our planet with sunlight -- the source of almost all of the energy on Earth. It is its very closeness that makes the Sun look like a giant yellow star. It is true that the Sun's diameter is about 100 times that of the Earth's. Nevertheless, the Sun is an average-sized star. When it reaches the end of its life as a main-sequence star, the Sun will become a red giant -- not a supergiant. A red giant has the same lifespan as a supergiant and collapses for the same reasons, but it does not explode. Instead, it shrinks into a star called a white dwarf, which is cold and can be smaller than Earth. The Sun is too bright for direct observation. Hence, astronomers have devised methods and instruments, such as the spectrometer, to study it.
Fortunately, the Sun is the exception; gazing directly at other stars is not a problem. Nonetheless, people who live in highly populated areas, such as cities, do not see the same night sky that ancient civilizations enjoyed. Modern technology has a price. The electric lights that allow us to see at night obscure most of the 6,000 stars that would otherwise be visible from Earth with the unaided eye.
In Walt Whitman's famous poem, the speaker, "tired and sick" of the scientific data, walks out of an astronomer's lecture. In the "mystical moist night-air," he looks up "in perfect silence at the stars." Our fascination with the stars may never end.
