If you follow the various crime dramas on TV, you know that once the villain’s DNA is found the crime scene analysts can tell everything about him: sex, age, height, weight, race, what he had for dinner last night and probably where he lives. Whether that is real science or not, light tells just about everything there is to know about a star.
In 1842 French philosopher Auguste Comte tried to categorize certain things that would forever remain unknown. Among those was “the chemical and mineralogical” makeup of stars. He would be proven wrong within two years of his death.
Light is a form of energy. Physicists think of light as being a wave, much like waves in water. The wavelength of light is measured by the distance from the crest of one wave to the crest of the next. The shorter the wavelength of light, the more energy it has and vice versa. Humans tend to think of light only in terms of those wavelengths that the eye can detect, which range from the longest wavelengths of red to the shortest of violet. Wavelengths longer than red are called infrared, while wavelengths shorter than violet are called ultraviolet. Scientists, however, often lump all electromagnetic radiation under the heading of “light.” Other forms of electromagnetic radiation are radio waves, which have wavelengths in the hundreds of meters (a meter is roughly a yard), to microwaves, exactly what are used in microwave ovens and are what the police really use in radar guns, and are still much longer than infrared light, to gamma and x-rays, which have very short wavelengths and thus high energy, which allows x-rays to penetrate solids. The higher energy of the shorter wavelength light, such as x-rays and gamma rays, can cause damage to humans. Gamma rays are one of the deadly emissions from a nuclear explosion.
Since light is a form of energy scientists can use light to determine the temperature of a star. An object heated to about 500 degrees Celsius (900 degrees Fahrenheit) begins to glow red, literally red-hot. At 3,000 C (5,400 F), it begins to emit white light. So by analyzing the spectrum of light given off by a star, science can determine its temperature.
In 1752 Scottish physicist Thomas Melvill noticed that different substances emitted different colors when burned. By carefully categorizing the colors emitted by the burning of various elements and comparing those to the light given off by a star the star’s composition can be deduced. Melvill wasn’t expecting this nor was he looking for it. His discovery illustrates the adage that most scientific discoveries are accompanied not by a cry of “Eureka!” but by a murmured “that’s funny.”
Each element has a distinctive color, which acts as its DNA, allowing it to be identified just by looking at it. This combination of light, color and atoms is called spectroscopy. The science of studying light emitted by objects is called spectroscopic emission. The opposite phenomenon, absorption of certain light wavelengths by a substance, also exists and that is called spectroscopic absorption. Spectroscopic absorption allowed scientists to determine the composition of the sun. By noting which wavelengths were absorbed and therefore not visible in the light emitted from the sun, scientists could determine what elements were present in the sun that absorbs those wavelengths.
In addition to determining the composition and temperature of stars from starlight, the star’s velocity can also be found. This stunning discovery was made by Thomas Huggins and his wife, Margaret, herself an accomplished astronomer and 24 years his junior. When Thomas, at 84, was too feeble to clamber around a telescope, his young wife, only 60, was able to take over.
Science had long known that stars appear to change position in the sky relative to Earth and to other stars. This movement across the sky is called proper motion. However, proper motion is incremental and even with advanced technology is difficult to detect. In addition, proper motion only tracks movement of a star laterally in the sky, as if all the stars were in the same plane. Proper motion can tell nothing about the movement of a star either toward or away from the earth.
Mr. and Mrs. Huggins were able to combine spectroscopy with a piece of physics known as the Doppler Effect, after Christian Doppler, the Austrian physicist who discovered it, to determine the velocity of stars. The Doppler Effect would become key in proving the Big Bang Theory.