SUMMARY OF KEY IDEAS
The Nature of Light
Photons, packets of vibrating electric and magnetic fields, all carry energy through space at the same speed, the speed of light (300,000 km/s in a vacuum, slower in any medium).
From longest to shortest wavelengths, radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-
rays, and gamma rays are all the forms of electromagnetic radiation. Photons behave sometimes as particles, sometimes as waves.
Visible light occupies only a small portion of the electromagnetic spectrum.
The wavelength of a visible-
light photon is associated with its color. Wavelengths of visible light range from about 400 nm for violet light to 700 nm for red light.
Optics and Telescopes
A telescope’s most important function is to gather as much light as possible. When possible, it also resolves (reveals details) and magnifies an object.
Reflecting telescopes, or reflectors, produce images by reflecting light rays from concave mirrors to a focal point or focal plane.
Refracting telescopes, or refractors, produce images by bending light rays as they pass through glass lenses. Glass impurity, opacity to certain wavelengths, and structural difficulties make it inadvisable to build extremely large refractors. Reflectors are not subject to the problems that limit the usefulness of refractors.
Earth-
based telescopes are being built with active optics and adaptive optics. These advanced technologies yield resolving power comparable to the Hubble Space Telescope.
Capturing Nonvisible Light: Nonoptical Astronomy
Radio telescopes have large, reflecting antennas (dishes) that are used to focus radio waves.
Very sharp radio images are produced with arrays of radio telescopes linked together in a technique called interferometry.
Earth’s atmosphere is fairly transparent to most visible light and radio waves, along with some infrared and ultraviolet radiation arriving from space, but it absorbs much of the electromagnetic radiation at other wavelengths.
For observations at other wavelengths, astronomers mostly depend upon space telescopes. Such satellite-
based observatories are giving us a wealth of new information about the universe and permitting coordinated observation of the sky at many wavelengths. Charge-
coupled devices (CCDs) record images on many telescopes used between infrared and X- ray wavelengths. By studying the wavelengths of electromagnetic radiation emitted and absorbed by an astronomical object, astronomers can learn about the object’s temperature, chemical composition, rotation rate, companion objects, and movement through space.
Blackbody Radiation
A blackbody is a hypothetical object that completely absorbs all the electromagnetic radiation that strikes it. The relative intensities of radiation at different wavelengths that it then emits depend only on its temperature. Stars closely approximate blackbodies.
Wien’s law states that the peak wavelength of radiation emitted by a blackbody is inversely proportional to its temperature—
the higher its temperature, the shorter the peak wavelength. The intensities of radiation emitted at various wavelengths by a blackbody at a given temperature are shown as a blackbody curve. The Stefan-
Boltzmann law shows that a hotter blackbody emits more radiation at every wavelength than does a cooler blackbody.
Identifying the Elements by Analyzing Their Unique Spectra
Spectroscopy—
the study of electromagnetic spectra— provides important information about the chemical composition of remote astronomical objects. Kirchhoff’s three laws of spectral analysis describe the conditions under which absorption lines, emission lines, and a continuous spectrum can be observed.
Spectral lines serve as distinctive “fingerprints” that identify the chemical elements and compounds comprising a light source.
Atoms and Spectra
An atom consists of a small, dense nucleus (composed of protons and neutrons) surrounded by orbiting electrons. Atoms of different elements have different numbers of protons, while different isotopes have different numbers of neutrons.
Quantum mechanics describes the behavior of particles and shows that electrons can only be in certain allowed orbits around the nucleus.
The nuclei of some atoms are stable, while others (radioactive ones) spontaneously split into pieces.
The spectral lines of atoms of a particular element correspond to the various electron transitions between allowed orbits of that element. Each orbit has a different energy level. When an electron shifts from one energy level to another, a photon of the appropriate energy (and, hence, a specific wavelength) is absorbed or emitted by the electron.
All different elements, isotopes, and molecules have different sets of spectral lines.
When an atom has more protons than electrons, or vice versa, it is said to be charged. An atom loses an electron when the electron absorbs a sufficiently energetic photon, which rips the electron out of orbit.
The motion of an object toward or away from an observer causes the observer to see all of the colors from the object blueshifted or redshifted, respectively. This effect is generically called a Doppler shift.
Motion across the sky, called proper motion, yields no Doppler shift.
WHAT DID YOU THINK?
What is light? Light—more properly “visible light”—is one form of electromagnetic radiation. All electromagnetic radiation (radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X- rays, and gamma rays) has both wave and particle properties. 
Which type of electromagnetic radiation is most dangerous to life? Gamma rays have the highest energies of all photons, so they are the most dangerous to life. However, ultraviolet radiation from the Sun is the most dangerous electromagnetic radiation that we commonly encounter.
What is the main purpose of a telescope? A telescope is designed primarily to collect as much light as possible.
Why do all research telescopes use mirrors, rather than lenses, to collect light? Telescopes that use lenses have more problems, such as chromatic aberration, internal defects, complex shapes, and distortion from sagging, than do telescopes that use mirrors.
Why do stars twinkle? Rapid changes in the density of Earth’s atmosphere cause passing starlight to change direction, making stars appear to twinkle. Seen from space, stars do not twinkle.
Which is hotter, a “red-hot” or a “blue- hot” object? Of all objects that glow visibly from heat generated or energy stored inside them, those that glow red are the coolest. 
What color does the Sun emit most brightly? The Sun emits all wavelengths of electromagnetic radiation. The colors it emits most intensely are in the blue-green part of the spectrum. Because the human eye is less sensitive to blue- green than to yellow, and because Earth’s atmosphere scatters blue- green wavelengths more readily than longer wavelengths, we normally see the Sun as yellow.