Summary of Key Ideas
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
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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.
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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.
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The Stefan-Boltzmann law shows that a hotter blackbody emits more radiation at every wavelength than does a cooler blackbody.
Discovering Spectra
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Spectroscopy—the study of electromagnetic spectra—provides important information about the chemical composition of remote astronomical objects.
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Kirchhoff’s three laws of spectral analysis describe the conditions under which absorption lines, emission lines, and a continuous spectrum can be observed.
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Spectral lines serve as distinctive “fingerprints” that identify the chemical elements and compounds comprising a light source.
Atoms and Spectra
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An atom consists of a small, dense nucleus (composed of protons and neutrons) surrounded by electrons. Atoms of different elements have different numbers of protons, while different isotopes have different numbers of neutrons.
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Quantum mechanics describes the behavior of particles and shows that electrons can only be in certain allowed orbits around the nucleus.
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The nuclei of some atoms are stable, while others (radioactive ones) spontaneously split into pieces.
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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.
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The spectrum of hydrogen at visible wavelengths consists of part of the Balmer series, which arises from electron transitions between the second energy level of the hydrogen atom and higher levels.
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Every different element, isotope, and molecule has a different set of spectral lines.
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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.
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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.
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The equation that describes the Doppler shift states that the size of a wavelength shift is proportional to the radial velocity between the light source and the observer.
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Motion across the sky, called proper motion, yields no Doppler shift.
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WHAT DID YOU THINK?
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.
How can we determine the age of space debris found on Earth? We measure how much the long-lived radioactive elements, such as 238U, have decayed in the object. Carbon dating is only reliable for organic materials that formed within the past 100,000 years. It cannot be used for determining the age of rocks and minerals on Earth or from space. These substances were formed more than 4.5 billion years ago. Radioactive carbon in them has long since decayed to stable isotopes.