A temperature of – 273° C ( or 0 K), at which all molecular motion stops; the lowest possible temperature.

Kirchhoff’s Laws: 

Kirchhoff’s three laws of spectral analysis de-scribe conditions under which different kinds of spectra are produced.

• A hot, dense object such as a blackbody emits a continuous spectrum covering all wavelengths.

Photons: 

An explanation of blackbody curves led to the discov-ery that light has particlelike properties. The particles of light are called photons.

• Planck’s law relates the energy E of a photon to its frequency or wavelength : E h hc/ , where h is Planck’s constant.

• A hot, transparent gas produces a spectrum that contains bright ( emission) lines.

• A cool, transparent gas in front of a light source that itself has a continuous spectrum produces dark ( absorption) lines in the continuous spectrum.

An atom has a small dense nucleus composed of protons and neutrons. The nucleus is surrounded by electrons that occupy only certain orbits or energy levels.

 • When an electron jumps from one energy level to another, it emits or absorbs a photon of appropriate energy ( and hence of a specific wavelength).

• The spectral lines of a particular element correspond to the var-ious electron transitions between energy levels in atoms of that element. • Bohr’s model of the atom correctly predicts the wavelengths of hydrogen’s spectral lines.

The Doppler shift enables us to determine the radial velocity of a light source from the displacement of its spectral lines.

The point in a telescope where the objective focuses light.

The range of radio wavelengths to which Earth’s atmosphere is transparent.

A telescope in which the principal optical component is a concave mirror. 

A telescope in which the principal optical component is a lens. 

• Chromatic aberration is an optical defect whereby light of different wavelengths is bent in different amounts by a lens. 

• Glass impurities, chromatic aberration, opacity to certain wave-lengths, and structural difficulties make it inadvisable to build extremely large refractors. 

Reflecting Telescopes: Reflecting telescopes, or reflectors, produce images by reflecting light rays to a focus point from curved mirrors. 

• Reflectors are not subject to most of the problems that limit the useful size of refractors. 

Angular Resolution: A telescope’s angular resolution, which indicates ability to see fine details, is limited by two key factors. 

• Diffraction is an intrinsic property of light waves. Its effects can be minimized by using a larger objective lens or mirror. 

• The blurring effects of atmospheric turbulence can be minimized by placing the telescope atop a tall mountain with very smooth air. They can be dramatically reduced by the use of adaptive optics and can be eliminated entirely by placing the telescope in orbit.

Sensitive light detectors called charge-coupled devices ( CCDs) are often used at a telescope’s focus to record faint images. 

Spectrographs:

 A spectrograph uses a diffraction grating to form the spectrum of an astronomical object. 

Radio telescopes use large reflecting dishes to focus radio waves onto a detector. 

• Very large dishes provide reasonably sharp radio images. Higher resolution is achieved with interferometry techniques that link smaller dishes together. 

Transparency of Earth’s Atmosphere: 

Earth’s atmosphere absorbs much of the radiation that arrives from space. 

• The atmosphere is transparent chiefly in two wavelength ranges known as the optical window and the radio window. A few wave-lengths in the near- infrared also reach the ground.

Telescopes in Space: 

For observations at wavelengths to which Earth’s atmosphere is opaque, astronomers depend on telescopes carried above the atmosphere by rockets or spacecraft. 

• Satellite- based observatories provide new information about the universe and permit coordinated observation of the sky at all wavelengths.

A starlike object that is not massive enough to sustain hydrogen fusion in its core.

A classification of a star of a given spectral type according to its luminosity.

The numbers of stars of differing brightness per cubic parsec.

A system for denoting the brightnesses of astronomical objects.

A grouping of stars on the Hertzsprung- Russell diagram extending diagonally across the graph from hot, luminous stars to cool, dim stars.

A star whose luminosity and surface temperature place it on the main sequence on an H- R diagram; a star that derives its energy from core hydrogen fusion.

Two stars that lie along nearly the same line of sight but are actually at very different distances from us.

The apparent displacement of an object due to the motion of the observer.

That portion of an object’s velocity parallel to the line of sight. 

A plot showing the variation of radial velocity with time for a binary star or variable star.

A classification of stars according to the appearance of their spectra.

A subdivision of a spectral class.

A binary star system whose binary nature is deduced from the periodic Doppler shifting of lines in its spectrum.

The distance to a star derived by comparing its apparent brightness to a luminosity inferred from the star’s spectrum.

A binary star whose binary nature is deduced from the presence of two sets of incongruous spectral lines.

The apparent displacement of a star due to Earth’s motion around the Sun.

Distances to the nearer stars can be determined by parallax, the apparent shift of a star against the background stars observed as Earth moves along its orbit. 

• Parallax measurements made from orbit, above the blurring effects of the atmosphere, are much more accurate than those made with Earth-based telescopes. 

• Stellar parallaxes can only be measured for stars within a few hundred parsecs.

A star’s luminosity ( total light output), apparent brightness, and distance from Earth are related by the inverse- square inverse-square law. If any two of these quantities are known, the third can be calculated. 

The Population of Stars: 

Stars of relatively low luminosity are more common than more luminous stars. Our own Sun is a rather average star of intermediate luminosity. 

The Magnitude Scale: 

The apparent magnitude scale is an alternative way to measure a star’s apparent brightness. 

• The absolute magnitude of a star is the apparent magnitude it would have if viewed from a distance of 10 parsecs. A version of the inverse-square law relates a star’s absolute magnitude, apparent magnitude, and distance.

Photometry measures the apparent brightness of a star. The color ratios of a star are the ratios of brightness values obtained through different standard filters, such as the U, B, and V filters. These ratios are a measure of the star’s surface temperature. 

Spectral Types: 

Stars are classified into spectral types ( subdivi-sions of the spectral classes O, B, A, F, G, K, and M), based on the major patterns of spectral lines in their spectra. The spectral class and type of a star is directly related to its surface tempera-ture: O stars are the hottest and M stars are the coolest. 

• Most brown dwarfs are in even cooler spectral classes called L and T. Unlike true stars, brown dwarfs are too small to sustain thermonuclear fusion. 

Hertzsprung- Russell Diagram: 

The Hertzsprung- Russell ( H- R) di-agram is a graph plotting the absolute magnitudes of stars against their spectral types— or, equivalently, their luminosities against surface temperatures. 

• The positions on the H- R diagram of most stars are along the main sequence, a band that extends from high luminosity and high surface temperature to low luminosity and low surface temperature. 

• On the H- R diagram, giant and supergiant stars lie above the main sequence, while white dwarfs are below the main sequence. 

• By carefully examining a star’s spectral lines, astronomers can determine whether that star is a main- sequence star, giant, super-giant, or white dwarf. Using the H- R diagram and the inverse-square law, the star’s luminosity and distance can be found without measuring its stellar parallax.

Binary stars, in which two stars are held in orbit around each other by their mutual gravitational attraction, are surprisingly common. Those that can be resolved into two distinct star images by a telescope are called visual binaries. 

• Each of the two stars in a binary system moves in an elliptical orbit about the center of mass of the system. 

• Binary stars are important because they allow astronomers to determine the masses of the two stars in a binary system. The masses can be computed from measurements of the orbital period and orbital dimensions of the system. 

Mass- Luminosity Relation for Main- Sequence Stars: 

Main-sequence stars are stars like the Sun but with different masses.

• The mass- luminosity relation expresses a direct correlation be-tween mass and luminosity for main- sequence stars. The greater the mass of a main- sequence star, the greater its luminosity ( and also the greater its radius and surface temperature).