7. Comparisons of the Sun with Other Stars

    • The stars that can be seen by any given observer depend on the observerís location on the Earth, specified by longitude and latitude, by the position of the star in the sky, denoted by its right ascension and declination, and by the time of night.

    • The location of the stars in the sky slowly changes by precession over a 26,000-year period.

    • The distance of a nearby star can be inferred by its angular displacement from a distant star observed over a six-month period. This angle is known as the annual parallax, and it is roughly equal to the ratio of the mean Earth-Sun distance, or AU, and the distance to the nearby star.

    • The apparent brightness of a star is the observed brightness when its radiation arrives at the Earth. There are many more faint stars of low brightness than very bright ones.

    • The luminosity is an intrinsic measure of a star. It is the amount of energy radiated every second, or the emitted power. There is a huge range in stellar luminosity, ranging from 9 million to 0.03 times the luminosity of the Sun.

    • The effective disk temperature of stars ranges from about 50,000 K to about 2,000 K.

    • Hot stars are blue, cool stars are red.

    • The line spectra of stars are described by the spectral sequence O, B, A, F, G, K, M that describes a progressive decrease in temperature from hot B stars to cool M stars.

    • Giant stars are hundreds of times bigger than the Sun and supergiant stars are thousands of times larger than the Sun.

    • The masses of stars vary from 100 to 0.1 times the Sunís mass. The mass of stars can be determined using Keplerís third law with observations of binary star systems.

    • The luminosity of a star depends on the fourth power of the starís mass. The most massive and luminous stars are supergiant stars.

    • All stars move. There is a radial motion, along the line of sight, and a proper motion, transverse to the line of sight. The Doppler shift in the wavelength of a spectral line can be used to measure the radial velocity. A bigger Doppler shift corresponds to a larger the radial velocity. A shift to longer, redder wavelengths corresponds to motion away from the observer; a shift to shorter, bluer wavelengths denotes motion toward the observer.

    • Millions of stars in globular star clusters move in all directions, supporting the cluster against collapse by the mutual gravitational attraction of its component stars.

    • High-speed stars, known as runaway stars, move at speeds of about 100 kilometers per second and originate from loosely bound stellar associations of hot, massive stars.

    • Young stars rotate rapidly, and older stars, including the Sun, rotate slowly. The rotation is slowed by intense stellar winds or stellar magnetic fields connected to the surrounding regions.

Copyright 2010, Professor Kenneth R. Lang, Tufts University