Chapter 11: Chapter 11: Characterizing Stars

How near is the closest star other than the Sun? How near is the closest star other than the Sun? Is the Sun brighter than other stars, or just closer? Is the Sun brighter than other stars, or just closer? What colors are stars? What colors are stars? Are brighter stars hotter? Are brighter stars hotter? What sizes are stars? What sizes are stars? Are most stars isolated from other stars, as the Sun is? Are most stars isolated from other stars, as the Sun is? WHAT DO YOU THINK?

You will discover… that the distances to many nearby stars can be that the distances to many nearby stars can be measured directly, while the distances to farther ones are determined indirectly the observed properties of stars on which the observed properties of stars on which astronomers base their models of stellar evolution how astronomers analyze starlight to determine a how astronomers analyze starlight to determine a star’s temperature and chemical composition how the total energy emitted by stars and their how the total energy emitted by stars and their surface temperatures are related the different classes of stars the different classes of stars the variety and importance of binary star systems the variety and importance of binary star systems how astronomers calculate stellar masses how astronomers calculate stellar masses

As the Earth orbits the Sun, a nearby star appears to shift its position against the background of distant stars. Our eyes change angle as we look at things that are different distances away.

Our eyes are adjusting for the parallax of the things we see. The closer the star is to us, the greater the parallax angle p.

Apparent Magnitude Scale Several stars in and around the constellation Orion labeled with their names and apparent magnitudes Astronomers denote the brightness of objects in the sky by apparent magnitudes. Stars visible to the naked eye have magnitudes between m = –1.44 and about m = +6.

The Inverse-Square Law The same amount of radiation from a light source must illuminate an ever-increasing area as the distance from the light source increases. The decrease in brightness follows the inverse-square law. This means, for example, that tripling the distance decreases the brightness by a factor of 9.

Temperature and Color The intensity of light emitted by three hypothetical stars is plotted against wavelength. The range of visible wavelengths is indicated. Where the peak of a star’s intensity curve lies relative to the visible light band determines the apparent color of its visible light. The insets show stars of about these surface temperatures.

Principal Types of Stellar Spectra

Classifying the Spectra of Stars Williamina Fleming (standing) Annie Jump Cannon

“Oh, Be A Fine Guy/Girl, Kiss Me!” Spectral Classes

Hertzsprung-Russell Diagram Luminosities of stars are plotted against their spectral types. Luminosities of stars are plotted against their spectral types. Luminosity and spectral type are correlated. Luminosity and spectral type are correlated. Main-sequence stars fall along the red curve. Main-sequence stars fall along the red curve. Giants are to the right and supergiants are on the top. Giants are to the right and supergiants are on the top. White dwarfs are below the main sequence. White dwarfs are below the main sequence.

Types of Stars and Their Sizes Stellar luminosities are graphed against the surface temperatures. Stellar luminosities are graphed against the surface temperatures. Dashed diagonal lines indicate stellar radii. Dashed diagonal lines indicate stellar radii. For stars of the same radius, hotter stars are more luminous than cooler stars. For stars of the same radius, hotter stars are more luminous than cooler stars.

Luminosity Classes Luminosity classes permit finer distinctions between giants and supergiants. Luminosity classes permit finer distinctions between giants and supergiants. Ia and Ib encompass the supergiants. Ia and Ib encompass the supergiants. II, III, and IV indicate giants of different brightness. II, III, and IV indicate giants of different brightness. V is the main-sequence stars. V is the main-sequence stars. White dwarfs do not have a luminosity class. White dwarfs do not have a luminosity class.

Binary Star System ζ Ursae Majoris is a binary system with stars separated by only about 0.01 arcseconds.

Center of Mass of a Binary Star System

The Mass-Luminosity Relation Luminosities and masses are plotted using logarithmic scales. Luminosities and masses are plotted using logarithmic scales. The more massive a star, the more luminous the star. The more massive a star, the more luminous the star.

Each dot represents a main-sequence star. Each dot represents a main-sequence star. The number next to each dot is the mass of that star in solar masses. The number next to each dot is the mass of that star in solar masses. Mass, luminosity, and surface temperature of main-sequence stars increase from lower right to upper left Mass, luminosity, and surface temperature of main-sequence stars increase from lower right to upper left The Mass-Luminosity Relation

Spectral Line Motion in Binary Star Systems The radial-velocity curves of the binary HD

Double-Line Spectroscopic Binary Kappa Arietis has spectral lines that shift back and forth as the two stars revolve about each other.

Light Curves of Eclipsing Binaries (a) a partial eclipse (b) a total eclipse (c) The binary star NN Serpens

WHAT DID YOU THINK? How near is the closest star other than the Sun? How near is the closest star other than the Sun? Proxima Centauri is about 40 trillion kilometers (25 trillion miles) away. It takes light about 4 years to reach the Earth from there. Proxima Centauri is about 40 trillion kilometers (25 trillion miles) away. It takes light about 4 years to reach the Earth from there. How luminous is the Sun compared with other stars? How luminous is the Sun compared with other stars? The most luminous stars are about a million times brighter and the least luminous stars are about a hundred thousand times dimmer than the Sun. The most luminous stars are about a million times brighter and the least luminous stars are about a hundred thousand times dimmer than the Sun. What colors are stars? What colors are stars? Stars are found in a wide range of colors, from red through violet, as well as white. Stars are found in a wide range of colors, from red through violet, as well as white.

Are brighter stars hotter than dimmer stars? Are brighter stars hotter than dimmer stars? Not necessarily. Many brighter stars, such as red giants, are cooler but larger than hotter, dimmer stars, such as white dwarfs. Not necessarily. Many brighter stars, such as red giants, are cooler but larger than hotter, dimmer stars, such as white dwarfs. What sizes are stars? What sizes are stars? Stars range from more than 1000 times the Sun’s diameter to less than 1/100 the Sun’s diameter. Stars range from more than 1000 times the Sun’s diameter to less than 1/100 the Sun’s diameter. Are most stars isolated from other stars, as the Sun is? Are most stars isolated from other stars, as the Sun is? No. In the vicinity of the Sun, two-thirds of the stars are found in pairs or larger groups. No. In the vicinity of the Sun, two-thirds of the stars are found in pairs or larger groups. WHAT DID YOU THINK?

Key Terms absolute magnitude apparent magnitude binary star center of mass close binary dwarf star eclipsing binary giant star Hertzsprung-Russell (H-R) diagram diagram initial mass function inverse-square law light curve luminosity luminosity class main sequence mass-luminosity relation OBAFGKM sequence optical double photometry radial-velocity curve red giant spectral types spectroscopic binary spectroscopic parallax stellar evolution stellar parallax stellar spectroscopy supergiant visual binary white dwarf