Neil F. Comins • William J. Kaufmann III. Discovering the Universe Ninth Edition. CHAPTER 11 Characterizing Stars. Interstellar dust illuminated by a pulse of light emitted from the red giant star, V838 Monocerotis, in the center of the image. WHAT DO YOU THINK?.
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Discovering the Universe
Interstellar dust illuminated by a pulse of light emitted from the red giant star, V838 Monocerotis, in the center of the image.
Our eyes change the angle between their line of sight as we look at things that are different distances away. Our eyes are adjusting for the parallax of the things we see. This change helps our brain determine the distances to objects and is analogous to how astronomers determine the distance to objects in space.
As Earth orbits the Sun, a nearby star appears to shift its position against the background of distant stars. The star’s parallax angle (p) is equal to the angle between the Sun and Earth, as seen from the star.
The closer the star is to us, the greater the parallax angle p. The distance to the star (in parsecs) is found by taking the inverse of the parallax angle p (in arcseconds), d=1/p.
(a) Several stars in and around the constellation Orion, labeled with their names and apparent magnitudes.
(b) Astronomers denote the brightnesses of objects in the sky by their apparent magnitudes. Stars visible to the naked eye have magnitudes between
m=–1.44 and about m=+6.
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, which means, for example, that tripling the distance decreases the brightness by a factor of 9.
The car is seen at distances of 10 m, 20 m, and 30 m, showing the effect described in the previous image.
This beautiful Hubble Space Telescope image shows the variety of colors of stars.
These diagrams show the relationship between the color of a star and its surface temperature. The intensity of light emitted by three stars is plotted against wavelength. The range of visible wavelengths is indicated. The location of the peak of each star’s intensity curve, relative to the visible-light band, determines the apparent color of its visible light. The insets show stars of about these surface temperatures. Ultraviolet (uv) extends to 10 nm.
The corresponding spectral types are indicated on the right side of each spectrum. The hydrogen Balmer lines are strongest in stars with surface temperatures of about 10,000 K (called A-typestars). Cooler stars (G- and K-type stars) exhibit numerous atomic lines caused by various elements, indicating temperatures from 4000 to 6000 K. Several of the broad, dark bands in the spectrum of the coolest stars (M-type stars) are caused by titanium oxide (TiO) molecules, which can exist only if the temperature is below about 3700 K.
(a) Williamina Fleming (standing)
(b) Annie Jump Cannon
The modern classification scheme for stars, based on their spectra, was developed at the Harvard College Observatory in the late nineteenth century. Female astronomers, initially led by Edward C. Pickering and Williamina Fleming, and then by Annie Jump Cannon, analyzed hundreds of thousands of spectra. Social conventions of the time prevented most female astronomers from using research telescopes or receiving salaries comparable to those of men.
On an H-R diagram, the luminosities of stars are plotted against their spectral types. Each dot on this graph represents a star whose luminosity and spectral type have been determined. The data points are grouped in just a few regions of the diagram, revealing that luminosity and spectral type are correlated: Main-sequence stars fall along the red curve, giants are to the right, supergiants are on the top, and white dwarfs are below the main sequence. The absolute magnitudes and surface temperatures are listed at the right and top of the graph, respectively. These are sometimes used on H-R diagrams instead of luminosities and spectral types.
On this H-R diagram, stellar luminosities are plotted against the surface temperatures of stars. The dashed diagonal lines indicate stellar radii. For stars of the same radius, hotter stars (corresponding to moving from right to left on the HR diagram) glow more intensely and are more luminous (corresponding to moving upward on the diagram) than cooler stars. While individual stars are not plotted, we show the regions of the diagram in which main-sequence, giant, supergiant, and white dwarf stars are found. Note that the Sun is intermediate in luminosity, surface temperature, and radius; it is very much a middle-of-the-road star.
These spectra are from two stars of the same spectral type (B8) and, hence, the same surface temperature (13,400 K) but different radii and luminosities: (a) the B8 supergiant Rigel (58,000 solar luminosities) in Orion, and (b) the B8 main-sequence star Algol (100 solar luminosities) in Perseus.
Dividing the H-R diagram into regions, called luminosity classes,permits finer distinctions between giants and supergiants. Luminosity classes Ia and Ib encompass the supergiants. Luminosity classes II, III, and IV indicate giants of different brightness. Luminosity class V indicates main-sequence stars. White dwarfs do not have their own luminosity class.
About one-third of the visible “stars” in our region of the Milky Way are actually double stars. Mizar in Ursa Major is a binary system with stars separated by only about 0.01 arcsec. The images and plots show the relative positions of the two stars over nearly half of their orbital period. The orbital motion of the two binary stars around each other is evident. Either star can be considered fixed in making such plots.
(a) Two stars move in elliptical orbits around a common center of mass. Although the orbits cross each other, the two stars are always on opposite sides of the center of mass and thus never collide. (b) A seesaw balances if the center of mass of the two children is at the fulcrum. When balanced, the heavier child is always closer to the fulcrum, just as the more massive star is closer to the center of mass of a binary star system.
Illustrated here are (a) a partial eclipse and (b) a total eclipse. (c) The binary star NN Serpens, indicated by the arrow,undergoes a total eclipse. The telescope was moved during the exposure so that the sky drifted slowly from left to right. During the 10.5-min eclipse, the dimmer but larger star in the binary system (an M6 V star) passed in front of the more luminous but smaller star (a white dwarf). The binary became so dim that it almost disappeared.
For main-sequence stars, mass and luminosity are directly correlated—the more massive a star, the more luminous it is. A main-sequence star of 10 solar masses has roughly 3000 times the Sun’s luminosity ; one with 0.1 solar masses has a luminosity of only about 0.001 solar luminosities. To fit them on the page, the luminosities and masses are plotted using logarithmic scales.
On this H-R diagram, each dot represents a main-sequence star. The number next to each dot is the mass of that star in solar masses. As you move up the main sequence from the lower right to the upper left, the mass, luminosity, and surface temperature of main-sequence stars all increase.
The diagramsindicate the positions and motions of the stars, labeled A and B, relative to Earth. Below each diagram is the spectrum we would observe for these two stars at each stage. The changes in colors (wavelengths) of the spectral lines are due to changes in the stars’ Doppler shifts, as seen from Earth.
This graph displays the radial-velocity curves of the binary HD171978. (The HD means that this is a star from the Henry Draper Catalogue of stars.) The entire binary is moving away from us at 12km/s, which is why the pattern of radial velocity curves is displaced upward from the zero-velocity line.
The spectrum of the double-line spectroscopic binary kappa Arietis has spectral lines that shift back and forth as the two stars revolve around each other. (a)The stars are moving parallel to the line of sight, with one star approaching Earth, the other star receding, as in Stage 1 or 3 of Figure 11-15a. These motions produce two sets of shifted spectral lines. (b) Both stars are moving perpendicular to our line of sight, as in Stage 2 or 4 of Figure 11-15a. As a result, the spectral lines of the two stars have merged.
center of mass
Hertzsprung-Russell (H-R) diagram
initial mass function