How Much do Stars "Weigh"? 183 - 190
Seeing Double!Roughly half of the stars that we see are members of multiple star systems. An inspection of any field of stars with even a modest telescope will reveal numerous "close pairs" of stars. It was once thought that this closeness was the result of coincident alignment. If you look carefully at the Big Dipper you will notice that the second star in the handle is actually two stars - Mizar and Alcor. In fact, these stars are about 3 light years from each other and though they are moving together as part of a common group do not orbit each other.
What Good are Binary Stars?Binary stars are our "laboratories" in which we can learn about:
Types of Binary Systems
2. Astrometric Binaries
3. Spectroscopic BinariesSystems that appear as a single star but which show variations in their spectra which can be attributed to the presence of more than one component. Spectroscopic binaries are subdivided into two categories:3a. Double-Line Spectroscopic Binaries
3b. Single-Line Spectroscopic Binaries
4. Eclipsing Binaries
What Binary Stars Can Tell Us:The study of binary star systems remains an integral part of fundamental stellar astronomy. By applying Kepler's and Newton's laws to the analysis of binary star orbits it is possible to determine the mass and basic dimensions of stars. This is our most direct and accurate way of determining stellar masses. Use the Eclipsing Binary Simulator applet show below to investigate the following:
To show you how binary stars can tell us some of the basic properties of stars consider the following worked example in which the mass of a binary system will be determined. Example 8.14 Use the following data for the star 61 Cygni to determine the mass of the system
You may recall from section 8.1 that 61 Cygni was the first star for which parallax had been detected. 61 Cygni is also a visual binary with both stars separated by about 30 seconds of arc. Since we know the star's parallax we can determine its distance. This data was critical in the determination of the separation of the two stars. Also, the because both stars are visible it is possible over the course of many decades to observe the stars orbiting each other. From this the period was determined. (Note - you don't need to watch for an entire 722 years to determine the period! Only a "few decades" will suffice!) Recall that Kepler's 3rd Law provides a crucial link between the size of orbits and the period of the orbits. If we use units of years for period and AU for star separation Kepler's 3rd Law assumes the following form: where m1 and m2 are the masses of the stars measured in solar units, "a" is the orbital separation in AU and P is the orbital period in years. This leads to This tells us that the total mass of the 61 Cygni system is 2.1 solar masses but it does not tell us the individual masses of the stars. In order to do this you must have more data which normally is provided by a spectroscopic analysis of the light coming from the binary system.
Unlocking the Secrets of the Stars - Looking at Spectra
Figure 8.8 shows the periodic variation in shape of the spectral lines that occurs when two stars in a single-lined spectroscopic binary orbit each other. These periodic variations contain a wealth of information and an important part of an astrophysicist's training is devoted to learning to tease out this information. By studying the way in which the lines vary (and if two sets of spectra are visible) it is possible to identify the two stars and to determine the mass of each star. (In Example 8.14 the best we could do was determine the total mass). In addition to mass spectral line variation can tell us surface temperatures, composition and even if the surface is vibrating. If you return to Figure 8.6 and study the combined spectrum you will see two distinct sets of lines always shifting in opposite directions. Sometimes the lines from one star shift toward the blue end of the spectrum - this indicates that the star is moving toward you. At the same time the other star's lines are shifting to the red and you can conclude that the star is moving away from you. By measuring the amount of spectral line shifting, and by using the Doppler effect formula, you can deduce the speed of each star. Example 8.15 In a double-lined spectroscopic binary system the a spectral line whose rest-frame wavelength is 434.1 nm is observed to be split into two lines, one at 433.9 nm and the other at 434.4 nm. Determine the velocity of each star with respect to your line of sight.
You can do the same calculation for the red-shifted component to conclude that the other star is moving away from your line of sight with a velocity of +207 km/s Practice
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To determine the mass and size of the stars
Chapter 9.5
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