Double Star Systems

Orbit of star A

Orbit of star A

Double Stars Orbits

Orbit of star B Center of mass

Figure 3.19. A binary star consists of two stars A and B orbiting a common center of mass, bound together by their mutual gravity.

Orbit of star B Center of mass

Figure 3.19. A binary star consists of two stars A and B orbiting a common center of mass, bound together by their mutual gravity.

A visual binary can be resolved with a telescope so that two separate stars can be seen. Over 70,000 visual binaries are known. Mizar in Ursa Major was the first binary star discovered, in 1650. Beautiful Albireo in Cygnus is a colorful yellow and blue star. You can see these and many others in a small telescope. (See Useful Resources and Web Sites for observers' guides.)

Many visible stars may have companions that are too faint to be seen. An astrometric binary is a visible star plus an unseen companion star. The presence of an unseen companion is inferred from variable proper motion of the visible star. Brilliant Sirius (Sirius A) in Canis Major was an astrometric binary from 1844, when its nature was detected, until 1862. Then its faint companion star (Sirius B) was observed.

A spectroscopic binary cannot be resolved in a telescope. Its binary nature is revealed by its spectrum. A varying Doppler shift is apparent in the spectral lines as the stars approach and recede from Earth. Almost a thousand spectroscopic binaries have been analyzed. The brighter member of Mizar (Mizar A) is a spectroscopic binary.

An eclipsing binary is situated so that one star passes in front of its companion, cutting off light from our view at regular intervals. An eclipsing binary regularly changes in brightness. You can see the famous eclipsing binary Algol, the Demon, in Perseus. Algol "winks" from brightest magnitude 2.2 to least bright magnitude 3.5 in about 2 days and 21 hours.

An optical double is a pair of stars that appear to be close to each other in the sky when viewed from Earth. Actually, one is much more distant than the other, and they have no physical relationship to one another.

Test your eyesight by finding both Mizar and Alcor (nicknamed "the testers"), the optical double in the handle of the Big Dipper in Ursa Major.

How does an optical double differ from a visual binary?_

Answer: The stars in an optical double are far apart and have no actual relationship to one another. The stars in a visual binary are bound together in space by their mutual gravity.



7"his self-test is designed to show you whether or not you have mastered the material in Chapter 3. Answer each question to the best of your ability. Correct answers and review instructions are given at the end of the test.

1. Refer to the chart on page 78. From the measured parallax, find the distance to Barnard's Star in (a) parsecs_; (b) light-years_.

2. Explain why the bright (dark) spectral lines of light emitted from (absorbed by) the atoms of an element are unique to that element._

3. Explain how a star's spectrum is formed.

4. List the following types of spectral lines in order as they appear in stars of decreasing temperature.

. (4) Neutral helium. . (5) Neutral metals. . (6) Ionized metals.

_(1) Very strong hydrogen lines.

_(2) Ionized helium.

_(3) Bands of titanium oxide molecules.

5. Match the following properties of a star that can be deduced from its spectrum with the appropriate method listed on the right.


Chemical composition.






Radial velocity.



Gas density, axial rotation,


magnetic field.

Doppler shift. Spectral type (class). Line shape. Characteristic lines.

Doppler shift. Spectral type (class). Line shape. Characteristic lines.

6. The proper motion of Sirius is 1.34" per year. Find how much Sirius will change its position on the celestial sphere in the next 1000 years._

7. Define space velocity.

8. Refer to Table 1.1. By using their apparent magnitudes, absolute magnitudes, and spectral classes, match one of the four stars to each description.

(1) Betelgeuse.






Most luminous.


Least luminous.








Most distant.

9. Label the following on the H-R diagram in Figure 3.20:

Absolute luminosity (Sun = 1).

(3) Spectral class.

(4) Absolute magnitude.

(5) Main sequence.

White dwarfs. Supergiants.

(11) Brown dwarfs.

10. What is the most basic property of a star that determines its location on the main sequence (its temperature and luminosity)?_

11. Use the H-R diagram to explain why, compared to our Sun, red giants must be very large and white dwarfs must be very small._

Figure 3.20. An incomplete H-R diagram.

12. Match:

Can be resolved with a telescope. Unseen companion inferred from variable proper motion of visible companion.

Binary nature revealed by its spectrum.

Changes in brightness regularly as one star blocks its companion from our view.

Member stars have no actual physical relationship to one another.

(1) Astrometric binary.

(2) Eclipsing binary.

(3) Optical double.

(4) Spectroscopic binary.

(5) Visual binary.


Compare your answers to the questions on the self-test with the answers given below. If all of your answers are correct, you are ready to go on to the next chapter. If you missed any questions, review the sections indicated in parentheses following the answer. If you missed several questions, you should probably reread the entire chapter carefully.

0".549 and


2. Each spectral line is light of a particular wavelength emitted (or absorbed) by the atom when one of its electrons jumps between a higher and a lower energy level (orbit). Each element has its own unique set of allowed electron orbits, so each element has its own characteristic set of spectral lines. (Sections 3.2, 3.3)

3. Stars are blazing balls of gas where many kinds of atoms emit light of all colors. This light, emitted from the star's surface, passes through the star's outer atmosphere. There, atoms of each element absorb their characteristic wavelengths, so a pattern of dark lines crosses the continuous band of colors—the star's spectrum. (Sections 3.3, 3.4)

5. (a) 4; (b) 2; (c) 1; (d) 3. (Sections 3.3, 3.5 through 3.10)

6. 1340", or roughly one third of a degree.

Solution: Proper motion = 1.34" per year. A degree is equal to 3600". (1.34" per year) x 1000 years (Section 3.9)

7. Velocity of a star with respect to the Sun. (Section 3.9)

(a) 3; (b) 1; (c) 1; (d) 2; (e) 4; (f) 3; (g) 4; (h) 1. (Sections 3.6, 3.7, 3.12 through 3.16)



Distance Apparent



(ly) Magnitude




522 0.45




11 0.40




262 0.98




9 -1.44


9. (a) 3;

(b) 1; (c) 4;

(d) 2; (e) 8; (f) 6;

(g) 5; (h) 9;

(i) 10;

(j) 7; (k) 11.

(Section 3.18)

10. Mass.

(Section 3.19)

11. Red giants are relatively cool but luminous; hence, they must have a large surface area radiating energy. White dwarfs are relatively hot but faint; hence, they must have a small surface area radiating energy into space. (Sections 3.18 through 3.20)

12. (a) 5; (b) 1; (c) 4; (d) 2; (e) 3. (Section 3.21)

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