Note the distances and luminosities of the three brightest stars in Cygnus, listed first in the chart below, and compare with 61-Cygni, listed last:
Star |
Apparent Visual Magnitude mv |
Distance |
Absolute Visual Magnitude Mv |
alpha-Cygni (Deneb) |
1.25 |
1,600 |
-7.2 |
gamma-Cygni (Sadr) |
2.23 |
815 |
-4.8 |
beta-Cygni (Albireo) |
3.07 |
410 |
-2.4 |
61-Cygni (A+B) |
5.18 |
11 |
+7.5 |
Using the equation
one can compare the relative apparent luminosities of the stars. Alpha-Cygni appears 2.47 times brighter than gamma-Cygni, and gamma-Cygni appears 2.17 times brighter than beta-Cygni. Using a very rough approximation, one can point out that while alpha-Cygni appears about 2 times brighter than gamma-Cygni, it is 2 times farther away (just the reverse of the layman’s first assumption). The same applies to the next pair: gamma-Cygni appears to be about 2 times brighter than beta-Cygni, even though it is 2 times farther away.
Data on 61-Cygni is included partly for historical interest: It was the first star to have its distance measured by parallax (Bessel, 1837). It is a very close neighbor to the sun (the 13th closest), but it appears only about 1/7 as bright as beta-Cygni even though it lies 37 times closer to us than beta-Cygni.
These distance-luminosity relationships demonstrate how radically different are the natures of the stars we see. In absolute terms, alpha-Cygni has a luminosity of more than 60,000 times the sun, gamma-Cygni more than 7,000, and beta-Cygni nearly 800. In contrast, 61-Cygni is only about 9% as bright as the sun.
[The data on the stars in Cygnus came from Skyguide, A Field Guide to the Heavens (Golden Press, 1990) and Astronomy: From the Earth to the Universe, by Jay Pasachoff, 1987, Appendix 7. What data I coudn’t find I derived from equations in my old college texts: Survey of the Universe, Menzel et al., Chapter 19 and Astrophysics and Stellar Astronomy, Swihart, Chapter I].