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Distance-Luminosity Relationships of Stars in the Constellation Cygnus

Three Bright Stars of Cygnus

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
(light years)

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 laymanís 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 coudnít 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].