Welcome to Visual Astronomy
Astronomical Observing
Based mainly on observation with binoculars

By Alain Dussault

The Color of Stars and Variable Stars


I refer you to the Sky & Telescope site at:


When you looked on a sky charts or on the Planisphere, you remark that the stars are identified
by either a name, a greek alphabet letter or by a number.

In a constellation, the stars are attributed a greek letter from Alpha, for the most brilliant star in
that constellation and so on by decreasing magnitude of stars. If a constellation contains
more than 26 stars, a number will be given to the stars also according to the decreasing magnitude.

Furthermore, each brilliant star has been given a name. So in general the first brilliant stars
of a constellation, bear a name. For example in Ursa Major (the Big Dipper), each star of the
Dipper bear a name like, Dubhe, Merak, Phecda, Megrez, Mizar, 60 Alcor, Alioth et Alkaid. In
general, they are Arab names.

Though at first sight, all stars look whites, a more meticulous observation show that some of them
have distinctive colors, although that certain red giant looks like that to the naked eye. The more
the color is dark, the more, the surface temperature of the star is high.

The aid of telescopes, has permit to refine the analytical technique of the stellar light. The passing
of the light of a star through a prism, give a specter, sort of a colored band like a rainbow.

To characterize stellar specters, astronomers have established seven principal spectral types,
designated by the letters O (Violet), B (Blue), A (White), F(White), G (Yellow), K (Orange) and
M (Red), in increasing surface temperature order of the star, which is the only observable part of it.

To remember them we used the expression "Oh Be A Fine Girl Kiss Me".

So the hottest surface temperature of a star is of a Blue color and a Red star is the coldest. Our
Sun is situated almost in the middle in this order of surface temperature.

The life evolution of Stars happen as per the main sequence on the HR Diagram or
Hertzsprung-Russell Diagram.

The following sites give examples of HR Diagram:


A DOS program, (SCLOCK20), simulate the evolution of the stars in this diagram and a Windows
(HRCAL20) show class informations on this diagram for stars.

You will be able to downloaded them form the site:


When at this site, used the search engine to find both programs.

Multiples Stars

Two third of the stars have other stars orbiting around them. We named them multiple stars.
There are multiple stars of 3 or 4 components and even more.

A pair of stars, in appearing near one to each other but having no physical bond between them,
constitute an optic binary stars or an optic double stars. This is not a true double stars.

By double stars, we usually mean two stars of the same system, that is one turn around the other.
The double star named Algol is the most popular, because it is accessible to binoculars.

For a double or multiple stars system , we use the terms like: separation, position, angle and period.

The term separation is the maximum distance that can separate the two stars, when they happen
to be side by side. This is measure in arcseconds.

By position angle, we mean the direction or angle that the second star make with the first. This is
measured in degrees toward the East. East has a P.A. of 90 degrees and West of 270 degrees.

The period is the time that take the second star to complete an orbit. This period can extend from
a few days to many centuries.

To be able to separate or resolute, a double or multiple stars, depend, of the degree of their
separation. The more the degree of their separation is small, the more the diameter of the
telescope must be great to be able to distinguish each of them. A telescope resolution can be
estimated by the following formula:

R = 14.4 / D

R = resolution in arsseconds.
D = diameter of the mirror in centimeters.

Note: Atmospheric turbulence (seeing), limit the theoretic separation of a telescope to a value
superior to R.

Variable stars.

For an article on variable stars, go to the site:


The light emit by certain starts vary in intensity on a certain lap of time. These stars are called
variable stars.

Eclipsing variables are double stars, where one pass periodically in front of the other. They are
named Algol type, because Algol was the first star of this type to be observed.

In this case, it is not possible to observe separately the two stars. It is only the variation in
luminous intensity that allow us to deduct of their double stars status. Typical period of an
eclipsing variable is only a few days and the minima can last from a few minutes to a few hours.

Other type of stars, short period or long period variables, are stars in which the variation of
luminosity is due to fluctuations in their internal and surface temperatures.

In the short period category, the Cepheids are among the more numerous and more known.
They are pulsating stars, of which the spectral class and surface temperature change from period
of a few days to a few months.

Long period variables of Myra type, after the star that bear that name, are red supergiants which
have a period superior to many hundred days and whose magnitude variations are very important.
Mira magnitude, vary from 3 to 10 in a period of 322 days.

The other variable stars are stars which exploded partially, like the Nova. They become suddenly
very bright and go back down to their normal magnitude.

Supernovas, on the contrary, have violent explosion and disintegrate almost completely at the
end of their life evolution.

Next Chapter, Comets, Oort cloud, Eclipses...

Preceding Chapter, Deep Sky or Celestial Objects. Definition of different objects

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Last update February 5th., 1998.