Space is big - really, really, really big. In fact, the distance to stars are so great that the ordinary units - miles and kilometers - are no longer practicable. The nearest stasr is 25 trillion miles (25,000,000,000,000 miles) away - a number too awkward to write, to remember, or to use.

Three units are commonly used in Astronomy: (1) the Astronomical Unit; (2) the Light Year; and (3) the Parsec.

  1. The Astronomical Unit is, by definition, equal in length to the distance from the earth to the sun - 93 million miles (93,000,000 miles). This unit, astronomically speaking, is a fairly small one, and it is used primarily in stating distances within the Solar System. Thus, the distance of Pluto is 40 Astronomical Units, or 40 X 93,000,000 miles = 3,720,000,000 miles. (A.U. is the proper abbreviation for Astronomical Unit.)

  2. Another unit of astronomical distance is the Light Year, defined simply as the distance traveled by a ray of light in a year. Knowing that the speed of light is 186,000 miles in one second, it is easy to compute that distance. 186,000 is multiplied by the number of seconds in a year:

186,000 X 60 (seconds per minute) X 60 (min's per hour) X 24 (hours per day) X 365¼ (days per year) = 5,880,000,000,000 miles (5.88 trillion miles) or ( 5.88 X 10^12 miles). Using this unit of distance, Aplha-Centauri is at a distance of 4.2 light years; or, the light by which we see the star has been traveling 4.2 years.

  1. The definition of the Parsec is based on a triangle as follows:

If (a), in a triangle, one of the angles is 90º; and if (b), one of the legs at the right angle is 93 million miles long; and if (c), the angle opposite it is one second; then, the length of the other 90º leg is equal to one Parsec. Thus, if angle B is the 90º angle, AB is 1 A.U. long, and angle D is 1 second of angle in size, then BD will have a length of 1 Parsec.

If (a), the angle B is 90º; (b), the side of AB is 1 A.U. (93,000,000 miles) long; (c), the angle D is 1 second, then: the side BD is equal to one Parsec.

Star Systems

Star Properties

Spectral Class - A one letter designation which decribes the color and temperatures of the star. The common classes, from hottest to coolest are: O, B, A, F, G, K , M. Classes O, B, & A are the hottest and range from white to blue-white in color. Classes F & G, medium temperature stars, are yellow, and are the most likely to have Earth-like planets. Classes K & M are cool red stars. Other, less common classes are W (Wolf-Rayet stars) which are white stars much hotter than O class stars; C class stars which are too cool to support Earth-like life and are dim red in color; and S class stars, the coolest of all, are reddish-brown in color. Each spectral class is also broken down into numerical subclasses; 0 (hottest) through 9 (coolest).

Temperature - The sirface temperature of astar directly relates to its spectral class, and even at the coolest, is messured in degrees of many thousands.

Mass - To qualify as a star an body must be at least 0.085 solar masses (1 solar mass is the size of our sun, Sol). There is no know upper limit to the saize a star can be. Bodies which are smaller than 0.085 solar masses do not ignite and become gas giants (like Jupiter) or brown dwarves.

Star Sizes




Ia, Ib


Hundreds of solar diameters



Tens of solar diameters



Several solar diameters


main sequence

Approx. 1 solar diameter


Sub dwarf

Less than 1 solar diameter


white dwarf

Approx. 5000 km

Brightness - The brightness of a star is deetermined primarily be its mass. The more massive the star, the brighter the light. The brightness is messured by it magnitude. The apparent magnitude of a star is based on the brightness of it's light as seen from Earth. The absolute magnitude is based on it's brightness as viewed from 10 parsecs away

<Next Section>

SmallHome.gif (1533 bytes)