Sourcebook

Space

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

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

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