Stars

Stars

Star Distances

If we represented the Sun by a globe two feet in diameter, the Earth could be shown by a pea on a circumference of a circle 215 feet in radius. On this scale a large pin’s head about one and a half miles from the sun globe would serve for Pluto, but Proxima Centauri , the star nearest to the Sun, would be 10,000 miles away.

Models of this kind help us to appreciate the immensity of interstellar space compared with the extent of the Solar system. They also emphasize the need for a unit of distance, like the light-year, equivalent to about six million million miles.

Proxima Centauri, a faint star in southern skies, is 4,3 light-years away. The Sun, on the other hand, is just over eight light-minutes away, so a rough comparison between the distance of Proxima Centauri and that of the Sun is one of 4,3 years with eight minutes.

Stars, photograph in public domain

Bright Sirius, another near neighbour of the Sun, is 8,7 light-years away, but Canopus, second brightest star in the night sky, is 300 light-years away.

At least 20 stars, most of them faint objects, lie within a radius of 11,5 light-years from the Sun, while 17 of the brightest stars lie within a radius of 400 light-years. Space, it seems, is remarkably empty of stars, so empty as to make the solar system appear overcrowded by comparison.

Differences is BrightnessAlthough the stars are at different distances from us, this fact alone does not account for their differences in apparent brightness. They also differ in intrinsic brightness, or luminosity. Some stars are many thousand times more luminous than the Sun, others are over a thousand times less luminous. The immense range arises largely from differences in surface area and surface temperature. Thus, if two stars are equal in size but different in surface temperature, the more luminous star will be the one with the greater surface temperature. Conversely, If they both have the same surface temperature, the more luminous star will be the larger of the two.

Temperature and Colour

The surface temperature of a star also determines its colour. Reddish stars, like Betelgeuse and Antares look red because they are fairly cool. The temperature of their surface is of the order of 3,500 degrees centigrade, or below that of the filament of an electric light bulb. The sun with a surface temperature of nearly 6,000 degrees centigrade, looks yellow. Rigel and Regulus, two blue-white stars, have surface temperatures of about 20,000 degrees.

Despite their red colour, Betelgeuse and Antares are among the 20 brightest stars in the sky. This is because of their immense size. Betelgeuse, with an average diameter of about 300 million miles, could easily contain the orbit of Mars, while Antares is only slightly smaller. Yet their great distances from us (650 light-years and 400 light-years respectively) cause them to look like points even in the largest telescopes.

Double Stars

Telescopes reveal that many stars which appear to be single objects to the standard eye consist of two, three or even more stars. Of course, two stars will look close together if they happen to lie in almost the same direction, but one of them may be hundreds of light-years beyond the other. Two stars that actually are close together are said to form a binary system. The components, tied together by the invisible bonds of gravitation, revolve about each other. Sirius, Alpha Centauri, Procyon, and Spica are binaries, although Alpha Centauri is more strictly a triple system since it includes a third member in the form of Proxima Centauri. At least, one third of the brighter stars consist of two or more suns.

Variable Stars

Many stars change in brightness and are therefore called variables. In some cases the star itself pulsates, that is, it varies in diameter (and hence in surface area) and also in surface temperature. Quiet often, as with Delta Cephei the light changes are remarkably regular, but they can be quite irregular, as in the caseof Betelgeuse. Rhythmic light changes are also characteristic of certain binaries. If the orbits of the component stars are presented edgewise or nearly edgewise, one star will partially eclipsed by the other. A well-known example of this type of variable, known as an eclipsing binary, is Algol, or Beta Persei.

Erupting Stars

Also among the variables are stars that undergo one or more sudden increases in brightness. The surges are probably due to great flare-like eruptions, which in extreme cases cause a star to shed practically all of its outer regions. In the latter event, the star, termed a nova, increases in brightness by a factor of 5,000 to 100,000 times within two or three days and then gradually fades to return to its former insignificant state. But these changes are mild compared with those associated with supernovae, or stars like the one seen by Tycho Brahe in 1572. These stars undergo titanic explosions in which practically all their material is ejected into space. At maximum brightness they can reach the luminosity of 100 million stars.

In some respects stars are like people. In general they have a common form, structure, and composition. They have various individual characteristics and traits, yet can be sorted into a relatively small number of different times. They tend to form associations: some go through life singly, others in pairs, and others in closely knit groups and clusters.