The New Zealand Railways Magazine, Volume 13, Issue 5 (August 1, 1938)

Other Stars

Other Stars.

The stars differ, one from another, in a most astonishing way in size, in density, in temperature and in luminosity, and to a much smaller degree in mass. A few examples will make this clear.

Size.—Antares, Alpha Herculis, and Mira Ceti are three celebrated giants,
(Photo., G. W. Ritchey, Yerkes Observatory.) Nebula about Nova Persei, Sept. 20th, 1901.

( Photo., G. W. Ritchey, Yerkes Observatory.) Nebula about Nova Persei, Sept. 20th, 1901.

having diameters of three or four hundred million miles. There would be room inside Antares for Mercury, Venus, the Earth and Mars to move in their orbits, with some 66 million miles to spare outside the orbit of Mars. The volume of this giant is about 110 million times that of the sun. We cannot give examples of the opposite extreme since the smallest stars must be quite invisible through the most powerful telescopes. But we have details of a number of very small stars that happen to be specially near to us. One of these, called van Maanen's star, is said to have a diameter little over 6,000 miles, so it is considerably smaller than our earth. This gives a range, amongst known stars, of over 66 thousand times in diameter and of nearly 300 millions of millions in volume.

Density.—In spite of its small size van Maanen's star turns out to have a mass about 48,000 times that of the earth. It is nearly 300,000 times as dense as the sun, whilst the sun is four million times as dense as Antares. So the range in density is over a million millions.

Temperature.—The effective temperature of the photosphere varies greatly from star to star, and the resulting differences in the spectra have led to a useful classification of the stars. In O type, or Wolf-Rayet, stars, the elements which make their presence known are at temperatures between 30,000 and 50,000 degrees, whilst in some M type stars, such as Mira at minimum, they may be at 1,800 degrees only. These are the temperatures that rule near the surface. All stars are much more intensely heated within. In the strange companion of Sirius, Eddington calculates that the central temperature is a thousand million degrees.

Luminosity.—When the distance of a star is known, its intrinsic luminosity can be deducted from its apparent brightness. The results are often surprising. Sirius, apparently the brightest star in the sky, is in reality only 27 times as luminous as the Sun, whilst Rigel has 18,000 and Canopus 77,000 times the solar brightness. The apparent supremacy of Sirius is due to the fact that it is only 8.8, whilst Rigel is 543 and Canopus 652 light years from us.

It can readily be imagined how badly we should fare if any one of these stars was substituted for our Sun. It would be almost as disastrous in another way if our Sun were a dwarf.

Though so faint in comparison with Canopus or any one of innumerable other stars, our Sun shines 11,000 times as brilliantly as Proxima Centauri and 50,000 times as brightly as Wolf's star. This gives a 3,850 million fold range in luminosity.

Mass.—Our knowledge of stellar masses is less extensive than that of the other physical characteristics, since the mass can be determined only in the case of binary stars. The range is believed to be actually somewhat restricted, and it is often assumed that the majority of luminous stars have masses between one hundredth of, and one hundred times, the mass of our Sun. Except in the case of eclipsing binaries, it is only the minimum, not the actual mass, that can be found. In a list of spectroscopic Binaries given by R. G. Aitken, in his book “The Binary Stars,” the minimum values for one pair are 113.2 and 44.9, and for another pair 75.6 and 63.3 times the mass of our Sun. The lower limit is, of course, quite indeterminable, since, with the exception of a few that are specially close to us, all the smaller stars are invisible; but a number of pairs are known in which each star is much less massive than the Sun.