In this article we will introduce you that what are stars and what are the types of stars.
We look at the heaven and see twinkling, glittering little points of light, known as stars. The ancient called them “fixed stars” because they apparently don not alter their position with respect to one another, in contrast to the wandering stars or planets.
Mr. Halley first proved that the bright stars, Sirius, Procyon, and Arcturus were gradually changing their position with reference to the neighboring stars. The apparent relative displacements are so small even in the course of centuries that the appearance of the constellation of light stars does not appreciably alter in the course of thousands of years.
The stars were divided by the ancient into groups or constellations (groups of stars) to which were given names of persons or objects famous in ancient mythology. This was done for the convenience of their location in the sky.
Number of stars
The number of stars in the whole sky down to a given limit of magnitude can be determined with sufficient accuracy by selecting a large number of sample areas, distributed over the sky in a uniform manner.
Seares and Van Rhyjin estimated that the total number of stars was about 30 thousand million-a finite number-and the number down to a magnitude limit of 28 million is about half of total. This number, although very uncertain, suffices to give a general indication of magnitude of the number.
Types of stars
Many stars, which appear single to the naked eye, are found, when examined in a telescope, to be double. They may appear to be double for two causes:
The two stars may be at very different distances but nearly at the same line of sight, in which case there is physical connection between them. Such pairs are termed as optical doubles.
They may be at the same distance and physically connected, revolving about their center of gravity under the action of their mutual attraction. Such pairs are termed binary system and are of interest to astronomers.
Variable stars are of great interest and importance. They are so called because their light varies. The number of known variable stars is several thousands and is being added to continually. Some of these stars vary irregularly while others are quite regular in variation. The periods of the light variations range hours to several hundred days.
There are some types of variable stars. One type which is particularly interesting is the cephoid variables. They intrinsically vary bright to the naked eye. They each vary regularly in brightness over a period of six days. The important thing about this kind of stars is that there is a fairly exact relation between the brightness and the rate of variation.
Cephoids are found in all parts of the sky. If their period of variation is known it is an easy matter to calculate their distance.
Novae or temporary types of stars
The name is applied to the star which experiences a sudden and usually considerable increase in brightness after which its light diminishes, at first somewhat rapidly, and then more slowly, but usually with small and irregular oscillations to a more or less steady value.
The majority of novae, before they outburst, were faint stars generally showing small fluctuations in brightness. The normal nova appears to undergo only one outburst. No typical nova has been known to have a second outburst, having occurred in the past some centuries earlier, cannot be excluded.
We do not know for certain the physical causes of which produce the outburst. But there is little doubt that these originate within the star itself. The phenomenon suggests that instability sets in and that in the comparatively rapid transition to a new equilibrium state, there is a large amount of energy released.
The increased radiation pressure causes the rapid expansion of the outer layers of the star which are finally thrown off from the star. Milne suggested that the nova stage is an unstable stage, which every star must pass through at a certain stage of its evaluation. The star begins suddenly to collapse; a large quantity of gravitational energy is released, which causes the outward expansion of the surface layer of the star. According to this theory the final stage of a nova is a white, dwarf star.
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