The aggregate of all existing things, that is to say, the whole creation embracing all celestial objects and space, is known as the universe. The universe is sometimes referred to as the Cosmos because of its apparent perfect orderliness (the Greek word Cosmos means orderly), and the study of the science of the universe is, therefore, called cosmology. Cosmogony applies to any of a large number of theories concerning the creation of the universe, and cosmography denotes description of the Universe.
Modern Astronomical theory and observation indicate that the universe is finite and expanding. The material of which the cosmos is composed is segregated into units, which are known as island universes. These are vast regions composed of gaseous matter at an extremely low density, interspersed with billions of stars. The island universes are disc-Shaped, separated from each other by immense distances amounting to millions of light years, and are observed to be rotating around an imaginary hub.
The island universe, to which the Earth (a mere satellite of a star) belongs in, is known as a galaxy, the diameter of this galaxy is about 100,000 light years, and its maximum thickness about 3,000 light years.
For describing distances such as the dimensions of galaxies and the distances between stars, Astronomers use (as we have done above.) a practical unit of distance known as a light-year. This is the distance traversed by light, which travels at the rate of 186,000 miles per second, in a year. The dimensions of galaxies are small compared with the distances, which separate neighbouring galaxies; Astronomical distances therefore have magnitudes wholly incomprehensible to most minds.
It appears that the principal physical feature of the universe is space occupied by gaseous material at an infinitesimally low density interspersed with countless millions of stars. A star comprises a vast quantity of material: at a relatively high density. Nuclear processes within a star result in its temperature being exceedingly high, and this in turn renders it self-luminous by virtue of the enormous quantity of electromagnetic energy embracing a wide range of frequencies (including those of heat and light). which it radiates. To terrestrial dwellers, the most prominent star is the Sun which, because of its proximity to the Earth (one of the Sun’s family of planets), renders all other stars invisible when it is above an observer's horizon. The Sun is situated about one-third of the diameter of the galaxy from the edge.
At night, when the sun is below the horizon and the air is clear and cloudless, the stars in all their glory provide one of the most majestic and awe-inspiring sights in Nature. The study of the stars must have begun as soon as there were men to observe, the science which treats of the celestial bodies - their distribution. motions, stars or star distribution - is called Astronomy, from the Greek meaning the law of the stars or star distribution. In ancient times, practical Astronomy - an art and science which served to provide the means of time keeping, direction, measuring and calendar making was referred to as Astrology. This science has since degenerated into the mere investigation or the aspects of the planets relative to one another and to the Sun, and their imagined influence on the destinies of men.
Astronomy is usually regarded as being the most ancient of the sciences, and having as many practical uses it is small wonder that Astronomy became the first science to be cultivated by mankind. Investigation into the histories of every ancient people reveals their rude attempts to discover the law’s governing the movements of celestial bodies. They would indeed be bewildered by the giant optical and radio telescopes the spectroscopes and the interplanetary space probes of modern Astronomy.
The celestial bodies fall conveniently into two broad classes: those relatively near the Earth, (the Sun, the planets (including the Earth) the satellites of the planets, comets and meteors). and those beyond the Solar System - the stars. Stars are classified into three main groups according to their size and nature. About three-quarters of the stars belong to the same class, which is the group known as the main-sequence stars, so named because when they are arranged in order of size, they are automatically arranged n order of colour. The other two classes are known as white-dwarfs and red-giants, the former on account of their extremely high temperature and comparatively small size, and the latter on account of their tremendous size and comparatively low temperature. The nearest star to the Earth is, of course, the Sun, which is an average-size main-sequence star.
When the night sky is viewed in the plane of the Galaxy (which, it will be remembered is disc-Shaped), more stars are observable than when the sky is viewed in a direction, which is at right angles to the plane of the galaxy. The immense number. of stars which are visible on a clear dark night in the plane of the galaxy appear as a white belt stretching across the night sky.
This rich region of stars, is known as the galactic arch or the Milky-Way. Practically all of the stars which are visible to the unaided eye belong to the galaxy, other island universes being so remote that only a few of them may be seen without telescopic aid. Island universes appear as diffuse regions of light, and until comparatively recently, it was thought that they were clouds of interstellar gas within the galaxy itself. Because of this they were given the name of nebulae.
Although a glance at the night sky suggests that there are innumerable stars visible, in fact the average number of stare which an ordinary observer can distinguish without a telescope on a good night is only about 2,000 and the total number of stars in the entire sky which could be seen with the naked eye is between 5,000 and 6,000. The stars, because of their varying sizes, types and distances, have different degrees of brightness. The brightness of stars are measured on the stellar magnitude scale, whereby bright stars are of low numerical magnitude and faint stars of high numerical magnitude.
The earlier Astronomers took it that the fainter a star was, the farther away it was, and as a general guide this is not too bad. However, stars do vary considerably in the amount of light they radiate and so it became necessary to have some means of means of measuring the actual amount of radiation given off by a star. To facilitate comparisons the actual brilliance of a star at a certain specified distance is known as the star’s absolute magnitude, while the relative brightness of a star is known as its apparent-magnitude. The absolute magnitude of a star is the stellar magnitude it would appear to have if it were at a distance of 32.6 light years from the Earth.
The apparent brightness of a star depends upon its intrinsic brightness as well as upon its distance form the observer.
Were all stars of equal intrinsic brightness it would be an easy matter to ascertain their relative distances because the intensity of light received from a luminous source falls off as the square of the distance of the source from the observer. This, however, is not the case: there is in fact a great range of intrinsic brilliance of stars, and the apparent brightness of a star gives no indication of its distance from the observer.
In the (apparent) stellar magnitude scale, the brighter stars are said to be of the first magnitude the magnitude number increasing as the apparent brilliance decreases.
Stars just visible to the naked eye are said to be of the sixth magnitude. This practice dates back to the Greek Astronomer Hipparchus (190-120 B.C.) on whose theories Ptolemy’s great Astronomical book the Almagest, written in the second century A.D, was based. However, the discovery by Sir John Herschel in 1830 that a first-magnitude star is about one hundred times brighter than a sixth magnitude star caused the Ptolemic grading to be modified slightly. Stars are currently graded according to the definition that a first-magnitude star is one from which the Earth receives one hundred times as much light as it receives from a sixth-magnitude star.
By this definition a second-magnitude star is one hundred times brighter than a seventh magnitude star, a third-magnitude star one hundred times brighter than an eighth magnitude star, and so on. Negative magnitudes are thus possible, because the star which is one hundred times brighter than a fifth magnitude star must be of magnitude 0, and a star which is the same amount brighter than a fourth magnitude star must be of magnitude -1. Stars, which have magnitudes intermediate between stars of integral-number magnitudes, have magnitude numbers, which are decimal quantities. The star Antares has a magnitude of 1.2. The star Capella has a magnitude of 0.2, while the brightest star in the heavens, Sirius, has a magnitude of -1.6. The planet Venus ♀can attain a magnitude or -4.4, while the Sun’s magnitude is -26.7. Stars as faint as the 23rd magnitude can be observed with the Mount Palomar telescope, these being no less than 6,300 million time fainter than 1st magnitude stars. Only the brightest stars (those listed on the daily pages of the N.A.) concern the navigator. The apparent stellar magnitudes of all navigational stars and planets are listed in the N.A. In practice the first-magnitude stars are those brighter than magnitude 1. These are, in order or brightness, Sirius, Canopus, Rigil Kentaurus, Vega, Capella, Arcturus, Rigel, Procyon, Achernar, Hadar; Altair and Betelgeuse. Only few navigational planets and stars have magnitudes of 2 and less.
There is no sharp line of demarcation in the apparent brilliance or stars of consecutive magnitudes. Moreover, because the estimation of stellar magnitudes depends upon optical comparison it is impossible to state the magnitude of a given star with absolute numerical precision.
Two stars are said to have the same magnitude when they appear to the eye to be of the same brightness. In measuring or comparing magnitudes visually, because of the uncertainty of human judgement, precise values or comparison cannot be obtained. One difficulty in estimating relative brightness of stars arises from the diversity of colours of stars. It is difficult enough to compare the magnitude of stars of the same colour, but when their colours are different, the difficulty is increased considerably.
Photometric Methods, of which there are several, are available for measuring the visual apparent magnitudes of stars. Photometric method involves comparing the brightness of a star with that of a standard light source or a standard selected star. It may be thought that by photographing a star field the relative brightness of the images of the stars on the plate or film will be the same as that of the same stars viewed visually, but this is unfortunately not so because of the variety of colours of stars. The photographic plate or film is more sensitive to blue light and less sensitive to red light when compared with the human eye. It follows, therefore, that for a given exposure a blue star will produce a larger image on the plate or film than a red star of the sane magnitude. The camera, therefore, cannot replace the human eye for determining the apparent magnitudes of stars. Star photographs, however, when compared with visual Observations, are invaluable for estimating the degree of colour of stars. The difference between the photographic magnitude and that observed visually is a function of the colour of the star called the colour index. Photographic magnitudes depend on the type of film or emulsion used, and are influenced by the use of coloured filters. When using photography for ascertaining and comparing star magnitudes, standard films and exposures are used.
Atmospheric conditions often add to the difficulty of estimating star magnitudes because atmospheric absorption of light from stars varies with the clarity and humidity of that part of the atmosphere through which the light rays pass. Under the apparent stellar magnitude scale, described above, the star Aldebaran is of magnitude 1.1 and is apparently much less bright than the Sun with magnitude -26.7. The importance of the concept of absolute magnitude will, however, be appreciated when the fact that Aldebaran is 68.46 light years away from the Earth is taken into account. If Aldebaran were placed at the specified distance for determining the absolute magnitude (32.6 light years) it would appear to have a magnitude of 0.5, and this is its absolute magnitude. The Sun has an absolute magnitude of 4.8 and so is inherently nearly a hundred times fainter than Aldebaran. From this it will be clear that the apparent stellar magnitude scale is a purely relative scale of apparent brightness as seen from the Earth; the absolute magnitude measures the actual amount of radiation given off by a star.