Astronomical object

Formation of universe and celestial bodies

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• published: 04 Nov 2015
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Formation of universe and other celestial bodies Celestial Bodies & the Universe All astronomical objects in the sky, such as the Sun, the Moon and the stars, are collectively known as celestial bodies. They are separated from each other by immense regions of emptiness called space. The space, along with the celestial bodies, make up what we call the Universe. The universe, in effect, is mostly empty space. Measuring distances in space The size of the universe is so beyond human comprehension that the common units of distance, like the kilometer, make little utility sense. Instead, distances in space are measured using the speed of light. The speed of light is the greatest speed that can be attained by any material object in the universe, and is about 300000km/s. This is equivalent to light being able to circle the Earth seven times in one second! Accordingly, light-second (light-sec), a unit of distance, is defined as 1 light-sec=300000km The distance that a light beam can travel in a year is known as a light-year (ly). Since there are 86400 seconds in a day, and there are 365 days in a year, a light year equals , or 9.46 trillion kilometers! Thus, Celestial Bodies Rotation and Revolution rotation.gif Fig 1: Rotation. (Adapted from: Purdue University, Theory of Tropics) A celestial body spins around an imaginary line through itself called an axis. This spinning motion is called rotation (see Fig. 1). The axis of rotation passes into and out of the body at two points known as the north (N) and south (S) poles. A celestial body may also move around another in a closed path called an orbit. This motion is referred to as revolution. Orbits may be oval paths or circular paths. The orbital revolution of a celestial body around another is the result of the pull of the gravitational force that one celestial body exerts on another. The larger a celestial body, the larger will be its gravitational force. Just as you can make a stone tied to a string revolve around your hand, a large celestial body can make a smaller body revolve around it in an orbit. Stars binary-stars-sirius-a-sirius-b.jpg Fig 2: The binary stars Sirius A (white) and Sirius B (blue). (Source: HEIC, Sirius A) Stars are huge balls of burning gases, mostly hydrogen and helium (see Fig. 2). However, they look like tiny points of twinkling lights in the night sky because they are extremely far from us. In fact, only a very small percentage of the actual number of stars is visible to the naked eye at night. Sagan (Cosmos, ch. VIII) vividly states that the total number of stars in the universe exceeds the total of all the grains of sand in all the beaches of our world. Stars are the only celestial bodies which generate heat and light. Our Sun is actually an average-sized star. It is the nearest star, and appears so big because it is only about 500 light-sec from Earth (which means that light from the Sun takes about 500 seconds, or about 8½ minutes, to reach us). Some stars, like our Sun, occur singly. Stars occurring in pairs, orbiting around each other, are called binary stars. In other cases, there are three or more stars grouped together. A good example of a binary star system is the Sirius system comprising Sirius A and Sirius B (Fig. 2). Sirius A, visible to the naked eye, is the brightest star in the night sky. It is also known as the Dog Star. It is about twice the size of our Sun. Its companion, Sirius B, is much smaller. The closest star to us (not counting the Sun) is Proxima Centauri, which is 4.3 light-years away. This implies that if, hypothetically speaking, Proxima Centauri suddenly increases its brightness, we will know about it only after 4.3 years—the time taken by light from Proxima to travel across space and reach us. Proxima actually belongs to the triple-star system called the Alpha Centauri System. Two stars in the system, Alpha Centauri A and Alpha Centauri B, are comparable in size to our Sun, while Proxima Centauri is much smaller. The stars are always present in the sky, but can be seen only at night when the Sun is not around to hide them with its brightness. Although the entire night sky rotates from east to west, the positions of the stars in the sky, in relation to each other, appear “fixed”. Stars do move relative to each other, but their immense distances, from us and from each other, give us the impression of their positions being fixed over the normal course of human history. Such positional shifts can be detected only when comparing the night skies over thousands of years.