## Celestial Bodies & the Universe

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# The Universe - 01

## 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

$1 light-sec = 300000 km$

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 $( 300000 × 86400 × 365 ) km = 9.46 × 10 12 km$ , or 9.46 trillion kilometers! Thus,

$1 ly = 9.46 × 10 12 km$

### Celestial Bodies

#### Rotation and Revolution

A celestial body spins around an imaginary line through itself called an axis. This spinning motion is called rotation (see Fig. ). 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

Stars are huge balls of burning gases, mostly hydrogen and helium (see Fig. ). 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 (Sagan 2002, 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 500light-sec from Earth (which means that light from the Sun takes about 500 seconds, or about 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. ). 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

#### Constellations

Since ancient times, humans have looked at the night sky trying to find patterns among the stars, and gave names to easily recognisable patterns. Such patterns of stars are called constellations. One of the most famous and easily recognised constellations is the Big Dipper (Fig. ), or the Plough, which is actually part of a bigger constellation called the Ursa Major or Great Bear. The Big Dipper is a collection of seven stars. In India, the Big Dipper is named the Sapta-rishi (Sapta:- seven; Rishi:- sages).

The Big Dipper is useful for locating the Pole Star (see Fig. ), or the North Star (called Dhruv Nakshatra in India) which indicates the direction north. Unlike other stars, the North Star does not rise or set—always remaining in the same position in the sky throughout the year—as the sky rotates, and so is a reliable direction finder.

Another famous constellation is the Orion, also known as the Hunter (Fig. ). It is considered the most beautiful constellation of all.

The stars belonging to a constellation are at different distances from us. However, since we lose the perception of depth because of the enormous distances, they appear to be equidistant, forming a fixed pattern of the constellation. If, hypothetically speaking, our Earth was taken to a distance of, say, 1000 light years from where it is now, the sky will appear very different, as most of the known constellations will lose shape from this new vantage point.

Nevertheless, over thousands of years, the constellations do change their shapes, as the relative movements of the stars become detectable over such large time spans.

Since prehistory, constellations had been useful for finding directions at night, and for keeping track of seasons as certain constellations are visible only in certain times of the year. Today, constellations are also useful in the tracking of artificial satellites and for locating stars. By 1930, astronomers had mapped out the boundaries of 88 constellations which, taken together, cover the entire sky, making it possible to assign any star to a constellation

#### Planets and Satellites

Planets are celestial bodies which revolve around stars, and may be rocky or gaseous. A planet does not shine by itself, but is lit and shines by the light of the star it orbits. Normally, the planets are much smaller than the stars they orbit. (see Fig. )

A number of stars have their own planetary systems with planets orbiting them. The space between planets within a planetary system is called interplanetary space.

The rotation of a planet on its axis creates the effects of days and nights. The star-facing side of the planet has its day. As the planet rotates, the daylight zone turns to the opposite side and comes under the night zone.

Satellites are celestial bodies which, apart from rotating on their axes, revolve around planets in orbits and, just like planets, do not generate their own light. A satellite too shines by the light of the star of its parent planet.

The planetary system to which we belong is the Solar System, which consists of the Sun, 8 planets (of which our Earth is the third planet), the planetary satellites (the Moon being a satellite of Earth), 5 dwarf planets (which includes Pluto), comets, the Asteroid belt, the Kuiper belt and the Oort cloud.

The word planet means "wanderer" in Greek. The Ancients had observed that a few celestial objects would change positions against the background of other "fixed" stars, and the Greeks applied the name "planets" to them. The Sun and the Moon were also counted among these "wanderers." The other "wanderers" that were seen were actually planets orbiting our own Sun. The motions of these "wanderers" could be made out against the fixed background of stars because of their proximity to Earth.

It is only recently that astronomers have been able to confirm the presence of planets around some nearby stars. The first extrasolar planet was found in 1995, and since then, many more have been detected. Since extrasolar planets cannot be observed directly through telescopes (as their suns swamps them with their brightness), astronomers use indirect methods for confirming their existence.

## The Universe

The Encyclopedia Britannica defines the Universe as '... everything that exists, including matter and energy'. The true size of the universe is not known and is difficult to even imagine—it is so immense that the most advanced telescopes made by us do not know its limits. The universe is also known as the Cosmos, and the study of the universe is Cosmology or Astronomy.

### Galaxies

A galaxy is a large collection of stars, gas and dust that move together as a group through the universe. The universe is populated by billions of galaxies. Galaxies are of many sizes, and even the smallest of them contain millions of stars. Most galaxies are either elliptical or spiral in shape.

The space between the stars within a galaxy is known as interstellar space, while the space between galaxies is called intergalactic space.

#### The Milky Way

Our Sun belongs to a galaxy called the Milky Way galaxy, or simply, the Galaxy. It is a spiral galaxy and contains more than 200 billion stars (see Fig. ). It has a disc like structure with most of the stars lying in the plane of the disc. The diameter of the Milky Way is about 150000 light years.

The spiral arms of a spiral galaxy contain a much larger number of stars as compared to the areas between the arms. Our Solar System is located in one of the spiral arms of the Milky Way, about two-thirds of the distance out from the centre of the galaxy (see Fig. ).

Just as our Earth orbits the Sun, the solar system travels around the centre of the Milky Way along with the other stars. It takes about 200 million years for the Sun to complete one rotation.

On very clear nights and away from city lights, a white band of hazy light can be seen crossing the skies. This is the band of the Milky Way (see Fig. ), which we in India call the Akash-Ganga. From our location inside the galaxy, the band results from the diffuse lights from the huge number of stars as we look along the plane of the galaxy (that is, when we view our own galaxy edge-on from the inside).

#### The Andromeda Galaxy

The Andromeda galaxy is a spiral galaxy and the nearest neighbouring large galaxy of the Milky Way (see Fig. ). It's diameter is double that of our galaxy, and lies at a distance of about 2.5 million light-years from us. It is even visible to the naked eye on a clear night as a small hazy patch.

Note that Andromeda is one of the very few galaxies beyond our Milky Way which can be seen with the naked eye—the rest of the sky is filled with stars belonging to our own galaxy.

### Origin of the Universe

Scientists believe that the universe began suddenly, about 13–14 billion years ago, in an event called the Big Bang. All matter at the time of the Big Bang was tightly packed into an infinitely hot and dense single point, called the singularity. Since the Big Bang event, the universe has been expanding and cooling. About 300,000 years later, it was cool enough for atoms of hydrogen (and some helium) to come into existence. As time went on, due to gravitational attraction, the atoms began to form groups of immense gas clouds spread across all space. Within each of these gas clouds, smaller regions of gases further clumped together gravitationally to form nebulae (singular—nebula).

Thereafter began the process of formation of stars. Each nebula, over time, gravitationally compressed itself to such an extent that the extreme pressure at the centre generated enough heat to start the process of nuclear fusion, igniting the ball of gas to give birth to a star. In this nuclear fusion reaction, hydrogen atoms fused to form helium atoms, giving out a lot of energy (as heat and light) in the process.

Thus, with time, each of the original group of gas clouds became a galaxy comprising a huge number of stars formed out of the gaseous nebulae.

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