The Universe: Still Expanding

The Universe

(Broadly, it is the celestial bodies and the space together comprise the universe)

Universe

We all must know what the term Universe denotes. But do we know what comprises of Universe? All objects that are a part of sky are called celestial bodies and these celestial bodies or heavenly bodies in a vast emptiness called Space. Objects like stars, planets, satellites, asteroids, meteors, comets, dust and gases are called celestial bodies. All these heavenly bodies within the space is termed as Universe.

The Stars

Stars are huge bodies of glowing gases. They give out huge flames and radiate a tremendous amount of heat and light. So they have heat and light of their own. The distances of stars are expressed in terms of light years. One light year is the distance traveled by light in one year at a speed of light which is about 300,000 km/s. Light year is a unit of distance and is equal to 300,000365246060 km, which is equal to 9,460,000,000,000 km or 9.461012 km. The color of a star is determined by its surface temperature. Stars which have low temperature appear red. Those with higher temperature appear white and those with very high temperature are blue. 

Stars

Stars are not scattered regularly in space: they occur in clusters, better described as galaxies. So, all stars including our sun move around some celestial body or a group of bodies with high speeds. However, when viewed from the earth the distance between any two stars does not seem to change in spite of their great speeds. This is because the stars are so far away from us that any changes in distance between them do not become perceptible in a few years. The stars appear to move from east to west. This is because the earth rotates from west to east about an imaginary axis that passes through its center. The earth completes one revolution in 24 hours, so the stars. A star, therefore, travel an angle of about one degree in four minutes. Since the stars rises four minutes earlier everyday, they are one degree higher in the sky than the previous day. In a month, they rise 30 degrees higher. In 6 months they move 180 degrees and will be close to the western horizon. Thus the constellations also move westwards.

However, there is one star, which appears stationary to us known as pole star or Dhruv tara. The North Star or Polaris indicates the north because it is seen directly above the North Pole. It lies close to the axis of rotation of the earth. So it does not appear to move like all other stars. In fact, all the stars appear to revolve around the Pole Star. The pole star is not visible from the southern hemisphere. Some of the northern constellations like Ursa Major may also not be visible from some points in the southern hemisphere. 

Constellations

Constellations

A group of stars forming various shapes is called constellation i.e. star patterns. Laghu saptarshi or Ursa Minor is one such constellation. Vrihat Saptarshi also known as Ursa Major, a group of seven stars is another constellation. It forms a part of the constellation of the Big Bear. It can be seen during summer time in the early part of the night. Orion or Mriga is another well-known constellation that can be seen during winter in the late evenings. The star Sirius, which is the brightest star in the sky, is located close to Orion. Cassiopeia is another prominent constellation in the northern sky. It is visible during winter in the early part of the night. A constellation does not have only 5-10 stars. It has a large number of stars. However we can see only the bright stars in a constellation with our naked eye. All the stars which make up a constellation are not at the same distance. They are just in the same line of sight in the sky.

Galaxies

Milky Way Galaxy

A galaxy is a system of stars that also contains a large number of gas clouds (nebula), some of which are quite huge. It is in these gas clouds that new stars are born. They are bound to one another by gravitational forces. A galaxy has billions of stars very close to one another. There are billions of such galaxies in the Universe. Some galaxies are spiral in shape and others are elliptical. However, there are galaxies which have no regular shape.

The galaxy in which our sun is located is called Milky Way galaxy. It contains several hundred billion stars of all ages, sizes and masses. It appears like a river of light running from one corner of the sky to the other. That is why it is called the Milky Way Galaxy. It is spiral in shape. In India it is known as Akash Ganga. The Milky Way Galaxy is rotating slowly about its centre. Its center is at an inestimable distance from us. The sun too, along with the solar system, rotates about the centre of the galaxy. It takes about 250 million years to complete one revolution. Our sun is just one of the hundred thousand million (1011) stars in the Milky Way Galaxy. The sun is located about 30,000 light years away from the center of the galaxy. 

Big Bang Theory

Big Bang explosion

In the earlier part of the twentieth century, Edwin Hubble (1889-1953), an American astronomer, showed that all other galaxies are rushing away from our galaxy. The velocity with which they are rushing away from us, increases as their distances increase from us. The motion of other galaxies away from us gives rise to the idea of the expanding universe. From this, if we take inference then we can say, If all the galaxies are moving away from one another at this moment, there must have been a time in the past when all the galaxies were together at one place. At that time, the whole universe occupied a small volume. It is believed that a huge explosion took place at that time, and since then, the universe is expanding. It has been estimated that this event occurred about 15 billion years ago. This means, the universe is about 15 billion years old. This event occurred that time and ultimately led to the present movement of galaxies away from one another is explained by the Big Bang theory.  Scientists believe this gigantic explosion formed small atoms which initiated the formation of the Universe. Then the atoms lead to the formation of molecules. Eventually a Nebula was formed.

Nebula

A Nebula

A nebula is a huge cloud floating in the Universe. It is made up of gases and dust. Temperature at the centre of the nebula is very high because the atoms and molecules of the gases attracted by the gravity of each other keep colliding. When the temperature rises over 10,000,000 Centigrade, nuclear chain reactions take place and a new star is born. Thus, a nebula is a birthplace of stars.

Stars too Die

White Dwarf Star

As we read above, stars are formed in giant clouds of dust and gas, and progress through their normal life as balls of gas heated by thermonuclear reactions in their cores. The ages of the stars range from millions to billions of years. A typical star, such as our Sun, radiates small amounts of X-rays continuously and larger bursts of X-rays during a solar flare. The Sun and other stars shine as a result of nuclear reactions deep in their interiors. These reactions change light elements into heavier ones and release energy in the process. The outflow of energy from the central regions of the star provides the pressure necessary to keep the star from collapsing under its own weight. When a star has used all hydrogen at its center, it starts fusing helium into carbon. If the mass of the star is only a few times the solar mass, the fusion process does not go beyond this. As the energy production inside the star stops, its core contracts under its own weight. Nuclear reactions outside the core cause the dying star to expand outward in the "red giant" phase before it begins its inevitable collapse. Depending on their mass, they reach the end of their evolution as a white dwarf, neutron star or black hole. 

A Neutron Star

If the star is about the same mass as the Sun and density becomes very high, it will turn into a white dwarf star. The size of a white dwarf star is comparable to that of a planet. The mass of such stars is always less than about 1.44 times the mass of the sun. This result was proved by S. Chandrasekhar. The maximum mass is, therefore, called the Chandrasekhar Limiting Mass. A white dwarf is considered as a dead star because it does not produce its own energy by fusion process. It shines by radiating the heat stored by it during its lifetime.

A Massive Black Hole

If it is somewhat more massive, it may undergo a supernova explosion and depending on their size, ends up as Neutron stars or Black holes. Generally a supernova explosion leave behind a neutron star. But if the collapsing core of the star is very great i.e. at least three times the mass of the Sun then nothing can stop the collapse. The star implodes to form an infinite gravitational warp in space i.e. a black hole (in which the gravitational pull is so high that not even light can escape out of them).

New Cycle

A Brown Dwarf Star

The cycle begins anew as an expanding super shell from one or more supernovas trigger the formation of a new generation of stars. The life cycles of stars follow patterns based mostly on their initial mass. These include

• intermediate-mass stars such as the sun, 
• with half to eight times the mass of the sun, 
• high-mass stars that are more than eight solar masses, and 
• low-mass stars a tenth to half a solar mass in size. 

The greater a star's mass, the shorter its lifespan is. Objects smaller than a tenth of a solar mass do not have enough gravitational pull to ignite nuclear fusion. They might become failed stars and known as brown dwarfs, so they never evolve.