Mechanics

# Life cycle of a star and black hole

## What are stars?

On the clear night we can see a cloudy band that stretches North to South across the sky.in fact we are seeing part of our own galaxy,the Milky way.there countless stars in our galaxy. We cannot see our galaxy as a whole, but scientists can see many other galaxies in the sky.

We see many twinkling lights in the night sky. Some of these lights come from objects in space called stars. The Sun is also a star. Beyond the solar system, billions and billions of stars are present in space. Every star is a ball of glowing gases which emits energy in the form of heat and light. Astronomers say that our Sun is a medium-sized star. Some stars are much larger and some are smaller then our Sun.

## Colours of Stars

We know that stars emit heat and light in different amounts, so, stars have  different temperatures. The colour of a star is related it to its temperature .

The coolest stars have about 2800⁰ C temperature at their surfaces and appear red. The hottest stars have 28000⁰C are higher temperatures and look blue. The stars with in-between temperatures have orange,yellow and white colors.

The sun is a yellow star. It has a temperature of 5,500 to 6000⁰C at its surface. Stars that are a little colder then the sun look orange. Stars that are a little hotter then the sun appear white.

See the table of colour and temperature of some stars:

 Name of star Colour Temperature in °C Betelgeuse red 2800 Arcturus orange 4100 Sun yellow 6000 Polaris yellow 5800 Vega white 9700 Algol blue 11700 Beta blue 28000

## Brightness of Stars

The brightness of star of a star depends on two factors:

1. Distance of the star from the earth.
2. Amount of energy the stars emits imagine you mare looking to stars that are exactly the same distance from the earth. The star which emits greater amount of energy will seem brighter then the other.

Now imagine two stars that emit equal  amount of energy. One is near to the earth and other is very far away from the earth. Which star will look brighter? The near one or farther one?

## Star Distances

The stars are very far away from us. They are also at great distances from each other. Distance s between stars are so great that they cannot to measured in kilometer. Instead, we use light-years to express the distance in the universe. A light-year is a measure of distance that light covers in one year with a speed of 300,000 kilometers per second. It seems that a light-year is a very long distance. The sun is our closest in our galaxy. The next closest star Proxima Centauri  is 4.2 light-years away us. We can also say that light of this star will take 4.2 years to reach the earth.

## The life of stars

Science has told us that the universe is finite, with a beginning, middle and a future. Stars behave life cycles too. A stars is also born, changes, and then dies. The lifespan of a star is measured in billions of years.

## Birth of a star

We have studied that great clouds of gasses and dust are present in galaxies. Each of these clouds is called a nebula. Stars are burn in nebula (singular nebula). A nebula collects more dust and gas during its travel through space. the gas and dust particles are packed into a hot spinning ball of matter. Such a ball of hot matter is called a protostar. With the passage of time,a protostar becomes hot enough produce great amount of energy. At this stage of protostar is called a star. A star like the Sun emits light and heat all the time.

## Death of a star

The matter of a star is converting into energy. This radiant energy is released into space. Our star (the Sun) is dying which is shown in above figure.

## Red Giant Stage

Our star (the Sun) has passed five billion years while emitting energy. After the next five billion years,the hydrogen in the core of the Sun may be used up. The Sun will star to collapse. The core will become denser and hotter and the Sun will swell in size. It will become a red giant. The Sun will be red giant for only about 500 million years.

## Dwarf Stage

By and by the Sun is the form of red giant will cool and gravity will make it collapse inward. Our star will become a white dwarf at this stage. Eventually, the Sun will become a burn-out black chunk of every dense matter. It will not emit light any more. This last stage of a star’s life is called a black dwarf.

## Formation of Black Holes (Life of a Massive Star)

Stars more than six times as massive as our Sun are called massive stars. A massive star has short lifespan then the Sun or other low-mass stars. Hydrogen in the core of a massive star is used up with a much fast speed. After only 50 to 100 million years, no hydrogen is left in the core of a massive star. At this time, the core collapses and the star becomes 1000 times greater then its original size. It is now called a super giant.(fig.12.12).

With the passage of time the super giant becomes so dense that it cannot bear the pressure of outer layer. The outer layer crash inward with a tremendous explosion, called supernova. At the time of supernova, the light of the star becomes much more than all other stars of the galaxy. Great shells of gases fly off the star. Only the tiny core of the star remains left. This core contains only neutrons, so it is called a neutron star. It is extremely dense. Some times after the supernova explosion the massive star becomes a black hole.

A black hole is so dense that nothing can escape from it due due to its very strong gravity. Even light cannot escape from a black hole and it is no more glowing. In fact the black hole is the last stage of the life cycle of a massive star.

## Looking at Stars

People have looked at the star for thousands of years. A telescope is a device that makes a far away object appear very close. Many more stars can be seen with the telescope than with the unaided.

A simple telescope has two lenses.

The objective lens collects light from a distant objects and bring that blight, or image, to a point or focus.

An eyepiece lens  takes the light from the objective lens and magnifies it.

Safety Tips For Observing the Sun:

The Sun emits dangerous radiation. Viewing directly into the Sun can damage our eye sight. Make sure the safety of your eye before viewing the Sun.

1. A pinhole or small opening is used to view the image of the Sun on a screen placed a half meter or more beyond the opening.
2. Use two or three sheets of X-Rays film for viewing the Sun.

Remember!  No filter is safe for use with any optical device, i.e. telescope, binoculars,etc.

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