LIFE OF A STAR

Grand star forming region R136

Stars are hot bodies of glowing gas that start their life in Nebulae. 
Stars form when enough dust and gas clump together because of gravitational forces. Nuclear reactions release energy to keep the star hot.
They vary in size, mass and temperature, diameters ranging from 450x smaller to over 1000x larger than that of the Sun. 
The colour of a star is determined by its temperature, the hottest stars are blue and the coolest stars are red. The Sun has a surface temperature of 5,500 degrees Celcius, its colour appears yellow.

THE RING NEBULA
A PLANETARY NEBULA SIMILAR TO WHAT OUR SUN WILL BECOME

The Sun and other stars use nuclear fusion to release energy.
Nuclear fusion involves two atomic nuclei joining to make a large nucleus. Energy is released when this happens

THE LIFE CYCLE OF A STAR

A star goes through a life cycle. This is determined by the size of the star. The diagram below summarises the stages :

Stars about the same size as our Sun

These follow the left hand path:

Main sequence star → red giant → white dwarf → black dwarf

Stars much bigger than our Sun

These follow the right hand path:

Main sequence star → red super giant → supernova → neutron star or black hole

NEBULA

Stars are born in a region of high density Nebula, and condenses into a huge globule of gas and dust and contracts under its own gravity

There are different types of nebula. An Emission Nebula e.g. such as Orion nebula, glows brightly because the gas in it is energised by the stars that have already formed within it. In a Reflection Nebula, starlight reflects on the grains of dust in a nebula.

PROTOSTAR

A  region of condensing matter will begin to heat up and start to glow forming Protostars. 

MAIN SEQUENCE STAR

If a protostar  contains enough matter the central temperature reaches 15 million degrees centigrade.At this temperature, nuclear reactions in which hydrogen fuses to form helium can start.
The star begins to release energy, stopping it from contracting even more and causes it to shine. It is now a Main Sequence Star.

Life Of A Small Star

RED GIANT STAR

A star of one solar mass remains in main sequence for about 10 billion years, until all of the hydrogen has fused to form helium.
The helium core now starts to contract further and reactions begin to occur in a shell around the core.
The core is hot enough for the helium to fuse to form carbon. The outer layers begin to expand, cool and shine less brightly. The expanding star is now called a Red Giant.

 Red giants have diameter's between 10 and 100 times that of the Sun.

RED DWARF NEAR A PLANET

Red Dwarfs are very cool, faint and small stars, approximately one tenth the mass and diameter of the Sun. They burn very slowly and have estimated lifetimes of 100 billion years. Proxima Centauri and Barnard's Star are red dwarfs.

WHITE DWARF

White dwarfs are the shrunken remains of normal stars, whose nuclear energy supplies have been used up. White dwarf consist of degenerate matter with a very high density due to gravitational effects, i.e. one spoonful has a mass of several tonnes. White dwarfs cool and fade over several billion years.

When it stops shining, the now dead star is called a Black Dwarf.

Life Of A Massive Star

RED SUPERGIANT

Massive stars have a mass 3x times that of the Sun. Some are 50x that of the Sun. While it takes billions of years in a small star to use up all the hydrogen, it takes only millions in a massive star 
The massive star then becomes a Red Supergiant. These stars have diameters up to 1000 times that of the Sun and have luminosities often 1,000,000 times greater than the Sun.

SUPERNOVA  REMNANT

In the next million years a series of nuclear reactions occur forming different elements in shells around the iron core. The core collapses in less than a second, causing an explosion called a Supernova, in which a shock wave blows of the outer layers of the star. The actual supernova shines brighter than the entire galaxy for a short time.

This is the explosive death of a star, and often results in the star obtaining the brightness of 100 million suns for a short time.

CROSS SECTION OF NEUTRON STAR

When a supernova explodes, protons and electrons to combine to produce a neutron star. Neutron stars are very dense. Typical stars having a mass of three times the Sun but a diameter of only 20 km. If its mass is any greater, its gravity will be so strong that it will shrink further to become a black hole. Pulsars are neutron stars that are spinning very rapidly.

BLACK HOLE CYGNUS X-1

Black holes are believed to form from massive stars at the end of their life times. The gravitational pull in a black hole is so great that nothing can escape from it, not even light. The density of matter in a black hole cannot be measured. Black holes distort the space around them, and can often suck neighbouring matter into them including stars.

Next time you look at a star, remember the many different things it will become during it's lifetime!