Stars begin their lives as clouds of dust and gas called nebulae. The gravity of a passing star or the shock wave from a nearby supernova may cause the nebula to contract. Matter in the gas cloud will begin to coalesce into a dense region called a protostar. As the protostar continues to condense, it heats up. Eventually, it reaches a critical mass and nuclear fusion begins.

This begins the main sequence phase of the star. It will spend most of its life in this stable phase. The life span of a star depends on its size. Very large, massive stars burn their fuel much faster than smaller stars. Their main sequence may last only a few hundred thousand years. Smaller stars will live on for billions of years because they burn their fuel much more slowly.

Eventually, the star’s fuel will begin to run out. It will expand into what is known as a red giant. Massive stars will become red supergiants. This phase will last until the star exhausts its remaining fuel. At this point, the pressure of the nuclear reaction is not strong enough to equalize the force of gravity and the star will collapse.

Most average stars will blow away their outer atmospheres to form a planetary nebula. Their cores will remain behind and burn as a white dwarf until they cool down. What will be left is a dark ball of matter known as a black dwarf. If the star is massive enough, the collapse will trigger a violent explosion known as a supernova.

If the remaining mass of the star is about 1.4 times that of our Sun, the core is unable to support itself and it will collapse further to become a neutron star. The matter inside the star will be compressed so tightly that its atoms are compacted into a dense shell of neutrons. If the remaining mass of the star is more than about three times that of the Sun, it will collapse so completely that it will literally disappear from the universe. What is left behind is an intense region of gravity called a black hole.

Main Sequence Stars – The main sequence is the point in a star’s evolution during which it maintains a stable nuclear reaction. It is this stage during which a star will spend most of its life. Our Sun is a main-sequence star. A main-sequence star will experience only small fluctuations in luminosity and temperature. The amount of time a star spends in this phase depends on its mass. Large, massive stars will have a short main sequence stage while less massive stars will remain in the main sequence much longer. Very massive stars will exhaust their fuel in only a few hundred million years. Smaller stars, like the Sun, will burn for several billion years during their main sequence stage. Very massive stars will become blue giants during their main sequence. 
Red Giants – A red giant is a large star that is reddish or orange in color. It represents the late phase of development in a star’s life, when its supply of hydrogen has been exhausted and helium is being fused. This causes the star to collapse, raising the temperature in the core. The outer surface of the star expands and cools, giving it a reddish color. Red giants are very large, reaching sizes of over 100 times the star’s original size. Very large stars will form what is called red supergiants. Betelgeuse in Orion is an example of a red supergiant star. 
White Dwarfs – A white dwarf is the remnant of an average-sized star that has passed through the red giant stage of its life. After the star has used up its remaining fuel. At this point the star may expel some of its matter into space, creating a planetary nebula. What remains is the dead core of the star. Nuclear fusion no longer takes place. The core glows because of its residual heat. Eventually, the core will radiate all of its heat into space and cool down to become what is known as a black dwarf. White dwarf stars are very dense. Their size is about the same as that of the Earth, but they contain as much mass as the Sun. They are extremely hot, reaching temperatures of over 100,000 degrees. 
Brown Dwarfs – A brown dwarf could also be called a failed star. During the process of star formation, some protostars never reach the critical mass required to ignite the fires of nuclear fusion. If the protostar’s mass is only about 1/10 that of the Sun, it will glow only briefly until its energy dies out. What remains is a brown dwarf. It is a giant ball of gas that is too massive to be a planet but not massive enough to be a star. They are smaller than the Sun but several times larger than the planet Jupiter. Brown dwarfs emit no light or heat. They could account for some of the dark matter suspected to exist in the universe. 
Variable Stars – A variable star is a star that changes in brightness. These fluctuations can range from seconds to years depending on the type of variable star. Stars usually change their brightness when they are young and when they are old and dying. They are classified as either intrinsic or extrinsic. Intrinsic variables change their brightness because of conditions within the stars themselves. Extrinsic variables change brightness because of some external factor, like an orbiting companion star. These are also known as eclipsing binaries. 
Binary Stars – Many stars in the universe are part of a multiple star system. A binary star is a system of two stars that are gravitationally bound to each other. They orbit around a common point, called the center of mass. It is estimated that about half of all the stars in our galaxy are part of a binary system. Visual binaries can be seen as two separate stars through a telescope. Spectroscopic binaries appear as one star and can only be detected by studying the Doppler shifts on the star’s spectrum. Eclipsing binaries are binary systems where one star blocks the light from another as it orbits its companion. 


A nebula is a cloud of gas (hydrogen) and dust in space. Nebulae are the birthplaces of stars. There are different types of a 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. The nebula surrounding the Pleiades Cluster is typical of a reflection nebula. Dark Nebula also exists. These are dense clouds of molecular hydrogen that partially or completely absorb the light from stars behind them e.g. the Horsehead Nebula in Orion.

Planetary Nebula are the outer layers of a star that are lost when the star changes from a red giant to a white dwarf.


A star is a luminous globe of gas-producing its own heat and light by nuclear reactions (nuclear fusion). They are born from nebulae and consist mostly of hydrogen and helium gas. Surface temperatures range from 2000ºC to above 30,000ºC, and the corresponding colors from red to blue-white. The brightest stars have masses 100 times that of the Sun and emit as much light as millions of Suns. They live for less than a million years before exploding as supernovae. The faintest stars are the red dwarfs, less than one-thousandth the brightness of the Sun.

The smallest mass possible for a star is about 8% that of the Sun (80 times the mass of the planet Jupiter), otherwise, nuclear reactions do not take place. Objects with less than critical mass shine only dimly and are termed brown dwarfs or a large planet. Towards the end of its life, a star like the Sun swells up into a red giant, before losing its outer layers as a Planetary Nebula and finally shrinking to become a white dwarf.


This is a large bright star with a cool surface. It is formed during the later stages of the evolution of a star like the Sun, as it runs out of hydrogen fuel at its center. Red giants have diameters between 10 and 100 times that of the Sun.

They are very bright because they are so large, although their surface temperature is lower than that of the Sun, about 2000-3000ºC. Very large stars (red giants) are often called Super Giants. These stars have diameters up to 1000 times that of the Sun and have luminosities often 1,000,000 times greater than the Sun.


These 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.


This is a very small, hot star, the last stage in the life cycle of a star like the Sun. White dwarfs have a mass similar to that of the Sun, but only 1% of the Sun’s diameter; approximately the diameter of the Earth. The surface temperature of a white dwarf is 8000ºC or more, but being smaller than the Sun their overall luminosity’s are 1% of the Sun or less.

White dwarfs are the shrunken remains of normal stars, whose nuclear energy supplies have been used up. White dwarves 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.


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. There are two general types of Supernova:-

Type I These occur in binary star systems in which gas from one star falls on to a white dwarf, causing it to explode.

Type II These occur in stars ten times or more as massive as the Sun, which suffers runaway internal nuclear reactions at the ends of their lives, leading to an explosion. They leave behind neutron stars and black holes. Supernovae are thought to be the main source of elements heavier than hydrogen and helium.


These stars are composed mainly of neutrons and are produced when a supernova explodes, forcing the 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 believed to be neutron stars that are spinning very rapidly.


Black holes are believed to form from massive stars at the end of their lifetimes. 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 neighboring matter into them including stars.


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