Explanation 2
Stars are born from clusters of interstellar matter
Outer space is not an absolute vacuum. In fact, it is filled with very thin clouds of hydrogen and helium, and dust-like interstellar particles. These are the raw materials of future stars. Clusters of interstellar particles attract more and more other particles, gradually increasing in size. Eventually, the cluster begins to contract by virtue of its own gravity. Then, when the core temperature has reached around 10 million degrees, a nuclear reaction begins. The period up until this point is known as the "contraction phase" and, in the case of a star with a mass similar to that of our Sun, takes about 500 million years.
The longest period in a star's life is as a main sequence star
Once the contraction phase is completed, the star becomes a fixed star, an "adult" so to speak, and enters the main sequence star phase. Stars in this phase produce energy as the result of a nuclear reaction that creates one helium atom from every four hydrogen atoms. This means that the amount of hydrogen in the star gradually decreases, while the helium increases. The main sequence star phase is the longest period in a star's life, and in the case of a star with a mass similar to that of our Sun, lasts for about 10 billion years. Our Sun is thought to be around 4.6 billion years old, which means that it is probably about halfway through its main sequence star phase.
As the outer layers expand from the helium core, the star becomes a red giant
The helium at the centre of the star continues to increase until a helium core is formed. Nuclear reaction then begins to spread outward. As the helium core grows heavier, the core's temperature also increases, and the outer layers begin to expand until the star becomes a massive red star known as a red giant. In the case of a star that is about the size of our Sun, the gases of the outer layer are expelled, and then contract, so that the star becomes what it known as a white dwarf. However, if a star has a much greater mass than our Sun, the final stages of its giant star phase end in a supernova explosion. The giant star phase is about one tenth as long as the main sequence star phase.
Some stars collapse under their own weight, causing supernova explosions
When a star's mass is about three times that of our Sun, after the red giant phase, it begins to collapse under its own weight, causing a supernova explosion that scatters it through space. Its brightness at this point will be 100 billion times that of the Sun. When this happens, it looks as if a bright new star has appeared in the night sky. Supernova explosions of some exceptionally massive stars leave in their wake neutron stars, called pulsars, and black holes. Sometimes the scattered clouds of gas and particles become the material of new stars.
Explanation 3
Stars are hot bodies of glowing gas that start their life in Nebulae. They vary in size, mass and temperature, diameters ranging from 450x smaller to over 1000x larger than that of the Sun. Masses range from a twentieth to over 50 solar masses and surface temperature can range from 3,000 degrees Celsius to over 50,000 degrees Celsius.
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 Celsius, its colour appears yellow.
The energy produced by the star is by nuclear fusion in the stars core. The brightness is measured in magnitude, the brighter the star the lower the magnitude goes down. There are two ways to measuring the brightness of a star, apparent magnitude is the brightness seen from Earth, and absolute magnitude which is the brightness of a star seen from a standard distance of 10 parsecs (32.6 light years).
- Small Stars- The Life of a Star of about one Solar Mass.
- Small stars have a mass up to one and a half times that of the Sun.
Stage 1 - 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.
Stage 2 - A region of condensing matter will begin to heat up and start to glow forming Protostars. If a protostar contains enough matter the central temperature reaches 15 million degrees centigrade.
Stage 3 - At this temperature, nuclear reactions in which hydrogen fuses to form helium can start.
Stage 4 - The star begins to release energy, stopping it from contracting even more and causes it to shine. It is now a Main Sequence Star.
Stage 5 - 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.
Stage 6 - The helium core now starts to contract further and reactions begin to occur in a shell around the core.
Stage 7 - 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.
Stage 8 - The helium core runs out, and the outer layers drift of away from the core as a gaseous shell, this gas that surrounds the core is called a Planetary Nebula.
Stage 9 - The remaining core (that’s 80% of the original star) is now in its final stages. The core becomes a White Dwarf the star eventually cools and dims. When it stops shining, the now dead star is called a Black Dwarf
- Massive Stars – The Life of a Star of about 10 Solar Masses
- Massive stars have a mass 3x times that of the Sun. Some are 50x that of the Sun.
Stage 1 - Massive stars evolve in a similar way to a small star until it reaches its main sequence stage (see small stars, stages 1-4). The stars shine steadily until the hydrogen has fused to form helium (it takes billions of years in a small star, but only millions in a massive star).
Stage 2 - The massive star then becomes a Red Supergiant and starts of with a helium core surrounded by a shell of cooling, expanding gas.
Stage 3 - In the next million years a series of nuclear reactions occur forming different elements in shells around the iron core.
Stage 4 - 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).
Stage 5 - Sometimes the core survives the explosion. If the surviving core is between 1.5 - 3 solar masses it contracts to become a tiny, very dense Neutron Star. If the core is much greater than 3 solar masses, the core contracts to become a Black Hole.