Nebula historically referred to extended space objects. The Andromeda galaxy was previously the Andromeda nebula before being found to be a galaxy. Nebulae are now space volumes of interstellar clouds and dust. The space volumes where the gas and dust are most dense we call diffuse nebulae; they are space volumes where stars are born.
H – Ⅱ diffuse nebulae regions are so called because a lot of hydrogen in them is in its heavy deuterium form. In this form of hydrogen its normal nuclear single proton is accompanied by a neutron so that its atomic mass is 2. Hence H – Ⅱ. Gasses in these areas are known as molecular clouds. They are a feature of spiral galaxies that contain much gas and dust in their spiral arms. The milky way, our galaxy, is just such a spiral galaxy and in our galaxy there are many such nebulae where stars are being born. The following are stages in a star’s life.
Stage 1: Molecular Cloud.
The process by which the gas and dust of a molecular cloud start to collect in clumps is called accretion. It is thought that the universe’s earliest molecular clouds were entirely of hydrogen and helium and that only later clouds contained tiny fractions of heavier elements, probably from star explosions and emissions. Isotopes and ionisations are thought to be common in space.
The process of accretion is one in which heavier accumulations become more massive as they attract more surrounding dust and gas and the more massive the clump becomes the greater becomes the pressure on its core. In this way the desire for external energies cause core materials to become increasingly agitated heat up.
Stage 2: Proto Stars.
Centres of these heated gas clumps are where proto stars are born. Being surrounded by gas they can at an early stage be observed by infra red telescopes that see the energies emitted by their cores. The star may spend a 100,000 years at this stage growing. Eventually the star starts radiating energy in excess of that it is taking on board and the cocoon of dust and gas around it gets dispersed by jets of what is called a T-tauri wind. The star has become a main sequence star and is now visible to optical telescopes.
Stage 3: Main Sequence.
The star spends 90% of its life at this stage. After a few orbits of its galaxy it will have been pulled by others, from its birth area and into a space of its own. Its core is now undergoing a fusion process, started by the pressures built up in the previous stage. The fusion process is outputting energy at the expense of the star’s mass energy.
The fusion process is not a burning process. It is a number of nuclear process by which hydrogen in the hot core of the star is converted (fused) to helium, the processes involving a loss of mass energy to radiated energy. The energy output is given by Einstein’s well known E = mc2 where c is the speed of light. There is no explosive release of energy because gravitational pressures on the core determine the rate of fusion, but the fusion energy releases then counter and diminish the gravitational desires for energy.
The processes by which stars convert hydrogen to helium vary according to star mass which is usually a comparison with our own sun’s mass (solar mass). Stars like our sun fuse hydrogen to helium via deuterium and if their core temperatures are above 15 million degrees Kelvin via beryllium, lithium and boron. The fusion in stars with masses over 1.2 solar masses may involve carbon, nitrogen and oxygen and in stars of over 3 solar masses fusion is almost entirely via these elements. .
You may think a more massive star will last longer but that is not the case because fusion energy releases are scattered energy releases and on a large star less controlling of gravity
Stage 4: Giants.
The stage of giant is not an automatic phase in a stars life. Small stars, with below 0.35 solar masses, never get beyond the above described stage. helium produced in their cores is convected throughout the star and they become red dwarf stars.
For larger mass stars when all the hydrogen in the core has fused to helium the star core starts to cool and rapidly contracts as gravity wins. But the layers outside the former core contain hydrogen, which now comes under pressure, heats up and starts to rapidly fuse to helium, becoming hotter than at the main stage. At about 108 Kelvin, the helium starts fusing to carbon.
The above happens as a helium flash in stars up to 2.57 solar masses but in a more controlled fashion in more massive stars. Star luminosity increases at this stage by by a factor of 1,000 to 10,000 and its outer parts swell so that the star becomes a sub giant, red giant or super giant. Betelgeuse is a super giant. Although the core and lower layers are now generating more energy than at the main stage the increase of size brings decreased surface radiation and such stars usually appear red.
In medium stars like our sun the heat is sufficient for the fusion of Helium into heavier oxygen and carbon. This process is much shorter than the hydrogen to helium fusion. Again, there is loss of mass and energy is output.
Stars, at main stage more than 5 times the mass of our sun will go on to fuse carbon and oxygen into neon, sodium, magnesium, sulphur and silicon and maybe into calcium, iron, nickel, chromium and others, with each stage shorter than its previous one.
Stage 5a: Planetary Nebulae.
When gravity is insufficient for further fusion, stars, that were at the main sequence stage up to 7 times the mass of our sun, now have their hot cores sending out increased photons. These photons push the outer carbon and silicon elements into space creating a glowing planetary nebula (it looks like a planet) with a white dwarf star at its centre.
Stage 5b: Supernova.
Stars that had masses above 7 times that of our sun (when at the main sequence) go the supernova route. When fusion ceases their iron cores implode from about earth size to about that of the size of a city in less than a second. Outer gasses are pulled in to strike the core and are compressed in the process. The compression causes the gasses to heat up to billions of degrees and within 15 minutes an explosion results. The super heated gasses carry heavy elements like gold, platinum and uranium into space. The star Betelgeuse is said to be near following this route.
Stage 6: Core Remnant of Supernova
The core remnant after the outer layer has been thrown off at the above supernova stage determines what happens next and is as per the following table.