Stars form within dense molecular clouds inside galaxies. These clouds of gas and dust obscure the first stages of stellar formation through optical telescopes. Fortunately current advances in radio and infrared astronomy now permit astronomers to peer within these clouds and gain a better understanding of the processes involved in starbirth. Computationally intensive computer simulations also permit them to model the processes and test the results against observations.
Gravity is actually the force in charge of stellar formation and the mass of material which forms at star mostly determines its life and fate.
Nebulae are merely clouds of interstellar dust and gas and show up either as dark areas blotting out background stars the so-called dark or absorption nebulae or as brighter clouds of gas that give off or reflect light. They are the most noticeable components of the interstellar medium.
Regardless of what you might think, space is not an ideal vacuum. The room between the stars is loaded with a tenuous array of material that gives the foundations of stars. This material is dust and gas and collectively is called the interstellar medium (ISM). The ISM gas is mostly hydrogen whilst the dust is approximately 1% by mass and contains carbon compounds and silicates. Dust is in charge of the interstellar reddening and extinction of starlight. The harder of the ISM a star's light moves through on its way to an observer on Earth the greater it gets scattered and absorbed, reducing the star's apparent brightness and reddening its look.
Properties of the ISM vary widely based upon its location inside a galaxy. At its most tenuous, in hot areas between denser clouds, it may possess a density of only 100 particles for each cubic metre, mainly ionized hydrogen atoms. In the interior regions of shells of gas encircling stars the density can be up to 1017 particles for each m3 although this is still a million times less dense compared to a normal vacuum on Earth.
Stars are hot bodies of glowing gas that begin their life in Nebulae. They differ in mass, size and temperature, diameters varying from 450x smaller to over 1000x bigger than that of the Sun. Masses vary from a twentieth to over 50 solar masses and surface temperature can vary from 3,000 degrees Celcius to over 50,000 degrees Celcius.
The colour of a star is based on its temperature, the coolest stars are red and the hottest stars are blue. The Sun features a surface temperature of 5,500 degrees Celcius, its colour looks yellow.
Stage 1: Stars are born in an area of high density Nebula and condenses into a massive globule of gas and dust and contracts under its own gravity.
Stage 2: An area of condensing matter will start to heat up and begin to glow building Protostars. If a protostar contains sufficient matter the central temperature gets to 15 million degrees centigrade.
Stage 3: At this temperature, nuclear reactions where hydrogen fuses to make helium can start.
Stage 4: The star starts to release energy, preventing it from contracting even more and causes it to shine. It is now a Primary Sequence Star.
Stage 5: A star of a single solar mass stays in main sequence for around 10 billion years, till all of the hydrogen has fused to make helium.
Stage 6: The helium core now begins to contract further and reactions begin to take place in a shell across the core.
Stage 7: The core is hot sufficient for the helium to fuse to make carbon. The outer layers start to expand, cool and shine less brightly. The expanding star is now referred to as a Red Giant.
Stage 8: The helium core runs out and the outer layers drift of far from the core as a gaseous shell, this gas that encompasses the core is known as a Planetary Nebula.
Stage 9: The rest of the core (thats 80% of the original star) is now in its last stages. The core turns into a White Dwarf the star at some point cools and dims. When it stops shining, the now dead star is referred to as a Black Dwarf.
Stage 1: Massive stars evolve in a simlar way to a tiny stars until it reaces its primary sequence stage. The stars shine steadily until the hydrogen has fused to create helium ( it takes billions of years in a small star, however only millions in a massive star).
Stage 2: The massive star then turns into a Red Supergiant and starts of with a helium core encircled by a shell of cooling, expanding gas.
Stage 3: In the next million years a number of nuclear reactions take place forming various elements in shells across the iron core.
Stage 4: The core collapses in less than a second, leading to an explosion known as a Supernova, where a shock wave blows of the outer layers of the star. (The actual supernova shines brighter than the whole galaxy for a short time).Stage 5: Occasionally the core survives the explosion. If the surviving core is between 1.5 - 3 solar masses it contracts to turn into a tiny, very dense Neutron Star. If the core is much more than 3 solar masses, the core contracts to become a Black Hole.