Emerald Robinson: Hi I'm Emerald Robinson; this What Is video has us seeing stars. For thousands of years man has gazed with wonder at the thousands of stars that light up our night sky. But it wasn't until recent history that we could prove that a star is actually a giant luminous ball of super heated gas, much like our own Sun. But as far as stars go, the Sun is considered average. There are stars that are brighter and dimmer, hotter and cooler.
A star forms when an interstellar cloud of dust, hydrogen, helium and other gases becomes so massive that its internal pressure isn't strong enough to offset its own gravity. This cloud collapses into itself and as it becomes smaller it starts to spin.
After about 100 million years of collapsing and spinning the temperature and pressure at its core become sufficient enough for nuclear fusion to begin, and a star is born.
A star spends about 90% of its lifetime fusing hydrogen into helium. This is the main sequence stage. Stars in this stage are commonly referred to as dwarf stars. The duration a star spends on the main sequence depends primarily on the amount of fuel it has to consume and the speed of its reactions; the larger the star the quicker the fuel consumption.
When a small to average sized star exhausts its supply of hydrogen it cools and expands into a red giant. Eventually the star will exhaust all of its helium, cease fusion reactions, and significantly shrink in size. The dull, tiny remnant left behind is called a white dwarf. When the dead star stops shining completely it becomes a black dwarf.
A larger star becomes a massive super red giant before eventually collapsing into itself. When this happens a massive explosion called a supernova occurs. Sometimes the core survives the explosion turning the star into either a radioactive neutron star or a black hole.
Scientists estimate between 200 to 400 million stars exists in the Milky Way galaxy and considering the countless galaxies in our known universe, nobody knows how many stars truly exist.