Main Sequence Star
When we think about stars, most of our minds immediately go to our Sun. The Sun is what's called a Main Sequence Star, an adult star characterized by being in its longest, most stable era of its life cycle.
Main Sequence Stars burn by fusing Hydrogen into Helium in their cores. But while Main Sequence Stars are the first thing to pop up when talking about what a star is, there's plenty more stars out there!
Dwarf Stars
Dwarf Stars, for example, are small stars that make up a LOT of the universe. They come in many different colors, with many different lights and many different temperatures.
Red Dwarf stars are cool and faint, having burned away a lot of their gasses over the years.
White Dwarf stars are the dense cores of stars that have burned out, being small, but VERY dense objects.
Brown Dwarf stars are "failed stars", too small to burn gasses like other stars.
Dwarf Stars are usually not very big or bright, but they're very old, some even billions of years old!
Giant Stars
Then, there's the big bright fiery stars! Giant Stars are big stars that burn a lot of gas, really quickly. They tend to not stick around as long as a Dwarf star, since by that time, a Giant Star forms from a star that has burned up a lot of its gasses in its previous life cycle era. Like Supergiants, and even larger Hyper-giants!
These Giant stars are much rarer than your usual Main Sequence or Dwarf Star, because they live so quickly, only around 10-50 million years compared to the billions Dwarf Stars do. But when a Giant finally burns out, that's a sight to see!
Supernovas
When a star reaches the end of its lifespan, when it's burned away all its gasses and masses, it collapses in on itself. This collapse, and the massive explosion that comes from it, is called a Supernova!
Supernovas release an IMMENSE amount of energy when they go off, in the form of a light that can outshine even entire galaxies, and sending cosmic dust flying into the space around it. This cosmic cloud can clump together into a new galaxy, cooling and acting as a starter for new stars to form.
Neutron Stars and Pulsars
There's a few extra special stars out there that behave in odd ways. Neutron Stars, for example, are incredibly dense, incredibly small stars left over from a Supernova event, formed from the core of the dead star.
Pulsar Stars are a special, highly magnetized, rotating Neutron Star that emits long beams of radiation from its poles. They're named Pulsars because they seem to "pulse" upon observation!
Spectral Classification System
All Stars are classified using something called the Spectral Classification System. Stars are organized by surface temperature and atmospheric pressure. Stars are given a letter, that gives it range from the hottest (blue) to the coolest (red) stars. From hottest to coldest:
O, B, A, F, G, K, M
Our Sun is a G-type star, while the brightest star in the sky, Sirius A, is an A-type. Even if a star is classified as "cold", even the coldest stars reach up to 3,900 degrees Kelvin, or over 6,500 degrees Fahrenheit /3,600 degrees Celsius! Stars are VERY hot.