Two factors result in so many different types of stars: the size of the clouds they are born from and what kinds of elements they contain.
The Jewel Box Cluster (NGC 4755) is host to a large collection of new and old stars. Credit: ESO/Y. Beletsky
- Stellar size and class diversification originates from the initial mass of the molecular cloud from which stars form.
- The elemental composition of the molecular cloud, specifically the abundance of elements heavier than helium, influences the mass, formation rate, and lifespan of resulting stars.
- The mass of a newly formed star dictates its main sequence lifespan and subsequent evolutionary path, influencing its eventual size, temperature, and brightness.
- Stars with higher concentrations of heavy elements evolve more rapidly than those with lower concentrations, contributing to the observed diversity in stellar properties.
What are the factors that result in so many different sizes and classes of stars?
Kenton Bowers
Pahrump, Nevada
To understand how we classify and differentiate the stars we see in the sky, we first need to understand how they are born and change throughout their lives.
Star formation begins with a massive cloud of cold gas and dust. The gas and dust then begin to clump together, and as a single clump grows in size, its gravity gets stronger. It then keeps accumulating the material around it until its gravity becomes so strong that it collapses in on itself, like a sandcastle built so tall that it can’t support its own weight. As that collapse happens, the inside of the now very large clump begins to heat up because of all the pressure from the tons and tons of material on top of it.
How and when this collapse starts depends on the overall mass of the gas and dust cloud and what types of elements are inside it. Compared to a smaller cloud, a more massive cloud can produce stars that are more massive — and make a larger number of them. A cloud with a lot of metals (defined by astronomers as elements heavier than helium) can make massive stars with high concentrations of those elements and make them quickly, while clouds with more hydrogen and helium tend to make smaller stars more slowly.
The baby star that is born from one of these clouds has a very specific size, mass, temperature, and elemental makeup based on the collapse. Once this star’s core gets hot enough to start fusing hydrogen into helium, we define it as a main sequence star, which includes those like our Sun. As the star gets older and evolves, its mass and elemental composition still play a big role. Bigger stars fuse elements in their cores much faster than smaller ones, causing them to run out of fuel sooner and therefore die earlier. They are astronomy’s version of “live fast, die young.” Stars with lots of heavy elements also tend to evolve more quickly than stars with fewer heavy elements, and can lose a lot of their mass throughout their lives. This combination of processes makes every star a little different (size, temperature, brightness, etc.), which leads to our classification of stars.
So, long story short, the two primary factors that result in so many different types of stars and dictate how we classify them are the size of the molecular clouds the stars are born from and what kinds of elements are inside of them.
Jessica Ranshaw
Astronomy Graduate Student, Indiana University Bloomington, Indiana
