Over the course of my last eight posts, we’ve covered just about everything there is to cover about our home galaxy–or, well, at least the basics.
We’ve explored how astronomers first discovered what that incredible, milky stream of dust across the night sky actually is. We’ve followed astronomers like the Herschels and Harlow Shapley as they tried to measure the size of our galaxy.
We’ve covered its structure–a thin disk of spiral arms, surrounded by an enormous, diffuse halo–and how truly massive this great wheel is.
Most recently, we delved into the composition of the Milky Way–that is, how much heavy elements its stars contain. We discovered that stellar compositions hint at how old certain parts of the galaxy are.
But there’s one question we haven’t answered yet, and it’s quite possibly the most important one of all.
How did the Milky Way actually become what it is today?
A protostar forms when one dense core of an interstellar cloud condenses enough so that gravity can overcome the repulsive forces between the particles, and collapse the cloud. A very cool object then forms in the cloud’s depths, visible only at infrared wavelengths—known as a protostar.
A protostar is compressed enough to be opaque no matter the wavelength—that is, no radiation can pass through it due to its density. However, what separates it from a “true” star is that it’s not compressed enough to generate energy by nuclear fusion.
Astronomers also define a protostar specifically as a young star that’s not yet detectable at visible wavelengths. In other words, protostars emit only longer-wavelength light—that is, infrared and radio waves.
You’d think that becoming a true star would be the next step for a protostar. But that’s not quite how it happens…
Astronomers have discovered that the clouds of gas and dust—the interstellar medium (ISM)—found between the stars are made of the same materials as the stars themselves. In fact, hydrogen is the most common element in both stars and the ISM, followed closely by helium.
But it would be more accurate to say that stars are made of the same material as the ISM, not the other way around.
This is because all of the stars formed out of material in the ISM at some point millions to hundreds of billions of years ago. And when they die, they return that material—what’s left of it—to the ISM.