Astronomers have a pretty solid idea of how stars are born. They begin within the dense, cold dust of an interstellar cloud such as this one. They heat up and get more luminous as they contract, and then drop in luminosity as they continue to contract steadily toward the main sequence.
I’m going to spend at least the next ten or so posts talking about the main-sequence portion of a star’s life cycle. Basically, we’re talking about a star’s adulthood.
You know what, while we’re at it, why don’t I draw up an analogy between a star’s life cycle and that of a human:
- When a human is a mere fetus developing within its mother, a star is a protostar.
- We say a star has been “born” when it crosses the birth line—basically, satisfies certain expectations for its temperature and luminosity for its specific mass—and becomes visible.
- After that, a star steadily approaches adulthood. A “child” star is referred to as a Young Stellar Object (YSO) or a pre-main-sequence star.
- An “adult” star is one that has begun to fuse atomic nuclei in its core for fuel. At this stage, the star has reached the main sequence.
- When a star runs out of fuel, it leaves the main sequence. We’ll cover this evolution in depth very soon.
I explained this process in depth in my last post. But I also posed the question: how do astronomers know all this? Where’s the evidence? Continue reading
What happens when a star is born?
A couple of posts ago, I explained how a protostar forms out of a dense cloud core within the interstellar medium. But…wait. What exactly is a protostar again?
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… Continue reading