Where is a protostar on the HR diagram?

Protostars on the HR Diagram: Where Baby Stars Hang Out

The Hertzsprung-Russell diagram – or HR diagram, as astronomers like to call it – is basically a stellar cheat sheet. It plots stars based on their brightness (luminosity) and temperature (color), giving us a way to understand how stars evolve over their lifetimes. Think of it as a cosmic family portrait, with each star finding its place based on its unique characteristics. But where do protostars, those wee little baby stars still in the making, fit into this grand scheme?

What Exactly Is a Protostar, Anyway?

Before we go hunting for protostars on the HR diagram, let’s get clear on what they are. A protostar is a star in its absolute earliest stage of life, still bulking up by gobbling gas and dust from its surroundings. Unlike grown-up, main-sequence stars that are busy fusing hydrogen in their cores, protostars are powered by the energy released as material crashes onto their surface and the swirling disk around them. It’s a messy, chaotic phase, a bit like a cosmic toddler learning to walk before it starts its main sequence.

So, Where Do We Find Them on the HR Diagram?

Now, here’s the thing: protostars are often shy. They’re usually hidden behind clouds of gas and dust, making them hard to spot with regular telescopes. But if we could plot them, a protostar would theoretically sit above and to the right of its eventual main-sequence home. Imagine a star map, and these baby stars are hanging out a bit higher and further to the right than their adult selves will be.

Why this odd location? Well, a couple of reasons:

• They’re Cooler: Protostars haven’t quite fired up their nuclear engines yet, so their surface temperatures are relatively cool, giving them a reddish hue.
• They’re Surprisingly Bright: Even though they’re cool, protostars are actually quite luminous. This is because they’re much larger than main sequence stars. A star’s luminosity depends not only on its surface temperature, but also on its size, the protostar will actually be more luminous than the Main Sequence star.

As a protostar shrinks under the force of gravity, its outer layers start to heat up, and its overall brightness dims. This causes it to slide down and to the left on the HR diagram, following a sort of stellar baby track.

Baby Star Highways: Hayashi and Henyey Tracks

These “baby tracks” aren’t just random wanderings; they follow theoretical paths called Hayashi and Henyey tracks. Imagine them as highways on the HR diagram, guiding protostars towards adulthood.

• Hayashi Tracks: Think of these as the local roads for smaller stars. Stars up to about three times the mass of our Sun follow these near-vertical paths, shrinking and dimming while staying at roughly the same temperature.
• Henyey Tracks: For the bigger, more ambitious protostars (more than half the Sun’s mass), it’s more like a highway. They start on the Hayashi track, then merge onto the Henyey track, which is more horizontal. This means they heat up while staying at a fairly constant brightness as they cruise towards the main sequence.

Finally, Growing Up: Joining the Main Sequence

Eventually, the protostar’s core gets so hot and squished that nuclear fusion finally kicks in. Boom! It’s a real star! At this point, it stops shrinking and settles down on the main sequence. Where it lands depends on its mass: heavyweights end up on the hot, bright upper left, while the lightweights chill on the cool, faint lower right.

Don’t Confuse Them with Red Giants!

Here’s a fun fact: a brand-new protostar and a puffed-up red giant star can briefly look like they’re in the same neighborhood on the HR diagram. The key difference? A red giant is at the top-right side, which indicates it’s cooler (red) but much more luminous due to its large size.

The Takeaway

Protostars have a special spot on the HR diagram, hanging out above and to the right of the main sequence. This location tells us they’re cooler and brighter than their main-sequence counterparts, reflecting their early, formative years. As they evolve, they follow specific tracks, eventually joining the stellar family on the main sequence. So, next time you glance at the HR diagram, remember those hidden protostars – the future stars of the cosmos, just waiting to shine.