Science

# What Is It? Hertzsprung-Russell Diagram

If you’re like me, you’ve sat outside on a warm summer night, watched the darkness rise, and count the stars as they popped onto the night sky seemingly out of nowhere. They look more or less the same, but if you’re in a really dark location, you start to notice that some are bigger and brighter than others.

That’s because all stars are not the same. They vary is size, age, composition, distance from Earth, etc. There is one star we know very well, the Sun. Other stars are a little more difficult to study, but over hundreds of years we’ve managed to glean a wealth of information about stars. This information has – in impressive fashion – been distilled onto plot called the Hertzsprung-Russell (or H-R) diagram.

To understand how an H-R diagram works, we need to know about the two most important property of stars: their luminosity and surface temperature. Luminosity is just the amount of energy emitted by the entire surface of the star per unit time, and the surface temperature is, well, the temperature of the surface of the star. (It turns out that the color of a star is pretty good indication of temperature.) These two stellar properties are are tied together; luminosity is proportional to the temperature of the star to the fourth power and to the square of the star’s radius. That means that a cool star can have a high luminosity of its radius is big enough. Conversely, a really hot star can have a low luminosity of it’s also very small.

Got it? Good. Now we can actually look at the wonder that is an H-R diagram:

Source: European Southern Observatory

OK, let’s look at this. You can see from the axes that an H-R diagram plots surface temperature vs. luminosity, with the temperature decreasing as you move to the right and the luminosity increasing as you go up the vertical axis. (Don’t worry about the O, B, A, F, G, K, and M. It’s a way to classify the light spectra of stars, which is a separate but related topic I won’t get into now.) There are other ways to plot this information, but I think this is the most straight forward and all H-R diagrams ultimately say the same thing.

It’s not necessarily obvious how one is supposed to read this, but this particular version of the H-R diagram has some handy labels we can use. You see kind of the spine of the graph? That’s what astronomers call the main sequence. When stars are born, they are born onto the main sequence. While stars are on the main sequence, they are fusing hydrogen in their cores. As stars age, they evolve off the main sequence and start fusing helium. Stars in this stage of their evolution are both luminous and cool. Which, according to the relation we established above, must mean that their radius is huge. Those stars have evolved into giants. For stars of a certain mass, once they complete the giant phase of their evolution, they whip around and become white dwarfs, very small, very hot, and not very luminous husks of their former selves.

But wait! There’s more! Stars also follow a handy mass-luminosity relation. The luminosity of a star is roughly proportional to its mass. (Well, it’s mass to some power, depending on the kind of star we’re talking about.) Look at the diagram and think about what that means. The higher up the luminosity axis you go, the more massive the star.

The luminosity also has implications for stellar lifetimes. It’s been shown that the time a star spends on the main sequence is about 10 Gyr (or 10 billion years) times the ratio of the stellar luminosity to the solar luminosity to the -5/7 (because nothing can just be easy). So we know from the mass-luminosity relation that less luminous stars are also less massive. Let’s take a star with a luminosity fraction of 0.1. The time it should spend on the main sequence is about 50 billion years. The universe is only about 14 billions years old. It’s been shown that no star under 0.9 solar masses has evolved off the main sequence in the history of the universe. Whoa.

So that’s it, an H-R diagram. If you know what you’re doing – which, now, you do – you can glean an impressive amount of information about the stars that populate the universe. Have fun.

Featured image credit: Hubble ESA via Flickr