Life on the Main Sequence: How Stars Evolve

main sequence stars
This stellar nursery in the Large Magellanic Cloud has stars that are living on the main sequence, a time when they fuse hydrogen in their cores. NASA/ESA/Hubble Heritage Team.

If you want to understand stars, the first thing you learn is how they work. The Sun gives us a first-class example to study, right here in our own solar system. It's only 8 light-minutes away, so we don't have to wait long to see features on its surface. Astronomers have a number of satellites studying the Sun, and they've known for a long time about the basics of its life. For one thing, it's middle-aged, and right in the middle of period of its life called the "main sequence". During that that, it fuses hydrogen in its core to make helium. 

Throughout history, the Sun has looked pretty much the same. This is because it lives on a very different timescale than humans do. It does change, but in a very slow way compared to the rapidity in which we live our short, fast lives. If you look at a star's life on the scale of the universe's age — about 13.7 billion years — then the Sun and other stars all live pretty normal lives. That is, they are born, live, evolve, and then die on timescales of tens of millions or a few billion years.  

To understand how stars evolve, astronomers have to know what types of stars there are and why they differ from each other in important ways. One step is to "sort" stars into different bins, just as you might sort coins or marbles. It's called "stellar classification". 

Classifying Stars

Astronomers classify stars by a number of their characteristics: temperature, mass, chemical composition, and so on. Based on its temperature, brightness (luminosity), mass, and chemistry, the Sun is classified as a middle-aged star that is in a period of its life called the "main sequence". 

Virtually all stars spend the majority of their lives on this main sequence until they die; sometimes gently, sometimes violently. So, what is the main sequence? 

It's All About Fusion

The basic definition of what makes a main-sequence star is this: it's a star that fuses hydrogen to helium in its core. Hydrogen is the basic building block of stars. They then use it to create other elements.

When a star forms, it does so because a cloud of hydrogen gas begins to contract (pull together) under the force of gravity. This creates a dense, hot protostar in the center of the cloud. That becomes the core of the star.

The density in the core reaches a point where the temperature is at least 8 - 10 million degrees Celsius. The outer layers of the protostar are pressing in on the core. This combination of temperature and pressure starts a process called nuclear fusion. That's the point when a star is born. The star stabilizes and reaches a state called "hydrostatic equilibrium". This is when the outward radiation pressure from the core is balanced by the immense gravitational forces of the star trying to collapse in on itself.

At that point, the star is "on the main sequence". 

It's All About the Mass

Mass plays an important role in simply driving the star's fusion action, but mass is quite a bit more important during the life of the star. The greater than the mass of the star, the greater the gravitational pressure that tries to collapse the star. In order to fight this greater pressure, the star needs a high rate of fusion. Therefore the greater the mass of the star, the greater the pressure in the core, the higher the temperature and therefore the greater the rate of fusion.

As a result, a very massive star will fuse its hydrogen reserves more quickly. And, this takes it off the main sequence more quickly than a lower-mass star.

Leaving the Main Sequence

When stars run out of hydrogen, they begin to fuse helium in their cores. This is when they leave the main sequence. High-mass stars become red supergiants, and then evolve to become blue supergiants. It's fusing helium into carbon and oxygen. Then, it begins to fuse those into neon and so on. Basically, the star becomes a chemical creation factory, with fusion occurring not just in the core, but in layers surrounding the core. 

Eventually, a very high-mass star tries to fuse iron. This is the kiss of death. Why?  Because fusing iron takes more energy than the star has, and that stops the fusion factory dead in its tracks.  The outer layers of the star collapse in on the core. This leads to a supernova. The outer layers blast out to space, and what's left is the collapsed core, which becomes a neutron star or black hole.

What Happens When Less-massive Stars Leave the Main Sequence?

Stars with masses between a half a solar mass (that is, half the mass of the Sun) and about eight solar masses will fuse hydrogen into helium until the fuel is consumed. At that point, the star becomes a red giant. The star begins to fuse helium into carbon, and the outer layers expand to turn the star into a pulsating yellow giant.

When most of the helium is fused, the star becomes a red giant again, even larger than before. The outer layers of the star expand out to space, creating a planetary nebula. The core of carbon and oxygen will be left behind in the form of a white dwarf.

Stars smaller than 0.5 solar masses will also form white dwarfs, but they won't be able to fuse helium due to the lack of pressure in the core from their small size. Therefore these stars are known as helium white dwarfs.Like neutron stars, black holes, and supergiants, these no longer belong on the Main Sequence.

Edited and updated by Carolyn Collins Petersen.