How Stars Change throughout Their Lives

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.

Stars are some of the fundamental building blocks of the universe. They not only make up galaxies, but many also harbor planetary systems. So, understanding their formation and evolution gives important clues to understanding galaxies and planets.

EarthSunSystem_HW.jpg
The Sun affects the solar system in many ways. It teaches astronomers how stars work. NASA/Goddard Space Flight Center

The Sun gives us a first-class example to study, right here in our own solar system. It's only eight 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 time, it fuses hydrogen in its core to make helium. 

Throughout its history, the Sun has looked pretty much the same. To us, it has always been this glowing, yellowish-white object in the sky. It doesn't seem to change, at least for us. This is because it lives on a very different timescale than humans do. However, it does change, but in a very slow way compared to the rapidity in which we live our short, fast lives. If we 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 over tens of millions or billions of 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 people might sort coins or marbles. It's called "stellar classification" and it plays a huge role in understanding how stars work. 

Classifying Stars

Astronomers sort stars in a series of "bins" using these 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". 

hertzsprung-russell diagram
This version of the Hertzprung-Russell diagram plots the temperatures of stars against their luminosities. The position of a star in the diagram provides information about what stage it is in, as well as its mass and brightness. European Southern Observatory

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.

Spitzer Space Telescope Pictures Gallery - The Starless Core That Isn't
The "Cores to Disks" Spitzer Legacy team used two infrared cameras on NASA's Spitzer Space Telescope to search dense regions of interstellar molecular clouds (known as "cores") for evidence of star formation. NASA/JPL-Caltech/N. Evans (Univ. of Texas at Austin)/DSS

The density in the core reaches a point where the temperature is at least 8 to 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", which 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. When all these conditions are satisfied, the star is "on the main sequence" and it goes about its life busily making hydrogen into helium in its core.

It's All About the Mass

Mass plays an important role in determining the physical characteristics of a given star. It also gives clues to how long the star will live and how it will die. 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. That determines how fast a star will use up its fuel.

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, which uses its fuel more slowly.

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 for that star. Why? Because fusing iron takes more energy than the star has available. It stops the fusion factory dead in its tracks. When that happens, the outer layers of the star collapse in on the core. It happens pretty quickly. The outer edges of the core fall in first, at the amazing speed of about 70,000 meters per second. When that hits the iron core, it all starts to bounce back out, and that creates a shock wave that rips through the star in a few hours. In the process, new, heavier elements are created as the shock front passes through the material of the star.
This is what's called a "core collapse" supernova. Eventually, the outer layers blast out to space, and what's left is the collapsed core, which becomes a neutron star or black hole.

T he Crab Nebula is a remnant left over after a massive star exploded as a supernova. This composite image of the Crab Nebula, assembled from 24 images taken by the NASA Hubble Space Telescope shows features in the filamentary remains of the star as its material spreads out to space. NASA/ESA/ASU/J. Hester & A. Loll

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.

A planetary nebula called the Southern Owl Nebula
Will the Sun look like this in the far distant future? This extraordinary bubble, glowing like the ghost of a star in the haunting darkness of space, may appear supernatural and mysterious, but it is a familiar astronomical object: a planetary nebula, the remnants of a dying star. This is the best view of the little-known object ESO 378-1 yet obtained and was captured by ESO's Very Large Telescope in northern Chile. European Southern Observatory

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.