SEED Science

Virtual Experiment - Build Your Own Star

Build Your Own Star

Use our star simulator to build your own star! You determine the fate of your star by setting initial characteristics. Then watch as its life story unfolds before your eyes. Here’s your guide to the Build Your Own Star controls and displays. But first, a little background…


Star Primer

All stars have a beginning and an end. But their life cycles vary. Some are short lived, while others remain bright for a long time. Some end up as white dwarfs, while other become neutron stars or black holes.

Star controls
You determine the fate of your star by setting initial characteristics…

Stars generate energy through a process called fusion. Atoms of lighter elements combine to form atoms of heavier elements. In the most common fusion process in the universe hydrogen combines to form helium. This is what is going on in the Sun right now. In some stars, helium fuses into carbon and oxygen. These elements may then fuse into still heavier ones.


There are two main factors that determine how the life of a star unfolds:

  • The mass of the star—how much stuff there is in it.
  • The proportion of the star that is made of metal.

For astronomers, the term “metal” does not just mean iron, copper and other elements that we normally think of as metals. Any elements other than hydrogen and helium are referred to as metal. (This may sound odd, but it is just the way the term "metal" is defined for astronomers.)

Star life stages
A star near the beginning and end of its lifecycle: Protostar (left) and Neutron Star (right).

Stars are giant balls of gas in a balance between gravity and heat. Gravity pulls the gas inwards towards collapse, while the pressure due to the internal heat pushes the star towards expansion. It is helpful to think of stars as being in a balancing act: on the brink of a gravitational collapse that is only prevented by the heat generated via nuclear fusion. Stars that are massive must burn fuel much faster than light stars in order to maintain the balance. Despite having more fuel they age much faster—in millions of years instead of billions.

The metallicity of stars is their proportion of elements other than hydrogen and helium. This has an effect on the balance between gravity and heat because it affects how easily the generated heat can push gas out. Higher metallicity means the star is more opaque—less transparent—to photons. So light and heat must work harder before escaping the star.


Vary Mass and Metal Content to Determine the Fate of Your Star


Mass and metal adjustments
Vary mass and proportion of metal content to see how your star's lifecycle changes.

Build Your Own Star allows you to vary these two factors using the sliders at the lower left of the display. The numbers for mass are solar masses. A star with a mass of 1.0 has the same mass as our sun. If you set the slider at 100 your star will have the mass of 100 suns.

The numbers for metal tell you what proportion of the star is made of metal—elements other than hydrogen and helium. For example, if you set this at 0.01 your star will be 1/100 or 1% metal. The metallicity of our Sun is about 0.02.


Stages in the Life of a Star


The Hertzsprung – Russell diagram plots visible stars along two dimensions.

Once you have determined mass and metallicity the fate of your star is set. You will see a list of the stages in the life of your star in the window at the upper right of the display. As the animation progresses, the current stage will be highlighted. But what are these stages? What do these terms mean? These names come from a tool used by astronomers that is known as the Hertzsprung – Russell diagram. It is named after two astronomers who, working independently, developed the diagram.

The Hertzsprung – Russell diagram plots visible stars along two dimensions. The vertical axis is the luminosity, or absolute brightness of the star compared to our Sun. A star with a luminosity of 1 is as bright as the Sun. Luminosity is not the same as the apparent brightness of the stars as we see them in the sky. A very luminous star that is far away will appear dim, while a low luminosity star that is nearby will appear bright. The scale is corrected for distance. Stars that are higher on the diagram are more luminous than those lower down. 


Stages in the lifecycle of a star.

The horizontal axis is the surface temperature in degrees Kelvin. Stars to the left are hotter than those on the right.

Most stars fall in an area called the Main Sequence. Some giant stars are very bright, but cool. Despite their low temperature they have such enormous surface area that they radiate a lot of energy and are hence as bright as hotter, but smaller, stars. White dwarfs are dimmer, but hot. Neutron stars and black holes also fall in this lower left area of the diagram.

Our Sun is a very ordinary star. It is found solidly in the main sequence.

The Hertzsprung – Russell diagram is a snapshot of the stars we now see, but it may also be used to describe the life cycle of a given star. Most stars spend the better part of their lives in the Main Sequence. Eventually they will diverge from this area. Then, their mass and metallicity will determine where they go, which path they follow in the Hertzsprung – Russell diagram.

Here is a summary of the stages that stars go through. Not all stars go through all the stages. Also notice that the images are at different scales, shown at the lower left of each picture. And, the size of a star at a given stage will depend upon the initial mass and metallicity.


A cloud of dust that pulls together due to gravity. It becomes hot enough for fusion to begin and a star is born. Once fusion begins the internally generated heat will balance the gravitational collapse and the star will stabilize.

Main Sequence

Stars spend most of their lives here, gradually growing brighter and hotter and hence moving from lower right to upper left in the HR diagram. They are fueled mainly by hydrogen fusing into helium.


Hertzsprung Gap

In the star’s core, most of the hydrogen has fused into helium. The denser helium forms a core, outside of which a layer of hydrogen continues to burn. At some point the helium core collapses while hydrogen fusion continues in the outer layers. The star begins to expand and the surface cools. It is only the surface temperature that is lower; the actual core temperature is higher when helium is fused than when hydrogen was fused.


Naked Helium Star

This stage occurs for very massive stars after the Hertzsprung Gap when the outer layers of the star are lost to stellar winds. Instead of becoming a giant, the compact helium core is all that is left.


Core Helium Burning

Temperature rises in the collapsing core. Helium fuses into carbon and oxygen.


Asymptotic Giant Branches

As the helium fuel is used up the core again collapses and the outer layers expand and cool. The star becomes a super giant.


Carbon/Oxygen White Dwarf

The outer layers expand off the star and the core remains as a white dwarf. This is the fate of lower mass stars.


Neutron Star

After several more rounds of fusion in which heavier elements are formed the core collapses further and becomes very dense. Radiative pressure from the core causes the explosive loss of the outer layers. We have a supernova. What is left is a dense neutron star. Some neutron stars are observed as rapidly rotating pulsars.


Black Hole

If the star is very massive, the collapse of the core is so great that it becomes a black hole instead of a neutron star.

Now it's time to Build Your Own Star…