Star Sizes

Our ultimate goal in this unit is to understand how stars are born, change over time, and ultimately die. This is really hard to figure out because stars “live” for billions of years. The only way for us to answer these kinds of questions is to survey millions of stars and observe them at various stages of their “lifecycles” - some newly born, others squarely in “mid-life” and others that have recently “died.”
Explore 18.1 A Stellar Census to learn about the distribution of stars in terms of brightness, luminosity, and mass, including the prevalence of low-mass stars and the discovery of brown dwarfs.
The two key variables we will measure to understand stars are their luminosity and temperature. As I stated in a previous lesson, we can use these two values to determine other stellar properties (such as a star’s mass and radius). But how? Well, let’s consider the mass of stars first. If we think back to our study of gravity and orbits, we saw that Newton’s Law of Gravity combined with Kepler’s Laws allows us to accurately determine the mass of any two objects that are orbiting one another. So, if we can find two stars that are orbiting each other, we can accurately measure their masses.
When astronomers measured the mass of many stars, they made a wonderful discovery… there is a clear relationship between the mass of a star and its luminosity. This means that if you measure the luminosity of a star (which is relatively easy to do), then you can accurately calculate its mass, even if there is nothing orbiting that particular star.
Review 18.2 Measuring Stellar Masses to understand how stellar masses are determined through the analysis of binary star systems and the mass-luminosity relation.
We can also independently measure the diameter of stars and likewise derive a relationship between luminosity, temperature, and star size. But how do we measure the diameter of stars that are too far away to be resolved in a telescope? Watch the video Eclipsing Binary Stars for an explanation.
Read 18.3 Diameters of Stars to learn about methods for measuring the diameters of stars, including the use of eclipsing binary systems and transits.
Our sun is SO incredibly huge, but there are other stars in our universe that are SO much bigger than the sun. It is truly incredible. Watch the Kurzgesagt video The Largest Star in the Universe to see a scale model that shows the largest stars compared to our own sun. (Here’s a link directly to the best part of the video.