By Robert Hazen, Ph.D., George Mason University
A major challenge in astronomy is that the universe is a three-dimensional volume, and yet, the backdrop of stars that we see looks to be two-dimensional. Astronomers tell us that stars are natural fusion reactors, but how can they know that? It turns out that there are at least six key pieces of information that we can glean about the stars from our astronomical observations.
Through astronomical observations, we can measure the distance to many stars; we can measure their composition and see what atoms they’re made of; we can measure their motion and observe how they’re moving with respect to the backdrop of stars; we can measure their temperature; we can measure their brightness; and we can often measure their mass. Let’s look at these six characteristics of stars.
Triangulating the Stars
The distance to an object can be measured in a number of ways, but the most basic is called parallax; that’s a form of triangulation. The Sun is the center of the orbit of the Earth. The Earth goes around the Sun with a diameter of orbit of about 186 million miles. So, over a six-month period, the Earth swings from one side of the Sun to the other, and then back again.
Our position with respect to the stars too keeps shifting back and forth, and as it shifts, closer stars appear to move more than the more distant stars. Therefore, for stars that are up to, perhaps, ten light-years away, we can actually see the variation in the position of the star, and measure its distance directly by triangulation.
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We also can use ‘standard candles’ as another way of measuring distance.
Stars and other objects in the heavens have a known brightness. If one held a candle in front of us, and we knew how bright that candle was, then when the person walked some distance away, the candle would get dimmer and dimmer, but we would be able to tell how far away it was just by measuring its brightness.
By the same token, we can tell how far away many stars are by measuring their brightness.
The Composition of a Star
We can often measure the composition of a star by looking at its spectrum. As we know, different chemical elements emit lines, bright lines, when electrons rise to higher energy levels and drop back down. They emit photons of characteristic wavelengths.
Thus, hydrogen has one set of characteristic wavelength, helium another, calcium another, and so on. So by looking at the characteristic spectrum of stars, we can actually determine the composition of those stars, even though they may be millions of light-years away.
Learn more about forces in the universe.
The Doppler Shift
The motion of a star can sometimes be detected by absolute motion over short periods of time. From year to year, some stars actually appear to move slightly, and so we can see how they’re moving in the skies. But there’s also a method called the Doppler shift, by which we can measure the movement of a star because of the change in the wavelengths of the characteristic lines.
Any object that emits waves will display this behavior of the Doppler Effect, whether it’s moving toward us or away from us. When an object moves towards us, the wavelengths come more quickly. They pile up, and so we see a shorter wavelength, a higher frequency, a blue shift of the light; and so the characteristic hydrogen lines are shifted towards the blue part of the spectrum. However, if the object moves away from us, the wavelengths are spread out towards the red end of the spectrum, and we see a red shift. The degree of the blue shift, or the red shift, tells us the relative velocity of the star to us.
Temperature of a Star
Determining the surface temperature of a star is pretty simple; it’s just basically the color of the star. We all have noticed this at some point: a very hot flame, such as an acetylene torch, looks blue-white. A candle flame is more like a yellow flame.
If we have a fire that’s slowly dying, with embers cooling down, they become orangish or reddish, and darker and darker. That’s a reflection of the temperature of the material. Redder materials are cooler, while blue-white stars are very hot stars, and we have all gradations in between.
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The brightness of a star, how bright it appears to be, combined with the knowledge of how far away the star is, gives us an indication of the total energy of the star.
Hence, brightness becomes a very important thing to measure. This is called the ‘apparent magnitude’, and the apparent magnitude is directly related to distance and energy of that star.
Mass of a Star
Finally, the mass of a star can often be determined just by looking at its motion, especially if we have a binary star system.
Most of the stars in the heavens include one star rotating around a second star, and that rotation motion can often tell us the mass. Yet, most of our information about the mass of stars comes from the theory of how stars form and how they burn.
Astronomy is no doubt a science. But it is also an art-the art of collecting, analyzing, and interpreting photons from space. It focuses on astronomical data, and what can we learn from them. That is perhaps the very nature of astronomy itself.
Common Questions about Understanding the Characteristics of Stars
We can often measure the composition of a star by looking at its characteristic spectrum.
Any object that emits waves will display the behavior of the Doppler Effect, especially if it’s moving toward us or away from us.
We can determine the temperature of a star by looking at its color. Redder materials are cooler, while blue-white stars are very hot stars; then there are gradations in between.