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Q & A: Measuring the properties of stars

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Most recent answer: 10/22/2007
Q:
How do they firgure out size, color, temperature, and brightness of a star? What are the stars size, color, temperature, and brightness of stars?
- Jenna
Aldrich , Warwick, RI USA
A:
Hi Jenna,

Great question! By measuring the properties of stars, astronomers can learn lots about how they work, about the environments in which stars are found (clusters, galaxies, dust clouds, etc.), and some things about the whole universe (such as its expansion rate). By studying stars that are older and younger than our sun, we can make some guesses as to how our sun will behave as it gets older.

The easiest thing to measure about a star is how bright it appears to us. Back in the old days, the best light detectors anyone had were their own eyes, and people ranked stars brightnesses comapring them to other stars. These days, we have electronic equipment, such as photometers and charge coupled devices (CCD's) which can read out with high precision exactly how much light is hitting them per second. A CCD is the same thing that's found in video cameras which reads out electrical pulses which indicate how much light strikes a tiny surface. It's important to collect only light from one star at a time (or do as best a job as possible), and to collect as much of it as possible, so to do a good job with this measurement, large telescopes are used.

The next easiest thing to measure about a star is its color. Some stars actually look red or blue or yellow to they eye, and again modern telescopes and equipment make the study of color more precise. They use diffraction gratings, finely etched parallel lines on metal, to split the light into its different colored components, in much the same way a prism does. Electronic equipment (it used to be film, but nowadays electronics is faster, more reliable, and more convenient for data storage) reads out the light thus split. By looking at absorption lines in the spectrum, astronomers can determine the chemical composition of the star's surface and immediate surroundings. By measuring the shift in the frequencies of light corresponding to the absorption lines, astronomers can calculate, using Doppler's law, how fast the star is moving towards us or away from us. This has been used to gauge the expansion of the universe, as well as to measure how fast galaxies are turning.

The temperature of the star can be deduced from the spectrum of light measured with the diffraction grating. It is known just how much light at each color you expect to get from a large, hot ball of gas like a star. Hotter stars are more blueish, and colder stars are more reddish. Some stars, called "brown dwarfs", are so cold they emit rather little light at all. This temperature is only the temperature of the "surface" of the star (that is, the photosphere). We cannot tell easily the temperature in the middle of a faraway star, although we can use our knowledge of how gases work under high heat and pressure conditions to make educated guesses.

To get the size of a star usually involves estimating the distance to the star. Stars that are not too too far away can have their distances measured using parallax. As the earth moves around the sun, we see the stars from a different vantage point every night. Observations made six months apart have the biggest difference in vantage point. If we compare where we see a nearby star, as seen in a background of very faraway stars, at one observation and six months later, we may find that its apparent position has shifted a tiny amount relative to the faraway background stars. By knowing the size of the earth's orbit and this little angle, we can estimate the distance to the star.

Some stars are close enough to be seen as a disk in high-power telescopes (I believe Betelgeuse is big enough and close enough to see the disk). Most stars appear as points of light in even the largest telescopes, however. To estimate the size of a star where the disk is visible, we simply make a triangle with one angle is the angular radius of the disk, and the side is the distance to the star. The far side is the radius of the star.

This only works for a couple of stars. A more common way to do it is to measure the brightness, the temperature, and the distance. We know what colors a hot gas will emit at a particular temperature, and we also know how much light energy will be emitted per unit time from the surface of the star. By knowing the distance to the star and how bright it seems to us, we can calculate the total light energy coming from the star per unit time (the "luminosity"). The energy put out by the star is proportional to its area -- the bigger the star is at its temperature, the more light energy it will emit. We can then deduce the size from that.

Most stars are too far away to use parallax to estimate their distances. Some special kinds of stars, called "Cepheid variable stars", are stars that periodically change their brightness. The time it takes for these stars to dim and get bright again depends on their intrinsic brightness, as explained . This is calibrated for nearby Cepheid variable stars and used for faraway ones. By knowing how intrinsically bright a faraway Cepheid variable star is, and by knowing how bright it appears to us, we can estimate how far away it is. If we know other stars are nearby the Cepheid variable, we can use the same distance estimate.

Tom

(published on 10/22/2007)

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