How Far is That Star, Part II
Distance is the holy grail of astronomy, since the distances to
objects in space, while certainly not obvious, give us the shape of the
universe. Astronomers spend a lot of time and effort trying to determine
how far away things are in space, and they use a real grab-bag of
different techniques.One method they use is called parallax, which yields the distances to the very nearest stars. Parallax is basically involves trangulation; see How Far is That Star, Part I for details. Parallax, however, is only good for stars within about a hundred light years, and 99.99999% of the stars in our galaxy are outside this range. What about the rest?
The distances to those stars, as well as stars in nearby galaxies, can be established via the fine art of "main sequence fitting".
The details are complicated (big surprise there!). But you don't need the details to get an understanding of how it works, so I'm going to shamelessly gloss over most of the spectroscopy involved in order to make the concept clear. In my opinion, the concept is most important; if you want details, pursue them on your own (which isn't hard... once you have the concept). Here goes...
The further a star is from you, the dimmer it appears. This is true for any source of light. (Well, sure! A car headlight is a lot dimmer if the car is a mile away than if you hunker down in front of the bumper and stare into the bulb!) So at first brush, it looks like the dimmer a star appears to us, the further away it must be. Bright stars are close, and faint stars are farther away. This simple idea would work perfectly if all stars had the same intrinsic brightness. They don't.
Let's define "intrinsic brightness". Astronomers like to say
"intrinsic luminosity" or maybe "absolute magnitude". I like to think of it as "wattage". A
100 Watt light bulb is brighter than a 60 Watt light
bulb. They both appear brighter if they are closer to you, and look
dimmer if they are farther away, but the 100 Watt bulb is always emitting
more light than the 60 Watt bulb. Imagine this: if the 60 Watt bulb was
close to you, and the 100 Watt bulb was very far away, the 60 Watt bulb
would seem to be the brighter one! Or they might appear to be the same
brightness.
Stars have wattages too, of course. They can vary tremendously, from tiny stars a hundred-thousandth as bright as the Sun to supergiant stars a hundred thousand times brighter than the Sun (the Sun's wattage appears to be medium). So how can we tell if a dim-looking star is really dim (low-wattage) and close, or possibly bright (high-wattage) and very far away? Is a bright star really bright (high-wattage), or does it just appear bright because it is very close?
Boy, are we stuck. Well, maybe not; if there was a way to study a star and figure out its wattage, we could get the distance. If we knew that a star that appeared bright was actually low-wattage, we would know that the star was close. Imagine studying a dim star and discovering that it was actually a high-wattage star. It would have to be really far away to look so dim! (If this sounds imprecise, it's because I'm blatantly skipping an explanation of the inverse-square law, which describes exactly how something gets dimmer as it gets farther away.)
"Okay, wise guy," you ask, "how do we figure out the wattage of a star?" Well, we have used parallax to determine the distances of thousands of stars. If you know the distance to a star, getting its wattage is easy. For example, a close star that looks dim must be low-wattage. A similar-looking star that we know is farther must have a higher wattage. More distant stars that look brighter? Even higher wattages.
So now what? Here's the kicker: by studying the stars within
parallax range, we notice a "wattage pattern" we can apply to all the
stars in the galaxy. This is the pattern:
Hot? Cool? We're talking temperatures, here, something that is easily observable from a star's color. Hot stars (say, 200,000 degrees Fahrenheit at the surface) emit more bluish, short-wavelength light. They actually look blue. These are high-wattage stars. Examples are Spica, Regulus, and Vega. Cooler stars (I say "cool" loosely, around 4,000 degrees) are reddish, and low-wattage. Examples are Proxima Centauri, Barnard's star, and 61 Cygni. (Ever seen a red star? Blue? No? Well, go outside and look! They've been there all your life.)
Blue stars are high-wattage. Red stars are low-wattage. Yellowish or white stars (like the Sun) fall in the middle. This pattern applies to about nine out of ten stars, stars astronomers say are part of the "Main Sequence" of stars. The freaky stars that are not on the Main Sequence may be red supergiants (like Betelgeuse) or white dwarfs. Thankfully, these oddballs are rare, and stand out spectroscopically.
Have you got it? Observe your mystery star, and see what color it is. From the color, get the temperature, and from the temperature, get the wattage. Then compare the wattage to how bright (or dim) the star appears to be in the sky. Bingo! You've got the distance to the star. This approach, good to plus or minus about 20%, can be used on any Main Sequence star we can resolve, whether in our galaxy or our closest neighbors, the Magellanic Clouds.
Steve White
Nightly Observing Program
Kitt Peak Visitor Center
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Updated: 01/10/2000
