How is the distance to the stars measured?

Asked by: Steve Meyers

Answer

There are many different methods used to find stellar distances, but the earliest one used on the nearest stars involves the principle of parallax. Hold a pencil upright in front of your face at about arms length. Now blink each eye in succesion. You will see an apparent shift in position of the pencil against the background. That shift is parallax.

You will also find that the nearer the pencil, the greater the parallax. As distance increases, parallax is less apparent. The effect is dependent on both the distance to the object and the distance between your eyes.

To notice parallax in distant objects, your 'eyes' need to be further apart. Surveyors accomplish this with two different measurements, from each location of the simulated 'eyes'. Measuring the distance between these two 'eye' locations, and the amount of parallax shift, distance can be calculated.

For stellar distances, two 'eyes' on the earth are still not far enough apart to create an apparent shift. This is limited by the earth's diameter--8000 miles. So the two observation points used are from opposite sides of the earth's orbit around the Sun. Since we know the diameter of the earth's orbit, and that it travels from one side to the other every 6 months, a 'nearby' star's position shift from six months ago against a background of further stars can be measured. Using the same calculations that a surveyor on earth uses, the distance to the nearer stars can then also be determined.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor

There are lots of ways, but historically the most accurate has been parallax. This is the same effect that makes distant trees appear to move more slowly than nearby ones when you drive past them in a car. Basically, as the Earth orbits the Sun, nearby stars appear to move a teeny bit as a reflection of our motion, while stars farther away move less. Since we know how big the orbit of the Earth is, we can use trigonometry to calculate the stars' distances.

I have info about all this at my website: Bitesize Astronomy, part of my Bad Astronomy website.
Answered by: Phil Plait, Astronomer (http://www.badastronomy.com)