Julia Kennefick, assistant professor of physics in the J. William Fulbright College of Arts and Sciences, replies:

First, we need to know the distance from the Earth to the Sun. We have determined this accurately by bouncing radio waves off of solar system bodies such as Venus and by knowing the geometry of the solar system.

For relatively nearby stars, out to about 1,500 light years (a light year is the distance light can travel in a year, about 9.5 trillion kilometers), astronomers use a technique called parallax. Parallax is the shifting in apparent position of a nearby object with respect to more distant objects as the observer changes her own position. Because we have two eyes, or two different perspectives, this technique is what allows us to see in three dimensions. In the case of stars, we use different positions in our yearly orbit around the Sun to watch how nearby stars move with respect to the more distant "fixed" stars. Then from measuring how much they move, which is measured as the small angle in a very long triangle, and knowing our own distance from

For relatively nearby stars, out to about 1,500 light years, astronomers use a technique called parallax.

the Sun - the side opposite that angle on the triangle - we can compute the distance to the star using trigonometry.

For more distant stars, this parallax angle is too small to measure accurately. Beyond about 1,500 light years, we have to rely on "standard candle" methods. A standard candle is an object whose luminosity, or power, we know. For instance, if we know that the light on our neighbor's porch contains a 60 Watt light bulb (its luminosity), we can determine its distance from us by measuring its apparent brightness. The closer we are, the brighter it will appear.

Luckily, there are several kinds of objects in the universe whose luminosity we think we know. One such object is a Cepheid variable star. We use the brightness of these stars to determine their distances and the distances to clusters of stars that contain them, such as globular clusters in our own Milky Way galaxy or even the distances to other nearby galaxies where we can see individual Cepheid variable stars.

Another standard candle being used today are the supernovae resulting from the explosion of white dwarf stars that have exceeded their mass limit. We can see these explosions almost all the way across the observable universe and so can compute the distance to the galaxies that contain them. These galaxies are further away than we expected, leading us to conclude that the universe is accelerating in its expansion.

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