SaddleBrooke Skygazers

How far are the stars?

John Lauder

When astronomers use their telescopes to look at stars, the distances are gigantic. For example, the closest star to Earth (besides our sun) is something like 24,000,000,000,000 miles away. That’s the closest star. There are stars that are billions of times farther away than that. When you start talking about those kinds of distances, a mile or kilometer just isn’t a practical unit to use because the numbers get too big. No one wants to write or talk about numbers that have 20 digits in them!

So to measure really long distances, people use a unit called a light year. Light travels at 186,000 miles per second. Therefore, a light second is 186,000 miles. A light year is the distance that light can travel in a year, or 186,000 miles/second * 60 seconds/minute * 60 minutes/hour * 24 hours/day * 365 days/year = 5,865,696,000,000 miles/year.

A light year is 5,865,696,000,000 miles!

Since radar and space probes cannot reach the stars, other methods are needed for finding stellar distances. Nearby stars are measured with parallax, which involves two measurements of the star’s exact position in the sky. The readings are taken on opposite sides of the earth’s orbit, six months apart. From this triangulation (or surveying) method, the star distance is determined. The parallax technique works for stars out to a distance of around 400 light-years. The satellite Hipparchus, launched in 1997, has been used for many of these measurements. Many thousands of stars fall within parallax range, including Arcturus (37 light years), Sirius (8.6 light years), and Spica (220 light years).

For the longer star distances, indirect methods are used. The Cepheid variable technique is useful out to millions of light-years. Cepheids are a category of stars whose actual brightness is well known. If a Cepheid appears dim its distance thus can be estimated. The method is similar to judging the distance to an oncoming car by observing its headlight in the distance. Cepheids are very bright stars, so they can be identified in faraway galaxies. The Cepheid method gives the distance to the Magellanic Clouds (180,000 light years) and also to the Andromeda Galaxy (2.9 million light years).

A third method for stellar distances uses brightness measurements. It turns out that a star’s color spectrum is a good indication of its actual brightness. The relationship between color and brightness was proven using the several thousand stars close enough to earth to have their distances measured directly. Astronomers can therefore look at a distant star and determine its color spectrum. From the color, they can determine the star’s actual brightness. By knowing the actual brightness and comparing it to the apparent brightness seen from Earth (that is, by looking at how dim the star has become once its light reaches Earth), they can determine the distance to the star.

Skygazer meetings are held at MountainView West Ballroom, at 7:00 p.m. on the second Sunday of the month and will begin again in January.