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# Interstellar Distance Measurement Help

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By — McGraw-Hill Professional
Updated on Oct 3, 2011

## Interstellar Distance Measurement—The Astronomical Unit

The distances to stars in our part of the Milky Way galaxy can be measured in a manner similar to the way surveyors measure terrestrial distances. The radius of the earth’s orbit around the sun is used as the base line.

### The Astronomical Unit

Astronomers often measure and express interplanetary distances in terms of the astronomical unit (AU). The AU is equal to the average distance of the earth from the sun, and is agreed on formally as 1.49597870 × 10 8 kilometers (this is sometimes rounded off to a figure of 150 million kilometers). The distances to other stars and galaxies can be expressed in astronomical units, but the numbers are large.

## The Light Year

Astronomers have invented the light year, the distance light travels in one year, to assist in defining interstellar distances so the numbers are reasonable. One light year is the distance a ray of light travels through space in one earth year. You can figure out how far this is by calculation. Light travels approximately 3.00 × 10 5 kilometers in one second. There are 60 seconds in a minute, 60 minutes in an hour, 24 hours in a day, and about 365.25 days in a year. So a light year is roughly 9.5 × 10 12 kilometers.

Let’s think on a cosmic scale. The nearest star to our Solar System is a little more than four light years away. The Milky Way, our galaxy, is one hundred thousand (10 5 ) light years across. The Andromeda galaxy is a little more than two million (2.2 × 10 6 ) light years away from our Solar System. Using powerful telescopes, astronomers can peer out to distances of several billion light years (where one billion is defined as 10 9 or one thousand million).

The light year is an interesting unit for expressing the distances to stars and galaxies, but when measurements must be made, it is not the most convenient unit.

## The Parsec

The true distances to the stars were unknown until the advent of the telescope, with which it became possible to measure extremely small angles. To determine the distances to the stars, astronomers use triangulation, the same way surveyors measure distances on the earth.

Figure 8-5 shows how distances to the stars can be measured. This scheme works only for “nearby” stars. Most stars are too far away to produce measurable parallax against a background of much more distant objects, even when they are observed from the earth at different times of the year as it orbits the sun. In Fig. 8-5, the size of the earth’s orbit is exaggerated for clarity. The star appears to be in slightly different positions, relative to a background of much more distant objects, at the two observation points shown. The displacement is maximum when the line segment connecting the star and the sun is perpendicular to the line segment connecting the sun with the earth.

Suppose a star thus oriented, and at a certain distance from our Solar System, is displaced by one second of arc when viewed on two occasions, three months apart in time, as shown in Fig. 8-5. When that is the case, the distance between our Solar System and the star is called a parsec (a contraction of “parallax second”). The word “parsec” is abbreviated pc; 1 pc is equivalent to approximately 3.262 light years or 2.063 × 10 5 AU.

Fig. 8–5. The distances to “nearby” stars can be determined by measuring the parallax resulting from the revolution of the earth around the sun.

Sometimes units of kiloparsecs (kpc) and megaparsecs (Mpc) are used to express great distances in the universe. In this scheme, 1 kpc = 1000 pc = 2.063 × 10 8 AU, and 1 Mpc = 10 6 pc = 2.063 × 10 11 AU. Units such as the kiloparsec and the megaparsec make intergalactic distances credible.

The nearest visible object outside our Solar System is the Alpha Centauri star system, which is 1.4 pc away. There are numerous stars within 20 to 30 pc of our sun. The Milky Way is 30 kpc in diameter. The Andromeda galaxy is 670 kpc away. And on it goes, out to the limit of the observable universe, somewhere around 3 × 10 9 pc, or 3000 Mpc.

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