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12.4 The Distance to Stars

The vast distances that separate stars and make them so hard to observe are awkward to express in ordinary units. Astronomers use units appropriate to these distances. The easiest to understand is the light year (abbreviated ly), the distance light travels in 1 year, which is about 6 million million miles (6 trillion miles), or 1016 meters (sometimes it is hyphenated as light-year). You can calculate this easily by recalling that the speed of light is 300,000 kilometers per second. A light year is just the speed of light times the number of seconds in a year, or 300,000 × (3600 × 24 × 365) = 9.5 × 1012 kilometers. Notice that although light years can be quoted in meters or kilometers, it& rsquo:s not strictly a metric unit; metric units were designed for terrestrial use and astronomers often use units that are better suited to the scale of the cosmos.

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Proxima Centauri, the closest star to the Sun, in X-ray light. Click here for original source URL.

Remember that the light year is a unit of distance, not time. The common mistake of using light year as if it were a unit of time is like saying that the ball game lasted for 2 miles. The confusion is exacerbated by the fact that in everyday life, people often quote distances in terms of time, as in "Phoenix is about 4 hours away." The Earth is only 12,800 kilometers in diameter, so light appears to travel instantly across even the longest distances on the planet. It would take light only 42 milliseconds to travel the length of the Earth's diameter. Light travel time begins to be noticeable within the Solar System. People old enough to remember the Apollo Moon landings will recall the stilted dialogs with the astronauts, due in part to the fact that the Moon is 1.3 light seconds aways. Signals took 1.3 seconds to get to the Moon and 1.3 seconds to get back. For the Mars rovers, it can take between 15 and 35 minutes for commands from NASA to reach the rovers, depending on where Earth and Mars are in their orbits. The Sun is 8 light minutes away from Earth. If the Sun were to instantly go dark (don't worry; this won't happen for billions of years), the light would go out in our sky 8 minutes later.

Over the vast reaches of space, however, light takes a substantial time to travel from one place to another. The nearest star beyond the Sun, Proxima Centauri (which is in orbit around Alpha Centauri), is about 4.3 light-years away. We see it as it was 4.3 years ago. The North Star, Polaris, is about 650 light-years away. The light from Polaris takes about 650 years to reach us, so we are seeing it now as it was during a dark time in the Middle Ages, when half the population of Europe died from the bubonic plague! If Polaris had suddenly exploded in 1950, we would not know it until about A.D. 2600. Deneb, the brightest star in Cygnus, is 2600 light years away, so the light we see now was emitted around the time Pythagoras was born. The most distant star visible to the naked eye is V762 Cassiopeia, which is 16,300 light years from the Earth. Light we see now left this star just after the end of the last Ice Age, at the dawn of human civilization. Distant light is old light.

Astropedia Image
A parsec is the distance from the Sunto an astronomical object which has aparallax angle of one arcsecond. (1 AU and 1 pc are not to scale (1 pc = 206265 AU)). Click here for original source URL.

Astronomers more commonly use an even larger unit of distance called the parsec (abbreviated pc). You should remember that a parsec is about 3¼ light-years; the exact conversion is 1 parsec = 3.26 light-years. In metric units, 1 parsec = 3 × 1016 meters. The term parsec comes from the fundamental way that astronomers measure distance by geometry. A parsec is the distance that produces a parallax shift of 1 arc second. Astronomers use the parsec and its multiples — kiloparsecs = 103 pc; megaparsecs = 106 pc — to express cosmic distances as they talk about more remote parts of the universe. You can convert parsecs to light-years roughly by multiplying by three. Another convenient fact to remember is that near the Sun (in what is called the Solar Neighborhood), stars are roughly a parsec apart. For instance, Alpha and Proxima Centauri, the closest stars to the Solar System, are about 1.3 parsecs away form the Sun.

One way to understand why astronomers use so many different units for measuring distance is to consider a situation closer to home. If you were doing a home remodeling project, you might use feet and inches to measure the dimensions of a room. If, instead, you were making a map of your neighborhood, those units would be too small and you might switch to measuring distances in blocks or miles. Now think about giving someone directions to your house from the other side of town. For a large town, it would be more reasonable to tell someone how many traffic lights or miles to travel, instead of using inches, feet, or blocks. The unit of measurement is chosen so that the scale of interest can be described in a few to few hundreds of the unit. A room is ten or more feet, a neighborhood is ten or more blocks and a city is ten or more miles.

It is possible to get a sense of the emptiness of space. A parsec is 40 million times the diameter of the Sun. Even though the stars can appear close to each other in the night sky, this is an illusion of projection. In three-dimensional space, stars are separated by distances millions of times their sizes. In a similar way, if you viewed a region of sparsely planted trees from the side, they would appear to be closely spaced. Only when viewed from above would the sparseness be obvious. A parsec is also about 200,000 A.U., or 2500 times the diameter of the Solar System. If we assume that other stars have planets too, then the these star systems fill only about 1/25003, or less than a ten billionth, of the volume of space. Space is fantastically empty.