![]() However, if we narrow the criteria somewhat then stars make excellent standard candles. Stars vary from class M dwarf stars that are hundreds of times less luminous than our sun, to the most brilliant of supergiants that are 10,000 times more luminous. Unfortunately, in the same way that not all candles or all light bulbs are equally luminous, we know that not all stars have the same luminosity. If all stars were exactly the same luminosity, then we would be in business stars would make excellent standard candles - find a nearby one, such as the sun, measure its luminosity, and you would know the luminosity of all stars. The apparent brightness, B, of the candle (the amount of energy per second per unit area), is the luminosity divided by the area illuminated, thus B decreases as the square of the distance of the observer from the source (candle): B = L / 4πr 2 Its luminosity, L (total energy emitted per unit second) is nearly constant, but the area, A, of the sphere that the candle illuminates is proportional to the square of the distance, r. As you move further away it will appear dimmer and dimmer. If you stand very near the candle it will appear quite bright. Think about lighting a candle in a dark room. To determine the distance to more distant objects we must use another technique. Although satellites are being designed which will measure parallaxes to a micro-arcsecond using interferometric techniques (in particular the Space Interferometry Mission), this will extend our reach only to the nearby Local Group of galaxies. Even this, however, allows us to measure distances only within our corner of the Milky Way galaxy. Recently the Hipparcos satellite measured the parallaxes to thousands of stars with a resolution of approximately 0.001 arcseconds, extending the reach of stellar parallaxes to about a thousand parsecs. This limits parallax measurements to stars within about a hundred parsecs from the sun. That means it can measure, to within 20 percent accuracy, the distances of stars that lie tens of thousands of light-years away.Angular displacements below a few percent of an arcsecond cannot be measured from the ground. The European Space Agency’s Gaia mission, currently underway, can measure parallax angles of just a few millionths of an arcsecond. That’s why a parsec has that value, and not any other.Īlthough astronomers often measure distant objects in parsecs or megaparsecs (1 megaparsec is 1 million parsecs), only nearby objects have parallaxes that we can actually measure. And a parsec is the distance - 3.26 light-years - that a star must lie from the Sun for its parallax angle to be exactly 1". ![]() The two different sightlines, one at each end of Earth’s orbit, create a triangle the parallax angle is defined as half the angle at the triangle’s apex. If you draw a simple diagram, you’ll see that the distance the star appears to move is related to the angle at which it is viewed. Translated to the stars in the sky, two photographs of the same nearby star taken six months apart will show it appearing to move against the background of more distant stars because Earth has moved to the other side of the Sun in its orbit. Your finger will appear to shift because each eye views it from a slightly different angle. Next, open your left eye and close your right. Close just your left eye and observe where your finger appears against the background. One of the simplest ways to see for yourself how this works is to hold your hand at arm’s length in front of your face and raise one finger. This is because as our planet moves, our viewpoint changes. Over the course of several months, nearby stars appear to move with respect to more distant objects - an effect called parallax. Earth circles the Sun, making one complete orbit per year. ![]() ![]() Question: Why is a parsec 3.26 light-years and not some other number?Īnswer: A parsec, or “parallax second,” is defined as 3.26 light-years because of how it is measured. ![]()
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