Distance
A long-standing problem in the study of planetary nebulae is that their distances are generally not well known. Many methods for estimating distances to planetary nebulae rely on making general assumptions, which may be very inaccurate for the object concerned.
In recent years, however, observations made using the Hubble Space Telescope have allowed a new method of determining distances. All planetary nebulae are expanding, and observations several years apart and with high enough angular resolution will reveal the growth of the nebula in the plane of the sky. This is typically very small—only a few milliarcseconds a year or less. Spectroscopic observations can reveal the velocity of expansion of the nebula along the line of sight using the Doppler effect. Then, comparing the angular expansion with the known expansion velocity, the distance to the nebula can be calculated.
Hubble Space Telescope observations of NGC 6543 several years apart have been used to calculate its distance. Its angular expansion rate is 3.457 milliarcseconds per year, while its expansion velocity along the line of sight has been found to be 16.4 km/s. Combining these two results implies that NGC 6543 is 1001 ± 269 parsecs (3×1019 m), or about 3300 light-years away from Earth.
Read more about this topic: Cat's Eye Nebula
Famous quotes containing the word distance:
“Why does the past look so enticing to us? For the same reason why from a distance a meadow with flowers looks like a flower bed.”
—Franz Grillparzer (1791–1872)
“You say I’m small? I certainly can relate, although it is a matter of perspective. The distance is deceptive, my friend, you stand too low.”
—Franz Grillparzer (1791–1872)
“The rage for road building is beneficent for America, where vast distance is so main a consideration in our domestic politics and trade, inasmuch as the great political promise of the invention is to hold the Union staunch, whose days already seem numbered by the mere inconvenience of transporting representatives, judges and officers across such tedious distances of land and water.”
—Ralph Waldo Emerson (1803–1882)