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:
“Though there were numerous vessels at this great distance in the horizon on every side, yet the vast spaces between them, like the spaces between the stars,—far as they were distant from us, so were they from one another,—nay, some were twice as far from each other as from us,—impressed us with a sense of the immensity of the ocean, the “unfruitful ocean,” as it has been called, and we could see what proportion man and his works bear to the globe.”
—Henry David Thoreau (1817–1862)
“Like the water, the Walden ice, seen near at hand, has a green tint, but at a distance is beautifully blue, and you can easily tell it from the white ice of the river, or the merely greenish ice of some ponds, a quarter of a mile off.”
—Henry David Thoreau (1817–1862)
“As distance shows a horse’s strength, so time reveals a person’s heart.”
—Chinese proverb.