Explanation:
The Great Orion Nebula and the Comet
This colorful image covers a familiar sight in the Northern Hemisphere—the famous star-forming region, the Great Orion Nebula. Not only that, but it also captures a delicate image of a passing comet. This image was taken this week through a remote telescope in New Mexico, with the right panel captured on September 26 and the left panel on September 27. Comet 217P/LINEAR, trailing a green tail, passes above the blue Running Man Nebula, near the top of both images. From the two images taken on consecutive nights, the comet is clearly visible moving across the night sky against the cosmic nebula and background starlight. In fact, on September 27, the comet was only five light-minutes away from us, while the background Orion Nebula is 1,500 light-years away. Comet 217P/LINEAR is too faint to be observed with the naked eye; it is a short-period comet with an orbital period of about 8 years. Its furthest point from the Sun is closer than Jupiter, and its perihelion is just beyond Earth’s orbit.
Translation from: http://www.skylook.org/apod/astropix.html
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