This is a picture of NGC 7662, a “blue snowball” planetary nebula. Planetary nebulae are white-dwarf stars formed after a red giant ejects its outer gaseous shell leaving behind just the white-dwarf. Scientists think that around 95% of stars in our Milky-Way galaxy (including the Sun) will end their lives in this way—as a red giant ejecting its outer shell and leaving behind the white-dwarf. Scientists estimate that our sun will reach this stage in its life in about five billion years. The temperature in the star in the middle of this planetary nebula is estimated around 75,000K. The nuclei of planetary nebula are incredibly hot—they are among the hottest stars known to scientists. NGC 7662 is found in the western part of the constellation Andromeda and is considered to be one of the brighter objects in the night sky.
The middle of NGC 7662 is a barely visible central star. This is the extremely dense and hot white-dwarf star. Around the central star is the ejected and expanding gaseous shell from the previous red giant star. NGC 7662 is calculated as having a linear diameter of approximately 1.5 light years. The spectrum of planetary nebulae gases consists of emission lines. Due to the filter and color contrast choices this picture depicts NGC7662 as being green, while other pictures of this same object are often blue. Many planetary nebulae produce spherical structures, but it appears that this star did not eject its shell in a completely spherical way—this leads to the crescent, non-spherical, white structure we see inside of the larger green object.
Bennett, Jeffrey, Megan Donahue, Nicholas Schneider, and Mark Voit. The Cosmic Perspective. 6th ed. San Francisco: Addison-Wesley, 2010. 543-44.
Balick, B. "NGC7662 Blue Snowball Planetary Nebula." astroimages.org. Web. 6 Dec. 2010. <http://www.astroimages.org/ccd/ngc7662.html>.
|Right Ascension (J2000)||23:25:54|
|Filters used||blue(B), green(V), red(R), and clear(C)|
|Exposure time per filter||1x60 seconds in C, 1x300 seconds in BVR|
29, 2010 (C)