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Astronomical Observatory: Cool Images

North America Nebula (NGC 7000), Andrew Butler

North America Nebula

The North America Nebula is an enormous emission nebula. From our perspective on Earth, the nebula would look over three times as big as the full moon if it were bright enough to be seen with our naked eyes! That is about the angular size of ones thumbnail held out at arm's length. Its linear size is roughly 50 light-years in height and 40 light-years in width. Even more mind-boggling is its distance from us – about 1600 light-years!

This object was possibly discovered by William Herschel on October 24, 1786, who noted its "faint, extremely large, diffuse nebulosity." His son, John Herschel, definitely observed it before 1833. On December 12, 1890, Max Wolf took the North America Nebula's first photographs.

This region of our galaxy is mostly composed of neutral hydrogen gas. A single star, only recently discovered as the culprit, is luminous and hot enough to emit intense ultraviolet light that can eject the electrons in the hydrogen atoms from the protons they are bound to. After the electrons recombine with the protons, the electrons emit light at a very specific wavelength, called H alpha. This wavelength happens to be in the red portion of the spectrum of visible light, specifically 656.2 nanometers (one nanometer is one-billionth of a meter).

Not only does the North America Nebula emit pleasing red light, but it is also a star-forming region. This is evidenced by a few open clusters of stars probably associated with the nebula. The most obvious of these is NGC 6997, which has formed near the "east coast" of the nebula (labeled on this image). William Herschel catalogued this cluster on the same date as his possible discovery of the nebula itself (October 24, 1786).

The North America Nebula is actually only a part of a larger gas cloud. A dark dust lane, which blocks light behind it from reaching our eyes, separates the North American Nebula from the Pelican Nebula, which can be seen to the right of the North American Nebula in this image taken by Adam Block. These two nebulae together constitute the entire gas cloud. (Note that the ionizing star mentioned above looks dim from our viewpoint because it lies behind the dust lane.)

The North America Nebula is in the prominent summer constellation Cygnus the Swan, sometimes dubbed the “Northern Cross.” The nebula is 3 degrees east of the brightest star in that constellation, Deneb. This star is part of the "summer triangle," a conspicuous triangle in the sky formed by three bright stars: Deneb in Cygnus, Vega in Lyra (the Lyre), and Altair in Aquila (the Eagle). The summer triangle is easily visible to the eye during summer, and can be seen in early spring through late fall.

Image processing details: This picture consists of 45 individual images mosaicked together (9 tall, 5 wide), covering 1.8° by 1.5° of sky altogether. Each field was exposed for 5 minutes in the H alpha filter 3 times. The individual images were then dark-subtracted and flat-fielded. The cosmic rays in them were removed using a professional image processing program called IRAF (Image Reduction and Analysis Facility). Then the 3 images of each field were median-combined to minimize the remaining noise. These steps were repeated for each of the 45 fields, which were then mosaicked together using MaxIm DL. This produced a grayscale image. A red transfer function was applied to this image using CCDSoft to be scientifically consistent with the known color of the nebula and to allow the eye to more easily pick out the brightest and faintest regions of the nebula.

Right Ascension (J2000) 20h 58m 48.0s
Declination (J2000) +44° 20' 00"
Filter used
Exposure time 300 seconds per image; 3 x 45 images
Dates observed

June 12, 15, 16, & 17
August 28, & 30

References:

Astronomy Picture of the Day. Eds. Dominique Dierick and Dirk De la Marche. 6 June 1996. NASA / Goddard Space Flight Center. 3 Dec. 2005.

Block, Adam. NOAO / AURA / NSF. The North American Nebula and Pelican Nebula. 3 Dec. 2005.

Comerón, F, and A Pasquali. "The Ionizing Star of the North America and Pelican Nebulae." (2005) Astronomy & Astrophysics 430: 541-48.

Fisher, Mark. Orion Arm. 3 Dec. 2005.

Kutner, Marc L. Astronomy: A Physical Perspective. 2nd ed. Cambridge, UK: Cambridge University Press, 2003.

Students for the Exploration and Development of Space. John Frederick William Herschel (March 7, 1792 - May 11, 1871). 3 Dec. 2005.

Students for the Exploration and Development of Space. NGC 7000. 3 Dec. 2005.

 

 

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