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Skull Nebula (NGC 246) & Fetus Nebula (NGC 7008).
Lindsay Elenbaas & Shannon Beezhold

Skull Nebula Fetus Nebula
Skull Nebula - Lindsay Elenbaas Fetus Nebula- Shannon Beezhold

Nebulas, what are they?
When a low mass star runs out of energy, it begins to cool and blows off its outer layers (1). These outer layers, consisting mostly of gases, then expand out into space creating a planetary nebula (5). At the center of a planetary nebula is a dying star, soon to become a white dwarf (1). Though the star is dying, it still emits energy typically in the form of UV light. The UV light from the dying star gives light to the gases and other materials that were blown off in this ejecting process and makes it visible for us to see. Although we refer to the emitted light as a planetary nebula it has relatively nothing to do with planets. When the first planetary nebulae were spotted their shape and color gave the appearance of the planet Uranus giving these nebulae their name (3).

The Skull Nebula
NGC 246, commonly referred to as the Skull Nebula, is a nebula that lies within the constellation Cetus. The Skull Nebula is approximately to the 12th magnitude and contains a lot of fluorine. The nebula (enhanced in the inset image for visibility) was discovered in 1785 by William Herschel and is roughly 1,600 light years away, hovering through space at approximately 80 kilometers per second (2). The leading edge of NGC 246 collides with matter within the galaxy, the interstellar medium, slowing down and condensing the gases on the edge whereas the trailing edge's gases inflate freely. The collisions of gases described help explain the reason for green lobes (not seen in the image) as the gases heat up and become unstable. The star in the middle of the nebula actually is two stars known as a binary star system. In a binary star system the two stars orbit each other (2).

The Fetus Nebula
The Fetus Nebula resides roughly 2800 light years away in the constellation Cygnus. A light year is the distance light travels in a year. This nebula is about 1 light year in diameter (5). It is unique because of the distinct fetus shape created by the inner shell within the outer shell. It is also exceptional because of the high surface brightness shining colors of blue, green, and purple (5). William Herschel first spotted the Fetus Nebula in the 18th century, however he did not classify it as a planetary nebula rather he classified it with shapely bright emission nebulae. It was later moved to the category of planetary nebulae. Faintly evident above in the Fetus Nebula photograph are two colorful shells of gas which blow off from the rather bright dying star in the middle of the nebula. There are two common explanations for why there are two distinctive shells. The first theorizes that the inner shell expands at a faster rate than the outer shell because of the mass loss of the dying star happening in an irregular rather than constant fashion. The second explanation states the two shells are there because of interactions with an “uneven interstellar medium” (4). In the view above there are no non-stellar objects, however there are two stars, unrelated to the Fetus Nebula, nearby in a binary system, one yellow and one blue.

How are they similar?
The Skull Nebula and the Fetus Nebula are similar in ways more than just their label of “planetary nebula.” The planetary nebula label itself speaks to the similarities as both surround dying stars (3). They both have hues in the blue and green shades. Both were discovered by William Herschel within 2 years of each other (2, 5). The Skull Nebula and Fetus Nebula are both on the visible light spectrum. The process through which their molecules go in order to be visible is ionization. The atoms gain or lose electrons causing them to ionize and thus giving off light (2). Lastly, both nebulae have remnant compact object(s) at the center of their gas (3, 4).

How are they different?
The Skull Nebula and Fetus Nebula differ in many ways. The Skull Nebula is much dimmer in the night sky while the Fetus Nebula shines brightly for all to see. However the Skull Nebula is much larger across with a diameter of 4-6 light years, compared to the 1 light year diameter of the Fetus Nebula (5). The Skull Nebula is also much closer at approximately 1,600 light years away, while the Fetus Nebula resides at 2,800 light years away (5).  The Skull Nebula is formed around a binary star system while the Fetus Nebula is formed around a single star. This will affect the rest of the life cycle of these stars leading to very different fates. The Fetus Nebula will eventually blow away and the dying star within it will be left behind as a white dwarf. In contrast, because of the binary system the Skull Nebula is formed around, once the Skull Nebula has been blown away the dying stars will become white dwarfs but their lives will not end there. The white dwarfs will turn on and off periodically and eventually these two white dwarfs will create a Type 1a Supernova due to the binary system they contribute to (1).

References:

(1)Bennett, Jeffrey O., Donahue, Megan, Schneider, Nicholas and Voit, Mark. The Cosmic Perspective Fundamentals. 2nd Edition. San Francisco, CA: Pearson Education, 2016. Print.

(2) Rector, Travis. "Gemini Observatory: Exploring the Universe, Sharing Its Wonders." Gemini Images a "Shocking" Skull of Gas. Gemini Observatory, Nov. 2005. Web.

(3) Palma, Christopher. "Planetary Nebulae and White Dwarfs." Planetary Nebulae and White Dwarfs. The Pennsylvania State University, 2014. Web.

(4) Ventrudo, Brian. "One Minute Astronomer." One Minute Astronomer. Mintaka Publishing Inc, 2015.

(5)Bonnell, Jerry and Nemiroff, Robert. "Astronomy Picture of the Day." APOD: 2008 August 25. NASA Web, 25 Aug. 2008

 

Object Skull Nebula Fetus Nebula
Right Ascension (J2000) 00:47:03.338 21:00:32.503
Declination (J2000) -11:52:18.94 +54:32:36.18
Filters used B (Blue), C (Clear), R (Red), V (Green) B (Blue), C (Clear), R (Red), V (Green)
Exposure time per filter B(150s x 3), V(130s x 2), and R (120s x 2); C (130s x 3)

B (180s x 2), V (120s x 4), and R (180s); C (120s x 4)

Image dimension 837x596 pixels; 18.14x12.9 arcminutes 406x304 pixels; 9.97x6.59 arcminutes
Date/time observed October 1, 2015, 5:33 UT October 1, 2015, 9:09 UT

 

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