Skip to Navigation | Skip to Content

Previous image Up to Astr111 Index Next image ASTR 110 Photography Projects, Fall 2016

Snowball Nebula (NGC 7662) & Fishhead Nebula (IC 1795)
Joshua Ashkinazi & Jabez Bang

NGC 7662 IC 1795
Snowball Nebula - Joshua Ashkinazi Fishhead Nebula- Jabez Bang

Introduction- Nebula: The two objects pictured above are examples of nebulae. Nebula finds its origin in the Latin word for “cloud” and these clouds are the very stuff our universe uses to form entire astronomical systems. These clouds are composed of gas and dust in space, consisting of hydrogen, helium gas and plasma. The clouds are created from an expanding shell of ionized gas ejected from old red giant stars. This shell-ejection occurs when a star can no longer support itself by fusion reactions in the core, where the gravity from the outer part of the star changes the structure of the star, and forces inner parts to condense and heat up. The instability in the core causes the outer layers of the star to eject away, in the form of a brisk stellar wind. William Herschel was a british astronomer that, from 1782 to 1802, conducted a survey of non-stellar objects, and ultimately found around 2400 objects defined by him as nebula. Herschel's discoveries were supplemented by his sister, Caroline Herschel, and his son, John Herschel later on. William published these findings as the General Catalogue of Nebulae and Clusters in 1864, however, the catalogue was later edited by John Dreyer, another astronomer, with supplemented work from other 19th century astronomers. Dreyer published the findings in 1888 as the New General Catalogue, the abbreviation of which is the most commonly used identifying label for nebulae (NGC). William Hershel’s work uncovered many important findings, and helped narrow down and classify what a nebula actually is, however, it was Edwin Hubble in 1924 that helped really defined what nebulae are.

Snowball Nebula (NGC 7662): The astronomical object pictured to the left is the planetary nebula NGC 7662, otherwise known as the Snowball Nebula. Discovered in 1784 by William Herschel, the planetary nebula is aptly named for its circular shape and bluish glow. A planetary nebula forms, in glorious fashion, during the end of the evolutionary cycle of a low-mass star—stars like our sun. During the asymptotic giant branch, a low-mass star sheds the surface layers accrued during its Red Giant phase, releasing a cloud of gas and dust into space, as the star begins to reveal its core. As a result of instability from core energy emission, the star pulsates, producing a powerful wind that pushes out the cloud — as explained before— creating the three shell-like layers seen in the Snowball Nebula. A recent survey by NASA's Chandra X-ray Observatory examined the diffuse X-ray emission from planetary nebulae, in which the x-ray emission formed in the same way as the compact shells, with sharp rims and surrounding faint halos. According to the survey, these compact shells are less than about 5000 years old, which would correlate with the time it would take for these pulsations to occur. In addition to the solar wind caused by pulsations, the hot core emits ultraviolet energy, giving those shells that fluorescent glow. There is also a lot of oxygen gas within the Snowball Nebula’s cloud, giving the nebula its blue color, so much so that the NGC 7662 is also known as the Blue Snowball Nebula. The blue color is also indicative of the incredible heat being emitted from the star. The Snowball Nebula is about 1,800 to 5,600 light years away with a diameter of about 20,000 to 50,000 AU. The nebula is found in the west portion of the constellation Andromeda, and is a popular astronomical object to view for casual observers as it can been seen with small telescopes in the Northern hemisphere during the fall and winter seasons.

Fishhead Nebula (IC 1795): The Fishhead Nebula is a bright emission nebula with various, and colorful dust clouds. Emission nebula are created when interstellar gas, filled with neutral hydrogen atoms, become ionized by young, bright, nearby stars using a lot of powerful ultraviolet photons. These hydrogen atoms, now in an excited state, emit their own light. As the hydrogen atoms return to their lowest energy state, the atom emits photons at a wavelength equivalent to the energy it takes for hydrogen to change energy states. The result is a wavelength, known as Hα, in the red end of the spectrum, giving the Fishhead Nebula that reddish hue. Emission nebula like these are commonly referred to as a HII region, indicating the highly ionized nature of the gas. The nebula is a site for star formation, as the stars required to ionize the gas live for very short times and were probably born in the cloud that it is now irradiating. The Fishhead Nebula is about 70 light-years across, with glowing gas and dark lanes of obscuring dust. It is located just over 6000 light-years away in the direction of constellation Cassiopeia. This small nebula is located in a much larger emission nebulae, known as the Heart Nebula (IC 1805) and the Soul Nebula (IC 1848). It is found in the bright knot region, known as NGC 896, at the end of the Heart Nebula, and was also discovered by Wilhelm Herschel in 1787.

Similarity: Both objects are a nebula that are found in constellations. Each nebula has hot, dense stars within the gas. Both nebulae were also discovered by the same person. These objects are both very bright, as they glow with brilliant colors in the night sky. Both objects are pushing out gas and dust that will, potentially, create new astronomical objects to be scattered among the vast expanse known as space. Both objects are also found in the night sky in the Northern Hemisphere during the winter season.

Difference: Primarily, they are two fundamentally different types of nebulae — planetary nebulae (Snowball) are stellar remnants, while emission nebulae (Fishhead) are stellar nurseries. While one is a graveyard, the other is a birthing-ground. The two nebulae are in different location. The Snowball nebula is located in the constellation Andromeda and the Fishhead nebula is located in the northern constellation Cassiopeia. There are difference in their structures and colors as well. The Snowball Nebula contains a lot of oxygen, which gives the nebula mostly blue color. The Fishhead Nebula also has some oxygen but it is mainly hydrogen and sulfur atoms which, creates green and red colors.


Object Snowball Nebula Fishhead Nebula
Right Ascension (J2000) 23:25:53.90 02:26:30
Declination (J2000) +42:32:00.8 +62:04:00
Filters used B (Blue), C (Clear), R (Red), V (Green) B (Blue), C (Clear), R (Red), V (Green)
Exposure time per filter B, V, R, and C (10s) B, V, R and C (300s)
Image dimension 402x304 pixels; 8.7x6.6 arcminutes 963x671 pixels; 20.9x14.5 arcminutes
Date/time observed October 7, 2016, 9:25 UT October 10, 2016, 09:28 UT



Secondary content.


Side content.