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NGC 404 & Caldwell 30 (NGC 7331)
Britt Botterman & Nick Aukerman

NGC 404 Eliptical Galaxy-Britt Botterman Caldwell 30 Spiral Galaxy-Nick Aukerman

This webpage will take a comparative look between two separate galaxies, while simultaneously focusing on defining and explaining the basic properties that constitute a galaxy.  In this report, Caldwell 30 (NGC7331) and NGC404 will be the two galaxies comparatively related. 

Before comparing these two galaxies, it is first important to define what constitutes a galaxy.  According to our textbook (The Cosmic Perspective Fundamentals), “A galaxy is a great island of stars in space, containing millions, billions, or even trillions of stars, all held together by gravity and orbiting a common center."  Secondly, it is important to understand the different types of galaxies for comparison.

A galaxy can be broken down into three types: spiral, elliptical, and irregular. This webpage will focus only on the first two, as they relate to our galaxies. There are fundamental distinctions concerning stellar populations between spiral and elliptical galaxies.  First, the spiral galaxy can be broken down into three sections.  The inner most section of a spiral galaxy is called the bulge. Importantly, this is the nucleus of a galaxy and is the center most area of any given galaxy.  Generally, the bulge typically tends to be filled with older stars and a smaller mixture of younger stars.  The nucleus of the galaxy comprises the highest density of stars in the galaxy.  Secondly, the ‘mid’ section of a spiral galaxy is called the disk.  The disk of a galaxy is what provides the spiral galaxy its name.  The disk itself is not perfectly defined, as it depends on the objects used to define the disk.  Within this section, there is a higher abundance of helium and hydrogen.  Therefore, younger stars are more frequent and abundant.  Conversely, there is a smaller percentage of older stars within the disk of a spiral galaxy.  The third outermost section of a spiral galaxy is called the halo.  Less visible to the human eye, the halo of a spiral galaxy is faint and takes a more spherical shape.  The halo has a significant decrease in the abundance of gas, such as hydrogen and helium.  Thus, the halo of a galaxy contains primarily older stars.  It would be more difficult in this section for newer stars to collect enough gas for gravity to come into play, would be necessary to begin the process of creating a star.    Although both galaxies were formed around the same time, NGC 404 being an elliptical galaxy, consists of much older and lower mass stars. Consequently, there are significant differences in the composition and configuration of this galaxy type.  Elliptical galaxies have a far greater number of stars within their structure, but there is less of a gaseous abundance within an elliptical galaxy, making it much more difficult for any newer star to develop. This in turn leaves the galaxy to be composed of aging stars.  In terms of its overall structure, elliptical galaxies are more plainly spherical or oblong.  Most noticeably, elliptical galaxies lack the gaseous arms displayed by spiral galaxies.   

NGC404 is an elliptical galaxy located about 10 million light years away in the Andromeda Constellation. It was discovered by William Herschel in 1784 and is often referred to as "Mirach's Ghost" due to its close proximity (7 arc-min) to the second magnitude star Mirach, making it very difficult to observe. NGC 404 is visible through small telescopes and through surface brightness fluctuation observation, is estimated to stretch a distance of 10+/-.7Mly.

During the picture taking of NGC 404, the picture itself had to be cropped in a manner so that NGC 404 could be more visible.  As mentioned, the star Mirach neighbors NGC 404, and the view that we perceive of the two objects causes an over exposure of light that affects a camera.  With the elimination of the additional light, the overall structure of NGC 404 came forth.

Caldwell 30, otherwise known as NGC7331, is a spiral galaxy located within the Pegasus (the Winged Horse) constellation.  Roughly forty-million light years away, the galaxy known as Caldwell 30 was discovered by William Herschel in 1784.  Although a spiral galaxy's bulge (the core) typically co-rotates with the disk, Caldwell 30’s core does the complete opposite.  Known as a retrograde bulge, the core of the galaxy rotates in the opposite direction of its disk creating a mystery as to how such an occurrence has manifested.  Light from older, cooler stars dominates the center bulge, while bluer, younger stars fill out the spiral arms of the galaxy.  This would suggest that star activity is more abundant outside of the core of the galaxy.  Caldwell 30 shares similar attributes with our Milky Way.  The overall size of the two galaxies and shape are strongly similar.  Secondly, both share a black hole with roughly the same dimensions at their center. Since our time observing Caldwell 30, there has only been one observed supernova within the galaxy, and that was documented by Milton Humason. 

Additionally, within the picture, Caldwell 30 is sharing a frame with a few additional galaxies.  Although these galaxies look close to one another in the picture frame located at the top of this webpage, their presence near one another is by chance.  In fact, the so-called neighboring galaxies are roughly 10 times distant from one another. This group is called the Deer Lick Group.  NGC 7331 is the brightest galaxy within this group.  Using the Aladin Sky Atlas, four other galaxies are named within the same picture.  Located in the far upper right corner, NGC 7326 is slightly visible to an observer.  The clearest distinction of NGC 7326 from neighboring stars comes from the cloud of gas that surrounds the body of the galaxy.  The other three remaining galaxies are located on the left side of Caldwell 30, practically forming an obtuse triangle.  Just to the left of Caldwell 30, the highest of the three galaxies is called, NGC 7336.  The mid of three galaxies is called, NGC 7335.  And the lowest of the three galaxies is called, NGC 7337.  To better locate NGC 7336, first find the mid of the three galaxies,  NGC 7335.  More observant to the viewer, the mid galaxy can act as your marker.  To find NGC 7336, go north of NGC 7335 roughly an inch, and then to the left by four centimeters.  To find NGC 7337, go south of NGC 7335 by roughly five inches, and to the left by an inch.  NGC 7337 will appear to orbit a neighboring star (but, in truth is separated by great distance in space), hence the galaxy is partially blocked out. 

Comparatively speaking, the apparent magnitude of NGC404 is greater than that of Caldwell 30’s.  Apparent magnitude refers to the brightness of a celestial object as seen by one on Earth.  The brighter the object appears the lower its magnitude.  NGC404 has an apparent magnitude of 11.2, while Caldwell 30 has an apparent magnitude of 10.4.  In terms of structure, the most noticeable difference comes from the lack of spiral arms of NGC 404. In terms of color and star formation, Caldwell 30 appears to emit bluer light within its spiral arms. This would indicate that younger stars are forming more frequently in the spirals of Caldwell 30. Additionally, the color itself can indicate the relative temperature and age of stars forming within a galaxy. Bluer colors are more indicative of hotter temperatures, and younger stars, while more yellow and brown colors suggest colder temperatures, and a greater abundance of older stars.

In conclusion, galaxies can be compared to each other based on a number of relative similarities and differences.  Such things as the presence of gaseous arms, the decrease in the presence of helium and hydrogen, the overall shape, and the presence of older and newer stars.  Even the presence of a halo can indicate the presence of mostly older stars.  These characteristics may even be helpful in further predicting the future of these galaxies as well. 

References:

NGC 404 characteristis. (n.d.). Retrieved December 4, 2015, from http://www.jpl.nasa.gov/news/news.php?feature=1926,http://iopscience.iop.org/article/10.1086/421358/pdf September - NGC 7331 (Caldwell 30) Galaxy in Pegasus. (n.d.). Retrieved December 4, 2015, from http://www.lvastronomy.com/observing-challenge/97-september-ngc-7331-caldwell-30-galaxy-in-pegasu

1. (n.d.). Retrieved December 4, 2015, from https://en.wikipedia.org/wiki/NGC_40
2. (n.d.). Retrieved December 4, 2015, from https://en.wikipedia.org/wiki/NGC_733
3. Types of Galaxies - Spiral, Elliptical & Irregular Galaxies. (2015, April 6). Retrieved December 4, 2015, from http://space-facts.com/galaxy-types
4. (n.d.). Retrieved December 4, 2015, from http://csep10.phys.utk.edu/astr162/lect/milkyway/components.htm
5. (n.d.). Retrieved December 4, 2015, from http://www.space.com/22382-spiral-galaxy.htm
6. NGC 7331, a spiral galaxy in Pegasus. (n.d.). Retrieved December 4, 2015, from http://annesastronomynews.com/photo-gallery-ii/galaxies-clusters/ngc-7331-by-robert-gendler
7. NGC 7331 and four background galaxies. (2015, September 25). Retrieved December 4, 2015, from http://www.physicspages.com/2015/09/25/ngc-7331-and-four-background-galaxies

 

Object NGC404 Eliptical Galaxy Caldwell 30 Spiral Galaxy
Right Ascension (J2000) 01:09:26.9 22:37:05.1
Declination (J2000) +35:43:06.003 +34:25:13
Filters used B (Blue), C (Clear), R (Red), V (Green) B (Blue), C (Clear), R (Red), V (Green)
Exposure time per filter C and B (300s), R (90s), V (120s) C and B (420s), R (150s), V (210s)
Image dimension 631x349 pixels; 13.75x7.6 arcminutes 901x980 pixels; 19.5x21.2 arcminutes
Date/time observed September 29, 2015 03:58:05' PM October 1, 2015, 10:45:46' PM

 

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