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NGC 925, NGC 7331, & NGC 628
Kat Jonker, Mary Shin, & Josh Parks

NGC925 NGC7331
NGC 925 - Kat Jonker NGC 7331 - Mary Shin

NGC628  
NGC 628 - Josh Parks  

Galaxies are groups of stars bound together by gravity. Smaller collections of galaxies are called groups, while larger ones are termed clusters. The three galaxies pictured here are spiral galaxies, which have distinct spiral shapes and can be either "barred" or "unbarred." Spiral galaxies tend to have diameters between 15-150 light-years (ly) and masses between 1 billion and 1 trillion times the mass of the Sun. Generally, both red and blue colors can be observed in spiral galaxies. This indicates that spiral galaxies contain both young and old stars. The old stars tend to congregate in the bulge, while younger stars, as well as nebulae where stars are forming, appear more frequently in the arms.

NGC 925 is a barred spiral galaxy in the constellation Triangulum. It is 30.3 ± 2.3 million ly from the Sun, and its apparent magnitude is 10.7. It is located in the NGC 1023 cluster/group, which resides in the Canes Venatici Cloud in the Virgo Supercluster. There is a 10 million solar mass cloud of neutral hydrogen attached to NGC 925 by a streamer, which is an elongated mass of luminous matter. It was discovered by William Herschel (who also wrote symphonies!), and it is part of the Hubble Space Telescope Key Project on the Extragalactic Distance Scale, a long-term project dedicated to determining the Hubble constant to within +/- 10% using Cepheid variable stars as distance indicators.

NGC 7331 is an unbarred spiral galaxy discovered by William Herschel in 1784. It is in the constellation Pegasus and 50 million ly from the Sun; its magnitude is 10.4. It is similar in size and structure to the Milky Way and is often referred to as "the Milky Way's twin". However, discoveries in the 2000s question the similarity of the structure of the Milky Way, which is now believed to be a barred spiral. There is only one supernova that has been identified in this galaxy: SN 1959D, a Type IIL supernova, which was discovered by Milton Humason and H.S. Gates.

NGC 628, also known as Messier 74, is a grand design spiral galaxy in the constellation Pisces. Grand design spiral galaxies feature large, well-defined arms that can be easily seen by observers. NGC 628 is also face-on, which means its flat side faces Earth and makes the spiral structure obvious to Earth-based viewers. It was discovered in 1780 by Pierre Méchain and then catalogued in Charles Messier’s list of astronomical objects. It is between 30 and 40 million light-years away and is moving away from us at 793 km/sec. Its diameter is about 95,000 light-years, approximately the same size as the Milky Way. It has an above-average number of star formation regions, which show up in observations as bright patches of UV radiation in the arms. Among Messier objects, it is quite dim (apparent magnitude 10.0), becoming difficult to spot with even small amounts of light pollution.

All three of these galaxies appear relatively dim from Earth, with apparent magnitudes around 10-11. They are, however, near enough to be observed with relatively simple telescopes, which allowed all three to be discovered in the eighteenth century. They all feature bluish spots in their arms, which come from places where young stars appear in high density and emit spectra that peak in the blue-to-UV range. They are all located in the Virgo Supercluster along with the Milky Way.

Though all three galaxies are spirals, NGC 928 and NGC 7331 are barred, which means their bulges are elongated, while NGC 625 is much closer to circular. NGC 7331 is unique in that is has retrograde bulge; i.e., its center rotates in the opposite direction of the rest of the disk. Multiple supernovae have been observed in NGC 628, while NGC 7331 has only shown us one, and none have been confirmed in NGC 925. Only NGC 628 is oriented face-on towards Earth, which allows us a better picture of its shape than the others. Neither NGC 7331 nor NGC 628 has a large hydrogen streamer like NGC 925, which implies that they haven't had gravitational interactions with nearby galaxies as recently.

References:

Frommert, Hartmut and Kronberg, Christine. "Messier Object 74." Students for the Exploration and Development of Space. <http://messier.seds.org/m/m074.html>.

NASA/IPAC Extragalactic Database.

"NGC 7331: a large spiral galaxy". Centro Astronómico Hispano-Alemán. <www.caha.es/ngc-7331-a-large-spiral-galaxy.html>.

"NGC 925". Skyhound. <observing.skyhound.com/archives/nov/NGC_925.html>.

"Seeing Double: Spitzer Captures Our Galaxy's Twin". Jet Propulsion Laboratory. <jpl.nasa.gov/news/news.php?release=2004-165>.

Wikipedia, "Messier 74".

Wikipedia, "NGC 7331".

Wikipedia, "NGC 928".

Object NGC-628 NGC-925 NGC-7331-
Right Ascension (J2000) 01:36:42 02:27:16 22:37:04
Declination (J2000) +15:46:60 +33:34:44 +34:25:01
Filters used B (Blue), C (Clear), R (Red), V (Green) B (Blue), C (Clear), R (Red), V (Green) B (Blue), C (Clear), R (Red), V (Green)
Exposure time per filter B (3x160s); C, V, and R (3x80s) B (3x160s); C, V, and R (3x80s) B (3x160s); C, V, and R (3x80s)
Image dimension 590x379 pixels; 12.8x 8.2 arcminutes 377x321 pixels; 8.2x 7.0 arcminutes 622x454 pixels; 13.5x 9.8 arcminutes
Date/time observed October 5, 2016, 07:54 UT October 5, 2016 9:49 UT October 7, 2016, 8:08 UT

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