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Observing Galaxies

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Observing Galaxies OBSERVING GALAXIES RONALD E. MICKLE Denver, Colorado 80211 ©2000 Ronald E. Mickle ABSTRACT The objective of this project is to visually, photographically or by a charged coupled device (CCD) image, observe galaxies within the capability of the telescope used and its ancillary equipment and to compare observations with normally published images and descriptions of those galaxies as relating to surface brightness, visibility of spiral arms, etc. Observations for this project were made visually and by CCD. Galaxies come in many shapes, sizes and degrees of brightness, all divided into four classes: spiral, barred spiral, elliptical and irregular. Many people are familiar with variable stars, which can change in brightness from a thousandth of a magnitude to as much as 20 magnitudes. It’s interesting that certain galaxies, too, can change in brightness. But unlike variable stars, which are observed from relatively close distances, galaxies are observed as far out as our instrumentation will allow, over 15 billion light years. We know today there are literally billions of galaxies. This paper humbly includes observations of 21 of these galaxies and an appraisal of the specifications of the two telescopes used and their capabilities. (Universe 1999; The Astrophysical Journal) 1. OBSERVATIONS AND COMPARISONS Surface brightness is defined as “the luminosity per unit area on the sky, usually expressed for optical data as magnitudes per square arcsec. It is a useful distance- independent property to use in the comparison of low-redshift galaxies (where relativistic corrections are unimportant) as the angular area subtended and the luminosity both decrease with the inverse square of the distance”. ( The Facts on File Dictionary of Astronomy) A commonly misunderstood concept to most amateur astronomers is why a galaxy with the same magnitude as a star is much more difficult find or see. For example, when observing a galaxy and using the published magnitude, the observer must remember that the surface brightness will not appear to be as bright as a star with the same magnitude. The galaxy M51 (Table 1), which has a magnitude of 8.0, appears much fainter than reference stars in its vicinity that have the same magnitude. Both visual and CCD imaging observations were made. Visual observations were made during six nights, between April 11 and May 23, 2002, for a total of 10 observations 1 (Table 1). During the 48 nights in the observing schedule, there were approximately 16 weather nights when no observations were possible. 1.1. VISUAL OBSERVATIONS Visual observations were made from Denver, Colorado, United States: 40°N and 105°W. 1.1.1. VISUAL OBSERVATIONS: EYEPIECE FIELD-OF-VIEW To aid in locating the galaxies, especially from an observatory in a suburban setting with light pollution, it was necessary to compute the field-of-view (FOV) for the eyepieces. Using the 32mm eyepiece with a FOV equal to 14.1 arcmin to locate M83, which subtends an angle of 13.1’ would have been difficult without first locating the correct star field. The installed encoders are not accurate enough to place a galaxy in the field of view. The drift method was used to determine the FOV (AAVSO). As an example, computing the FOV for the 32mm Plössl eyepiece consisted of locating a star on or near the celestial equator. The star was placed just outside the east side of the eyepiece field of view and the Right Ascension of the telescope disengaged. This allowed the star to “drift” into view from east to west. The time it took the star to drift from the east side of the eyepiece and exit the west side was measured in seconds using a stopwatch. Nine measurements were made for the 32mm eyepiece. This time was converted to arcseconds by using the Earth’s rotational speed of 1o /4-minutes. This is proportional to the FOV in arcseconds (unknown)/FOV in seconds using the drift method. The formula is expressed as: 1o/4’ = D/m D = FOV in arcseconds, UNKNOWN. m = Measured drift of star across eyepiece in seconds. For the 32mm eyepiece: o D = FOV in arcseconds. 1 /4’ = D/m m = Nine measurements averaged 3600s/240s = D/56.31s to 56.31 seconds. 240(D) = 3600/56.31 1o = 3600 seconds. D = 202,716/240 4’ = 240 seconds. D = 844.65 arcseconds ⇒ 844.65 arcs/60 = 14.1 arcminutes (Variable Stars) 2 1.1.2. VISUAL OBSERVATIONS: EYEPIECE MAGNIFICATION AND FOV The following table shows the magnification and FOV, as explained in section 1.1.1. for each eyepiece measured. (Variable Stars) Eyepiece Power (x) Field-of-View (arcmin) 25mm 312x (not available) 32mm 244x 14.1’ 40mm 195x 14.7’ 65mm 120x 28.9’ 80mm 98x 19.9’ Focal Length 7800 mm I have been observing for more than twenty years, and still consider the task of identifying the star field correctly a bit tricky. There were times that I spent up to two hours properly identifying a star field. My wife says I should get a life! 1.1.3. VISUAL OBSERVATIONS: 20” REFRACTOR All visual observations were made using the University of Denver’s (DU) historic Chamberlin Observatory Alvan Clark-George Saegmuller 20-inch f/15 refractor telescope. The Clark 20” is capable of yielding images beyond 16.5 Mv. Of course, this is limited by the sky conditions and its location in Denver, Colorado. The five galaxies, all Messier objects, were chosen due to familiarity, relative brightness and ease of recognition. Some visual observations for this project followed the Public Night outreach at Chamberlin Observatory, co-sponsored by DU’s Physics and Astronomy Department and the Denver Astronomical Society (DAS). As Lead Observer during Public Nights, I am responsible for opening and closing the observatory, and presenting the lecture to the public prior to moving upstairs to the telescope for viewing. As with any celestial object, visual observations do not bring out the same detail as photographic or CCD images, but having the photograph available to reference during the observation aids in seeing certain detail, which otherwise could be overlooked. Table 1. Visual Observations Designation Coordinates Date/Time1 Mv2 mmddyyyyhhmm M51 11h 29m, 47° 11’ 042220020440 8.0 052220020450 3 M66 11h 20m, 12° 58’ 051420020351 10.0 042920020415 M82 9h 56m, 69° 40’ 041920020530 9.19 042920020435 M83 13h 37’, -29° 52’ 041920020630 8.0 042020020548 M104 12h 40m, -11° 37 042020020518 9.50 052420020450 1 All times Universal Coordinated Time. 2 Published magnitudes using Starry Night Pro Astronomy Software. For quicker and more accurate identification of each galaxy, a graduated approach, starting with the widest FOV eyepiece, and moving to the narrower FOV, was used. Note also that the 32mm eyepiece is a super Plössl of good quality. 1.1.4. COMPARISON TO PUBLISHED IMAGES: VISUAL M51 was observed on two separate nights (Table 1). The seeinga condition was logged as III with a note of a bright sky. As mentioned earlier, Chamberlin Observatory is located a short distance from downtown Denver, therefore, light pollution is a constant hindrance. M51, also known as the Whirlpool Galaxy, is an Sc class spiral galaxy located in the constellation Canes Venatici, 8.5 Mpc (million parsecs) from Earth. The galaxy is approximately 20 kiloparsecs in diameter, with an angular width of 0.21201°. When compared to published images some spiral could be seen, but this was after allowing the eyes to totally dark-adapt. The spiral arms of M51 contain hot, blue stars and are star forming regions, but no color was visible. NGC 5195, the companion galaxy to M51, was barely visible. I question whether or not it would have been noticed if not for prior knowledge and a photograph to reference. (Universe 1999, Sky & Telescope, Astronomy) M66 was observed on two separate nights (Table 1). The seeing condition was logged as III with intermittent haze. M66 is an Sb class spiral galaxy located in the constellation Leo, approximately 10.7 Mpc from Earth. The galaxy has an angular width of 0.14663°. When compared to published images, no spiral could be seen. The 65mm lens was used first to locate M66, then replaced with the 40mm and 32mm. The 40mm yielded the best a For visual observations, “seeing” is defined by the Antoniadi Scale of I - V, with I=perfect seeing and V=appalling. 4 view, allowing some nebulosity to be seen, but no detail structure. No color was visible. (The Facts on File Dictionary of Astronomy, Starry Night Pro) M82 was observed on two separate nights (Table 1). The seeing condition was logged as III with haze on one of the nights. M82, also known as the Cigar Galaxy, is an Irregular class galaxy located in the constellation Ursa Major, 3.5 Mpc from Earth. The galaxy has an angular width of 10.5’. (Starry Night Pro) When compared to published images, nebulosity could be seen. As with M66, the 40mm eyepiece yielded the best view. No detail structure was observed, nor was color visible, even though M82 is a starburst galaxy with prolific star forming regions. (Universe 1999, Astronomy) M83 was observed on two separate nights (Table 1). The seeing condition was logged as II-III, with intermittent clouds on one night. M83 is an SBb class galaxy located in the constellation Hydra, 4 Mpc from Earth. The galaxy has an angular width of 13.1’. (Starry Night Pro, The Facts on File Dictionary of Astronomy) When compared to published images, nebulosity and spiral structure could be seen. Details of the spiral were visible with the 65 mm eyepiece, but better views were possible with the 40mm and 32mm. Some blue color was visible after the eyes were dark adaptive.
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