Observational Astronomy Using Skynet

Transcription

1 Casey Long Observational Astronomy Using Skynet Introduction to Observational Astronomy Everybody has been an observational astronomer at some point in their lives. For most people this consists of simply observing the brightest stars and planets in the night sky. Professionals have access to cutting edge technology allowing them to probe deeper and deeper into the sky, unlocking a world of galaxies and nebulae. With such a vast number of observable objects, effective communication between fellow astronomers is necessary. The two most important properties for identifying an object are name and location. To standardize these properties, astronomers have developed coordinate systems and naming schemes to promote easy communication. Celestial Coordinate Systems For casual stargazers, it is convenient to use the horizontal coordinate system for defining the location of celestial objects. In this system, an object s location is described by its Altitude and Azimuth. Altitude is a degree measure from 0 (the horizon) to 90 (directly overhead) of the height of the object relative to the ground. Azimuth is a degree measure of direction from North (0 ), to East (90 ), to South (180 ), to West (270 ). This system is useful because the observer needs no special equipment to make a fairly good estimate at where an object is. The problem with this system, however, is coordinates differ for different observers as well as over time. That is, horizontal coordinates are only valid in one location and at one time for a given celestial object. To overcome this obstacle, the equatorial coordinate system can be used. In this system, Earth s equator and poles Long 1

2 are projected onto the sky. A measure called Declination (DEC) (from 90 (S) to +90 (N)) measures an object s position relative to the celestial equator, much like lines of latitude on Earth s surface. Due to Earth s rotation, longitudinal coordinates cannot be projected as simply. An arbitrary starting point synonymous with Earth s Prime Meridian must be defined as a constant reference point. This point was chosen to be the location of the Sun during the vernal equinox. From this point, Right Ascension (RA) is measured from 0 to 360, although this is often reported as sidereal time from 0 to 24 hours (approximate rotation of the Earth in one day). The advantage of this coordinate system lies in its consistency for all observers at all times. Although star locations will slightly change due to the procession of the equinoxes, this effect is extremely subtle. Recalibrations are necessary only once every 50 or so years, even for distant objects. Astronomical Catalogues While many major stars and celestial objects have common names, there are simply too many to make this system practical for naming everything in the night sky. Most visible stars, even those with common names, are referred to by their constellation preceded by a Greek character indicating its brightness relative to other stars in the constellation. For example, the star commonly known as Antares is also known as α Scorpii because it is the brightest star in the constellation Scorpius. For deep sky objects, much less are known by common names, making catalogues even more necessary. The famous Messier Objects (M1 M110) were catalogued by comet hunter Charles Messier in 1781 to help differentiate potential comets from fixed objects. The list contains a collection of galaxies, star clusters, Long 2

3 and nebulae that are among the brightest and easiest to see deep sky objects. However, this list is extremely limited and more extensive catalogues were needed to keep track of all the observable deep sky objects. The New General Catalogue of Nebulae and Clusters of Stars includes 7,840 objects designated by the initials NGC followed by a four digit number. The even larger Catalogue of Principal Galaxies contains 73,197 galaxies designated by the initials PGC followed by a five digit number. Many objects are included in multiple of these catalogues and can be referenced with many names. For example, the famous Andromeda Galaxy is also known as M31, NGC 224, and PGC While many other astronomical catalogues exist, these three were sufficient for referencing all of the objects I was looking for. Observing Once you know what an objects name is, and where it is located in the sky, you can observe it using appropriate equipment. For bright stars and planets, horizontal coordinates are generally enough to find objects with the naked eye. For deep sky objects, telescopes are usually necessary. Most advanced telescopes are computer controlled to improve accuracy and easy use. For these systems, equatorial coordinates are preferred for their consistency. When taking images, a computerized system has additional benefits. Due to Earth s rotation, an object s position in the sky is always changing slightly. Telescope mounts with computercontrolled motors keep the telescope focused on the object during long exposure shots. This eliminates streaks and blurs that would distort stationary photographs over long exposures. Long 3

4 Skynet System The Skynet system is composed of an extensive collection of computeroperated telescopes from around the world. Users log on to the Skynet website where they electronically request images to be taken. The program is maintained by staff at UNC and generously allows students from other universities and high schools to utilize the service. Thanks to some inside connections, I was lucky enough to get to try the program out myself. The following is an overview of the regular procedure I used to collect images. Image Taking Process The first step for getting images was picking a target in the night sky. Using the free software Stellarium, I was able to see what the night sky looked like in Chile and could target specific objects visible to the telescopes. After finding an object of interest that was visible this time of year, I logged onto the Skynet website. Using the Observation Manager, finding objects coordinates was very easy. The following screenshot shows what a typical search would return. In this example the Andromeda Galaxy (M31) was searched. Long 4

5 Screen Shot of SKYNET Observation Manager After searching for an object, the computer does much of the heavy lifting. It searches its archives and finds accurate coordinates using the equatorial coordinate system. It defaults to a maximum airmass of 3. Airmass is a relative measure of how much atmosphere light has to penetrate. By definition, the airmass at the zenith (straight up) is equal to 1. Basically this restricts the telescopes from taking images of objects too close to the horizon. At these low points in the sky, light must travel through a significantly greater amount of atmosphere distorting images. Another default is the maximum sun elevation, set at 18. In short, this ensures the picture is taken at night. There are a variety of filter options, but for galaxies, an open filter usually suffices, which is what I chose for all observations. After searching for an object, the following graph appears: Long 5

6 Observation Manager M31 Visibility Graph This graph shows the visibility of the given object from the locations of various telescopes. Most of my images were taken from the Prompt telescopes labeled CTIO in the above graph (red line). For this example, the Andromeda Galaxy is not visible for any of the telescopes. Lines are only plotted for nighttime hours, and if they do not go above the 20 elevation/3.0 airmass barrier, they will not return good images. So for this time of year, M31 simply does not get high enough in the sky during dark hours to allow image taking. The following is a graph of the Sombrero Galaxy s (M104 s) visibility, showing what a visible object s graph looks like. Long 6

7 Observation Manager M104 Visibility Graph As you can see, this galaxy is visible for most of the night, especially for the Prompt telescopes. If an object is sufficiently visible and all the parameters are set, pressing Next yields the following screen. Long 7

8 Telescope Selection Screen On this screen, you can choose which of the available telescopes you want to use to take images. My first choice was always the Prompt Telescopes, as they reportedly return the best images. If multiple telescopes are chosen, the first to become available during your specific visibility window is used to capture your desired images. After selecting your telescopes, pressing Next brings you to the exposure page. Add Exposures Page Long 8

9 On this page, you can select various exposure times as well as the number of exposures. For particularly bright objects, or if a bright star was in an objects field of view, a message appears setting a maximum exposure time. This is mainly to protect light sensitive instruments from overexposure to bright objects. Even for faint objects, the absolute maximum exposure time allowed is 80 seconds for the Prompt telescopes. In my earlier observations, I often selected a wide range of exposure times until I got a feel for appropriate exposure times for a given object. For example, my first target was the Sombrero Galaxy. It is roughly 30 Mly away and has an apparent magnitude of about I decided to take images at exposure times of 20, 40, and 60 seconds. These times yielded the following images. M104 20s M104 40s M104 60s The Sombrero Galaxy at 3 different exposure times As you can see, there is not a dramatic difference, however the 20 second exposure definitely has the lowest contrast. The halo around the galaxy is most well defined with 60 seconds of exposure. For most objects I simply chose 60 seconds, since this gave the best results. For extremely bright objects, like the Orion Nebula, I was restricted to shorter exposure times. Conversely, for the very faint Hoag s Object, I chose to use closer to the maximum allowable 80 second exposure. Long 9

10 After this page, you are brought to a confirmation page where you can either send your request for observation, or cancel if you realize a mistake in your inputs. Once a request is submitted, your observation will automatically happen at the earliest possible time. Most requests were processed the night after submission, unless cloud cover or high telescope use delayed them. Telescopes Most of my observations were completed on Prompt telescopes (Prompt1 Prompt5). This set of smaller telescopes is part of the Cerro Tololo Interamerican Observatory (CTIO) in central Chile. Nestled in the Andes at over 2,000 m, the site is an ideal location for telescopes. Its elevation minimizes the amount of distorting atmosphere, while its isolation removes unwanted light pollution. Prompt Telescopes Long 10

11 All of my images were taken from Prompt telescopes except for the Whirlpool Galaxy, which needed to be taken with an ARO telescope due to visibility constraints. Basic properties for all telescopes used to take images are outlined in the chart below: Telescope ARO 30 Prompt1 Prompt3 Prompt4 Location ARO CTIO CTIO CTIO Field of View (arcminutes) Pixel Scale (arcseconds per pixel) Max Exposure Time (s) Basic Telescope Properties Field of view is simply a measure of how zoomed in the telescope can get on a given object. Measures are reported as angular fields of view. The pixel scale is basically a measure of resolution. It reports how many arcseconds each pixel takes up. Exposure time is how long the telescope actively accepts light for a given image. Celestial Objects Beyond the easily visible stars and planets lies a world of star clusters, galaxies and nebulae filling the sky in all directions. Most of my observations were galaxies of different shapes and sizes. Imaging nebulae works best with special filters and at non visible wavelengths not available through the Skynet system. Star clusters are fairly easily observable, but aren t quite as unique and interesting as galaxies. Long 11

12 Elliptical/Lenticular Galaxies Most galaxies can be described as one of three types based of their observed appearance. The simplest are elliptical galaxies, which feature continuous, even star distributions over an elliptical shape. These can be further divided by ellipticity, from nearly circular, to extremely ovoid. Under the Hubble Sequence Classification System, elliptical galaxies are given a number from 0 to ~7 based off their shape. For example a galaxy classified as E0 would be very circular while one classified as E6 would be much more elliptical. Galaxies may also be classified as lenticular. Like ellipticals, these do not have any distinct spiral distributions, however they contain a bright central bulge of stars, which taper to a thin disk at their outer reaches. There is often some overlap between elliptical and lenticular galaxies. The following four pages (13 16) consist of elliptical galaxies in increasing order of ellipticity. Page 17 has Centaurus A, the lenticular/giant elliptical galaxy that has much debate over its classification. Each observed object contained in this report is displayed on its own page following the same format. Centered at the top of the page is my image from one of Skynet s telescopes captioned with the telescope name, date of capture, and exposure time. The middle table gives basic properties and relevant information for each object. The bottom picture is a professionally taken photo from larger telescopes and/or satellites. Some are purely visible light, but many include other spectrums to enhance the images. They are provided to give a clearer view of the object and as a standard to compare my images to. Long 12

13 Prompt 1 April 19, 2011 Exposure Time = 60s Catalogue Designations M89, NGC 4552 Type Elliptical Galaxy (E0) RA 12:35:39.9 DEC +12:33:21.7 Distance 50 ± 3 Mly Apparent Magnitude Long 13

14 Prompt 4 April 19, 2011 Exposure Time = 60s Catalogue Designations M87, NGC 4486 Common Name Virgo A Type Supergiant Elliptical Galaxy (E0) RA 12:30:49.4 DEC +12:23:28.0 Distance 53.5 ± 1.63 Mly Apparent Magnitude Long 14

15 Prompt 1 April 19, 2011 Exposure Time = 60s Catalogue Designations M49, NGC 4472 Type Elliptical (E4)/Lenticular Galaxy RA 12:29:46.8 DEC +08:00:01.5 Distance 55.9 ± 2.3 Mly Apparent Magnitude Long 15

16 Prompt 4 April 19, 2011 Exposure Time = 60s Catalogue Designations M59, NGC 4621 Type Elliptical Galaxy (E5) RA 12:42:02.3 DEC +11:38:49.0 Distance 60 ± 5 Mly Apparent Magnitude Long 16

17 Prompt 1 April 8, 2011 Exposure Time = 60s Catalogue Designations NGC 5128 Common Name Centaurus A Type Lenticular/Giant Elliptical RA 13:25:27.6 DEC 43:01:08.8 Distance Mly Apparent Magnitude Long 17

18 Spiral Galaxies Spiral galaxies make up most of the rest of the observable galaxies. They contain the central bulge found in lenticular galaxies, and in addition have a spiral formation outside this core. Spiral galaxies can be further classified based off a number of criteria. One of the most visually distinguishable classifications is between barred and unbarred galaxies. Barred galaxies have a nuclear bar that passes through the central bulge connecting spiral arms on either side, while unbarred galaxies lack this. Galaxies in between these two extremes are sometimes referred to as weakly barred spiral or intermediate spiral galaxies. Other observable measures such as tightness can be used to describe the shape and distribution of stars. Galaxies with particularly well defined arms are often called grand design spiral galaxies. The next five pages (19 23) feature a wide range of spiral galaxies, starting with barred and gradually transitioning to unbarred. Pages 24 and 25 show examples of two grand design galaxies, the latter of which is an example of an interacting galaxy pair. This image contains the much larger Whirlpool Galaxy with the smaller NGC 5159 at the end of one of its arms. These companion galaxies are of much interest to astronomers and astrophysicists for their insight into galactic structure and interactions. Radio astronomy has proven that the two are in fact interacting, not just two galaxies along the same visual line from our perspective. The Whirlpool Galaxy is one of the most recognizable and unique of the easily observable galaxies. Long 18

19 Prompt 3 April 13, 2011 Exposure Time = 70s Catalogue Designations M83, NGC 5236 Common Name Southern Pinwheel Galaxy Type Barred Spiral Galaxy RA 13:37:00.9 DEC 29:51:56.7 Distance 14.7 Mly Apparent Magnitude Long 19

20 Prompt 1 April 8, 2011 Exposure Time = 60s Catalogue Designation NGC 6744 Type Intermediate Spiral Galaxy RA 19:09:46.1 DEC 63:51:27.1 Distance 31 ± 5.2 Mly Apparent Magnitude observatory.com/images/galaxies/ngc6744.jpg Long 20

21 Prompt 1 April 8, 2011 Exposure Time = 60s Catalogue Designations M64, NGC 4826 Common Name Black Eye Galaxy Type Tightly Bound Spiral Galaxy RA 12:56:43.7 DEC +21:40:57.6 Distance 24 ± 2 Mly Apparent Magnitude Long 21

22 Prompt 4 April 6, 2011 Exposure Time = 60s Catalogue Designations M104, NGC 4594 Common Name Sombrero Galaxy Type Unbarred Spiral Galaxy RA 12:39:59.4 DEC 11:37:23.0 Distance 29.3 ± 1.6 Mly Apparent Magnitude Long 22

23 Prompt 4 April 13, 2011 Exposure Time = 60s Catalogue Designations M99 Type Unbarred Spiral Galaxy RA 12:18:49.6 DEC +14:24:59.4 Distance 50.2 ± 5.5 Mly Apparent Magnitude lrgb%20cropped.jpg Long 23

24 Prompt 4 April 13, 2011 Exposure Time = 70s Catalogue Designations M100, NGC 4321 Type Grand Design Spiral Galaxy RA 12:22:54.9 DEC +15:49:20.6 Distance 55 Mly Apparent Magnitude Long 24

25 ARO 30 April 13, 2011 Exposure Time = 60s Catalogue Designations M51, NGC 5194 / NGC 5159 Common Name Whirlpool Galaxy Type Interacting, Grand Design Spiral Galaxy RA 13:29:52.7 DEC +47:11:42.9 Distance 23 ± 4 Mly Apparent Magnitude Long 25

26 Irregular Galaxies Certain rare galaxies do not fit the common mold of spiral, elliptical, or lenticular types. These are clumped together as irregular galaxies. Most are asymmetric groupings with little or no distinctive pattern or shape. However, some have a clear pattern and shape, but simply do not fit under any of the three common categories. A subset of this group are known as ring galaxies. Intrigued by their shape I decided to take aim at Hoag s Object, a ring galaxy ~600 million light years away. I was not expecting much, but was surprised to find a faint tiny ring with a dot in the middle, the same shape I was expecting after seeing more precise photos online. Still skeptical, I decided to check if it was about the right size relative to the field of view of the image. The image was taken by Prompt1, whose field of view is 10 arcminutes (or 600 arcseconds). The outer diameter of Hoag s Object is about 45 arcseconds, so theoretically it should take up roughly 7.5% of the width of the image. Taking measurements on my computer screen (since relative values are all that really matter), the total width of the image was roughly 9.5, while Hoag s Object s diameter was about 3/4. Dividing gives 7.9% which agrees very well with the projected value. Given its similar size and shape, I think the image is in fact Hoag s Object. While not visually stunning, I was very surprised it came out at all and was excited with the results. (Note: the image on the next page was cropped to focus in on Hoag s Object, so the ~7.5% ratio does not hold) Long 26

27 Prompt 1 April 8, 2011 Exposure Time = 75s Catalogue Designations PGC Common Name Hoag s Object Type Ring Galaxy RA 15:17:12.8 DEC +21:35:03.1 Distance 600 ± 30 Mly Apparent Magnitude Long 27

28 Globular Clusters Globular clusters are gravitationally bound star systems that orbit around galaxies. Omega Centauri (page 29) is the largest of the many globular clusters orbiting our Milky Way Galaxy. With an apparent magnitude of 3.7, it is easily visible in the southern hemisphere, though individual stars are not resolvable so it appears more or less as a fuzzy star. Nebulae Nebulae are collections of dust and ionized gases loosely bound by gravity. Over time, if large enough masses of gas collapse, stars and planets can form. These stellar nurseries are some of the most visually stunning observable objects in the universe however specific equipment is required to get high quality images. I decided to target the Orion Nebula since it is very bright and nearby. There are an estimated ~700 stars in formation at several different stages of stellar evolution. Its lack of any definitive boundaries makes it an example of a diffuse nebula, the most common nebula type. Long 28

29 Prompt 4 April 9, 2011 Exposure Time = 60s Catalogue Designations NGC 5139 Common Name Omega Centauri Type Globular Cluster RA 13:26:47.3 DEC 47:28:46.1 Distance 15.8 ± 1.1 kly Apparent Magnitude Long 29

30 Prompt 4 April 12, 2011 Exposure Time = 15s Catalogue Designations M42, NGC 1976 Common Name Orion Nebula Type Diffuse Nebula RA 05:35:17.3 DEC 05:23:28.0 Distance 1,344 ± 20 ly Apparent Magnitude images orion nebula/ Long 30

31 Conclusion I have always been very interested in observational astronomy but have never gotten the chance to look deeper than my eyes or handheld binoculars had to offer. This project was a great opportunity to delve into some of the amazing objects that lie out of sight. It has only increased my interest in the field. Before obtaining my first image (M104), I was highly skeptical of how well the pictures would turn out. On the whole, I was blown away with the results. A lot of variables contributed to the quality of the images. First and foremost, the distance to an object along with its apparent magnitude played a big part. Additionally the time of year plays a role. Best images will come from objects directly overhead during the darkest hours in the middle of the night. These objects have the least interference from the sun, and have minimal atmospheric distortion. While conditions were not always ideal, most of the images were at least comparable to professional photos in terms of shape and light distribution. Experiencing the image taking process first hand has given me a whole new appreciation of the field of observational astronomy. Its amazing to think that just plugging some numbers into a website can control a telescope to take an image of any celestial object, let alone a galaxy over 500 million light years away. The Skynet system is very user friendly and I am very grateful to have been given a chance to work with it. Overall, collecting images was an informative, rewarding process that increased my interest and understanding of observational astronomy. Long 31

32 Observation Log Date Object Type RA DEC Filter Exposure 4/6 M104 (Sombrero Galaxy) Galaxy 12:39: :37:23.0 Open 20 4/6 M104 (Sombrero Galaxy) Galaxy 12:39: :37:23.0 Open 40 4/6 M104 (Sombrero Galaxy) Galaxy 12:39: :37:23.0 Open 60 4/8 NGC 6744 Galaxy 19:09: :51:27.1 Open 10 4/8 NGC 6744 Galaxy 19:09: :51:27.1 Open 20 4/8 NGC 6744 Galaxy 19:09: :51:27.1 Open 30 4/8 NGC 6744 Galaxy 19:09: :51:27.1 Open 60 4/8 NGC 5128 (Centaurus A) Galaxy 13:25: :01:08.8 Open 5 4/8 NGC 5128 (Centaurus A) Galaxy 13:25: :01:08.8 Open 15 4/8 NGC 5128 (Centaurus A) Galaxy 13:25: :01:08.8 Open 25 4/8 NGC 5128 (Centaurus A) Galaxy 13:25: :01:08.8 Open 60 4/8 M64 (Black Eye Galaxy) Galaxy 12:56: :40:57.6 Open 10 4/8 M64 (Black Eye Galaxy) Galaxy 12:56: :40:57.6 Open 20 4/8 M64 (Black Eye Galaxy) Galaxy 12:56: :40:57.6 Open 30 4/8 M64 (Black Eye Galaxy) Galaxy 12:56: :40:57.6 Open 60 4/8 PGC (Hoag's Object) Galaxy 15:17: :35:03.1 Open 25 4/8 PGC (Hoag's Object) Galaxy 15:17: :35:03.1 Open 50 4/8 PGC (Hoag's Object) Galaxy 15:17: :35:03.1 Open 75 4/9 NGC 5139 (Omega Centauri) Globular Cluster 13:26: :28:46.1 Open 30 4/9 NGC 5139 (Omega Centauri) Globular Cluster 13:26: :28:46.1 Open 60 4/12 M42 (Orion Nebula) Nebula 05:35: :23:28.0 Open 1 4/12 M42 (Orion Nebula) Nebula 05:35: :23:28.0 Open 5 4/12 M42 (Orion Nebula) Nebula 05:35: :23:28.0 Open 15 4/13 M51 (Whirlpool Galaxy) Galaxy 13:29: :11:42.9 Open 30 4/13 M51 (Whirlpool Galaxy) Galaxy 13:29: :11:42.9 Open 60 4/13 M51 (Whirlpool Galaxy) Galaxy 13:29: :11:42.9 Open 30 4/13 M51 (Whirlpool Galaxy) Galaxy 13:29: :11:42.9 Open 60 4/13 M83 (Southern Pinwheel Galaxy) Galaxy 13:37: :51:56.7 Open 70 4/13 M100 Galaxy 12:22: :49:20.6 Open 70 4/13 M99 Galaxy 12:18: :24:59.4 Open 30 4/13 M99 Galaxy 12:18: :24:59.4 Open 60 4/19 M59 Galaxy 12:42: :38:49.0 Open 60 4/19 M89 Galaxy 12:35: :33:21.7 Open 60 4/19 M87 Galaxy 12:30: :23:28.0 Open 60 4/19 M49 Galaxy 12:29: :00:01.5 Open 60 Long 32

33 Works Cited Celestial Object Information SKYNET System Various links and pages from: Skynet Authorship Policy Images and data obtained from images taken by a user with Skynet may only be used by that user or by others designated by that user. However, at least the first three people from the Skynet builders list and at least the first two people from each used telescope's builders list must be included as authors on any publications, unless waived by the Director of the Skynet Robotic Telescope Network (currently Reichart) in writing. Builders Lists Skynet: Daniel E. Reichart, Kevin M. Ivarsen, and Joshua B. Haislip Prompt: Melissa C. Nysewander, Aaron P. LaCluyze Long 33