observational astronomy using skynet - duke universitymkruse/phy105_s11/projects...observational...

Casey Long

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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

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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,

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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 2557. 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 computer‐

controlled 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.

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Skynet System

The Skynet system is composed of an extensive collection of computer‐

operated 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.

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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:

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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.

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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.

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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

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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 8.98. I decided to take images at exposure

times of 20, 40, and 60 seconds. These times yielded the following images.

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.

M104 – 40s M104 – 20s M104 – 60s

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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 http://static.panoramio.com/photos/original/23791558.jpg

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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) 16.3 10 10 10

Pixel Scale (arcseconds per pixel)

.96 .59 .59 .59

Max Exposure Time (s) 300 80 80 80

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.

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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.

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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 10.73

http://www.sciencephotolibrary.com/images/download_lo_res.html?id=828200463

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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 9.59

http://upload.wikimedia.org/wikipedia/commons/0/07/Messier_87_Hubble_WikiSky.jpg

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Type Elliptical (E4)/Lenticular Galaxy RA 12:29:46.8 DEC +08:00:01.5


http://upload.wikimedia.org/wikipedia/commons/4/4b/M49a.jpg

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Type Elliptical Galaxy (E5) RA 12:42:02.3 DEC +11:38:49.0


http://upload.wikimedia.org/wikipedia/commons/0/04/Messier59.jpg

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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 10‐16 Mly Apparent Magnitude 6.84

http://hubblesite.org/newscenter/archive/releases/1998/14/image/d/format/web/

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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.

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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 7.54

http://www.eso.org/public/images/eso0136a/

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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 9.14

http://www.capella‐observatory.com/images/Galaxies/NGC6744.jpg

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Common Name Black Eye Galaxy Type Tightly Bound Spiral Galaxy RA 12:56:43.7 DEC +21:40:57.6


http://hubblesite.org/gallery/album/entire/pr2004004a/web/

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Prompt 4 ‐ April 6, 2011 ‐ Exposure Time = 60s


Common Name Sombrero Galaxy Type Unbarred Spiral Galaxy RA 12:39:59.4 DEC ‐11:37:23.0



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Catalogue Designations M99

Type Unbarred Spiral Galaxy RA 12:18:49.6 DEC +14:24:59.4


http://www.freewebs.com/skyimager/m99‐lrgb%20cropped.jpg

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Type Grand Design Spiral Galaxy RA 12:22:54.9 DEC +15:49:20.6

Distance 55 Mly Apparent Magnitude 10.1

http://www.eso.org/public/images/eso0608a/

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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



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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)

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Catalogue Designations PGC 54559

Common Name Hoag’s Object Type Ring Galaxy RA 15:17:12.8 DEC +21:35:03.1


http://apod.nasa.gov/apod/ap100822.html

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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.

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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 3.7

http://hubblesite.org/newscenter/archive/releases/2008/14/image/a/format/web/

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Common Name Orion Nebula Type Diffuse Nebula RA 05:35:17.3 DEC ‐05:23:28.0

Distance 1,344 ± 20 ly Apparent Magnitude 4.0

http://www.adventuresinastrophotography.com/2007/10/17/first‐images‐orion‐nebula/

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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.

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Observation Log

Date Object Type RA DEC Filter Exposure 4/6 M104 (Sombrero Galaxy) Galaxy 12:39:59.4 ‐11:37:23.0 Open 20 4/6 M104 (Sombrero Galaxy) Galaxy 12:39:59.4 ‐11:37:23.0 Open 40 4/6 M104 (Sombrero Galaxy) Galaxy 12:39:59.4 ‐11:37:23.0 Open 60 4/8 NGC 6744 Galaxy 19:09:46.1 ‐63:51:27.1 Open 10 4/8 NGC 6744 Galaxy 19:09:46.1 ‐63:51:27.1 Open 20 4/8 NGC 6744 Galaxy 19:09:46.1 ‐63:51:27.1 Open 30 4/8 NGC 6744 Galaxy 19:09:46.1 ‐63:51:27.1 Open 60 4/8 NGC 5128 (Centaurus A) Galaxy 13:25:27.6 ‐43:01:08.8 Open 5 4/8 NGC 5128 (Centaurus A) Galaxy 13:25:27.6 ‐43:01:08.8 Open 15 4/8 NGC 5128 (Centaurus A) Galaxy 13:25:27.6 ‐43:01:08.8 Open 25 4/8 NGC 5128 (Centaurus A) Galaxy 13:25:27.6 ‐43:01:08.8 Open 60 4/8 M64 (Black Eye Galaxy) Galaxy 12:56:43.7 +21:40:57.6 Open 10 4/8 M64 (Black Eye Galaxy) Galaxy 12:56:43.7 +21:40:57.6 Open 20 4/8 M64 (Black Eye Galaxy) Galaxy 12:56:43.7 +21:40:57.6 Open 30 4/8 M64 (Black Eye Galaxy) Galaxy 12:56:43.7 +21:40:57.6 Open 60 4/8 PGC 54559 (Hoag's Object) Galaxy 15:17:12.8 +21:35:03.1 Open 25 4/8 PGC 54559 (Hoag's Object) Galaxy 15:17:12.8 +21:35:03.1 Open 50 4/8 PGC 54559 (Hoag's Object) Galaxy 15:17:12.8 +21:35:03.1 Open 75

4/9 NGC 5139 (Omega Centauri) Globular Cluster 13:26:47.3 ‐47:28:46.1 Open 30

4/9 NGC 5139 (Omega Centauri) Globular Cluster 13:26:47.3 ‐47:28:46.1 Open 60

4/12 M42 (Orion Nebula) Nebula 05:35:17.3 ‐05:23:28.0 Open 1 4/12 M42 (Orion Nebula) Nebula 05:35:17.3 ‐05:23:28.0 Open 5 4/12 M42 (Orion Nebula) Nebula 05:35:17.3 ‐05:23:28.0 Open 15 4/13 M51 (Whirlpool Galaxy) Galaxy 13:29:52.7 +47:11:42.9 Open 30 4/13 M51 (Whirlpool Galaxy) Galaxy 13:29:52.7 +47:11:42.9 Open 60 4/13 M51 (Whirlpool Galaxy) Galaxy 13:29:52.7 +47:11:42.9 Open 30 4/13 M51 (Whirlpool Galaxy) Galaxy 13:29:52.7 +47:11:42.9 Open 60

4/13 M83 (Southern Pinwheel Galaxy) Galaxy 13:37:00.9 ‐29:51:56.7 Open 70

4/13 M100 Galaxy 12:22:54.9 +15:49:20.6 Open 70 4/13 M99 Galaxy 12:18:49.6 +14:24:59.4 Open 30 4/13 M99 Galaxy 12:18:49.6 +14:24:59.4 Open 60 4/19 M59 Galaxy 12:42:02.3 +11:38:49.0 Open 60 4/19 M89 Galaxy 12:35:39.9 +12:33:21.7 Open 60 4/19 M87 Galaxy 12:30:49.4 +12:23:28.0 Open 60 4/19 M49 Galaxy 12:29:46.8 +08:00:01.5 Open 60

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Works Cited

Celestial Object Information

http://en.wikipedia.org/wiki/Sombrero_Galaxy

http://en.wikipedia.org/wiki/NGC_6744

http://en.wikipedia.org/wiki/Black_Eye_Galaxy

http://en.wikipedia.org/wiki/Hoag's_Object

http://en.wikipedia.org/wiki/Messier_83


http://en.wikipedia.org/wiki/Orion_Nebula

http://en.wikipedia.org/wiki/Centaurus_A


http://en.wikipedia.org/wiki/Omega_Centauri

http://en.wikipedia.org/wiki/Whirlpool_Galaxy





SKYNET System

Various links and pages from: http://skynet.unc.edu/index.php

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

observational astronomy using skynet - duke universitymkruse/phy105_s11/projects...observational...

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