Galaxies Classification

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Materials Required:

  • Computer and internet access
  • Calculator
  • Ruler
  • Pencils and pens, eraser
  • Digital camera and/or scanner
  • Print out Galaxy Image Prints      document

Total Time Required: Approximately 2-3 Hours 

Part 1. The Local Group

In Table 1, you will find a list of most of the galaxies in the Local Group. This group is made up of our own galaxy, the Milky Way, and its closest neighbors. (Note: 1 kpc = 1 kiloparsec = 1000 parsec; 1 parsec = 3.26 light years)

    

Table 1. Galaxies of the Local Group

 

Name


Distance (kpc)


Diameter (kpc)


Name


Distance (kpc)


Diameter (kpc)

 

Milky Way


-


40.0


IC 1522


610


1.5

 

Sculptor


60


0.3


WLM


610


2.1

 

Large Magellanic Cloud   (LMC)


60


6.1


Andromeda I


675


0.6

 

Carina


90


0.2


Andromeda II


675


0.6

 

Draco


90


0.2


Andromeda III


675


0.9

 

Ursa Minor


90


0.3


M32


675


1.5

 

Sextans I


90


0.9


NGC 185


675


1.8

 

Small Magellanic Cloud   (SMC)


90


4.6


NGC 147


675


3.1

 

Fornax


150


0.9


NGC 205


675


3.1

 

Leo II


185


0.2


M31 (Andromeda Galaxy)


675


61

 

Leo I


185


0.3


IC 1613


765


3.7

 

NGC 6822


520


2.5


M33 (Triangulum Galaxy)


765


14.0

 

DDO 210


920


1.2

Below is a visual representation of our Local Group of galaxies. You can access and zoom into this image to see galaxies more clearly using this link Local Group.

Local Group VisualizationFigure 1. Local Group Visualization.

Type and label your answers to the below questions in your lab report.

  1. Using Table 1 above and noting the      diameters of the galaxies, which five galaxies in the      Local Group are the largest? In a few sentences, compare the sizes of the      other galaxies in the Local Group to the two largest ones. 
  2. Which three galaxies      have the largest angular size (not including the Milky Way)?  These      galaxies are the ones that look the largest in the sky. Explain how you      get your answer. 
  3. By hand and with pencil,      create a scale drawing of the Milky Way and the      Large Magellanic Cloud, showing their relative sizes and      the distance between them in kiloparsecs (the galaxies      can be represented by circles). Write down the scale you      use , and your calculations to find the scaled-down sizes      and distance. (An example of a good scale to use would like something      like: 5 mm = 10 kpc, also see this site Basic-Mathematics.com for more      information on scaling.) You will photo your drawing and insert it into      your lab report.

Figure 2. Local Supercluster Print

Local Supercluster Print

Image courtesy of Palomar Sky Survey Photographs

As you have already seen, galaxies can vary a lot in size. Now, we will look at how their shapes are different. Two basic types of galaxies are the spiral galaxies and the elliptical galaxies. Spiral galaxies have a disk-like structure,and a central bulge. Our own Milky Way is a spiral galaxy. Elliptical galaxies appear round, looking a lot like a football or an egg.

Examples of both types of galaxies can be found in the research-quality Palomar Sky Survey. Photographs, Figure 2, of the part of the sky that includes the center of the Local Supercluster. This cluster is in the direction of the constellation Virgo, so it is sometimes called the Virgo cluster. The Local Supercluster is 15 million parsecs away from the Local Group, and it contains thousands of galaxies.

4.Go to this site, ESO image, which is also an image of the Virgo Cluster that you can zoom in to better see some of the galaxies. Within this image locate:

  • two elliptical galaxies with      different ellipticities,
  • a spiral galaxy seen face-on,
  • and a spiral galaxy seen edge-on.

Make a diagram on a sheet of paper to show where these galaxies are on the picture, and try to sketch their appearances in a similar-sized box. Note: it is best to create a type of grid system and note if the galaxy is, for example, to right, middle, etc (Make sure to label each type of galaxy on your sketch. Photo and insert your signed and dated sketch into your lab report.)

Part 2. The Local Group.

Now that we have a handle on how to classify galaxies, we can take a look at how galaxies are grouped in the Universe. This may give us some clues as to how they form and take on so many different shapes.

Galaxies are found throughout the universe, from our next door neighbors — the Magellanic Clouds and Andromeda — all the way out to the edge of the visible universe 13 billion light years away. Nobody knows for sure, but it is estimated that there are 100 billion galaxies or more in the visible universe, and many more beyond that. Galaxies live in a variety of environments. Sometimes large numbers of them are packed close together in clusters, such as the Coma Cluster; sometimes they gather in smaller numbers called groups, like the Local Group that contains our Milky Way; and sometimes they are isolated far from one another in the field.

Clusters, groups, and some isolated galaxies can all be part of even larger structures called superclusters. At the largest scales in the visible universe, superclusters are gathered into filaments and walls surrounding vast voids, often described as resembling large soap bubbles. This structure often is referred to as the “cosmic web.”

The Coma Cluster, which is centered about 320 million light-years away, contains several thousand individual galaxies. The cluster has a roughly spherical shape and is about 20 million light-years across. (For comparison, the Milky Way is 100,000 light-years across). That many galaxies in a relatively small space makes the Coma Cluster one of the richest and densest galaxy clusters in our region of the universe. (download and print out the ‘Galaxy Image Prints’ for this part of the lab) On these pages you will be asked to count different types of galaxies.

Use the labels on this picture as an example of how to count the various objects.

Image of the Coma Cluster

I) Ellipticals or Lenticulars: It can be hard to tell these apart. If you know it’s either an E or S0/SB0, it is okay to guess.

II) Spirals and Barred Spirals: It can be hard to tell these apart. If you know it’s either an S or SB, it is okay to guess.

III) Irregular galaxy:  Irregular galaxies have no definitive shape.

IV) Uncertain:  An edge-on view of a galaxy that could possibly be an S0, SB0, S, SB, or IR.

There are too many possibilities, so do not count these.

V) Star: Any object that has “crosshairs” sticking out of it is a foreground star in the Milky Way galaxy, so do not count these.

?) Don’t count small, faint objects like these.

Count and clearly label the number of galaxies of each morphological type in the four images on the ‘Galaxy Image Prints’, sign and date your images, photo them and attach to your lab report. Create or copy the below tables into your report, typing in the numbers you found. Use the guidelines on the previous page to help you.

On the Galaxy Image Prints, the images on the top of each page (A&C) show the dense central core of the Coma Cluster, and the images on the bottom of each page (B&D) show galaxies out in the field. Fill in the two tables that you create in your lab "The Cluster" and "The Field" using the numbers you counted on these pages:

   

The Cluster

 

Coma Cluster


Ellipticals


Lenticulars


Regular & Barred Spirals


Total #

 

Image A

 

Image C

 

Sum of A & C


(e)


(f)


(g)


(h)

   

The Field

 

The Field


Ellipticals


Lenticulars


Regular & Barred Spirals


Total #

 

Image B

 

Image D

 

Sum of B & D


(i)


(j)


(k)


(m)

Using a calculator, find the percentages of each galaxy type in the cluster versus the field. Do this by noting the numbers in the Sum row of each table and following the calculations shown below. Show your work for each calculation.

   

In the Cluster and   In the Field

 

In the Cluster: 


In the Field:

 

% of Ellipticals =[(e)/(h)] x 100              


% of Ellipticals = [(i)/(m)] x 100               

 

% of Lenticulars = [(f)/(h)] x 100 


% of Lenticulars = [(j)/(m)] x 100

 

% of Spirals = [(g)/(h)] x 100


% of Spirals = [(k)/(m)] x 100

  1. Where did you find a higher      percentage of spiral galaxies, in the cluster or in the field?
  2. On a sheet of blank paper, create      a sketch of the Image C print, a pair of galaxies that seem to be      colliding or disrupting each other. Describe (type out) the features      that lead you to believe this. What do astronomers believe these galaxies      will look like in the future when the collision is completed? (Label each      type on your signed and dated sketch.) Photo and insert your sketch into      your lab report.

Part 3. Additional Research.  

Galaxies differ outwardly in their appearance but also differ internally with their stellar composition, colors and available gases for new star formation.  For example, as you have seen with this lab our galaxy, the Milky Way, is a larger spiral galaxy than other galaxies of the Local Group which is a reflection of its age as compared to the other galaxies of the Local Group.  Our galaxy is middle-aged in its evolution.  The classification of galaxies as spiral, elliptical, or irregular is also an indication of the evolutionary stage of each galaxy in general terms of young, middle-aged, or older galaxies.  

  1. Describe the general evolutionary      stage of spiral, elliptical, and irregular galaxies as related their being      best described as young, middle-aged, or older galaxies.  Provide      ‘specific evidence’ which then supports your decision (include citation(s)      in proper APA format).  Answers should include 3 paragraphs (1 per      galaxy type) with a minimum of 50 words for each paragraph.  

NOTE: You must provide a reference list showing the source(s) that you used, including our own textbook, in proper APA citation format.




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