Create a Hubble Tuning Fork diagram


In this activity you will create stunning colour images of galaxies and add them to the Tuning Fork template to recreate the famous Hubble image.


Students work with real data files for a number of galaxies collected from the LCO data archive. They will use Photoshop software to make these files into colour images by combining different filters (B,V and R) for each galaxy. Finally, they will correctly classify each galaxy and arrange them into the classic Tuning Fork template to recreate the famous Hubble image! 

Learning Objectives

  • Understand that the Universe is made up of a range of objects, including galaxies
  • Understand that a galaxy is a gravitationally bound collection of stars, cosmic gas and dust and other astronomical objects
  • Realise that there are several galaxy classes that are identified by their varying shapes and sizes and be able to identify galaxies of various class
  • Understand that the Hubble Tuning Fork diagram is a useful way to catalogue galaxies, but does not demonstrate the evolutionary track of galaxies
  • Demonstrate an ability to obtain and manipulate real data files using Photoshop software 


Start by downloading the data files attached and making color images of the galaxies. 

Each student requires a computer with Adobe Photoshop software installed.


Southern Pinwheel galaxy

The Southern Pinwheel Galaxy, or M83, is a barred spiral galaxy.

Galaxies are huge collections of stars, gas and dust held together by gravity. They are fundamental building blocks of the Universe. Some are simple, while others are very complex in structure. Our Galaxy, the Milky Way, is known as a spiral galaxy as it has spiral arms that wind their way around the center of the galaxy (or bulge).

In 1926, astronomer Edwin Hubble realised that there are several types of galaxy and developed a classification scheme, known as the Hubble tuning fork diagram.

The diagram is roughly divided into two parts: elliptical galaxies (ellipticals) and spiral galaxies (spirals). Hubble gave the ellipticals numbers from zero to seven, which characterise the ellipticity of the galaxy, i.e "E0" is almost round and "E7" is very elliptical.

The spirals were assigned letters from "a" to "c," which characterize the compactness of their spiral arms. "Sa" spirals, for example, are tightly wound whereas "Sc" spirals are more loosely wound.

The spiral galaxies are sub-divided into two groups - normal spirals and barred spirals. The most important difference between these two groups is the bar of stars that runs through the central bulge in barred spirals. The spiral arms in barred spirals usually start at the end of the bar instead of from the bulge. Barred spirals have a "B" in their classification. An "SBc" is thus a loosely wound barred spiral galaxy.

"S0," or lenticular galaxies, are in the transition zones between ellipticals and spirals and bridge these two types.


Hubble found that some galaxies are difficult to put in the context of the tuning fork diagram. Those include irregular galaxies which have odd shapes, dwarf galaxies which are very small and giant elliptical galaxies which are very large elliptical galaxies residing in the centers of some clusters of galaxies.

For a time the Hubble tuning fork was thought to be an evolutionary sequence - that galaxies might evolve from one type to another progressing from left to right across the tuning-fork diagram. Hence "Sa" and "SBa" galaxies were called "early-type," while "Sc" and "SBc" were called "late-type."

Astronomers still use this nomenclature today, though the initial concept was later found to be an over-simplification. Galaxy evolution is a far more complex process than Hubble imagined, involving the conditions of the galaxy's initial collapse, collisions with other galaxies, and the ebb and flow of internal star birth.



In this activity you will search the B, V and R FITS files for each of the galaxies listed below. These can either be procured on the LCO data archive, or by carrying out the observations yourself. Once you have downloaded these files, you will combine them to make colour images, using Photoshop. Finally, you will classify your galaxies and add them to the Tuning Fork template to recreate the famous Hubble image. 

When you make astronomical observations you end up with FITS files. A grey scale FITS file is produced for each filter used in your observation. Colour images are a composite of observations taken with a red, a green and a blue filter. With Photoshop you can add the colour to the images and combine them with images from other filters to produce a full colour image.

Full Description

  1. Start by downloading the B (blue), V (green, or ‘visual’) and R (red) FITS files for each of the galaxies listed in the table below. Click on the galaxy name to be taken to the LCO data archive.

Note: When downloading pictures from the LCOGT website the green file will be called Green FITS. If you are making observations the green file will be called Visual and when selecting a filter it will be “V”. 

2. The data archive is found at

3. Simply input the name of your galaxy into the ‘Object’ field and select the filter your require from the 'Filter' drop-down menu.

4. On the observation page linked above, download the three FITS files and name them by their filter colour (this will become helpful later on).

5. Once you have collected a FITS file for all three filters for each of your galaxies, follow the steps below to make colour images of these galaxies using Photoshop. 

Saving and editing images of your galaxies using FITS Liberator

  1. You will need to ensure that FITS Liberator is installed on your computer, this can be used to change the downloaded files into something Photoshop can read. (If you do not currently have FITS Liberator on your computer, you can download the application from the NASA/ESA website at
  2. Launch FITS Liberator, and open your FITS files by going to File > Open. To open more than one at a time press Shift while you select the files. (Note: If your downloaded files appear as .fits.fz then you will need to rename them to just .fits to be able to open them in FITS Liberator.)
  3. Set the white and black level sliders in the histogram graph to give a good amount of contrast without too much black or too much saturation.
  4. Experiment with the different Stretch functions to enhance fainter parts of the image. Different stretch functions produce different results with each image so see what works. (In general, x^1.5, x^2, x^3, x^4, x^5, and exp(x) don’t do anything.)
  5. When you are happy with the way the image looks, tick the 8-bit button, which is below the Autoscale button (If you forget you can always change it later in Photoshop by clicking Mode then pressing 8-bit).
  6. Click Save As and then add your file and save it to somewhere you will remember.
  7. Open the next image into FITS Liberator and repeat until all images are edited. Import the images into Photoshop by opening Photoshop and clicking import or by dragging and dropping the images onto the Photoshop icon.

ngc1300 fits liberator

Adding colour to your images using Photoshop

  1. The next stage is to begin to add some colour information into these images. To do this launch Photoshop, open one of the colour images for a selected galaxy and go to: Image>Mode>RGB Color. Do the same for each of the three images of that galaxy.
  2. When all three images are open, select the red image. Then go to Image > Adjustments > Hue/Saturation.
  3. This will open a window with three sections.  First, tick the Colorize button in the lower-right corner.  Now enter the following starting values. For the red filter image enter Hue: 0  Saturation: 100  Lightness: -50. Leaving Hue alone, adjust Saturation and Lightness until you have a high contrast, crisp image. (These settings can be changed later.
  4. Then select your visual (green) image. Go to Image > Adjustments > Hue/Saturation, and enter Hue: 120  Saturation: 100  Lightness: -50 and adjust as necessary.
  5. Finally, select your blue image, Image > Adjustments > Hue/Saturation and enter 
Hue: 240  Saturation: 100  Lightness-50 and adjust as necessary.


Combining images in Photoshop

  1. Now that your galaxies are coloured, the files for each need to be combined into a single image.  To do this, you copy and paste the visual and blue images on top of the red image for each galaxy.
  2. So, select your visual image in Photoshop, and go to Select > All then Edit > Copy. You can also create a duplicate layer by right clicking on the layer then clicking duplicate then drag and drop the layer onto the red image, or layer from background by right clicking then clicking layer from background then drag and drop.
  3. Select your red image, and select Edit > Paste.
  4. You should see a Layers window to the right of your screen, if you do not, go to Window > Layers.  This window shows that a new layer has been added to the red image, it will be called Layer 1. Change the layer name to red if you wish.
  5. Now, follow the same process with your blue image. Select > All, Edit > Copy.   Then go back to the red image (with the visual image copied on top of it) and select Edit > Paste.
  6. You now have one image that combines the red, green and blue images.

Blending mode

  1. The next step is to blend the images so the colours mix.
  2. Above the list of layers in the Layers window, there is a drop down menu option, Normal. This controls the mixing mode of the layers.
  3. Highlight the blue image in the Layers window to select it. Then change the mode from Normal to Screen. You may also want to try other presets here, especially Lighten, or Linear Dodge.
  4. The blue layer will then blend with the visual layer and your image will turn a cyan color.  Select the visual layer and change the mode to Screen or Lighten or Linear Dodge, etc. This blends all the layers, and you have a complete RGB image.


Aligning layers

1. Before your basic color image is complete, remove haloing of the stars by aligning the layers.

2. With your combined RGB image open in Photoshop, select the Zoom Tool from the toolbar, and zoom into a star. As you can see, the layers are not quite aligned, and this causes color halos to form around stars.

3. Select the Move Tool. The idea is to align the visual and blue layers with the red. To do this, make the blue layer invisible, by clicking on the eye icon next to it in the Layers window.

4. Then, with the visual layer highlighted, click and drag with your mouse on the main image, to move the visual layer.

5. Align it as closely as you can to the red layer, using the star as a reference.  For finer movements, you can use your keyboard arrows.

6. When you are happy with the visual layer, make it invisible and make the blue layer visible again. Highlight the blue layer, and click and drag the image to move it over the red image in the same way.

7. When you are happy with the blue layer, make all the layers visible again.

Cropping, flattening and saving your images

1. Zoom out so you can see the whole image again by going to View > Fit on Screen.  Moving the layers may have created roughness on the edges of the image. You can remove these by cropping them out.

2. Select the Crop Tool, and draw a square around the regions of the image you wish to keep, then double click on the image.

3. The final step is to flatten the image. This merges the three red, visual (green) and blue layers into a single RGB image.

4. Go to Layer > Flatten Image (or right click then flatten)

Note: Make sure you are happy with the layer alignment, as you will no longer be able to move the individual layers after this.

5. Now you can save the image. Go to File > Save as. Enter a new name for the file, e.g. NGC1300_rgb and click Save.


Making a Hubble Tuning Fork Diagram

The final step is to download the HTF_template.jpg file and open this in Photoshop. Add all of your newly developed colour images to the template. Ensure that put the correct galaxy next to the correct class label.



Check each student has accurately classified their galaxies and placed them in the correct place on their Hubble Tuning Fork diagram. The accuracy of their diagram can be used to evaluate the students’ understanding of the topic. More detailed feedback can be obtained by asking the students to correctly classify a number of additional galaxies; this can be done using galactic images from the LCO archive.

Additionally, play a game of Messier Bingo after finishing the activity. Each time a galaxy appears, ask the group to classify it and . Messier Bingo can be found at:

  • 12-16
  • All ages

Medium (30-60 mins)

Type of Activity
  • Fun

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