Constellations are groups of stars that appear to form a pattern on the sky. Ancient societies labelled these based on what they looked like, for example Orion the Hunter.

In this activity, you will find the constellation that is unique to your plate.

Activity resources

In the link below you can find slides with background information, a printable list of instructions, and redshift-distance rulers.

Step 1: Finding the brightest objects on your plate

By accessing the SDSS database through an SQL query you can obtain a table with information about the 20 brightest objects on your plate.

1. Go onto the SDSS SQL search tool by clicking here.

2. Delete all the text in the box and copy and paste the text below, replacing XXXX with your 4-digit number plate. 

select top 20 spa.ra, spa.dec, spa.dered_r as magnitude, spa.class, spa.z as redshift, soa.xfocal as x, soa.yfocal as y 
from specphotoAll as spa
join specObjAll as soa on soa.specObjID = spa.specObjID
where spa.plate = XXXX and spa.dered_r > 0
order by spa.dered_r asc

3. Tick the “CSV” option for Output Format. 

4. Click Submit. 

5. Your browser should automatically download a file with all 20 brightest objects. You can get data for more of your plate’s objects, ordered by apparent brightness, by increasing the number “20” in the first line (up to “1000”, for all of the objects on the plate).

IMPORTANT: The y and x values returned by the query are in mm.

Step 2: Plot the location of the Objects

Next, we will use a spreadsheet to plot the positions of the 20 brightest objects of your plate. The example used here is done in Google Sheets, which requires a free Goggle Account, but you can use MS Excel, or any other spreadsheet software available to you.

1. Create a new Sheet, and import the .csv file that you downloaded in step 5 above using File -> Import. You should see your data neatly arranged in a table.

2. Select the last two columns (x and y) and create a “Scatter Plot” by doing Insert->Chart and choosing “Scatter Chart”. x and y are the coordinates in mm of the positions of each object on the plate, with the origin being located at the centre of the plate.

3. You can remove the grid lines and axes ticks by double clicking on the graph to open the Graph Editor and unticking the box “Major gridlines” for both the x- and y-axis. You should get a plot like the one shown below.

4. Resize the chart to make the plot area square

5. You can save the chart as a picture to use in other activities, e.g. asking pupils to design a constellation.

Step 4: From 2D to 3D.

To turn a constellation from a 2D pattern on the plate to a 3D map, you can use estimate the distance to each object. We can use the “redshift” column to estimate redshift using the Hubble Law. At these redshifts, the Hubble Law is a poor approximation for the real distance, and a better distance estimate can be found using the redshift-distance rulers supplied in the OneDrive folder. Using redshift to get a distance will work for objects outside of our own galaxy only (objects classed as GALAXY or QSO), that are moving away from us due to the expansion of the Universe.

We can’t estimate the distance to the stars with the data provided. However, their distance will be so small compared to the distance to a galaxy or a QSO that, on this 3D map, they can be thought of having a distance of zero.