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Another way to overcome the problem of intrinsic differences between galaxy magnitudes is to look at a huge
sample of galaxies. With a large enough sample, intrinsic differences of galaxy brightness should average out,
meaning that the trend of the Hubble Diagram should truly reflect the expansion of the universe.
Go to the Search Form tool (link opens in a
new window). From the public home page, go to SkyServer Tools, then Search, then Search Form.
Click Launch the Search Form tool. (Note: this Search Form is different from the Imaging Query Form;
the Search Form is designed for public and student use.) The principle of the Search Form tool is that you choose from menus and put values in textboxes
to write a sentence in English. Then you click a button to generate an SQL query that represents your
English sentence. Make the selections on the Search Form tool that are appropriate for making a Hubble diagram,
forming this sentence (the bold items are the selections you make from the menus):
Show me galaxies in the region anywhere with magnitudes (unspecified) and
colors (unspecified) for only objects with spectra, with redshifts (unspecified). Please
return 1,000 objects with object IDs, RA and Dec, magnitudes, and redshift. |
Click Generate Query to translate this sentence into SQL. The SQL statement that the tool generates
looks like this:
| select top 1000 p.objid, p.ra, p.dec, p.u, p.g, p.r, p.i, p.z, s.z as redshift |
| from galaxy p |
| join specObj s on s.bestObjID = p.objID |
Change the Output Format to CSV (comma-separated value). Click Submit Query to SkyServer. A new window will open where you can save your
query results to your computer. Make a graph of redshift against magnitude. You will see a graph with 1,000 points on it.
The graph will form a wide swath of points. Galaxies of a given luminosity lie on a straight line within
this swath.
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