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Add a Trendline to the Diagram

Can you really draw a straight line through your data? When scientists try to figure out what data mean, they often talking about making a "model": in this case, the model you are using to relate magnitude to redshift is a straight line. Scientists often speak of the "fit" - how well the model fits the data. The fit can be described with a percentage that shows how close the points lie to the place where they should lie if the model were true. Because every experiment has some error and every observation has some uncertainty, the fit is never 100% accurate. Generally, astronomers consider a fit above 90% to show that the model probably really does describe the data.

Optional Exercise 1: Find the fit of a straight line model in your Hubble diagram. Excel (or another graphing program) can find the fit automatically using a "trendline." The program tries to find a straight line that passes as close to all the data points as possible, then measures how far each point falls from this straight line.

See SkyServer's Graphing and Analyzing Data tutorial to learn how to make a trendline in Excel.

Multiply the R-squared value by 100 to find the fit as a percentage. What is this number? Is a straight line a good fit to your data?