Magnitudes and Distances

One of the easiest ways to compare galaxies is to compare their magnitudes. Magnitude is a measure of how bright a star or galaxy looks to us – how much light from that star or galaxy reaches Earth. In magnitude, higher numbers correspond to fainter objects, lower numbers to brighter objects; the very brightest objects have negative magnitudes.

The scale is set up so that if object A is 2.51 times fainter than object B, then object A’s magnitude will be higher by one number. For example, a magnitude five galaxy is 2.51 times fainter than a magnitude four star. The sun has magnitude -26. The brightest star in the Northern sky, Sirius, has magnitude -1.5. The brightest galaxy is the Andromeda Galaxy, which has magnitude 3.5.


The faintest object you can see with your eyes has a magnitude of about 6. The faintest object the SDSS telescope can see has a magnitude of about 23. SDSS measures magnitudes in five wavelengths of light: ultraviolet (u), green (g), red (r), near infrared (i), and infrared (z).

The image to the right shows the difference in brightness between a magnitude 16 galaxy and a magnitude 19 galaxy in the SDSS’s green (g) wavelength. The magnitude 16 galaxy is (2.51 x 2.51 x 2.51 =) 15.8 times brighter than the magnitude 19 galaxy.

Question 2: Why can magnitudes be used as a substitute for distances?
Explore 4. In this exercise, you will find the magnitudes of six galaxies in SkyServer’s database. The table below shows the object IDs and positions (right ascension and declination) of the six galaxies. The Object ID is SkyServer’s way of identifying stars or galaxies. Right ascension and declination are a way of measuring an object’s position in the sky, similar to latitude and longitude on Earth.

To find a galaxy’s information, click on its object ID in the table below. A SkyServer tool called the Object Explorer will open in a new window, displaying the galaxy’s data.

Use this SkyServer Excel workbook to keep track of your data.

Object ID





















Look at the galaxy’s close-up picture in the main frame of the Object Explorer. Just to the right of the picture, you will see a data table containing values for u, g, r, i, and z (the data are under the labels). These are the magnitudes of the galaxy.

Save each galaxy in your online notebook by scrolling to the bottom of the left-hand frame and clicking “Save in Notes.” Write down each galaxy’s magnitude in the wavelength of green light, which is given by “g.”

Launch Explore

So now you have a way of estimating how far away a galaxy is (at least relative to other galaxies). But how do you find the other variable for your graph, the speed at which galaxies are moving? You need to use a measurement called redshift. Click Next to find out what redshift is, and how to measure it.