Colors of Objects

A pale gray galaxy with a bright blue galaxy nearby

The galaxy UGC 05205 with a
bright blue companion galaxy

The Sloan Digital Sky Survey camera looks at millions of objects. The camera measures how bright they are in five different colors of light. To see these different colors, the camera looks through five different colored filters. The first filter is u, or ultraviolet. Ultraviolet light is the invisible light that causes sunburns. The next two filters are green and red, called g and r, which are colors you see every day. The last two filters are “infrared light” filters called i and z. You can’t see infrared light, but you can feel it by putting your hand over a burner on a stove or sitting in front of a fireplace.

The brightness of an object is called its magnitude. The higher the magnitude, the fainter the object! That scale might seem backwards to you, but astronomers have used it for thousands of years.

You can use SkyServer to look up the magnitudes of any of the 14 million objects the SDSS has seen. Click the Launch Navigate button to open the SkyServer Navigate Tool. The tool will open in a new window. You will see a screen like this:


The boxes ra and dec in the top left of the tool give a location in the sky that you want to see. The Search button goes to that location.

You can zoom in or out in the image with the zoom bar below Get Image. Click the plus sign to zoom in or the minus sign to zoom out. You can also move around in the sky by clicking the NWSE buttons around the image.


When you click on any star or galaxy you see in the image, a green square will come up around it. A close-up of the object will appear on the right, and the object’s data will appear above the close-up. Look closely at the data box at the top of the Navigation tool window. The data box should look like this (probably with different numbers):

The ra and dec values tell you where in the sky this object is. The type tells you what the SDSS computers think the object is (and the computers aren’t always right). u, g, r, i, and z tell you the object’s magnitudes through each of the SDSS’s five filters – ultraviolet (u), green (g), red (r), and infrared (i and z).


The star shown to the left is the same star whose data are displayed in the box with green text just above. The star looks blue. Its color can be measured scientifically by looking at differences in magnitudes between filters. For example, one way to measure the color is to look at the difference between the star’s u and g values. This star has a u-g color of 19.14 – 18.05 = 1.09. This number means the star gives off more green light than ultraviolet light (remember that the magnitude scale runs backwards). You could also measure the star’s g-r color, which would be 18.05 – 18.29 = -0.24. The star gives off more green light than red light.