

NGC 3982: A Step in the Cosmic Ladder
The beautiful spiral arms of NGC 3982 are easy to trace in this colorful cosmic portrait. The arms are lined with pink star forming regions, young blue star clusters, and obscuring dust lanes. NGC3982 isn’t just a pretty face, though.

Astronomers are particularly interested in NGC 3982 as a step on the extragalactic distance ladder. Astronomical distances are hard to measure. Astronomers have a variety of different tools for measuring distances each of which is a rung on the extra galactic distance ladder. NGC 3982 is a particularly useful galaxy for measuring distances because it contains two of these tools: a stellar explosion called a type 1a supernova and Cepheids variable stars.
SN 1998aq (left) found in NGC 3982. Comparison of Hubble NGC 3982 image (right).
Ground image with SN courtesy of H. Dahle
In 1998, the light from a supernova in NGC 3982 (named "SN 1998aq") reached Earth. This supernova event, know as a type 1a, resulted from a binary system where a white dwarf star was taking mass from its companion star. When the white dwarf had collected enough mass and was no longer able to support itself, the star detonated in a violent and extremely bright explosion.

Cepheids are stars that vary in brightness and they can be used to determine the distance of a galaxy by following their periodic changes. Astronomers use Cepheids to find the distance to type 1a supernovae then use type 1a supernova to find the distance to even more distant objects. Astronomers have used these distance markers to determine that the universe is now accelerating in its expansion and contains mysterious “dark energy.”