Fourth Moon Discovered Around Pluto

NASA announced the discovery of a fourth moon around Pluto last week! Astronomers were using the Hubble Space Telescope to look for rings around the dwarf planet when they discovered the new moon. It's currently designated P4 until they come up with a better name for it. The Hubble image above shows Pluto and its four moons. The big black bar down the center is used to block most of the light Pluto reflects so faint objects around it, like the moons, can be seen. P4 is only 8-21 miles wide, whereas Charon is 648 miles wide and Nix and Hydra are more like 20-70 miles wide. Pluto is a member of the Kuiper belt, along with 70,000+ other objects, so it's not surprising that Pluto has another moon that went undetected for years. It's difficult to see such small objects in space, even with Hubble! NASA's new horizon mission is currently on its way to Pluto to study the planet and its moons. It will be another ~5 years before New Horizons makes it there, but once it has arrived it will no doubt send back unprecedented information about Pluto and other outer solar system objects

Image Credit: NASA, ESA, and M. Showalter (SETI institute)

Doppler Shift

Astronomers use a phenomenon called the Doppler shift to measure how fast objects are moving towards or away from us in outer space. But how does the Doppler shift work? Here's the common example. You pull over to the side of the road because you hear an approaching ambulance. As the ambulance gets closer, the siren sounds like it's being emitted at a higher pitch. This happens because the sound waves are compressed in the air as the ambulance approaches, making their wavelength shorter and pitch higher. The opposite occurs after the ambulance passes; the sound waves stretch and increase in wavelength and the siren sounds lower pitched. 

Light waves experience this same phenomenon of squeezing and stretching as an object moves. Relative to the Earth, objects that are moving away from us have their light waves stretched to longer wavelengths, making the object appear more red. If an object is moving towards earth, it's light waves are compressed to smaller wavelengths making the object appear more blue. We can measure how Doppler shifted an object is by observing spectral lines emitted or absorbed by different elements in the moving object. In the image above, some absorption lines were red shifted, meaning this object is moving away from us.  The larger this change in wavelength (called "red shift" or "blue shift"), the faster the object is moving. Some objects are moving so or are so far away that their lines are shifted out of the visible spectrum and into other portions of the spectrum not normally seen! Astronomers have viewed many ancient galaxies whose light appears today in the radio, but was originally emitted in the visible spectrum.

Image Credit: Wikipedia commons