Is this one galaxy or two? Astronomer Art Hoag first asked this question when he chanced upon this unusual extragalactic object. On the outside is a ring dominated by bright blue stars, while near the center lies a ball of much redder stars that are likely much older. Between the two is a gap that appears almost completely dark.

How Hoag’s Object formed remains unknown, although similar objects have been identified and collectively labeled as a form of ring galaxy. Genesis hypotheses include a galaxy collision billions of years ago and the gravitational effect of a central bar that has since vanished.

This image, taken by the Hubble Space Telescope in July 2001, reveals unprecedented details of Hoag’s Object and may yield a better understanding. Hoag’s Object spans about 100,000 light years and lies about 600 million light years away toward the constellation of the Snake (Serpens). Coincidentally, visible in the gap (at about one o’clock) is yet another ring galaxy that likely lies far in the distance. Image Credit: NASA, R. Lucas (STScI/AURA)

NASA’s Kepler spacecraft has discovered the first confirmed planetary system with more than one planet crossing in front of, or transiting, the same star.

The picturesque, but snow-capped volcanoes, composing the Islands of the Four Mountains in Alaska’s Aleutian Island chain look suspiciously like an alien world in this August 2010 image from the ASTER camera aboard NASA’s orbiting Terra satellite.

The islands contain restless Mt. Cleveland, an active volcano currently being watched to see if it emits an ash cloud that could affect air travel over parts of North America. A close look at Mt. Cleveland, seen near the image center, shows red vegetation (false color), a white snow-covered peak, a light plume of gas and ash, and dark lanes where ash and debris fell or flowed.

Millions of volcanoes have likely been active over the turbulent history of the Earth’s surface, while about 20 volcanoes are erupting even today, at any given time. Image Credit: NASA