Spiral galaxy NGC 4921 presently is estimated to be 320 million light years distant. This image, taken by the Hubble Space Telescope, is being used to identify key stellar distance markers known as Cepheid variable stars.

The magnificent spiral NGC 4921 has been informally dubbed anemic because of its low rate of star formation and low surface brightness.

Visible in the image are, from the center, a bright nucleus, a bright central bar, a prominent ring of dark dust, blue clusters of recently formed stars, several smaller companion galaxies, unrelated galaxies in the far distant universe, and unrelated stars in our Milky Way Galaxy. Image Credit: NASA, ESA, K. Cook (LLNL)

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)

Researchers do not yet know what is lighting up IRAS 05437+2502, a small, faint nebula that spans only 1/18th of a full moon toward the constellation of the Taurus. Particularly enigmatic is the bright upside-down V that defines the upper edge of this floating mountain of interstellar dust.

This ghost-like nebula involves a small star-forming region filled with dark dust that was first noted in images taken by the IRAS satellite in infrared light in 1983. This recently released image from the Hubble Space Telescope shows many new details, but has not uncovered a clear cause of the bright sharp arc. Image Credit: NASA, ESA, Hubble, R. Sahai (JPL)