Hubble Heritage Archive: Hoag’s Object

The text and images in this article were originally published on September 5, 2002, and reflect information about Hoag’s Object available at that time.

HOAG’S OBJECT: A WHEEL WITHIN A WHEEL

A nearly perfect ring of hot, blue stars pinwheels about the yellow nucleus of an unusual galaxy known as Hoag’s Object. This image from NASA’s Hubble Space Telescope captures a face-on view of the galaxy’s ring of stars, revealing more detail than any existing photo of this object. The image may help astronomers unravel clues on how such strange objects form.

The entire galaxy is about 120,000 light-years wide, which is slightly larger than our Milky Way Galaxy. The blue ring, which is dominated by clusters of young, massive stars, contrasts sharply with the yellow nucleus of mostly older stars. What appears to be a “gap” separating the two stellar populations may actually contain some star clusters that are almost too faint to see. Curiously, an object that bears an uncanny resemblance to Hoag’s Object can be seen in the gap at the one o’clock position. The object is probably a background ring galaxy.

Ring-shaped galaxies can form in several different ways. One possible scenario is through a collision with another galaxy. Sometimes the second galaxy speeds through the first, leaving a “splash” of star formation. But in Hoag’s Object there is no sign of the second galaxy, which leads to the suspicion that the blue ring of stars may be the shredded remains of a galaxy that passed nearby. Some astronomers estimate that the encounter occurred about 2 to 3 billion years ago.

This unusual galaxy was discovered in 1950 by astronomer Art Hoag. Hoag thought the smoke-ring-like object resembled a planetary nebula, the glowing remains of a Sun-like star. But he quickly discounted that possibility, suggesting that the mysterious object was most likely a galaxy. Observations in the 1970s confirmed this prediction, though many of the details of Hoag’s galaxy remain a mystery.

The galaxy is 600 million light-years away in the constellation Serpens. The Wide Field and Planetary Camera 2 took this image on July 9, 2001.

Featured Image Credit: NASA and the Hubble Heritage Team

Fast Facts about Hoag’s Object

About this Object

Object Name: Hoag’s Object 
Object Description: Ring Galaxy
Position (J2000): R.A. 15h 17m 17s
Dec. +21° 34′ 45″
Constellation: Serpens
Distance: 600 million light-years (180 million parsecs)
Dimensions: The ring has a diameter of roughly 75,000 light-years (23,000 parsecs) to the inner edge and 120,000 light-years (37,000 parsecs) to the outer edge.

About the Data

Instrument: WFPC2
Exposure Date: July 9, 2001
Exposure Time: 3.6 hours
Filters: F450W (B), F606W (V), F814W (I)
Principal Astronomers: Hubble Heritage Team: K. Noll, H. Bond, T. Borders, C. Christian, L. Frattare, F. Hamilton, J. Lee, Z. Levay, P. Royle (STScI)
R. Lucas (STScI) 

About this Image

Image Credit: NASA and The Hubble Heritage Team (STScI/AURA)
Release Date: September 5 , 2002

Origin Story on Hoag’s Object

Hoag’s Object, An article by Ray Lucas (STScI)

Additional Images:

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