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Starts With A Bang

Mostly Mute Monday: Top 10 Bizarre Galaxy Pairs From Hubble

Image credit: Arp 230; Credit: ESA/Hubble & NASA; Acknowledgement: Flickr user Det58.

Spirals and ellipticals rule the Universe, but every so often, something far more intricate shows up.


“For me, the study of these laws is inseparable from a love of Nature in all its manifestations.” –Murray Gell-Mann

Galaxies normally come in one of two types: spirals or ellipticals. But sometimes, they exhibit structures far rarer and more complex.

Arp 87. Image credit: NASA, ESA, Hubble Space Telescope; Processing: Douglas Gardner.

From 1961–1966, Halton Arp gathered views of 338 pairs of unusual, interacting galaxies, creating the Atlas of Peculiar Galaxies.

Arp 274, a trio of star-forming galaxies. Image credit: NASA, ESA, M. Livio and the Hubble Heritage Team (STScI/AURA).

When spirals interact, intense star formation follows, with the arms often becoming distended.

Arp 148. Image credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University).

Occasionally, collisions result in a ringed structure.

Arp 116, dominated by the giant elliptical Messier 60. Image credit: NASA/ESA Hubble Space Telescope.

Some only interact slightly, leaving the larger structures unperturbed.

Arp 273, with both galaxies clearly affected by the interaction. Image credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA).

Others show a spiral appearing to unwind, as tidal forces from a secondary galaxy pull material away from the galaxy’s core.

Arp 194, with the interstellar “star bridge” connecting the two galaxies. Image credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA).

Sometimes a “bridge” of blue stars forms, as one galaxy rips material off of another, with the intergalactic gas collapsing to create starbursts.

Arp 284. Image credit: ESA, NASA; Acknowledgement: A. Gal-Yam (Weizmann Institute of Science).

The Arp catalog illustrates galaxies in many different stages of a collision:

  • prior to their first close pass,
  • in the collision process,
  • subsequent to an interaction but before merging,
  • and in the final merger stages.
Arp 240. Image credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University).

Unlike ellipticals, spirals are easily disturbed, often becoming destroyed entirely by such an interaction.

Arp 142. Image credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA).

Galaxies as distant as a million light years can be disturbed by a larger neighbor.

A fuller-frame image of Arp 142; notice the smaller blue galaxy in the process of being destroyed atop the frame. Image credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA).

All these will eventually settle into lone ellipticals, a process taking billions of years.


Mostly Mute Monday tells the story of a single astronomical phenomenon or object in visuals, images, video and no more than 200 words.

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