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Starts With A Bang

Mostly Mute Monday: Top 10 Bizarre Galaxy Pairs From Hubble

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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