The Strange Double Nucleus of the Cocoon Galaxy

James Maynard
Feb 7 · 3 min read

A galactic collision resulted in the creation of a strange pair of galaxies, and the larger one holds a second hidden core, a new finding reveals.

Roughly 30 million light years from Earth, the Cocoon Galaxy (also known as NGC 4490) is home to two galactic cores, a new study shows. Optical observations clearly show one core which has long been known to astronomers. However, a second core was recently found by astronomers using radio telescopes, hiding in clouds of gas and dust.

This pair of galaxies are the products of an ancient collision between a spiral galaxy and a barred spiral galaxy (the shape of the present-day Milky Way, but much smaller). The pair, drawn together by gravity, collided, triggering a period of active star building in each cluster of stars.

A nut-shaped collection of star with bands of red.
A nut-shaped collection of star with bands of red.
The galaxy NGC 4490, seen by the Hubble Space Telescope. Red areas are regions of active star formation. Image credit: ESA/Hubble/NASA

“One nucleus is visible in the optical, while the other is only visible at infrared and radio wavelengths. We find the optical nucleus and the potential infrared visible nucleus have similar sizes, masses, and luminosities. Both are comparable in mass and luminosity to other nuclei found in interacting galaxy pairs and much more massive and luminous compared with typical non-nuclear star-forming complexes, researchers wrote in Astrophysical Journal.

This event radically altered the shape of NGC 4490 (the larger of the pair of galaxies), while leaving the smaller of the two with a semblance of its former spiral shape. This galactic pair is small — together possessing a total of just 20 percent as much mass as the Milky Way.


An Out-of-this-World Hobby

Three men around a laptop computer with an image of a galaxy on their screen.
Three men around a laptop computer with an image of a galaxy on their screen.
Iowa State astronomers — Left to right, Charles Kerton, Curtis Struck and Allen Lawrence — have revealed a rare double-nucleus structure in a nearby and well-known galaxy. Image credit: Christopher Gannon/Iowa State University

In 2011, a former electronic engineer from Green Bay, Wisconsin, Allen Lawrence, enrolled in astronomy classes at the University of Wisconsin-Madison.

The same 30-year-long passion for astronomy that led Lawrence to carry his 20-inch telescope to star parties around his home city soon drove him to ask around his school for research opportunities. He was offered a chance to study one of two galactic targets, and the enthusiastic student selected NGC 4490 and its diminutive partner, NGC 4485.

After examining infrared images of the larger galaxy taken by NASA’s Spitzer Space Telescope, Lawrence saw what he thought was an unusual site — a double nucleus in the galaxy. These pairings are rare, particularly in small galaxies.

The galaxy NGC 4490 seen by the Hubble Space telescope. Image credit: Hubble/ESA | .gif by The Cosmic Companion.

Each of the galactic cores have similar sizes, masses, and brightness to such structures seen in other pairs of galaxies.

“I saw the double nucleus about seven years ago. It had never been observed — or nobody had ever done anything with it before,” Lawrence said.

Astronomers using optical telescopes have long seen one core of NGC 4490, and researchers using radio instruments have likely previously spotted the other, although no one had ever realized both cores existed within the galaxy.

Using telescopes studying targets over a wide range of wavelengths, called multi-messenger astronomy, can reveal information about astronomical bodies that would remain hidden to astronomers using a single telescope.

A panoramic view of NGC 4490. Video credit: ESA Hubble

Analysis of the data suggests that the Cocoon Galaxy may be the largest remnant of this ancient collision between families of stars. The formation of NGC 4490 in such a galactic encounter could explain a massive plume of hydrogen gas enveloping the galaxy.

The galaxies have already experienced their closest approach, and are currently 24,00 light years from each other, racing in opposite directions.


James Maynard is the founder and publisher of The Cosmic Companion. He is a New England native turned desert rat in Tucson, where he lives with his lovely wife, Nicole, and Max the Cat.

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The Cosmic Companion

Exploring the wonders of the Cosmos, one mystery at a time

James Maynard

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Writing about space since I was 10, still not Carl Sagan. Mailing List/Podcast: https://thecosmiccompanion.substack.com

The Cosmic Companion

Exploring the wonders of the Cosmos, one mystery at a time

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