Date: June 24th, 2012

Title: Encore: It’s a Smash Up – Interacting Galaxies

Podcaster: Brian Gray

Organization: Wilderness Center Astronomy Club:

This podcast originally aired on April 23, 2009

Description: The Universe has grand spirals and unassuming ellipticals. But the universe also has train wrecks that attract the gaze of many astronomers. Distorted shapes and vigorous star formation illustrate the power of tidal forces.

Bio: I have been interested in astronomy for my entire life, and I enjoy showing the sky to others. I am currently living in Massillon, Ohio, and I belong to a wonderful astronomy club.

Today’s Sponsor: This episode of “365 days of Astronomy” is sponsored by – Expanding your horizons in astronomy today. The premier on-demand telescope network, at dark sky sites in Spain, New Mexico and Siding Spring, Australia.


Hello, my name is Brian Gray, and I am a member of the Wilderness Center Astronomy Club located in Wilmot, Ohio. Our web site is . We have a public star watch on the first Friday of every month at 8:00 PM with a planetarium show and, when weather permits, observing.

On one level the Universe is filled with apparent serenity and majesty. Orderly spiral galaxies and unassuming ellipticals adorn the inky blackness of the Universe. On the other hand there are also collisions, disruptions, and chaos. Galaxies squeezed, twisted, and sometimes consumed by their neighbors. The Mice, the Antennae, the Grasshopper, the Siamese Twins, the Tadpole, and the Whale are a small sample of what happens when galaxies interact. Professional astronomers look at interacting galaxies to determine the ways that gravity can affect how galaxies evolve while the amateurs look to marvel at the beauty of distorted spiral arms and enhanced star formations.

The gravity that seeded the formation of galaxies in the first place is also the agent that can alter them. As one galaxy approaches another, material in the second galaxy that is closest to the intruder will feel a greater force than the material on the other side. This gravity gradient induces distortions in the affected galaxy. A tidal bridge reaching out to the other galaxy may form on the near side while a tidal tail streams off in the opposite direction. If both galaxies have loosely bound disks, the above scenario could happen to both at the same time.

If one galaxy is smaller than the other, it may lose a large portion of its contents to the larger while causing various degrees of distortions to the relative giant. Depending on the sizes, masses, velocities, duration of the encounter, and friction, the interacting galaxies may pass each other once or they may engage multiple times and eventually merge to form a larger galaxy while some material is ejected into the surrounding areas.

The distances between stars are vast compared to their sizes, so when galaxies do overlap each other the number of direct collisions between stars is probably very small. But the gas and dust that provide the raw materials for the stellar nurseries do impact each other. Regions of material are compressed setting off periods of vigorous star formation. Other portions may be funneled toward the center where a supermassive black hole may lurk. As some of this material spirals into the central black hole, the increased activity causes the galactic core to become more lively. Some Active Galactic Nuclei such as those found in the centers of Seyfert galaxies, blazars, and quasars may be a result of interacting galaxies.

One notable example of interacting galaxies is the duo of M81 and M82 located in the constellation Ursa Major. M82, the Cigar Galaxy, is known as a starburst galaxy because of its recent eruption of star formation triggered by an encounter with M81. M51 and NGC5195 are another set of notable partners in the cosmic ballroom. The spiral arms of M51 have been affected by its smaller companion and a tidal bridge connects the pair.

In many large galaxy clusters, the central region is dominated by the larger ellipticals while the spirals patrol the outskirts. This dichotomy is probably a result of interactions and mergers occurring more frequently in the center where the concentration of galaxies is greater. Merged galaxies tend to exhaust their supply of gas quickly leaving a collection of older stars behind as the massive stars expire and little material is left to form new stars.

The merger process also works to randomize the orbits of the stars within the new systems and, therefore, creating elliptical galaxies. Meanwhile the galaxies in the outer regions suffer fewer disturbances and have a better chance of retaining their more orderly arrangements. For example, in the Virgo Cluster, the central region is dominated by ellipticals such the giant M87 while the spirals are more likely to be found in the suburbs.

Even in our own local neighborhood, interactions are occurring on an ongoing basis. The Milky Way is surrounded by several satellite galaxies that are being stripped of material as they orbit the center of our Galaxy. This material is the source of some of the Milky Way’s stellar streams that lace above and below the disk. Also in a few billion years we will be up close and personal with our own big neighbor, the Andromeda Galaxy.

Currently, the Andromeda Galaxy is about two and a half million light years away. But that distance is shrinking at a rate of about 120 kilometers per second. After one or more passes, the two large spiral galaxies will eventually consolidate into a larger elliptical galaxy. Depending on the exact details, our solar system will be a resident of this new metropolis or possibly released into the void.

Thank you for listening to the 365 Days of Astronomy.

End of podcast:

365 Days of Astronomy
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