Why Does This Galaxy Have Such an Odd Shape?

NGC 7714, Markarian 538, Image credit: NASA / ESA / A. Gal-Yam, Weizmann Institute of Science.

This Galaxy, NGC 7714, has an odd shape.  In fact we call it a ‘Peculiar Galaxy.’  Why doesn’t it have the characteristic spiral arms if it is indeed a spiral? Why doesn’t it look more diffuse and football shaped like an elliptical galaxy?

The reason is that like millions of other galaxies in the Universe, it has recently collided with a nearby companion galaxy.

Now using the term ‘collided’ is not really accurate.  In reality the two galaxies are interacting via gravity.  During a ‘collision,’ stars in the interacting galaxies don’t physically hit each other.  The galaxies are incredibly large, but the stars that make them up are incredibly small, and so it is very uncommon for any stars to actually touch each other during the event.

What does happen is that the stars interact gravitationally, completely destabilizing the shape of the galaxy.  There can be a physical interaction with clouds of gas and dust in the galaxies, if they interact head-on.  When this happens there is a rapid amount of star formation on a massive scale.  This is known as a starburst, and it can make the colliding galaxies substantially brighter in only a few million years.

For the above NGC 7714, the galaxy it interacted with isn’t even in the above image.  The interaction was purely gravitational, but the shape of the galaxy is still distorted, and the shifting gas and dust has triggered lots of star formation in the galaxy from its own molecular clouds.  The interaction happened between 100 and 200 million years ago, which is actually very recent astronomically speaking.

Kudos to you if you got it right before reading!

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