Saturday, July 30, 2022

In astronomy, perspective can make weird jokes

In everyday life we are usually able to recognize the depth of the scene we are observing and determine which object is near and which is far. In astronomy, however, our eyes are not able to recognize the depth and we see all the celestial objects as if they were at the same distance projected on the celestial vault. This applies both if we are observing with the naked eye and if we are looking at an image taken by a telescope, such as this photo of Arp 194.

At first glance it would seem that what we are seeing is a group of interacting galaxies all located at the same distance plus some background galaxies which, being smaller and less luminous, can be inferred to be located further away.

Analyzing the system in detail, it appears that it consists of two or three interacting galaxies at the top that are connected by a tidal bridge of stars to the galaxy at the bottom. However, the data obtained by the Hubble Space Telescope has revealed that the galaxy below is not part of the system, but is actually located much further from Earth than those on the top. The trail of stars that seems to connect the two galaxies is not in fact a tidal bridge, but only a twisted spiral arm that by pure chance overlapped with the other galaxy, generating this magnificent optical illusion.

Credit: NASA, ESA, Hubble.

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