science & techESA/Hubble

Galaxies that look like close neighbours can be hundreds of millions of light-years apart

the gist

Hubble's picture of the week shows a group of spiral galaxies that look like neighbours but are not. The large spiral NGC 1356 is about 550 million light-years away, while the smaller LEDA 95415 is roughly 840 million light-years away, so they are around 300 million light-years apart despite appearing to touch. Meanwhile IC 1947, which looks remote on the left, is only about 500 million light-years away and is much closer to NGC 1356 in three-dimensional space. A flat image shows only angular separation across the sky, not distance from Earth. Astronomers need distance measurements to place galaxies in three dimensions.

so what

It is a useful reminder that astronomical images are projections, and that apparent closeness can be misleading in the sky.

fun fact

A light-year is about 9.46 trillion kilometres.

ESA/Hubble · CC BY 4.0

Image: ESA/Hubble & NASA, J. Dalcanton, Dark Energy Survey/DOE/FNAL/NOIRLab/NSF/AURA Acknowledgement: L. Shatz