What it is
The Heart Nebula, catalogued as IC 1805, is a vast cloud of glowing gas in the constellation Cassiopeia. Its shape earned the name, and the bright knot near the top right is its own small nebula, NGC 896. Just beside it in the sky sits the Soul Nebula, its well-known neighbour.
How far away
About 7,500 light-years from Earth. The light in this photo left the nebula long before the first cities were built. The cloud spans roughly 200 light-years, and it covers more of the sky than several full Moons side by side.
What it's made of
Mostly hydrogen, with traces of sulfur and oxygen. At its centre is Melotte 15, a young cluster of hot, massive stars. Their radiation makes the surrounding gas glow, and their stellar winds carve the dark pillars and hollows you can see in the cloud.
The colours here use the Hubble palette, which maps the light of sulfur, hydrogen and oxygen to red, green and blue. They show what the gas is made of, not what your eye would see.