The Heart Nebula is a region of ionized gas in the constellation of Cassiopeia. It's one of the larger nebulae in the sky, and you could fit four full moons across this frame with room to spare. We can't see it with our naked eye because it's too dim for our limited night vision to see without a telescope - not because it's small!

The gas cloud is being ionized by the cluster of hot, young stars that recently formed in the center of the nebula, in that little knot of red and orange surrounded by blue. The radiation from these massive stars (some of which are 50 times the mass of our Sun) is enough to excite the gas and make it glow in its own right. This is why it's called an 'emission nebula' - the gas emits its own light.

The Heart nebula lies about 7,500 light years away, which is relatively close in cosmic terms. Human civilization was in the Bronze Age and the pyramids were 2,000 years from being built when the light in this image left the nebula on its way to us.