The Flame and Horsehead Nebulae lie about 1400 light years away from Earth and form part of the Orion Molecular Cloud Complex, which is one of the closest and most active regions of stellar formation in the night sky.

The Flame Nebula is an emission nebula with a cluster of roughly 800 newly formed stars at its centre. The innermost stars are approximately 200,000 years old whilst stars towards its edge are around 1.5 million years old. Much of the glow of the Flame Nebula is caused by the binary star Alnitak, the easternmost star of Orion’s Belt, seen here as the very bright star above the Flame.

Alnitak (also known as Zeta Orionis) is a complex star system. Alnitak A is a close binary star, consisting of Alnitak Aa and Alnitak Ab. Alnitak Aa is 20 times larger, 33 times more massive and 20,000 times brighter than the Sun, emitting energetic ultraviolet light that causes the hydrogen gas nearby to become ionised. It is when the electrons recombine with the ionised hydrogen that the glow occurs. Altinak B is a much smaller star and completes one orbit around Alnitak A every 1500 years. It is far too close to Alnitak A to be resolved in this image. The nearby star on the left is Alnitak C, which is much dimmer and is probably not gravitationally connected to the other stars, happening to be on a similar line of sight. The angular separation between Alnitak A and Alnitak C is almost 1 arcminute.

The Horsehead Nebula is an absorption nebula with a heavy concentration of cold dark dense clouds of gas and dust that absorb light from the bright background nebula and result in almost complete opacity, whereas the neighbouring cloud concentration is much lighter, resulting in almost complete transparency. The Horsehead Nebula is only visible because of the silhouette it makes against the brighter nebula behind it with the pinkish red glow being predominately the result of the ionisation of hydrogen gas from the nearby star Sigma Orionis, the complex star system visible above the Horsehead.