Alnitak and friends
Alnitak (Zeta Orionis), the bright star in the picture, which is the leftmost star in Orion's belt, is a multiple star.
The primary star is a hot blue supergiant with an absolute magnitude of -6.0 and is the brightest class O star in the night sky with a visual magnitude of +2.0. It has two bluish 4th magnitude companions.
The Horsehead Nebula (top right of pic) is a dark nebula. The nebula is part of the much larger Orion Molecular Cloud Complex. It is approximately 1500 light years from Earth.
The red or pinkish glow originates from hydrogen gas predominantly behind the nebula, ionized by the nearby bright star Sigma Orionis. Magnetic fields channel the gases leaving the nebula into streams, showing as streaks in the background glow. A glowing strip of hydrogen gas marks the edge of the massive cloud, and the densities of nearby stars are noticeably different on either side.
The heavy concentrations of dust in the Horsehead Nebula region and neighbouring Orion Nebula are localized, resulting in alternating sections of nearly complete opacity and transparency. The darkness of the Horsehead is caused mostly by thick dust blocking the light of stars behind it.
The Flame Nebula (NGC 2024, just below Alnitak in the pic), is an emission nebula. It is about 900 to 1,500 light-years away.
Alnitak shines energetic ultraviolet light into the Flame and this knocks electrons away from the great clouds of hydrogen gas that reside there. Much of the glow results when the electrons and ionized hydrogen recombine. Additional dark gas and dust lies in front of the bright part of the nebula and this is what causes the dark network that appears in the center of the glowing gas.
At the center of the Flame Nebula is a cluster of newly formed stars, an estimated population of 800 stars.
Several of the other stars in the pic show reflection nebulae where starlight is reflected off inert dust and gas, illuminating it.