The Horsehead nebula, a dark nebula close to Orion's belt strikes an imposing figure against the background nebulosity.
It is part of a much larger region of dust and gas at the bottom of the image and is illuminated by the bright gas behind it, casting a shadow to its bottom left on the dust in front of it.
The red light comes from the Hα emission of hydrogen caused by a particular electron transition. The energy for this is the star at the top of frame Sigma Orionis which is a hot blue star releasing UV photons which ionise the hydrogen gas around it, causing it to emit light. The red line seen is the very edge of this cloud.
Though not apparent in this image the gas is channeled into streams and filaments by the magnetic field of Sigma Orionis.
Also visible to the bottom left of the Horsehead are NGC 2023 and the Flame Nebula. NGC 2023 is one of the largest reflection nebulae discovered at around 4 light years across, illuminated by the reflection of light from bright stars at its centre. On the other hand the flame nebula is an emission nebula, this time its ionisation is caused by Alnitak the brightest star in the frame and part of Orion's Belt
This image is a combination of 19 60 second exposures using a super fast celestron RASA telescope and a OSC camera