Diffuse nebulae are large interstellar clouds of gas and dust. If they have enough mass, the cloud can collapse together because of their gravity and stars will be born. If these stars are hot enough, they send out ultraviolet radiation, that let the clouds glow. After a few million years the gas and dust inside the nebula is used for the forming of new stars and maybe planets. The gasses that aren’t used yet, will be blown away by solar winds.
A diffuse nebula can become visible because the gas and dust reflect the light of nearby stars. We call these nebulae “reflection nebulae”. Reflection nebulae are mostly blue. An example of a reflection nebula is the Merope Nebula (see image below) that you can find around the Plejades.
A diffuse nebula can also send out light on it’s own. We call these nebulae “emission nebulae”. An emission nebula is very hot, because young stars inside the nebula heats the gas and dust around it. The nebula sends out this energy trough radiation. Emission nebulae can have different kind of colours, but the most common colour is red, because emission nebulae often contain hydrogen. The Trifid Nebula and the North-America Nebula are examples of emission nebulae.
If no light gets trough the gas and dust inside the nebula, and if it doesn’t reflect any light, we call the nebula an “absorption nebula” or “dark nebula”. These nebulae can be visible trough our telescopes tough, because the stars in the night sky shines on the dark nebula. This light doesn’t get trough the nebula, but shines around it. The Horsehead Nebula and the Cole Sack Nebula are examples of dark nebulae.
Some nebulae to observe for yourself
One of the most familiar nebulae is the Orion Nebula (see image on top) in constellation Orion. The Orion Nebula (M42) is an emission nebula and is located at 1.600 licht-years from earth. It is about 5 to 6 licht-years in size. The stars in this nebula are still young and hot and they have a gas- and dust-disk around them. Perhaps in the future planet-systems will arise from this disk.
The Orion Nebula has a magnitude of 4.0 and is already visible with the naked eye on bright, dark nights. There is a greenish colour visible with a binocular of small telescope (6-8cm).
There are a few reflection nebulae in the Plejades in constellation Taurus. The most familiar one is NGC 1435, better known as the Merope Nebula. You can find this nebula around the star Merope. The Plejades aren’t born from clouds of dust and gas, but they are just passing by. Within a few thousand years the nebula will leave Merope, if it survives the close encounter.
You can see the Plejades, with a magnitude of 1.6, already with the unaided eye. The Merope Nebula itself gets visible with at least 10cm telescopes.
The Horsehead Nebula
Barnard 33, better known as the Horsehead Nebula, is a diffuse nebula. It blocks the light of the emission nebula IC 343 behind it. But a few stars are bright enough to shine trough the nebula.
The Horsehead Nebula is located at a distance of 1.600 light-years and has a size of 3 light-years.
You can find the nebula in constellation Orion. With a magnitude of 8.4 it is only visible in dark nights, with a large telescope and no light pollution.
The Lagoon Nebula
The Lagoon Nebula (M8) is an emission nebula and it’s located in constellation Sagittarius. In the middle of this nebula is an open cluster: NGC 6530. The nebula itself is discovered in 1747, almost 70 years after the open cluster. Because the stars are heating the gas inside the nebula, it starts to glow. The Lagoon Nebula is 5.200 light-years away from the earth end has a size of 150 light-years.
This emission nebula has a magnitude of 6.0. If you observe in southern countries (Holland and Belgium are located to much north) you can see this nebula with the naked eye as a fuzzy spot. With a binocular or small telescope there is a dark stripe visible inside the nebula. The combination of open cluster and nebula makes the Lagoon Nebula a beautiful appearance.