Nebulae: An overview
Pillars of Creation Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas, and plasma. The word nebula is derived from the Latin word for cloud. Originally the word nebula was used to describe any extended astronomical object including galaxies. The older use of the word survives in modern usage in some confusing ways. Various star clusters and galaxies are still referred to as nebulae. Strictly speaking, the word nebula should be reserved for clouds composed of gas and dust. There are several types of nebulae.
Omega Emission Nebula
The first type of nebula is called an emission nebula. These are clouds of ionized gas that emit various colors. The most common color expressed by an emission nebula is red due to the ionizing hydrogen. There are other colors as well, but none as abundant as red due to the great amount of hydrogen within these clouds. The most common cause for the ionization of the gas is a result of high-energy photons from a nearby hot star, as in the case of emission nebulae located in H II regions where there are new stars being born or dying stars which are in the process of expelling their outer layers exposing the hot core. The majority of emission nebulae are not caused by a single star as the star would have to be massive to ionize a significant part of the cloud. Instead, the ionization occurs due to an entire cluster of high temperature stars.
Witch Head Reflecting Nebula
The next type of nebulae is the reflecting nebula. These are similar to emission nebulae in the regard that they are both visible due to light. Within a reflecting nebula, there is not enough energy to ionize the gas to form an emission nebula. However, there is enough to scatter the light and make the dust within the cloud visible. Therefore, the reflecting nebula takes on the color characteristic of the star that the nebula is reflecting. There are around 500 known reflection nebulae. Many times, emission and reflecting nebulae occur within the same dust cloud where one part of the cloud is hot enough to ionized the gas, but there are sections where the temperature is lower and the light is reflected.
Horse Head Dark Nebula
The final type of nebulae is the dark nebula. A dark nebula is a nebula that is so dense that it obscures the light that attempts to pass through it from emission nebulae or background stars. The extinction of light is caused by the interstellar dust located at the coldest, densest places within the nebula. Large dark nebulae are associated with Giant Molecular Clouds, whereas smaller dark nebulae are called Bok globules. Within these nebulae, stellar events such as the formation of new stars occur as the gravitational forces act within the clouds.
Nebulae offer a spectacular celestial show as they interact with outside energy forces. Much research is still being invested into these phenomena, and the more we discover helps us to discern better the origins of our own corner of the universe.