Types of Nebulae

Nebulae are one of the most breathtaking sceneries that the sky offers to our eyes, either with a telescope, a pair of binoculars, or, if we are lucky, with our naked eyes. We have gathered in our gallery tens of professional astrophotos of nebulae. These images show a great diversity in the characteristics of these celestial clouds. Let’s take a closer look.

A Brief History

The term “nebula” has not always had its current meaning. Before the emergence of telescopes, astronomers used the word “nebula” to make the difference between stars and non-stellar objects. Many of the previously-thought “nebulae” were open star clusters. Thus, the meaning of the word “nebula” changed a bit to mean “a deep-sky object.

By the middle of the 19th century, with the emergence of telescopes, mainly large telescopes, along with photography, it has become possible to distinguish real nebulae from stellar clusters and galaxies.

Since then, astronomers have been able to classify these clouds of gas and dust according to various criteria. For example, we can classify them according to whether they block or emit and reflect light. The two major classes are then dark nebulae and diffuse nebulae.

Different Classes of Nebulae

We can classify them as well according to their spectra.

The three major classes are then:

Emission nebulae: they reemit the light they receive from high-energy radiations of neighboring stars. Subcategories of emission nebulae were defined, such as:

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  • HII regions: active star formation regions, mainly composed of ionized hydrogen due to their proximity to high-energy radiation sources (hot stars). Our gallery holds several images of HII regions, such as these images of the Heart Nebula, and NGC 7000 (the Great Wall Nebula)

• • Planetary nebulae: an expanding disk-shaped cloud that a star (turning from a red giant into a white dwarf) has ejected at the end of its life. Note that they have nothing to do with planets. The word “planetary” is a misnomer that originates from the wrong interpretations that these objects were gas giants, such as Uranus. Check our gallery for two planetary nebulae together on the same image (HFG1 and Abell 6)

• • Supernova remnants: the matter that was ejected by a massive star right after its explosion at the end of its life. The Full Veil Nebula is an example of supernova remnants that we have in our gallery.

• • Wolf-Rayet bubbles: the envelope of matter that surrounds the Wolf-Rayet stars (hot stars from the main sequence that eject high-velocity stellar winds). See this image from our gallery of the Crescent Nebula (NGC 6888).

• • Herbig-Haro objects: bright knots of emission that form when narrow high-velocity jets of gas (emitted by forming stars) interact with their surroundings.

• Reflection nebulae: the dust composing these nebulae reflects the light coming from neighboring stars. Along with emission nebulae, their spectra show emission lines. Follow this link to see the Pleiades (M45), holding probably the most famous reflection nebula.

• Dark nebulae: they absorb the light from surrounding sources. Their spectra show absorption lines. The gas absorbs the stellar background light, while dust absorbs it and “reddens” it. For example, you can find in our gallery one of the most famous dark nebulae: the Horsehead Nebula, also available here. In both images, the Horsehead Nebula is taken with the Flame Nebula, which is an emission nebula.

Lifetime of Nebulae

On a cosmic scale, several and relatively fast transformations occur to all these nebulae. Supernova remnants and planetary nebulae will continue their expansion in space, enriching the interstellar medium with heavy elements. Soon, that is in a few thousand years, they would have lost their brightness. As for HII regions, or star-forming regions, they have enough energy to shine for another few hundred thousand or even a million years from now. Their brightness strongly depends on the life of the hot and massive stars that were born inside them and keep illuminating them. 

Giant molecular clouds are stellar nurseries. Their lifetime is about several tens of million

Conclusion

Nebulae have various types and origins. Some nebulae are associated with the birth of new stars. Some others are associated with the death of a star. The rich science behind their study gives us insight on the formation and fate of our Solar System.