Charles Messier, for instance, compiled his famous Messier Catalogue<\/a> of \u2018nebulae\u2019 in 1781, listing 103 objects, many of which we now know to be distant galaxies<\/a> or star clusters<\/a>.<\/p> You will often see historic astronomical records refer to the Andromeda Galaxy<\/a> as the ‘Andromeda Nebula’, for example.<\/p> Today, the term ‘nebula’ is more rigorously defined, and refers to clouds of dust and gas in the interstellar medium: the space between stars inside a galaxy.<\/p> Those clouds can form in a variety of ways, though.<\/p> So when we talk about a nebula, we may actually be talking about a supernova remnant, or about a planetary nebula.<\/p> A planetary nebula<\/a> has nothing to do with planets: it’s formed when a Sun-like star, at the end of its life, transitions from a red giant to a white dwarf<\/a>.<\/p> The majority of nebulae fall into the category of \u2018diffuse nebulae\u2019, so-called because they are vast objects, sometimes several hundred lightyears<\/a> across.<\/p> And diffuse nebulae, in turn, can be broken down into reflection, emission and dark nebulae.<\/p> The difference between them? It\u2019s all about illumination, and the clues are very much in the names!<\/p> Emission nebulae are also known as HII regions, because they contain vast quantities of ionized hydrogen.<\/p> These are giant clouds of gas and dust that have at least one star at their heart.<\/p> It is the light emitted by this star that causes the nebula as a whole to become visible.<\/p> Simply put, an emission nebula is a nebula that glows because a powerful close-by star is ionizing the gas, causing it to glow.<\/p> Emission nebulae are also associated with regions of star formation, so in some cases it can be the light emitted by many powerful young stars that causes them to glow.<\/p> Famous examples of an emission nebula include the Crescent Nebula<\/a>, the Bubble Nebula<\/a>, the Orion Nebula<\/a>, the Lagoon Nebula<\/a>, the North America Nebula<\/a> and the Cat’s Paw Nebula<\/a>.<\/p> Reflection nebulae, on the other hand, are giant clouds of gas and dust WITHOUT a star at their heart.<\/p> They do, however, lie close enough to a star (or a group of stars) that starlight is reflected by the material that makes up the nebula.<\/p> This makes the nebula visible and is what gives the reflection nebula its name.<\/p> Famous examples of a reflection nebula include M78<\/a>, the Iris Nebula<\/a>, the Running Man Nebula<\/a> and the Witch Head Nebula.<\/p> Some nebulae are even a mixture of emission nebulae and reflection nebulae, such as the Flaming Star Nebula<\/a> and the Trifid Nebula<\/a>.<\/p> Lastly \u2013 and you\u2019ve probably guessed this bit already \u2013 dark nebulae<\/a> are giant clouds of gas and dust that don\u2019t have a star at their heart, and aren\u2019t close enough to one to reflect its light, either.<\/p> As a result, dark nebulae aren\u2019t illuminated or visible, and in fact appear on the night sky as dark smudges that block our view of whatever other objects may lie beyond them.<\/p>What is a nebula?<\/strong><\/h2>
![\"Planetary](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2019\/10\/Death_Sun_like_star-a9a2df0-e1570445844939.jpg\")
Emission nebula<\/strong><\/h2>
![\"The](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2021\/03\/01.DavidWills_OrionAndRunningMan-1e3468c.jpg\")
![\"The](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2024\/05\/stars-lagoon-nebula-social-1024x538.jpg?fit=800%2C420\")
Reflection nebula<\/strong><\/h2>
![\"M78](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2024\/02\/15.M78_JaredBowens-1024x587.jpg?fit=800%2C459\")
![\"The](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2023\/04\/13.FlamingStarNebula-c48bf3d.jpg\")
Dark nebula<\/strong><\/h2>
![\"The](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/48\/2019\/10\/horsehead-nebula-c56e934.jpg\")