Emission nebulae are clouds of ionised gas that, as the name suggests, emit their own light at optical wavelengths. Their mass generally ranges from to 10, solar masses and this material can be spread over a volume of less than light year to several hundred light years.
One of the most common types of emission nebula occurs when an interstellar gas cloud dominated by neutral hydrogen atoms is ionised by nearby O and B type stars.
These extremely hot and luminous stars give off vast quantities of high-energy ultraviolet UV photons which break the neutral hydrogen atoms into hydrogen nuclei and electrons. These later recombine to form neutral hydrogen again, but this time in an excited state. The first detailed observations, though, waited for famous scientist Christiaan Huygens in Huygens, by the way, also was the first to come up with a standard formula for centripetal force, which he published in So a big year for Huygens.
About 50 years later, Edmond Halley wrote about six different nebulae. For that, he got a comet named after him. Not really, he was a very accomplished scientist who served as the second Astronomer Royal in Britain. But, famous names flocked to nebulae over the years.
Edwin Hubble helped categorize nebulae based on the spectra of light they produce, also discovering that nearly all nebulae are associated with stars and are illuminated by starlight. Some nebulae are bright enough to be seen with the naked eye.
Many others are visible through telescopes, depending on how many stars are around them to illuminate the dust clouds that form the nebulae. Through advanced space telescopes like the Spitzer and Hubble Space Telescopes, we have plenty of gorgeous imagery of nebulae. Most of these are formed through infrared and false-color images but they all are striking. Nebulae have been divided into five major categories. These are emission nebulae, reflection nebulae, dark nebulae, planetary nebulae, and supernova remnants.
Emission and reflection nebulae tend to be fuzzy in appearance and lack any noticeable shape or structure. They are also known as diffuse nebulae. Emission Nebula - An emission nebula is a cloud of high temperature gas. Within this type of nebula, a star energizes the atoms in the cloud with ultraviolet radiation. As these atoms fall back to lower energy states, they emit radiation. The process is similar to that of a neon light.
This causes the nebula to glow. Emission nebulae tend to be red in color because of the abundance of hydrogen. Additional colors, such as blue and green, can be produced by the atoms of other elements, but hydrogen is almost always the most abundant.
A fine example of an emission nebula is the Orion Nebula M Reflection Nebula - A reflection nebula differs from an emission nebula in does not emit radiation of its own. It is a cloud of dust and gas that reflects the light energy from a nearby star or group of stars. Reflection nebulae are frequently the sites of star formation. They usually tend to be blue in color because of the way that the light is scattered. Blue light is scattered more efficiently. The Trifid Nebula M20 in Sagittarius is a good example of a reflection nebula.
Dark Nebula - A dark nebula is a cloud of dust that blocks the light from objects behind it. They are very similar to reflection nebulae in composition and look different primarily because of the placement of the light source. Dark nebulae are usually seen together with emission and reflection nebulae. The Horsehead Nebula in Orion is probably the most famous example of a dark nebula. It is a dark region of dust in the shape of a horse's head that blocks the light from a much larger emission nebula behind it.
Planetary Nebula - A planetary nebula is a shell of gas produced by a star as it nears the end of its life cycle. Magnetic fields and rotation rates of the stars may also provide some disturbance of the star to create the interesting shapes.
Dark nebula are caused by giant clouds of dust blocking light from stars behind it. They come in many different shapes and can be seen with the naked eye when looking toward the constellation Sagittarius which is the center of our galaxy. The first picture in the series above shows the dark blotch that you'll notice if you were to look. Looking into the infrared wavelengths can penetrate through the dust to show stars within the clouds and stars behind them.
This is due because infrared light can penetrate through the dust. These are similar in nature to a diffuse nebula but due to our perspective, we are behind the cloud of dust or it is too thick. Otherwise, we may see something similar to the Orion Nebula. Diffuse Nebulae - The birthplace of stars.
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