synonymDiffuse nebula(nebula) generally refers to the bright Cepheus nebula
The Cepheus bright nebula (also known as diffuse nebula) can only be seen near bright stars, and it is these bright stars that illuminate these nebulae.The nebula is illuminated in two ways.NowOrion nebula And the nebulous matter in the Pleiades cluster as an example.The spectral image of the Orion Nebula shows a bright line spectrum, which indicates that the gas in the nebula is in a low-pressure state and is emitting light.This nebula is calledEmission nebula。However, spectral images of nebular matter in the Pleiades cluster show that.These nebulae reflect the light of the embedded stars.The spectrum of these nebulous materials is absorption spectrum, which is the same as that of the brightest stars in the cluster.Therefore, this nebula is calledReflection nebula。
Nebula contains almost all extended objects except planets and comets.They are mainly composed of hydrogen, followed by nitrogen, and also contain a certain proportion of metal elements and non-metallic elements.In recent years, the research also found that it contains organic molecules and other substances.Nebula is a nebulous object composed of gas and dust in interstellar space.The density of matter in the nebula is very low. If we measure it by the standard on the earth, some places are vacuum.However, nebulae are very large in size, and often cover dozens of light years.Therefore, general nebulae are much heavier than the sun.
The shape of nebulae is varied.Nebula and star have "blood relationship".The gas ejected by the star will become part of the nebula, and the nebular matter will be compressed into stars under the action of gravity.Under certain conditions, nebulae can transform each other.
At first, all nebulous objects in the universe were called nebulae.Later, with the development of astronomical telescopes and the improvement of people's observation level, the original nebulae were divided into three types: star clusters, galaxies and nebulae.
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On the evening of August 28, 1758, a man namedmessier French astronomy enthusiasts in the survey of the sky searching for comets suddenly found a cloud like patch with no position change between stars.Messier judged from experience that this spot is similar to a comet in shape, but it has no position change between stars, so it is obviously not a comet.What kind of celestial body is this?Before revealing the answer, Messier recorded in detail such discoveries (up to 1784, there were 103).The cloud like patch in Taurus discovered for the first time is listed as No. 1, namely M1, and "M" is the abbreviation of Messier's name.
The nebula celestial sequence established by Messier is still in use today.His records of unknown celestial bodies（Messier star list）Published in 1781, it attracted the attention of the famous British astronomer William Herschel.After long-term observation and verification, Herschel named these nebulous objects nebulae.
Other nebulae
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Because the resolution of early telescopes was not high enough,Extragalactic galaxyAnd some star clusters look like clouds, so they are also called nebulae.Hubble measuredAndromeda Galaxy After the distance, it is confirmed that some nebulae are actually star systems similar to our Milky Way.Due to historical habits, an extragalactic galaxy is sometimes called a nebula, such as the Magellanic Cloud and Andromeda Nebula.
Interstellar matter nebula
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When we mention space, we often think that there is nothing there, dark and silent vacuum.In fact, this is not completely right.The vast space between stars may be silent, but far from being a real "vacuum", there are various substances.These substances include interstellar gas, dust and particle flow, which are called "interstellar matter".
Interstellar matter is closely related to the evolution of celestial bodies.Observations have confirmed that interstellar gas is mainly composed of hydrogen and helium, which is the same as the composition of stars.In fact, stars are formed by "condensation" of interstellar gas.Interstellar dust is some small solid materials, including carbon compounds, oxides, etc.
The distribution of interstellar matter in space is uneven.Under the action of gravity, the gas and dust in some places may attract each other and become dense, forming a cloud.People call them "nebulae" figuratively.According to the shape, the nebulae in the Milky Way can be divided intoDiffuse nebula、Planetary nebulaAnd so on.
Compared with stars, nebulae are characterized by large mass, large volume and small density.The mass of an ordinary nebula is at least equal to thousands of suns, and its radius is about 10 light years.
Nebulas are often named according to their location or shape, such as:Orion nebula , Lyra Nebula
