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A '''nebula''', from the [[Latin]] word for ''[[cloud]]'', is a cloud of interstellar [[dust]] and [[gas]]es.<ref>[http://coolcosmos.ipac.caltech.edu/ask/224-What-is-a-nebula- What is a nebula] from coolcosmos.ipac.caltech.edu</ref> Nebulae have been observed both within the [[Milky Way]] [[galaxy]] and in other galaxies. These large clouds of mostly [[hydrogen]] gas can form [[star]]s and [[planet]]s. Secular scientists believe our [[solar system]] formed about 4.6 billion years ago from a nebula in the [[nebular hypothesis]].<ref name=creation.com>[https://creation.com/nebular-hypothesis Nebular hypothesis] from creation.com</ref> However this approach has several issues.<ref name=creation.com/>

A '''nebula''', from the [[Latin]] word for ''[[cloud]]'', is a cloud of interstellar [[dust]] and [[gas]]es.<ref>[http://coolcosmos.ipac.caltech.edu/ask/224-What-is-a-nebula- What is a nebula] from coolcosmos.ipac.caltech.edu</ref> Nebulae have been observed both within the [[Milky Way]] [[galaxy]] and in other galaxies. Nebulae consist of mostly [[hydrogen]] gas though other [[element]]s such as [[oxygen]] and [[nitrogen]] as well as [[organic compound]]s like [[cyanide|hydrogen cyanide]].<ref name=herschel>[http://herschel.cf.ac.uk/results/complex-molecules-orion-nebula Complex molecules in the Orion Nebula] from herschel.cf.ac.uk</ref> Secular scientists believe nebulae can form [[star]]s and [[planet]]s in a way known as the [[nebular hypothesis]], and that our [[solar system]] formed about 4.6 billion years ago in this way.<ref name=creation.com>[https://creation.com/nebular-hypothesis Nebular hypothesis] from creation.com</ref> However this approach has several issues and flaws.<ref name=creation.com/><ref>[https://answersingenesis.org/astronomy/cosmology/pierre-simon-laplace-the-nebular-hypothesis/ Pièrre Simon Laplace: The Nebular Hypothesis] from answersingenesis.org</ref> [[Creation scientist]]s suggest that [[God]] created nebulae along with other astronomical object on the fourth day of [[Creation week]] (Genesis 1:16).<ref name=aig-splendor>[https://answersingenesis.org/astronomy/the-splendor-of-gods-creation/ The Splendor of God’s Creation] from answersingenesis.org</ref>

Some well-known nebulae are the [[Orion nebula]], the [[Crab nebula]], and the [[Horsehead nebula]].

Nebulae often feature prominently in commercially available collections of deep-space images, since their colours and shapes are interesting, varied and curiously attractive. These images are often from those taken by telescopes in space like the [[Hubble Space Telescope]]. Some well-known nebulae are the [[Orion nebula]], the [[Crab nebula]], and the [[Horsehead nebula]].

Nebulae often feature prominently in commercially available collections of deep-space images, since their colours and shapes are interesting, varied and curiously attractive. These images are often from those taken by the [[Hubble Space Telescope]]

 

Later on in the 18^th century, [[astronomer]]s were searching for [[comet]]s in the night sky. At that time, both nebulae and comets appeared as fuzzy region when viewed through their telescopes. As a result some astronomers mistook nebulae for comets. To counter this, astronomers began to catalogue nebulae to avoid confusion. The most well known catalogue is the [[Messier catalogue]], a collection of over 100 objects noted by [[Charles Messier]] and published in 1781.<ref name=britannica/> It is important to note that not all these objects are nebulae. Some objects like [[galaxies]] were originally referred to as nebulae as they possess a similar appearance through the telescopes available at that time.

 

In the 19^th century, the astronomer [[William Herschel]] and his son [[John Herschel]] discovered thousands of nebulae, far more than anyone else.<ref name=britannica/> In the years between 1786 and 1802, William Herschel with help from his sister Caroline discovered at least 2,500 clusters, nebulae and [[galaxies]]. John Herschel discovered a further 1,700 objects. These discoveries form a major components of the [[New General Catalogue]] created by John Dreyer in 1888.<ref name=britannica/> Together, these form 4,630 of the 7,840 objects in the catalogue.<ref>[https://www.space.com/17432-william-herschel.html William Herschel] from space.com</ref>

 

The invention of [[photography]] in the late 1800s transformed the study of nebulae and astronomy in general. It allowed details that could not be seen with the naked eye to be seen. In 1880, the first photograph of the [[Orion nebula]] was taken, though reasonable quality images were not produced for another three years.<ref name=britannica/> Photography also allowed for the easy study of the [[spectrum]] of astronomical objects, the resolving of an objects light into different [[wavelength]]s. This made it easy to distinguish between the glowing gas of a nebula and a collection of unresolved [[star]]s. The spectra of stars tend to consist of a broad spectrum punctuated by dark absorption lines whereas nebulae, especially [[emission nebula]]e tend to have lines only at specific wavelengths. This technique was applied to the Orion nebula in 1864 to demonstrate it was composed of [[hydrogen]].<ref name=britannica/> At this point, the distinction of nebulae and galaxies started to become clear. As galaxies are composed of vast numbers of stars, their spectra resembles that of a star's.

 

 

==Properties and Structure==

Nebulae come in a variety of shapes and size, ranging from less than 1 [[light year]] across to hundreds of light years across. There are several different classifications of nebulae including [[diffuse nebulae]], [[planetary nebulae]] and [[dark nebulae]]. Diffuse nebulae are further split up into [[emission nebula]]e and [[reflection nebula]]e. All nebulae however consist of large clouds of gas and dust. Temperatures inside nebulae vary depending on the type but can be anywhere from 10 [[Kelvin|K]] to as high as 10 million K.<ref name=britannica/> Large nebulae can contain over ten thousand [[solar mass]]es of material.<ref name=aig-splendor/>

 

Most nebulae have a very similar composition, with around 90% of the [[atom]]s in the nebulae being [[hydrogen]] and much of the remaining 10% being [[helium]].<ref name=britannica/> This lead to nebulae containing [[ionization|ionized]] hydrogen being called "H II regions," as "H II" is commonly used to denote to singly ionized hydrogen in astronomy. Other elements like [[nitrogen]], [[oxygen]] and [[neon]] are often found as well at around two parts per thousand.<ref name=britannica/> Compounds and [[organic molecule]]s such as [[cyanide|hydrogen cyanide]], [[formaldehyde]] and [[methanol]] have also been observed in the [[Orion nebula]].<ref name=herschel/> Interstellar dust composed of [[carbon]] and [[silicate]]s accounts for a fraction (0.7%) of the material in nebulae.<ref name=britannica/> Typical densities in nebulae vary from hundreds to millions of hydrogen atoms per cubic centimetre.

 

*[[Diffuse nebulae]] - These are large but thin clouds of gas and dust.<ref name=seds-diffuse>[http://www.messier.seds.org/diffuse.html Diffuse Nebulae] from messier.seds.org</ref> These are sub classified into two types, [[emission nebula]]e and [[reflection nebula]]e. Emission nebulae, also known as H II regions, occur when high energy [[radiation]] from nearby large stars causes the nebulae to glow, normally a red color. If there are no large stars, then there may be a reflection nebulae, where the white/blue light of stars is reflected off dust.<ref name=seds-diffuse/> This produces a blue nebula. Examples include the [[Lagoon nebula]] and the [[Eagle nebula]].

*[[Planetary nebulae]] - These nebulae tend to be faint, fairly small (perhaps a few light years across) and are often circular or ring shaped with a shell-like structure.<ref name=seds-planetary>[http://www.messier.seds.org/planetar.html Planetary Nebulae] from messier.seds</ref> These were named "planetary nebulae" due to their resemblance to [[planet]]s when viewed through a telescope. They are often found to be expanding at a few tens of kilometres per second. Examples of planetary nebulae include the [[Ring nebula]], the [[Cat's eye nebula]] and the [[Butterfly nebula]].

*[[Dark nebulae]] - Also called molecular clouds, these nebulae are characterised by interstellar dust in the foreground, obscuring stars or a bright nebula in the background.<ref name=britannica-dark>[https://www.britannica.com/topic/molecular-cloud Molecular Clouds] from britannica.com</ref> Some times called absorption nebulae, dark nebulae tend to be very dense and cold. Due to their dark nature, they can only really be seen via far-infrared and [[microwave]] techniques and have a temperature of 7-15 K.,ref name=britannica-dark Well known examples include the [[Horsehead nebula]] and the [[Coalsack nebula]].

*[[Supernova]] remnants - These are the remnants of supernovae, composed of gas emitted during the supernova explosion.<ref name=seds-supernova>[http://www.messier.seds.org/snr.html Supernova Remnants (SNR's)] from messier.seds.org</ref> They are often large and impressive. Only a handful have been observed in the [[Milky Way]], but others have been seen in other [[galaxies]]. Examples include the [[Crab Nebula|Crab nebula]] and [[SN 1604]].

==External links==

==External links==

*[http://www.godandscience.org/love/tourjs.html Some spectacular pictures from the Hubble Telescope, which include many nebulae]

*[http://www.godandscience.org/love/tourjs.html Some spectacular pictures from the Hubble Telescope, which include many nebulae]

*[https://www.spacetelescope.org/images/archive/category/nebulae/ Some more images taken by the Hubble Space Telescope]

*[https://www.spacetelescope.org/images/archive/category/nebulae/ Some more images taken by the Hubble Space Telescope]

==References==

==References==

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