Nebula

Updated: Jan 31

Our universe is full of magnificent and fascinating celestial bodies. It is as colorful as a dream of a child. One thing that makes us stare up in our spectacular sky is a Nebula. In layman's terms, one can describe it as an "interstellar cloud”. It is comprised of cosmic dust, hydrogen, helium, and molecular clouds. They are vast in size, even hundreds of light-years in diameter. In earlier times, many diffused clusters and galaxies were identified as nebulae but with advancing technology, the classification of nebulae can generally be determined by their composition, location, emission spectrum, and other physical properties.


Some nebulae form from gas that is already present in the interstellar medium. They are generally the coldest and densest phase of the interstellar gas and are interestingly lighter than any vacuum created on the earth. Others are produced by stars during their evolution as planetary nebula or as supernova remnants after their death (of massive stars).


Planetary nebulae are the remnants of the final stages of stellar evolution for mid-mass stars (varying in size between 0.5-~8 solar masses). Evolved asymptotic giant branch stars expel their outer layers due to strong stellar winds, thus forming gaseous shells while leaving behind the star's core in the form of a white dwarf. If we talk about the star closest to us that is our Sun, it is expected to form a planetary nebula about 12 billion years after its formation, implying that we can expect a planetary nebula after 7.4 billion years from now.


Diffused nebula can be termed as the most arresting wonder of our universe and it is also responsible for the formation of stars such as in the "Pillars of Creation" in the Eagle Nebula. These forms as a molecular cloud and collapse under their weight thus spawning a variety of stars. Well, if you want to know the recipe for creating stars just take a look at this non-trivial equation: Molecular clouds + Gravity = Star. Though it's not accurate enough, there are a lot of parameters to consider like crossing the Chandrasekhar mass limit for a stable 'normal' star. Isn't it amazing that we can explain star formation (which itself is so complex) using this equation?


Another interesting fact is that all colorful nebula that is associated with stars get their illumination originally from starlight (refer to emission spectrum). Edwin Hubble discovered that the emission spectrum of nebulae is nearly always associated with B-type spectral stars and O-type main sequence stars. Nebulae have different visibility at optical wavelengths and are bright sources of infrared emission (due to dust in nebulae).


There is also one more type of nebula which was discovered recently which is, PPN (Protoplanetary nebula). The PPN is a temporary episode during the star's rapid evolution. It strongly emits infrared radiation and is a kind of reflection nebula.


This dark nebula is so dense that it obscures the visible wavelengths of light from the object behind. This obstruction of light is caused by interstellar dust grains present in the densest parts.



After all this information, you might wonder “Where can I see nebula in the sky?” Well, you need a clear sky for sure and can find the Orion nebula (brightest in our sky) though it's a tough job without a telescope. Also, be careful! Do not repeat the mistake of our early astronomer that is, to misidentify the Milky Way galaxy and other clusters and galaxies as nebula just because of their nebulosity. Not every cloudy colorful body is a nebula.

Here are some example of nebulae, We must tell you you will love them at their sight.

Eagle Nebula:

These striking image of “Pillars of creation” is the part of Eagle nebula which is located 7,000 light years away from Earth in the constellation Serpens. It was captured by Hubble telescope. Its name is apt as it is part of active star-forming region.

Data Source: . Credits: NASA, ESA and the Hubble Heritage Team (STScI/AURA)

Image source: https://www.nasa.gov/sites/default/files/thumbnails/image/pillars_of_creation.jpg


Helix Nebula

Helix Nebula which is popularly known as “ the Eye of God”. It is planetary nebula and located 700 light years away from Earth in the Aquarius Constellation. It has a dying star at its center. It threw off its outer layers as gaseous wind and transformed into white dwarf. The image provided is in multi-wavelength. Doesn’t it look quite creepy?

Data source: https://viewspace.org/interactives/unveiling_invisible_universe/star_death/helix_nebula

Image source:

https://www.nasa.gov/sites/default/files/thumbnails/image/archives_helix.jpg


Carina Nebula:

Thanks to emission spectrum of various elements, which themselves were formed due to Supernova explosions of massive stars, we can see such eyes soothing images. It is 7,500 light years away from Earth in the Sagittarius-Carina arm. The Chandra X-Ray vision has detected more than 14,000 stars in the region. The Carina Nebula is a good example of how very massive stars rip apart the molecular clouds that give birth to them. Eta Carinae is one of the brightest massive star which can be found in its core.

Data source: https://www.nasa.gov/multimedia/imagegallery/image_feature_2081.html

Image source: https://esahubble.org/images/heic0707g/ and N. Smith and NOAO/AURA/NSF



Horsehead nebula:

Horsehead nebula is a dark nebula embedded in the Orion Nebula which is 1344 light years away from earth in Orion Constellation. The horse’s prominent “jaw” is actually shaped by intense radiation from a nearby star blowing. This star forming region spreads across hundreds of light-years and covers much of the Orion constellation itself. Astronomers have learned the stellar nursery has already given birth to young stars, some even with protoplanetary disks.

Data source: https://astronomy.com/news/2020/10/the-horsehead-nebula-in-orion-an- unbridled-look

https://www.nasa.gov/multimedia/imagegallery/image_feature_89.html

Image Credit and Copyright: Jean-Charles Cuillandre (CFHT), Hawaiian Starlight, CFHT

Image source: https://www.nasa.gov/multimed