Dark nebulae
Introduction
A dark nebula refers to an interstellar cloud that tends to be very dense to an extent of obscuring light from reaching objects lying behind it. The cloud’s tendency to obscure the passage of light is attributed to by interstellar dust particles that occupy the coldest as well as densest regions of the molecular clouds. Large and complex clusters of dark nebulae are usually linked to large molecular clouds. According to Garfinkle (2008), the dark nebulae appear so as a result of millions of small-sized dust particles covered with extremely cold carbon monoxide as well as nitrogen, which tend to obscure light at observable wavelengths. In most instances, dark nebulae do not have clearly distinguished outer boundaries and they at times appear in convoluted serpentine forms. Very large nebulae can be seen using naked eyes where they appear as dark cloud patches that lie against a bright background of Milky Way (William, 2014). This paper explores the various attributes associated with dark nebulae including the size, length of its existence, basic characteristics, composition, how it can be observed, how long it will last, if there can be life in it, if it is beautiful as well as how it formed.
Attributes associated with dark nebulae
Dark nebulae comprise of types of clumps as well as clouds that mainly appear opaque due to millions of dust particles engulfed within the clouds. While these clouds do not have clearly distinguished outer boundaries, the various convoluted serpentine forms in which they may vary in size with the smallest ones ranging between one to fifty solar masses, which exist in a region estimated at one light year across the Milky Way. On the other hand, the size of the largest dark nebula is estimated to be over one million the size of the sun (Garfinkle, 2008). These contain the greatest content of interstellar medium. They occupy a region estimated at about one hundred and fifty light years across the Milky Way and they have an average density estimated at one hundred to three hundred molecules/cm3 as well as an internal temperature ranging between ten to a hundred Kelvin (William, 2014).
The dark nebulae have been in existence for more than three centuries with the first observations being made by early astronomers in 1784. According to Garfinkle (2008), one of the earliest astronomers, William Herschel, reported to have made observations in the Milky Way where he discovered various regions linked to luminous nebulae. The real nature of these dark regions were however not apparent to the astronomers as they believed that the regions were either devoid of stars or were holes in the Milky Way that were filled stars. Latter discoveries through photographic works of more recent astronomers, including Barnard, Wolf and Ross among others, showed that the dark regions were actually background stars that were obscured by non-luminous dust clouds.
The dark nebulae have unique basic characteristics that do not only distinguish them from other astronomical objects but they as well retain them in their distinguished hole-like regions for a long time. According to William (2014), the dark nebulae have an inner magnetic field that support and prevent them from falling as a result of disruptive collision from neighboring clouds. Similarly, the chemistry as well as the physical conditions prevailing in these clouds tends to differ significantly from those of the bordering low-density interstellar region. Hydrogen on the outer part of a dark nebula tends to be neutral and can allow a certain amount of light to penetrate through. Dust however gets more intense deeper within the cloud thereby blocking a great deal of ultraviolent radiation as well as making the cloud to become darker and cooler (Garfinkle, 2008). Star formation, which is the most significant interstellar event occurring in the dark nebulae takes place within the inner regions of the cloud through condensation. Dust grains within the cloud play an important role in emitting infrared radiation that eventually escape into the space thereby extracting energy from the cloud. Energy emission occurs when the cloud contracts in its gravity, which enhances radiation of half of the gravitational energy while the other half is used in heating the gas (William, 2014). The following image demonstrates how a dark nebula looks like:
The dark nebulae is composed of micron-sized pieces of dust that are coated with carbon oxide as well as frozen nitrogen, which form a thick mass that obscures any possible passage of light in observable wavelengths. The clouds are also rich in other elements such as molecular hydrogen, ammonia, atomic helium, formaldehyde, cyclopropenylidene as well as molecular ion, which are all moderately transparent (Garfinkle, 2008). The various elements constituting to the dark nebulae form the basis for star and planet formation hence the dire need to understand them so as to understand the star formation process. A dark nebula is not made of a smooth density but it instead constitutes of a chaotic mixture of smaller clouds that tend to pull themselves together into “protostars” that are destined to form individual stellar systems (William, 2014).
The process of dark nebula formation takes place when segments of the interstellar medium, comprising of gas and dust particles, go through a gravitational collapse. This gravitational collapse leads to matter clumping together to form regions of greater density. The outcome of this activity can attribute to the formation of stars at the heart of collapsing materials. The created stars’ ultraviolent radiation can then cause the neighboring gas to become visible particularly at optical wavelengths (Garfinkle, 2008).
According to William (2014), dark nebulae can be visible through varying means depending on the size of an individual cloud. The giant dark nebulae, labeled B111, can be seen with naked eyes and they often appear as dark patches or less luminous spots lying on the Milky Way. Smaller clouds such as B110 as well as B113 can be viewed using binoculars or a telescope where they appear as spots of dust that stand in a striking contrast against a starry background. Astronomical objects, such as infrared light, can be used to look deeper into the dark nebulae. The objects pierce through the dense cloud thereby converting the dense dark background into a tenderly glowing landscape.
Although the dark nebulae are often associated with a lot of darkness and may hardly exhibit their outer boundaries, they sometimes reflect parts of neighboring stars, which cause them to appear as extremely beautiful structures. According to Garfinkle (2008) the star situated at the heart of the inner fields of the cloud, known as the Iris Nebula, exhibits flowery images as well as images that look like opening petals. They as well exhibit an impressive symmetry, which tend to be attractive to cosmic observers. The cloud has a bright reflection of dominant blue color, which is an attribute of dust as well as grains replicating the starlight. The image below demonstrates how a beautiful dark nebula looks like:
The various attributes associated with the dark nebulae indicate that there is no possibility that life could thrive there. According to William (2014), life can only emanate through chemical connection that allow complexity that may eventually result to some form of mutation or from offshoots of past life that evolve into new and successful orientation. The fact that the basic composition of the dark nebula includes dust particles coated with frozen nitrogen indicates that there are no possible chemical reactions that can lead to any form of mutation that may eventually generate life. Similarly, there is no any documented evidence indicating that life has ever existed in the region, which generates further doubts relating to whether life can possibly be found in the dark nebulae (Garfinkle, 2008).
The dark nebulae are only temporary structures that will elapse in about five million years. Prior to this erosion, astronomers will have sufficient time to learn its various aspects not only using visible as well as infrared light but also using other varieties of light, such as ultraviolet, radio as well as the x-ray. They will also use the various colors available in the electromagnetic range of light to discover hidden details (William, 2014).
Conclusion
The dark nebula is a unique astronomical structure that is made up of huge masses of gases and dust particles that tend to obscure passage of light. The clouds vary in size as well as duration of existence. Their basic characteristics range from their physical to chemical composition. They are made up of dust, frozen nitrogen, ammonia, helium, ionized hydrogen as well as carbon oxide. They have been in existence for more than three decades and are expected to last for about five million years. They form as a result of gravitational collapse and the stars lying at the heart of their inner regions helps to illuminate their beautiful features.
References
Garfinkle, D. (2008). Three Steps to the Universe: from the Sun to Black Holes to the Mystery of Dark Matter. Chicago: University of Chicago Press.
William, V. (2014). Putting a New Spin on Galaxies: Horace W. Babcock, the Andromeda Nebula and the Dark Matter Revolution, Journal of the History of Astronomy, 45(2):111-156.