The Interstellar Medium, Star-formation, and Gas Recycling
In order to truly understand some of the most important astronomical phenomena, it is necessary to be familiar with the interstellar medium, (ISM). The interstellar medium is essentially what makes up the space between stars, planets, etc. The interstellar medium is comprised of 99% gas and only 1% dust. This dust is unbelievably small and is made of Carbon, iron compounds, ice, or a variety of silicates. Approximately 75% of the gas is Hydrogen compared with 25% Helium and is a mix between uncharged atoms and positively or negatively charged particles. Most of the interstellar gas is neutral hydrogen. However, since it is neutral and is not excited like ionized particles and giving off light, it is difficult to observe. Because of Hydrogen's occasional electron spin flip, this neutral it is possible to view using radio waves when when looking through the 21cm bands these transitions emit. Except in the cases of nebulae, the interstellar medium makes up only a miniscule fraction of the density of the air on Earth. In nebulae, the ISM is either in the form of freezing clouds or, when close to stars, incredibly hot ionized particles of Hydrogen since the particles are hit with mass amounts of UV radiation as new stars form. The ISM creates stunning displays in space as light from stars radiates off the amorphous clouds as seen below.
Picture taken from wiki
But how did all this material get there? The Hydrogen and Helium were formed from the big bang. The dust, however, comes from the deaths of stars. As a star approaches the end of its life, Hydrogen to Helium fusion comes to an abrupt end in the core. Extremely large stars go through a series of fusion processes that heat it until iron is created and cannot be fused to give off any more energy so the star dies and ejects much of its matter into space. Once smaller stars are no longer fusing H to He in their cores they go through a chain of events and end their lives as white dwarfs. At this point the star has shed its outer layers and they spread adding dust to the ISM as we know it today. This dust is necessary in the ISM because it provides a base upon which hydrogen in from the medium may accumulate and form molecules.
Areas where these molecules are more densely packed are known as nebulae. There are three types of nebulae. The first, emission nebulae, a clouds of ionized gas. As UV radiation is given off by nearby stars it excites the electrons in the gas and when they are excited, they emit photons which produce beautiful pink light displays such as in the image seen below of the eagle nebula.
Picture taken from wiki
The second kind of nebulae is known as dark nebulae. In these areas, the gas and dust is so densely packed together that light from stars or other objects cannot pass through. An example of this is the horsehead nebula as seen below.
The last type of nebula is known as reflection nebula. In these regions, light from nearby stars reflects off of gas particles of the ISM and as it is reflected, has a bluish hue. These nebulae are also sites for star creation. Both the Pleiades and the lower left region of the horsehead nebula where blue light is seen are excellent examples of reflection nebulae.
Picture taken from wiki
Areas of the ISM where star formation happens in ares known as molecular clouds. The matter that makes-up these freezing cold clouds is pulled closer together under its own gravity. As these particles come closer to each other, they heat up, giving off more energy and dart around at higher speeds. At these increased temperatures they are able to collide and stick together forming more massive objects through a process known as accretion. The cloud contracts more at an ever increasing rate as gravity becomes stronger each second and forms a truly dense object. The entire time, the temperature of these gases increases, and when it hits 10 million degrees Kelvin, Hydrogen fusion begins and this massive object is now officially a star on the main sequence. After billions of years this star dies and disperses its matter across the universe to one day undergo the same processes that led to its creation. This continuum of star death creating a massive cloud of dust and gas which will one day form a new star is known as gas recycling since the gas is constantly being reused to create an endless number of stars throughout the existence of the universe.
Picture taken from wiki
The second kind of nebulae is known as dark nebulae. In these areas, the gas and dust is so densely packed together that light from stars or other objects cannot pass through. An example of this is the horsehead nebula as seen below.
The last type of nebula is known as reflection nebula. In these regions, light from nearby stars reflects off of gas particles of the ISM and as it is reflected, has a bluish hue. These nebulae are also sites for star creation. Both the Pleiades and the lower left region of the horsehead nebula where blue light is seen are excellent examples of reflection nebulae.
Picture taken from wiki
Areas of the ISM where star formation happens in ares known as molecular clouds. The matter that makes-up these freezing cold clouds is pulled closer together under its own gravity. As these particles come closer to each other, they heat up, giving off more energy and dart around at higher speeds. At these increased temperatures they are able to collide and stick together forming more massive objects through a process known as accretion. The cloud contracts more at an ever increasing rate as gravity becomes stronger each second and forms a truly dense object. The entire time, the temperature of these gases increases, and when it hits 10 million degrees Kelvin, Hydrogen fusion begins and this massive object is now officially a star on the main sequence. After billions of years this star dies and disperses its matter across the universe to one day undergo the same processes that led to its creation. This continuum of star death creating a massive cloud of dust and gas which will one day form a new star is known as gas recycling since the gas is constantly being reused to create an endless number of stars throughout the existence of the universe.
Works Cited
"Interstellar Gas Cloud | COSMOS." Interstellar Gas Cloud | COSMOS. Swinburne University of Technology, n.d. Web. 05 Dec. 2016. <http://astronomy.swin.edu.au/cosmos/I/interstellar+gas+cloud>.
Smith, Gene. "University of California, San Diego Center for Astrophysics & Space Sciences." The Interstellar Medium. CASS/UCSD, n.d. Web. 05 Dec. 2016. <http://casswww.ucsd.edu/archive/public/tutorial/ISM.html>.
"What Is the Interstellar Medium?" What Is the Interstellar Medium? N.p., n.d. Web. 28 Nov. 2016.
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