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Molecular Clouds. Ekta Patel PHYS 2070 October 5 th , 2010 Instructor: J.West. Overview. Composition & make-up Size Brightness Location & Distance Origin/Age/Evolution How they are studied Collapse of a molecular cloud Research Study #1 Research Study #2 Works Cited. - PowerPoint PPT Presentation


Molecular CloudsEkta PatelPHYS 2070October 5th, 2010Instructor: J.West

1Composition & make-upSizeBrightnessLocation & DistanceOrigin/Age/EvolutionHow they are studiedCollapse of a molecular cloudResearch Study #1Research Study #2Works Cited

OverviewKorista, 2010

This is just a brief overview of my presentation. Ill start off discussing the composition of a molecular cloud, basically what it is. I will then be discussing the three different classes of molecular clouds and get into some details regarding each and what special characteristics they have separating them into their own class. Ill talk a little bit about how researchers study molecular clouds and the importance of that as well as what happens when a molecular cloud collapses! Ill end of with 2 research studies I found particularly interesting in this field!2Composition & Make-upUntil the 70s regarded as abnormally dense interstellar clouds.A dense cloud containing ~90% hydrogen (H2) & 10% helium.Molecules are abundant not atoms.10-20 Kelvin.Densities as high as 1012 particles/m.Main constituents: MoleculesDust grainsions

Molecular clouds are composed of approximately 90% hydrogen and about 10% Helium. Molecular clouds are a type of interstellar clouds. Interstellar cloud is the general name given to a mass accumulation of gas, plasma and dust within a galaxy. Molecular clouds are found in very cold interstellar regions, usually around 10-20 Kelvin, where the density can climb as high as 1012 particles/m3. Until the 70s astronomers used to consider these dense regions as abnormally dense interstellar clouds but now they are recognized as belonging to an entirely new class of interstellar matter. In these regions you will find gas particles in molecular form rather then atomic, and thus this predominance of molecules leads these highly dense areas to be known as molecular clouds.

3 of the main constituents of M.C.s: Molecules- There are over 100 known molecules, amongst which CO and H20 are very important. Their rotational lines are the main way the gas within the molecular cloud is kept cool, which balances the heating from cosmic rays and external UV photons. If CO and H20 were not present to cool the molecular cloud, their temperature could climb to approximately 100 K turning them into diffuse clouds.Dust Grains: They are the solid particles which condense in a stars atmosphere and then blow into interstellar space. Once a molecular clouds density exceeds approximately 10^5 cm^-3, dust grains are thought to become the main cooling system for the cloud. Essentially, they begin the task of what CO and H20 were doing before the cloud became so dense. Dust grains are also important as they are believed to begin the catalysis of H2 from H. Ions: Ions are found in trace amounts, but enough so that they can couple the motions of the magnetic field to that of a neutral gas. This causes waves and turbulence and it is this turbulence and magnetohydrodynamic wave motion that is believed to support a molecular cloud from caving in on itself.

3SizeMost massive object in our galaxyTypically 100-106 solar massesMC that exceeds 106 are Giant Molecular Clouds100 pc acrossIrregular shape: blobs & filamentsOccupy same space as a diffuse cloud

Wikipedia. 2010M.C are the most massive object within our own Milky Way Galaxy. Their masses are can be found to be greater than, but are typically 10^6 solar masses. When the mass of a m.c. exceeds 10^6, it is referred to as a Giant Molecular Cloud (GMC). Given that they are the largest object in the galaxy, they stretch great lengths. One molecular cloud can reach upto 100 pc across in length!

They dont have any specific or special shape to them. They usually have what are called blobs and filaments. Even though their mass so much greater than that of a diffuse cloud, both molecular and diffuse clouds occupy the same amount of space, due to the fact M.C.s have such high densities. 4Brightness

Typically noticed due to the fact that they block starlight from background starsLight present from star forming region dependent upon location:Within field of viewOpposite sideEmbedded within molecular cloud

Daou, et. Al. 2009Molecular clouds typically are noticed due to the fact that they block the starlight from background stars. Thus brightness of a molecular cloud would be an irrelevant question to answer.

However, in the molecular cloud Orion, the star-forming region happens to lie on our side and we are able to detect the star-light from the newly formed stars. If the star-forming regions are on the opposite far side or deep within the molecular cloud, we wouldnt be able to detect even this.

5Location & DistanceClosest are within a few hundred pcOrion Molecular CloudTypically found within the plane of the galaxyMost clouds clumped in spiral arms of the diskUsually around latitudes of 100o of the galactic centre

Daou, et. Al. 2009

Examination of infrared and radio maps of interstellar dust has revealed that molecular clouds do not exist on their own as individual objects. Instead they make up massive molecular cloud complexes which are typically around 50 pc across, but can reach as high as 100pc. These molecular cloud complexes contain enough gas to make a million stars like our sun. Currently astronomers know of about 1000 such giant complexes in our own Galaxy.

The closest molecular clouds to us are about a few hundred parsecs from the sun. The molecular cloud in the Orion constellations are evident based on the lack of background starlight, and their spectral emissions of molecular lines.

**Cue-Card Notes**


Certain number of stars are formed stars heat surrounding gas molecules break up gas dissipatesM.C.s colliding with galaxies experience it a little differently-collision causes rapid star formation which uses up most of the gas, before it can get hot and dissipate-Short life Jeans mass

Once a certain number of stars have been made, these stars in turn heat up the surround gas of the M.C. so much that it begins to break apart the molecules and the gas dissipates.

Molecular clouds in colliding galaxies experience death a little differently. The collision causes stars to form an accelerating rate and the gas pressure is increased by 100- 1,000 times. The hot surrounding gas causes rapid star birth via triggering shock waves. This leads to the formation of several hundred thousand stars which end up using most of the gas in the molecular cloud. Thus they molecular cloud is essentially used up before it gets too hot and breaks apart.

According the the Jeans mass, molecular clouds are short-lived. Even the smaller ones, with masses around 10 solar masses, are short-lived because they are in a state of gravitation collapse.

The stars found within the Orion molecular cloud which is one of the closest and thus best studied M.C.s are only a few hundred thousand stars, making them baby stars compared to others stars such as our sun, measured in billions of years.

Amino acid glycine has been found, which holds many implications for the future, an amino acid is an essential building block for proteins, which is the foundation for DNA, which in turn defines life as we know it on earth. The fact that A.A. are produced out in space, specifically a molecular cloud shows that evaluations and other life forms may exist out there.

7How They Are StudiedUse of spacecraft measures UV radiation of stars

Observation of other molecules effective in studying the interior of molecular clouds

Rotation Properties aids in spectral identification

Wikipedia. 2010Studying molecular clouds is a really difficult situation, since there are not that many ways of obtaining measurements and data, which also contributes to why the physics behind molecular clouds is also poorly understood. As mentioned earlier, molecular hydrogen (H2) primarily makes up a molecular cloud. However, it does not emit or absorb radio radiation. It emits short-wavelength ultraviolet radiation, but this is still not very helpful when trying to study the structure of the cloud. Another option would be to use 21-cm observations, but they only detect gas in the atomic form of hydrogen, and we are dealing with molecular hydrogen, so again that too will not help us. So even though theorists had expected an abundance of hydrogen in cold and dense regions of interstellar space, but it was seemingly difficult to prove. Spacecraft was then used to measure the ultraviolet radiation of stars found near the edges of a molecular cloud, and thus confirmed the presence of molecular hydrogen.

Since hydrogen is not an option to study for astronomers, they then turned to observation of other molecules in order to study the interiors of molecular clouds. Through this process, molecules such as as carbon monoxide, hydrogen cyanide, water, formaldehyde and about 150 other complex molecules are now known to exist in interstellar space. Even though these molecules are only found in very small amounts, not even comparable to hydrogen, they act as tracers for the clouds structure and physical properties. When chemical reactions take place in the cloud, these different molecules are formed, and by observing them, we can also confirm the presence of hydrogen.

Each of these molecules has its own rotation properties that is distinct and can also aid in studying areas with diverse physical properties. Given variations in temperatures and densities, one molecule may be useful in providing data on one particular region, and a different molecule will be useful at a different temperature and density. The information gathered via this method allows astronomers to have a reference point to compare their results with for diff


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