Fast Facts About Molecular Clouds

Molecular Cloud


Stars are born in nurseries with a collection of siblings that range in mass but share chemical compositions and dynamic histories. The source of stellar nurseries is a molecular cloud. These structures can span sizes of tens to hundreds of light years across and masses of thousands to millions of solar masses. In this article I review five fast facts about these originators of newborn stars.

Jackie Faherty, astrophysicist, Senior Scientist with the American Museum of Natural History and Editor at The Daily Galaxy.

Fast fact #1 –Molecular clouds are primarily Hydrogen but we trace it with CO

Hydrogen is by far the most abundant element in the Universe.  As a result, we find Hydrogen everywhere.  Molecular clouds are no exception to that rule. They are an accumulation of primarily molecular Hydrogen (H2) gas — two Hydrogen atoms that share their electrons.   Interestingly, molecular Hydrogen is notoriously difficult to detect in the Universe as it leaves very little imprint on the electromagnetic spectrum.  It might be everywhere, but finding a signature of it with modern observational Astrophysics tools is not easy.  However, Astronomers have discovered that carbon monoxide (CO) molecules — which are far easier to detect and are the second most abundant molecule in these clouds– are an excellent proxy for H2.  Consequently, we primarily trace the Hydrogen in molecular clouds by observing the amount of CO that is present.  

Fast fact #2 –Molecular clouds form and evolve due to the turbulent nature of stellar and galactic evolution

The progenitors of molecular clouds are hard to pinpoint.  Perhaps there are flows of neutral atomic Hydrogen (HI) moving through the Galaxy that get shoved into a spot where they can accumulate and form molecular Hydrogen? Perhaps many smaller clouds of HI get pushed together and the same effect happens?  What we do know is that molecular clouds are primarily H2 accumulated from motion within the Galaxy.  There are several theories on the major contributing factors to their formation and the majority of them involve shock waves (from e.g. Supernova or massive stellar winds) that propagate through the Galaxy.  Shock waves will push material around (collecting and compressing it) and ultimately lead to an accumulation of material in a given location.  Multiple shocks might amass the material and an alignment of a mean magnetic field line of a given location with the shock trajectory might explain how some molecular clouds build up in size. 

Fast Fact #3 –Molecular clouds ultimately give way to star formation

Once a molecular cloud has amassed thousands to more than a million solar masses in gas and dust, turbulence drives instabilities and ultimately leads to gravitational collapse. Shock waves are constantly pulsing through the Galaxy.  Initially they might be responsible for gathering the gas and dust together to form the molecular cloud but ultimately they lead to the fragmentation of pieces which become stars.  As a shock wave passes through a molecular cloud, it can trigger compressions which are strong enough to create the dense central engines of stars.  Molecular clouds will fragment into large pieces (the higher mass stars) and smaller pieces (the smallest stars and brown dwarfs).  Thus, molecular clouds are the precursor locations for star forming regions.  Oftentimes the gas and dust is still very present in a star forming area.  Therefore the molecular cloud that led way to the stars is still present.  For instance, the Taurus molecular cloud hosts a star forming region with hundreds (if not more) of newly formed stars.   

Fast fact #4 –Molecular clouds appear as the absence of stars when you look at the Milky Way 

Molecular clouds are “cold” and “dark” places.  They have typical temperatures of ~10 degrees Kelvin.  By definition they are an accumulation of molecular Hydrogen so they can be dense (~100 particles per cubic centimeter), span tens to hundreds of light years in size and hence have structure in the Galaxy.  All of this adds up to molecular clouds being objects that are big and a bit bulky but so cold that they are invisible to your eye.  However they tend to be in areas where there is a lot of light shining in their vicinity.  Specifically we often find molecular clouds tracing the spiral arms of the Galaxy or the Milky Way disk.  When you look toward the plane of the Galaxy you might find yourself staring at an area that appears to have the absence of stars.  When you see that, you may very well be staring at a giant molecular cloud.  One such famous example is called the Coalsack nebula.  It’s an apparent dark area, stretching several degrees in an otherwise richly filled part of the Milky Way (within the constellation of Crux).  The Coalsack nebula lies ~600 light years away.  There are millions of stars beyond the Coalsack that map out our Galaxy but none of their light can penetrate the gas and dust of the molecular cloud.  So instead, it appears as a dark patch of sky.  The next time you look at the Milky Way, search for these patches.  They won’t stay dark forever.  At some point turbulence will collapse the cloud and stars will be born. 

Fast fact #5 –The Radcliff wave is a giant gas structure that seems to be a connection of a number of molecular clouds

Astronomers have recently discovered a large structure of gas and dust following the pattern of a sinusoidal wave cresting 500 light years above and below the plane of the Milky Way.  This massive structure stretches ~8800 light years and connects several famous molecular clouds such as Taurus, Orion, Perseus, and Cepheus.  Interestingly the solar system passed through the wave ~13 million light years ago and will do so again in another 13 million years.  To date, no influence on the solar system has been correlated with past Radcliff wave intersections but researchers are still investigating.  The origin and propagation of the Radcliff wave remains a mystery.  Likely it is the impact of a major shockwave passing through the Galaxy.  A collision with a smaller, dwarf Galaxy (e.g. the Sagittarius dwarf Galaxy) might trigger such a shockwave.

Image credit: Hubble Space Telescope

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