Date: November 27th, 2012

Title: Astronomy Word of the Week : ISM

Podcaster: Dr. Christopher Crockett

Organization: United States Naval Observatory


Description: The popular image of space is a place of nothingness, devoid of all matter. But, between the stars, our galaxy is filled with a complex network of gas and dust.  The astronomy word of the week is: ISM.

Bio: Dr. Christopher Crockett is an astronomer at the United States Naval Observatory in Flagstaff, Arizona. His research involves searching for planets around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.

Today’s Sponsor: This episode of 365 days of Astronomy is sponsored by – Expanding your horizons in astronomy today. The premier on-demand telescope network, at dark sky sites in Spain, New Mexico and Siding Spring, Australia.

Additional sponsorship for this episode of 365 days of astronomy was provided by Clear Skies Observing Guides, a Modern Day Celestial Handbook. skies observing guides, or CSOG, is a new concept in visual amateur astronomy. The observing guides contain thousands of objects to observe through amateur telescopes, with matching tours for GOTO telescopes and matching AstroPlanner plan-files. CSOG allows you to target deep-sky objects and carbon stars you never observed before, night after night. Wishing astronomers around the world: Clear skies..! ”


The space between the stars is not empty.

Sure, it’s much emptier than anything here on Earth.  But it’s definitely not empty.

The interstellar medium, or ISM, makes up one-sixth of the mass of our Galaxy.  It contains the raw ingredients for making planets, asteroids, and stars.  Though tenuous—there is only about one atom for every cubic centimeter—there is enough material here to build entire galaxies.

99% of the ISM is gas.  About three-quarters of that gas is hydrogen, the fuel that powers stars for most of their lives.  One quarter is helium.  Almost all that hydrogen and helium was formed in the first three minutes after the Big Bang.  Only a couple percent of the gas is every other element on the periodic table.  Carbon, oxygen, magnesium, iron, uranium—all of it formed in the cores of long-dead stars.

The other 1% of the ISM is “interstellar dust”.  The dust consists of ices, carbon compounds, and silicate grains.  It is most likely formed around red giant stars.  Like polluting factories, these stars blow “atomic soot”—carbon, oxygen, silicon—into space, carried aloft by strong stellar winds.  Escaping the warm environments of these stars, the soot collects in cold, dark clouds.  There, shielded from the radiation of nearby stars, the atoms can collect and build complex chains.  In these clouds, astronomers have found amino acids—the building blocks of proteins.  The stuff of life is everywhere!

The ISM is complex.  It exists in tenuous balance with the stars. The ISM creates stars, and the stars give back to the ISM.  This is a place of cosmic recycling on a galactic scale!

The cycle starts in cold, dark clouds: the sites of star formation.  The outermost layers of these clouds shield the interiors from ionizing radiation that bathes the galaxy.  Tens to hundreds of light-years across, these clouds house enormous quantities of molecular hydrogen.  All it takes is a nudge from outside—a passing star cluster, a nearby supernova, the sweep of a galactic spiral arm—and the cloud becomes unstable.  Pockets of ever-increasing density grow, driven by the cloud’s own gravity.  From these dark cocoons, stars are born.  Upon ignition, they blow away the remaining material and light up the cloud.  The Orion Nebula, the Lagoon Nebula, the Witch Head Nebula: all of them clouds of gas and dust lit up by nearby young stars.

At the other extreme is the ionized gas.  Gas heated to millions of degrees where the electrons have been ripped from their atoms.  Shock heating from powerful supernova explosions excites the gas.  It responds by glowing with x-ray radiation.  Some of this gas is even blown free of the galaxy, into intergalactic space.

Between supernova shocks, young stellar winds, galactic magnetic fields, and turbulent motion, the ISM has a rich and complex structure.  Filaments of gas, dense pockets of hydrogen, and expanding voids connects the network of material interleaving the galaxy.

Take, for example, the neighborhood around our Sun.  You would never know it by looking at the sky on a clear, dark night, but our solar system is caught in a web of complexity.  Most of the ISM is invisible to human eyes.  Most of it is revealed in other parts of the electromagnetic spectrum: the cold, dark gas emits radio waves while the superheated plasmas glow with x-rays.  By putting together observations at all these wavelengths, astronomers have been able to draw a picture of what the space around the Sun looks like.

We are currently drifting near the edge of a cloud of gas 60 light-years across: the Local Interstellar Cloud.  The cloud itself is being blown away by a nearby grouping of young stars: the Scorpius-Centaurus Association.  About 400 light-years away, it is the closest region of massive star formation to the Earth.  Supernovae in the Association have evacuated the space around the young stars to form a bubble in the ISM.  Our Sun and Local Cloud sit in another bubble–the Local Bubble–adjacent to it.  Our Local Bubble has extremely low densities—only about one particle for every 10 cubic centimeters.  About 300 light-years across, our bubble is also the result of a supernova explosion near the Earth sometime in the last 10-20 million years.

And this is just a sampling of what surrounds us.  The ISM reminds us, once again, that the galaxy is far more complex than appears at first glance.  Without advances in technology that have opened up huge swaths of the electromagnetic spectrum to our study, much of the Universe would remain invisible to us. But even our humble eyes can catch glimpses of the hidden Universe.  On a dark night, where you can see the band of the Milky Way arching overhead, look closely.  The band of light is not uniform.  There are pockets where the light is missing.  They are the dark clouds of the ISM, blocking the light from more distant stars!  These are the sites of star formation, future stellar nurseries.  And you don’t need a telescope to see them.  Just a dark sky and a willingness to look up.  The clouds of dust and gas that fill the galaxy are the wombs of humanity.  And it is to those clouds we will one day return!

End of podcast:

365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by the New Media Working Group of the International Year of Astronomy 2009. Audio post-production by Preston Gibson. Bandwidth donated by and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at or email us at Until tomorrow…goodbye.

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