defining-a-molecular-cloud.html

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         title="Permalink to Defining a Molecular Cloud">Defining a Molecular Cloud</a></h2>
 
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    <abbr class="published" title="2012-09-20T20:02:00-06:00">
      2012/09/20
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    <address class="vcard author">
      By <a class="url fn" href="https://keflavich.github.io/blog/author/adam-keflavichgmailcom.html">Adam (keflavich@gmail.com)</a>
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    <p>I'm going to try to answer a few questions [this post is in-progress and
will be updated]:</p>
<ol class="arabic simple">
<li><a class="reference external" href="http://molecularclouds.blogspot.com/2008/08/defining-molecular-cloud.html#one">Why are molecular clouds called molecular clouds?</a></li>
<li><a class="reference external" href="http://molecularclouds.blogspot.com/2008/08/defining-molecular-cloud.html#two.htm">What distinguishes a molecular cloud from similar objects (e.g.
cores, clumps, HII regions)?</a></li>
<li><a class="reference external" href="http://molecularclouds.blogspot.com/2008/08/defining-molecular-cloud.html#three.htm">What do molecular clouds look like?</a></li>
<li><a class="reference external" href="http://molecularclouds.blogspot.com/2008/08/defining-molecular-cloud.html#four.htm">How do astronomers look at molecular clouds?</a></li>
<li>Why are astronomers look at molecular clouds (why are they
interesting)?</li>
<li>What do molecular clouds have to do with star formation and planet
formation?</li>
<li>How are molecular clouds involved in galactic evolution?</li>
</ol>
<p>1. Molecular clouds are regions in space with densities high enough and
temperatures low enough that molecules can form. The most common
molecule is Hydrogen (H:sub:<cite>2</cite>), simply because hydrogen is the most
common atom. The next most common is Carbon Monoxide, CO, that is
~10<sup>4</sup> times less common. It is the easiest to observe, though,
because H<sub>2</sub> can only emit light and energy in 'forbidden'
transitions that don't happen very often.
2. Size, temperature, and composition all distinguish molecular clouds
from other nebulae.</p>
<dl><dt><p>An H II region is very hot - in it, hydrogen atoms are ionized by
radiation from a nearby O-type star. In regions hot enough to ionize
hydrogen, all molecules are destroyed unless they have already collected
into larger dust particles.</p>
<dt><p>A core is a much denser bunch of gas than a molecular cloud. Cores are
dense enough that their outsides shield their insides from the radiation
of the rest of the universe and their insides cool to very low
temperatures. At low temperatures, the pressure supporting the cloud
against collapse is lower, and stars can form via gravitational
collapse.</p>
<dt><p>I don't really know what clumps are... they're supposed to be something
intermediate between cores and clouds, but I don't know what their
distinguishing features are.</p>
</dl><p>3. It depends on what wavelength you're looking in. In the optical,
where our eyes can see, they look dark - they're seen because they
absorb light from behind them. In the infrared, the hot ones glow, but
the cold ones are still invisible. If you go all the way to the
millimeter, all molecular clouds glow, no matter how cold, but in order
to see them there has to be a lot of material. I'll add some pictures
here later.
4. Lots of ways. Optical telescopes aren't the best choice, though. With
near-infrared images, we can detect molecular clouds by counting stars
and noticing when there aren't as many in some regions. This technique
is called &quot;NICE&quot; and works because there are so many stars in the sky
and dust in molecular clouds includes some, but not all, of their light.
In the far infrared, we can see hot dust glowing, but this is very
difficult because our atmosphere is opaque in the infrared - it's like
trying to look through a brick wall. We need telescopes in space to be
able to see anything at these wavelengths. In the millimeter, there are
some 'holes' in the atmospheric absorption, sort of like color filters,
that we can see through to detect the coldest emission.</p>
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