Astronomical Beauty of Hydrogen

If you enjoy chemistry, physics and math, it’s difficult not to love astronomy. Where else can you find an extraterrestrial interplay of this trio of intellectual endeavours?

Uncompounded, neutral hydrogen ( H2 ) occupies an extremely small part of the earth’s atmosphere. It is continuously produced naturally by tectonic and microbiological processes, thanks to iron-rich mafic rocks and iron-hydrogenase enzymes, respectively. Hydrogen gas is less dense than any other so it rises above the rest of the atmosphere’s components. Since hydrogen molecules have the same kinetic energy as air molecules, due to their lower molar mass, H2 ‘s molecules move 3.7 times faster than nitrogen and four times faster than oxygen. Although their molecules’ average speed of 1.8 km/s¹ at 10 °C is still inferior to Earth’s escape velocity of 11.2 km/s, the low mass of hydrogen’s charged products helps them accelerate to higher speeds by mechanisms in the ionosphere. Thus most of the lightweight gas leaves our planet.

Once hydrogen atoms join their own kind in space, they find plenty of company.  In the universe hydrogen is by far the most common element. It is the fuel of main sequence-stars; half of the galaxy’s hydrogen is in suns. The rest is found elsewhere in far lower densities in three forms:

(1) In molecular clouds the simplest element can be found bonded to its own kind as H2. For these clouds to exist, dust must shield them from ultraviolet rays, which are otherwise capable of breaking the single bond between atoms. But H2 is difficult to detect. Due to the perfectly symmetrical nature of the molecule, it cannot absorb infrared(IR) and change vibrational states. But luckily, His usually associated with the detectable and asymmetric carbon monoxide.

molecularcloud1
The above map of the Milky Way was made from the IR detection of carbon monoxide. It simultaneously gives us the location of molecular hydrogen and of molecular clouds in the galaxy. Source:Dame, Hartmann, Thaddeus/NASA

If a molecular cloud becomes large enough, thanks to gravity,  the cloud can become astronomically fertile and lead to the birth of a star. This is what’s happening in the so-called “Pillars of Creation”, part of Messier Object 16, commonly known as the Eagle Nebula.

Pillars
The Pillars of Creation, part of the Eagle Nebula, include molecular hydrogen.  https://en.wikipedia.org/wiki/Eagle_Nebula
nhi_allsky
Taken from http://www.astro.utu.fi

(2) In so-called HI regions, hydrogen can exist in free, atomic form (H). The spin of its lone proton (really a measurement of quantum angular momentum) is normally opposite that of its only electron. In a rare event, a collision between atoms can excite the hydrogen atom from the described spin state to one where the spin of the electron flips. When this state revert to its more common and stable one, energy in the ” twilight-zone” between microwaves and radio is emitted. Since there are so many free hydrogen atoms over large areas of space, despite the rarity of the events, there are enough of them to be observable with a radio telescope. Apparently small radio telescopes (1-3m) for observations of the 21 cm hydrogen
line can be built for about $300.

(3) An HII region is a thin plasma of charged hydrogen atoms and unbound electrons. It forms in the vicinity of certain stars whose radiation is capable of ionizing hydrogen gas. The star’s surface temperature and size will determine the size of the spherical HII region around it—the so-called Strömgren sphere. Some of the most beautiful sights of astronomy include HII regions. When the excited electron in each atom temporarily returns to a previously charged one, electromagnetic energy of select wavelengths is released. We don’t see the ultraviolet, but a transition from the third(n=3) to second(n=2) energy level leads to a prominent red emission at 656 nanometres, while n=4 to 2 and n=5 to 2 transitions create blue (486 nm) and blue-violet (486 nm), respectively. Each of the following familiar nebulae includes HII regions: Messier objects M8 (Lagoon Nebula), M16 (Eagle Nebula), M17 (Omega Nebula),  M42 (Orion Nebula), and M20 (Trifid Nebula), presented below in clockwise order.

 

Additional Sources:
The International Encyclopedia of Astronomy. Patrick Moore, editor

https://mwmw.gsfc.nasa.gov/cgi-bin/mw/mw5_zoom2.pl?370,18

http://astronomy.swin.edu.au/cosmos/M/Molecular+Cloud

http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/h21.html

http://www.spaceacademy.net.au/spacelab/projects/hlineobs/hlineobs.htm

https://phys.org/news/2016-07-oceans-large-overlooked-source-hydrogen.html

Footnote:

¹hydrogen’s average molecular speed can be calculated by equating 0.5 mv(kinetic energy) to 1.5 RT, where m is the molar mass in kg/mole while R = 8.31 J/mole K. Earth’s escape velocity in the absence of friction is obtained by equating kinetic energy to potential energy GMm/r, where G = gravitational constant and M and r are the mass and radius of the earth.