Star Stuff

by Martin Houde

Caltech Submillimeter Observatory

"We are star stuff" was one of the favorite
phrases of the late and renowned astrophysicist Carl Sagan.
But what should we understand from such a statement? 

In the first place, stars and the planetary systems that
accompany some of them (along with their inhabitants, such
as humans) originate from the gas and dust that permeate
the interstellar space. In other words, we and our local
star, the Sun, are ultimately built from the same raw material.
But the connection does not stop there. 

During the major part of their life, most stars act as giant
factories for the transformation of the simplest form of
atoms, i.e., hydrogen, into more complicated and heavier
ones like helium, carbon, nitrogen, oxygen, ...
In fact, these are just some of the by-products of the different
thermonuclear processes that happen deep within the interiors
of stars which allow for the production of the energy needed
to maintain them through adulthood. But near the end of
their life, once this transformation process is over, stars
eventually give a significant amount of their constituent
material back to the interstellar medium. The more massive
stars do so through gigantic explosions, called supernovae,
while the more typical stars, like our Sun, expel their
material in a less violent fashion through phenomena called
stellar winds. Once this material has found its way back
in the interstellar medium, it is available for the formation
of a new generation of stars and planetary systems. 

In other words, the universe has all the aspects of a huge
recycling enterprise where the stars act as the major players
and we as mere beneficiaries of the whole process. In fact,
without the presence of stars almost all of the matter in
the universe would be in the form of hydrogen. There would
basically exist none of the more complicated atoms needed
for the emergence of life (such as carbon and oxygen) and
there would ultimately be no humans! Incidentally, supernovae
are also responsible for the synthesis of all the elements
heavier than iron (copper, silver and gold are just a few
examples). So, rocky planets like our Earth could not exist
as such without the essential contribution of supernovae
to the replenishing of interstellar space. We are indeed
made from star stuff.

The process of star formation, however, cannot take place
just anywhere in space. Young stars and protostellar cores
are observed in neighborhoods where there is a significant
condensation of gas and dust. These regions are called molecular
clouds since their main component, hydrogen, exists in its
molecular phase. This implies that in order to study how
stars form and the conditions favorable to this, astronomers
must turn their attention to these objects. 

But by terrestrial standards, these clouds are very peculiar
beasts. First, they are cold, very cold; less than 100 deg. Kelvin
(or about -280 deg. F) and, second, they
are very tenuous. Although their density can vary a lot
from one cloud to another and even within a given cloud,
it is not unusual to find regions that have less than a
thousand particles per cubic centimeter (or about seventeen
thousand particles per cubic inch), much less than the best
vacuum achievable with today's technology. Because of these
and other of their characteristics, left to themselves,
molecular clouds will not shine at visible wavelengths like
stars do. They will, in fact, appear dark and opaque. Astronomers
must thus use different techniques to observe these objects.

This is one of the many things that we do at the Caltech
Submillimeter Observatory (CSO) where we spend a fair amount
of our time studying molecular clouds at millimeter and
submillimeter wavelengths using a state-of-the-art 10.4
meter (34 feet) telescope. These wavelengths are much too
long for the human eye to detect and the signals emanating
from the molecular clouds arrive to us in the form of radio
waves, not unlike the ones received by your everyday television
set. 

The CSO was dedicated on the summit of Mauna Kea in late
1986 and has since become one of the premier facilities
for submillimeter radioastronomy in the world. One of the
many areas where the CSO has distinguished itself through
the years is with its large molecular lines surveys. These
consist of studies where one tries to discover and identify
the different molecular species that exist in molecular
clouds. Up to now, using the data compiled from every radio
telescope in operation today or that were in the past, more
than a hundred species have been identified, and we are
still counting. The list comprises all kinds of molecules,
from the exotic kind to the better known ones like carbon
monoxide, ammonia and even vinegar!

But submillimeter astronomy is not a simple business. It
is hard work that requires exceptional meteorological conditions
to achieve the high quality observations needed to bring
advancements to this particular branch of science. Luckily
for the astronomical community, Mauna Kea is one of the
best sites in the world and is also host to two other major
submillimeter observatories besides the CSO (the James Clerk
Maxwell Telescope and the Harvard-Smithsonian Submillimeter
Array). Using these facilities and the clear skies of Mauna
Kea, astronomers all over the world are trying hard to bring
a better understanding of the workings of the universe and
this recycling business it seems to be involved in.