Among the most pressing of these questions is how stars are formed. Stars are born from the material between other stars. In some regions of space, the density of gas and dust is much higher than the norm, and atoms are sufficiently shielded from destructive high-energy photons to interact with other atoms to form simple molecules. In the highest density central regions of such molecular clouds, material is so well shielded that delicate, complex molecules can form. It is from these molecular cloud cores that new stars (including our own Sun) and planets are born. These are the places of creation; understanding them may help astronomers understand the conditions from which the Earth, and life has evolved.
Because the star forming process occurs behind so much intervening dusty material, visible-light telescopes cannot see what is happening. By moving to the infrared, sub-millimeter and millimeter wavelength regions, where the effects of this obscuration are nearly negligible, astronomers can begin to directly probe regions where stars are actively being born.
Understanding the star forming process has many astronomical consequences. Knowing what physical conditions are needed to form molecular cloud complexes is important in understanding the star-forming evolution of galaxies, both in the current age and, perhaps most importantly, when galaxies were first forming. The understanding of how a single star forms from a molecular cloud, then, has even cosmological implications.
The current working model for star formation may be represented by four conceptually distinct stages of development that are the culmination of the last several decades of theoretical and observational efforts. Within molecular clouds, there are cores of material which are denser than the surrounding cloud. Dense molecular condensations form within these molecular cores as the loss of magnetic support through ambipolar diffusion allows gas and dust to contract gravitationally. Eventually these condensations become sufficiently centrally concentrated to undergo dynamical collapse; the inner regions form an evolving protostar and surrounding disk while the outer regions form an extended infalling envelope of material (the arrows). At some point, a bi-polar wind breaks out along the rotational poles of the system, while material continues flowing inward along the equatorial regions. The visual extinction toward the central protostar is typically tens to thousands of magnitudes at this point, effectively obscuring it from scrutiny at optical wavelengths. Over time, the angle occupied by the wind broadens, removing surrounding material and halting the inward flow of material. At this point, the system becomes detectable at near-infrared and even optical wavelengths as a star plus a disk, commonly recognized as a T Tauri system.
Molecular clouds themselves harbor a number of fundamental myseteries which submillimeter data and analysis is expected to shed light upon. Our understanding of the evolution, and (kinematic/chemical) structure of a giant molecular cloud is incomplete. Submillimeter measurements will uncover the warmer, denser components of a molecular cloud as well as the UV-illuminated "surfaces" of clouds where the delicate interplay and feedback between massive stars and molecular clouds can be explored. This will be crucial to improving our understanding of how molecular clouds form and are disrupted by their environments (i.e. the stars they give rise to).
Submillimeter observations can test our understanding of interstellar chemistry in many important ways. Submillimeter observations can help identify heating and cooling processes in molecular clouds (i.e. the thermal balance). Pivotal neutrals and ions will be measured, such as H2D+, NH, NH2, H3O+, and metal hydrides like MgH, NaH and AlH+. The UV-radiated surfaces of molecular clouds harbor a rich photochemistry that serves as an excellent testing grounds for theories of molecule formation and dissociation. Evolved (dying) stars undergo periods of brief but potent mass loss. This ejected material forms dust and molecular gas which allows astronomers to probe dust formation, the enrichment of matter by stellar nucleosynthesis, and stellar winds.
Star formation takes place in other galaxies too -- and often on a much larger and more violent scale than ever imagined! Galaxy interactions can funnel large quantities of molecular gas into the centers of galaxies, triggering giant outbreaks of massive star formation called starbursts. With submillimeter interferometers we will be able to identify single molecular clouds in the nearest galaxies, but even with single antennas we can study the properties of ensembles of star-forming clouds as a whole and thereby determine the global molecular properties of that galaxy. As in the Galaxy, submillimeter observations will extend our diagnosing power to warmer, denser regions that are actively forming stars. Line emission from these galaxies is expected to peak at submillimeter wavelengths, and will allow high-resolution studies of the large scale distribution of dense gas in galaxies. Since galaxies glow in visible light due to stars, and stars owe their existence to molecular gas, these studies will provide information on galaxy evolution, dynamics, and morphology.
A microwave-sensitive telescope like the HHT can also probe the edge of the observable Universe directly, which is of interest to cosmologists. One of the great success stories of the Big Bang model has been the detection of the Cosmic Microwave Background Radiation (CMBR). This radiation stems from about 1 million years (virtually a blink of an eye, cosmologically speaking) after the Big Bang, when the Universe had expanded and cooled to an extent that atoms could form; leaving light waves to travel freely through space. Until that point, the Universe was ionized, and too dense and hot for light to travel far without interacting with matter. The so-called "last scattering surface" marks the epoch when the Universe became "transparent" and not "opaque". The radiation from this epoch has been shifted in wavelength due to the continued expansion of the Universe, now appearing in the microwave region of the spectrum, and is observable with millimeter and submillimeter antennas/receivers.
This is an image of the CMBR, taken by COBE. The irregularity apparent
here is somewhat exaggerated; the scaling in this plot shows
irregularities smaller than one-ten-thousandth of a degree (K)! These
bumps are the seeds from which galaxies are thought to have formed.
The existence of the CMBR is not new, and it has been well characterized by orbiting microwave satellites like the Cosmic Microwave Backround Explorer (COBE) on large spatial scales of many degrees. We now know that this background radiation is very smooth, but that there are "irregularities" in the radiation that many believe are the density perturbations, the seeds, if you will, that later formed galaxies. The characterization of this microwave background on small angular scales can help theorists differentiate between theories of galaxy and structure formation. "How did we get here?" is a question that permeates much astronomical thought!
