From observing stars at various stages of development, scientists believe they have a pretty good general idea of the stellar life cycle:
The Molecular Cloud Forms
Various stages of star formation in giant galactic nebula NGC 3603. The cold molecular clouds at right are stellar nurseries. The stars at left have blown away their dusty surroundings.
In the space between stars (the interstellar medium or ISM), a vast cloud of gas and dust collects and contracts under its own gravitational pull. It's made mostly of hydrogen generated at the beginning of the universe, and also contains helium and possibly other elements manufactured by stars. It may span hundreds of light years.
Some parts of the cloud become more dense than others - either by chance or by the intervention of a shock wave, which could be induced by such events as collision with another cloud, the supernova explosion of a massive star, or the gravitational attraction of a passing galaxy.
The Cloud Cores Collapse
The densest parts of the cloud (known as "cores") collapse, each one forming a protostar surrounded by a slowly rotating disk of gas and dust. This infall can happen very quickly, shrinking the protostar from the size of our entire solar system to that of Mercury's orbit in only six months.
The Protostar Blows Away Its Dusty Envelope
Even as it contracts, the protostar develops a strong wind which gradually blows away the gas and dust in which the protostar and its disk are enveloped.
The Star Emerges
Eventually, all that remains is the newly-formed star and its surrounding disk, which may clump together to form planets.
The Main Sequence and Beyond
Nuclear fusion in the star's core turns hydrogen into helium, releasing a copious amount of energy that pushes outward and counterbalances the inward gravitational pressure of the gas. This is known as the "main sequence," and can continue for billions of years.
When a star about the size of our sun uses up the hydrogen in its core, the core collapses further, generating additional heat. At sufficient temperature, nuclear fusion ignites in a shell of hydrogen around the core, creating outward pressure that causes the star's outer envelope to expand and cool. The star has become a red giant. Its core, meanwhile, continues to collapse until it reaches the temperature at which nuclear fusion ignites the helium, turning it into carbon and oxygen.
A more massive star goes on to stages in which it generates heavier and heavier elements. The most massive stars continue until finally they have iron cores.
The Star Dies, Enriching the ISM
Whatever its mass, every star eventually runs out of fuel for nuclear fusion.
The core of a red giant becomes a white dwarf, while its swollen outer envelope drifts off into space, contributing whatever elements it contains to the ISM.
Very large stars end in a supernova, an extremely powerful explosion that scatters the elements it has manufactured - which could be everything up to iron - into the ISM. The force of the explosion converts some of those elements to still heavier elements, and that's the origin of all naturally-occurring elements heavier than iron. The star may be completely destroyed, or leave behind a neutron star or - in the case of the most massive stars - a black hole.
The ISM, enriched with the material manufactured in the star and its explosion if any, goes on to form a new generation of stars. And the process repeats, creating more and more heavy elements.
What We Don't Know
This is the basic outline of a star's life, but scientists have to admit that they're short on details to explain the various stages.
In particular, the second step is literally shrouded in mystery. The earliest stages of cloud collapse, and the star-forming process that occurs within the collapsing cloud core, haven't been observable because dust blocks the light to which most telescopes to date have been sensitive.
Though it comprises only about 0.7% of the ISM's mass, dust has a profound impact on it. Its main effect is to absorb stellar radiation, especially in ultraviolet and optical wavelengths.
But far-infrared and submillimeter wavelengths shine through. And the absorbed light is reradiated in those longer wavelengths as well. That's what Herschel is designed to observe. Herschel will enable scientists to detect and describe the fundamental process of protostar collapse better than ever before.
Scientists will have to depend on reradiation from cold dust for much of the information they seek about star formation. They will use Herschel to learn about the dust and its relationship to the rest of the ISM and to the energy sources whose radiation the dust absorbs, processes, and re-emits.
Herschel will give astronomers their first chance to study at high spatial resolution the physical and chemical conditions that exist in the cold phases of the ISM, observations that are essential to understanding how the ISM changes before it begins to form a star.
Such studies will make it possible to understand what a galaxy's far-infrared/submillimeter spectrum says about the way in which it makes stars. It should also resolve the long-standing question of why the gas-to-dust ratio appears to be 10 times higher in the Milky Way than in other galaxies.
About 20 large molecular clouds are known to exist within 1,000 parsecs (3,258 light years) of the Sun, and star formation appears to happen somewhat differently from one cloud to another. Herschel will observe a large number of regions to help scientists build a more complete theory of star formation.
Astronomers believe the importance of dust in star formation applies as much to the very distant, very early universe as it does to our local universe. So Herschel will also conduct deep space surveys to determine the role of dust at high redshift, enabling scientists to deduce the total rate of star formation.
Another little-understood aspect of star formation is the phase in which outflowing jets develop at the protostar's poles (which looks much like the process by which supermassive black holes power quasars!).
It is widely held that this phenomenon serves to dissipate angular momentum from the infalling material. But real understanding will require that the physics and chemistry of the gas is studied in detail, in the far-infrared and submillimeter wavelengths through which it can be observed.
As stars go through their stages of nuclear fusion, and ultimately the end of that process, they blow off much of their mass, thereby enriching the ISM with the elements they've manufactured. But the complete process isn't well understood.
The total amount of mass of a depleted star and its surrounding nebula of gas and dust should at least equal the mass of the star at its peak. But, using the instruments available so far, scientists haven't been able to find all of the mass that stars lose to their surroundings. Herschel promises to make much more complete measurements possible.
Cloud cores have to be kept cold in order to be able to collapse to the point where they burst into nuclear fire.
If that seems counterintuitive, think of a hot air balloon. It starts out as a big collapsed bag, lying on the ground. As it is heated, the air inside expands, decreasing in density and swelling the balloon. Finally, the denser outside air pushes it up, away from the earth's gravitational pull.
Similarly, as the cloud core collapses under its own gravity, it heats up from the friction of its molecules colliding with one another. Its rising temperature makes it want to expand, in opposition to the gravitational force making it contract. It needs somehow to radiate away its heat if it is to continue the contraction process to the point where its temperature rises enough to trigger nuclear fusion.
Only those cloud cores able to maintain extremely low temperatures (around -260° C) can keep contracting until they reach a "point of no return," where the gravitational pull becomes irresistible until the outward push of nuclear fusion stops the collapse.
Hydrogen, as it turns out, isn't very good at radiating heat. So the cloud needs a cooling agent - something much more efficient as a radiator. Water, carbon and a few other substances may serve that purpose, even though the cloud may contain them only in trace amounts. Dust helps too, by shading the gas from warming radiation as well as by absorbing radiation and reradiating it away.
Herschel will trace the role of water and other possible cooling agents for a more comprehensive picture of this process. And on the way, it may shed light on a perplexing question: How did the very first stars form without the assistance of cooling agents that had not yet been manufactured?