From observing stars at various stages of development, scientists
believe they have a pretty good general idea of the stellar life
1. The Molecular Cloud Forms
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
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.
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.
2. 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.
3. 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.
4. The Star Emerges
Eventually, all that remains is the newly-formed star and its
surrounding disk, which may clump together to form planets.
Protoplanetary disks in the Orion nebula.
5. 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
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.
6. The Star Dies, Enriching the ISM
When a star like our sun casts off its shell, it
creates a nebula that can take a variety of shapes.
Whatever its mass, every star eventually runs out of fuel for
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
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
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.
Thousands of newly-forming stars are hidden within these vast,
dark clouds of gas and dust.
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
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
A new star, obscured in this view by its dusty disk, fires
enormous jets from its poles. The glow between the two jets
is light from the star reflected by dust above and below the
plane of the disk.
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
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.
Heat makes gas expand.
As a cloud collapses, it generates heat, which creates an expansion
force that could stop the collapse.
But if it contains substances that radiate heat away (by emitting
IR photons), the core can continue collapsing until nuclear fusion
ignites and it becomes a star.
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
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?