Act One

(The setting: A vast cloud of molecular hydrogen gas (H2) and some dust. Enter some compressive disturbance and the cloud begins to collapse.)

Scene One: The Initial Collapse

• Something triggers the collapse of a huge nebula. The nebula measures many light years across and contains several thousand solar masses of material. Average densities are on the order of 10^3 particles/cc and typical temperatures are a "chilly" 10 K - 50 K!
• gravitational potential energy is released - heats up the cloud. This turns out to be a trick that a star will use many times during its life.
• material piles up at the cloud center, density rises in the center.
• the core begins to collapse faster than the outer envelope.

 

Scene Two: Fragmentation and Further Collapse

• After several million years the collapsing cloud begins to fragment into numerous smaller clouds that each continue to collapse. The following descriptions apply to each of these perhaps thousands of "cloudlets".
• the increasing frequency of collisions between H2 molecules is the equivalent of heating up the cloud. The dust grains also heat up and begin to re-radiate this energy in the Infrared region.
• the core is warmed up to several hundred degrees K, the collapse slows down dramatically. Densities are now approaching 10^6 particles/cc and the cloud is about 100 times the size of our solar system.

Scene Three - It's Heating Up!

• material from the envelope continues to rain onto the core causing it to collapse slowly under the added weight.
• the core continues to heat.
• the contracting cloud is also spinning. In order to conserve angular momentum (the "figure skater effect") the spin rate increases and the cloud flattens out into a disk. Rapid spinning begins to inhibit collapse.
• the magnetic field of the collapsing cloud becomes more concentrated as the cloud collapses and begins to "stiffen" - pushing back and further inhibiting collapse.
• when the core reaches 2000 K the H2 molecules break apart. This takes a lot of energy - in order to pay the bill - the core collapses again. Eventually the core reaches approximately 10 000 K and the cloud begins to resemble a star. At about 30 times the size of our Sun the cloud has become a Protostar.

Scene Four: A STAR IS BORN!

• the core collapses from a diameter of about 4 AU to the size of the sun, heats up considerably and becomes hot enough for nuclear fusion to begin.
• the outer envelope - like a womb - shields the entire event from view at optical wavelengths. The dust in the envelope heats up and becomes a very large region glowing brightly in the infrared part of the spectrum.
• the outer envelope is blown away and a pre-main sequence star emerges. This is a violent process - a rapidly spinning and still collapsing outer disk collides with a now expanding shell from the newly formed star. Motion above and below the rotational plane of the nebula is easier and hence material "squirts" away at right angles to the disk.

Act Two - Scene One

(The setting: Astronomers from around the world are using an "armada" of telescopes to study star birth)

Scene One: The IR-View

• An astronomer adjusts the Infrared detector on her telescope and directs to the Serpens star forming region. An intense knot of IR radiation bursts through.

• Researchers using the HST are producing exquisite images of star formation. Some of these images support the basic ideas sketched in the previous passages while others are extending our understanding in surprising and un-anticipated ways. The images show clearly the jet like structures (bi-polar flows) ejected from the collapsing stellar cloud and protostar.

• Associated with jets are bright knots of light called Herbig-Haro objects. We think that these are the result of shock-wave heating produced by supersonic matter ejected along the jets. As this matter plows into dust and gas left over from the collapsing nebula it heats the gas and produces these bright, fast moving "blobs".

• Another fascinating set of images are those taken in the heart of the Orion Nebula. Small condensations called proplyds are thought to be actual images of protostars surrounded by their accretion disks.

Scene Four: An Orion Nebula Spectacular:

• A colleague in West Germany monitors "bright" microwave emissions from the same area. This is clear evidence - a star has recently formed. Furthermore, observations of tracer molecules like formaldehyde or carbon monoxide give evidence that regions in this "stellar" nursery continue to collapse.

Act Three - Scene One

(The setting: An astronomer is lecturing to his first year class. They are having trouble paying attention until he shows them an HR plot. Instantly they realize the import of what he is telling them (they are very bright).)

The HR diagram IS ALIVE! The salient features on the HR diagram are numbered along the HYASHI track or path that the star's changing appearance produces:

• the protostar now has a photosphere but has not yet achieved core temperatures high enough to fuse hydrogen. At this stage the star has a radius about 30 times that of our Sun. It is cool but its enormous size allows it to shine very brightly. We will probably not get a good view, however, because gas and dust shroud the protostar.
• the star continues to collapse and heat. It will also be spinning rapidly and possibly ejecting matter. The star may enter what is called the T-Tauri phase for a short period. The shrinking size will reduce the luminosity of the star.
• Core contraction has finally produced high enough temperatures and collision rates to support thermonuclear reactions in the core. The star settles onto the main sequence where it will spend about 90% of its life.

Seeds: Chp11, all
Kaufmann: Chp 20; pgs 364-383