(This article originally appeared in the Q3 1991 STAR newsletter. -Ian)

Forming Type OB Stars

Small stars (less than 10 solar masses) form throughout a giant molecular cloud (GMC). The really big hot stars, type B2 and up, containing over 10 solar masses, form through a different route. These stars can't form in the small cores located throughout the cloud; fragmentation breaks these cores into too small of pieces. Instead, large stars form in large dense clumps at the edges of the GMC. And more important, these large dense clumps are stable; without an external trigger they don't collapse.

Disturbing the Core

The massive clumps on the edges of the cloud are inherently stable. Because of magnetic fields and ultraviolet photons from outside the cloud, they maintain equilibrium between their gravity and internal pressure. They are also warm, with temperatures between 80 and 100 degrees Kelvin, as opposed to 10 to 30 degrees in the dense cores deep inside the cloud. Unless something disturbs the clump, it doesn't collapse. This is the hardest part about forming OB stars; the process is very hard to get started. But, as we shall see, once it gets going, it's self-seeding; the first bunch of OB stars force the formation of a second. Once OB star formation starts, the cloud will be disrupted within 50 million years.

The core disturbance can come from several places: a supernova shock, collision with another GMC, or the spiral density wave. Whatever causes the disturbance - the end result is the same, a collapsing clump and formation of OB stars.

There can also be combinations of these three effects. Before the cloud can recover from the effects of a supernova, it may encounter the spiral density wave. With the cloud already partially disrupted, a lessor shock will force the clump over the edge and into collapse. Because of the large disturbance created by a cloud-cloud collision, the cloud can't reach a steady state before encountering the spiral density wave. Thus, even if previous spiral arm crossings and collisions didn't trigger star formation, the combination of the two will.

Spiral Density Wave

When the cloud enters the spiral density wave, the front of the cloud slows as it tries to travel through the denser regions. The back part continues to move forward, compressing the cloud. Entering a spiral arm is a violent event resulting in a burst of massive star formation. It's this star burst which illuminates the dust in the spiral arms, giving them their visual shape.

There is some evidence that the GMC actually forms from smaller molecular clouds when they encounter the spiral density wave. This would mean the GMC and its dense clumps form rapidly. And, just as rapidly, the dense clumps collapse into OB star associations.

Cloud-Cloud Collision

The GMC may collide with another GMC. Strong shocks travel through the cloud, dissipating the energy of the collision. These shocks compress the clump enough for it to become unstable and start to collapse.

As clouds dissipate kinetic energy gained in the collision, the gas in the collision area heats to over a thousand degrees. The increase in gas temperature increases the gas pressure on the clump, forcing the clump inward. As the area cools down below 100 degrees, the clump becomes unstable, and the collapse into stars begins.

Supernova

A strong supernova shock can upset the stability of the clump. A strong supernova is required because in the clump the shock radiates strongly. This rapidly weakens the shock. Weak shocks are dissipated too quickly to have much effect on the clump. Also, the entire cloud is surrounded by a halo of neutral hydrogen. This also radiates away the shock energy. The shock forces the clump into a disk which collapses. With weak shocks, the clump's internal pressure and magnetic field combine to oscillate it back into a sphere before it can collapse.

The cloud needs to be close to the exploding star for a strong shock. But this is a rare event. Since there are initially no OB stars in the cloud, it must come from another cloud. But an OB star doesn't live long enough to separate from the cloud of its birth. So, the cloud must be close to a cloud-cloud collision with a cloud containing OB stars.

The Collapse into an OB Star

The collapse of the clump into large stars is very much the same as the collapse into a smaller star. It goes through all the same steps in the same order. The major difference is the time required for the collapse. When the Sun formed, a million years passed before the protostar grew hot enough to fuse hydrogen. In large stars, the protostar heats up in only 10,000 to 100,000 years.

During the collapse, fragmentation occurs within the clump. Normally four or five OB stars form, existing as a loose group of stars, called an OB star association. Often, they break down equally with two type O stars and two type B stars. Smaller stars don't form because of the clump's higher temperatures and the disruption caused when the large stars start to fuse hydrogen. The remaining material gets blown away by the strong stellar winds before another star forms. One example of an OB star association is the Trapezium in the Orion nebula. Evidence exists for extensive fragmentation before the first OB star forms. The clump starts with several hundred solar masses and less than 100 solar masses end up in the stars.

After the first few stars turn on, star production in the clump stops. OB stars have a stellar wind many times stronger than the stellar wind of small stars. A type O stellar wind has speeds up to 2000 km/sec, while a T Tauri star's wind is only about 300 km/sec. The strong wind breaks up and blows away the material which was collapsing into low mass stars. Any small stars found around OB stars are older. These stars formed long before the clump collapsed into an OB star and their close location is by chance. They became visible because the OB star association blew away obscuring dust and gas.

Blister Model

The strong stellar winds blow the gas and dust away from the OB star association, forming an HII region. The HII region may be depicted as a sphere expanding evenly out into space, but remember, it's sitting close to the edge of the cloud. Expansion is faster into the interstellar medium than into the GMC. The result is a large blister, slightly indented, appearing on the side of the GMC.

Studies have found that most HII regions are at the edges of molecular clouds. The Orion Nebula is the prime example of the blister model, with the blister facing us. The bright HII area behind the Horsehead Nebula is another blister, although that one is on the side of the cloud.

Chain Reaction Effect of OB Stars

When something has finally disturbed a clump, it collapses into an OB star association. The stellar wind and the intense radiation from the OB stars form an HII region. The HII region removes the dependence on external forces to produce big stars because it, coupled with supernovas, trigger more star formation. After the first OB star association forms, the process becomes self-seeding.

The expanding HII region accomplishes two major things: (1) it sweeps away all the low mass clumps, stopping star formation around the OB star association and (2) it compresses other large clumps, helping to trigger star OB star formation. What ends up happening is a trail of OB star associations going from old to young which appears as you approach the cloud. Moving at an average speed of 5 km/sec, star formation moves across and destroys the GMC.

When the HII region reaches a clump, the clump is compressed just like during a supernova shock. Both density and temperature increase from the HII region's shock. After the shock passes and the clump starts to cool, it becomes unstable. The clump begins to bounce back to its preshock density but the temperature drops faster than the density decrease. The higher density means the clump surpasses the critical mass needed for collapse. This is the same as happened when the first group of OB stars formed, only this appears as a more intense compression.

New OB star associations form at intervals of a few light years, corresponding to a few million years worth of expansion of an HII region. The interesting thing about this time interval is that it is also the life time of a supermassive star, those greater than 40 solar masses. By limiting fragmentation of the clump, the HII region allows stars this large to form. Whether the HII region is strong enough to trigger star formation by itself or if a supernova is also required is currently an unknown. Probably it depends on the clump itself. Some clumps are close to critical mass and take little prodding to collapse. Others need substantial density increase before they collapse.

OB stars, living such short lives, spend almost 20 percent of their life inside the cloud. It takes that long to blow away the gas and dust remaining after the clump's collapse. The rest of their lives are spent in close proximity to the cloud. Drifting at 3 km/sec, they don't stray far. With lifetimes measured in a few million years, they can't travel far. Only the dying remnants, the white dwarf, neutron star or black hole, travels away from the cloud mixing with the smaller stars.