The havoc of star-formation feedback is not confined to high-mass clusters. Low-mass clusters like NGC 1333 (a nebula and stellar nursery about 1,000 light-years away) contains only hundreds, rather than thousands, of stars. Even though no massive stars form in such clusters, the stars there all produce protostellar jets from their accompanying disks, and these, too, can play a dramatic role in shaping a cluster’s fate. Spitzer images of NGC 1333 and other low-mass clusters show them threaded with graceful arcs of jet material that extend from one side of these stellar nurseries to the other. In low-mass clusters, feedback from the jets may play the same disruptive role as do winds and UV radiation from big stars in high-mass clusters. Unfortunately, we do not yet have enough information to say whether most stars in the galaxy were formed in high-mass or low-mass clusters. If we knew the answer, we could move closer to estimating how many solar systems like our own exist.
The combined effects of jets, winds, radiation, and supernova explosions show that nature may enforce a kind of celestial family planning. Stars can beget other stars, but they can also shut down the birthing process. “We can see the jets from different stars interacting with each other,” Arce says. “Those outflows can either trigger the formation of new stars or disperse gas that would have been part of the star-formation process.”
The impact of this stellar birth control is still hotly debated, however. Some astronomers believe that such internal processes are not the key players in inhibiting star formation. They say factors external to the cluster, such as shearing forces produced by the galaxy’s rotation, disrupt the cluster more intensely and prevent stars from forming. But images from Spitzer have provided stunningly beautiful evidence to support the case for internal processes. Recent computer simulations have shown how hundreds of jets can act as hypersonic swizzle sticks stirring a cluster’s gas into turbulent motion that inhibits new stars from forming. And new studies give added weight to the idea of stellar feedback. “With the Complete Survey we nearly tripled the number of outflows we could see impinging on the cloud,” Goodman says.
As astronomers have investigated the feedback mechanisms that operate in high- and low-mass clusters, there remains a mystery related to the biggest potential star-forming regions of all. Both low- and high-mass clusters form within larger complexes of gas and dust called giant molecular clouds. Some of these clouds are dominated by high-mass clusters, others by low-mass clusters, and some have both. A typical cluster will extend across a few light-years. Its parent cloud can stretch across 300 light-years and contain enough matter to make a million stars. But a million stars do not form. Instead, star formation across a giant molecular cloud is a rather anemic process, and relatively few stellar nurseries arise. Only 10 percent of the mass of the cloud, on average, is converted through gravitational collapse into stars. The rest of it never collapses and eventually disperses into the tenuous interstellar medium throughout the galaxy. What is preventing the vast clouds from collapsing under their own weight?
Some astronomers believe the answer rests with the powerful magnetic fields that are known to thread the clouds. Others think galactic shock waves that ripple through them keep the bulk of their gas from collapsing. Still others say the stellar feedback that astronomers see operating in high- and low-mass clusters is sufficiently powerful to disrupt even the much larger molecular clouds.
“You have to ask why only 10 percent of the cloud turns into stars,” Bally says. “But we know that star formation adds energy to the system through winds, radiation, and jets. The supernova blast waves, of course, just add insult to injury.”
“If all these stars create turbulence,” Goodman says, “then that turbulence acts like a kind of heat that keeps the cloud inflated.”
The mechanism that prevents giant molecular clouds from collapsing—whatever it may be—has a lot to do with why we exist in the first place. Stars are factories that convert lightweight elements into heavier atoms. These atoms include carbon, oxygen, nitrogen, and all the other elements that are essential for life as we know it. When a star dies, this material is cast into space. When later generations of stars form, some of that material congeals into rocky planets like Earth. If galaxies raced through their fuel reserves, creating lots of first-generation stars early on, few stars with rocky planets would be born later. The odds for life would be much worse. Earth might very well not even exist.
By looking out to star-forming nurseries across the galaxy, astronomers have shown us something innate and strangely familiar about the birth of our own star and planet. Psychologists know that in human families the role of siblings can be just as important as that of parents. Thanks to the Spitzer telescope, the same might now be said of the heavens.
