ASTRONOMY Cloudy with a Chance of Making a star is no easy thing By Erick T. Young f there is anything you think astronomers KEY CONCEPTS would have fi gured out by now, it is how ■ Although astronomers’ I stars form. The basic idea for how stars theory of star forma- form goes back to Immanuel Kant and Pierre- tion has advanced sub- Simon Laplace in the 18th century, and the de- stantially in recent tails of how they shine and evolve were worked years, it still has serious out by physicists in the fi rst half of the 20th cen- holes. Stars form out tury. Today the principles that govern stars are of gaseous clouds that taught in middle school, and exotica such as collapse, yet where dark matter dominate the headlines. It might do those clouds come from and what makes seem that star formation is a problem that has them collapse? been solved. But nothing could be further from the truth. The birth of stars remains one of the ■ In addition, standard most vibrant topics in astrophysics today. theory treats stars in In the simplest terms, the process represents isolation, neglecting their interactions the victory of gravity over pressure. It starts and blowback on their with a vast cloud of gas and dust fl oating in in- natal clouds. terstellar space. If the cloud—or, more often, a dense part of such a cloud called a core—is cool ■ Astronomers are mak- and dense enough, the inward pull of its gravity ing progress on fi lling in these gaps. For in- overpowers the outward push of gaseous pres- stance, they have seen sure, and it begins to collapse under its own how massive stars can weight. The cloud or core becomes ever denser trigger the collapse of and hotter, eventually sparking nuclear fusion. clouds and how new- The heat generated by fusion increases the inter- born stars fl ing one an- nal pressure and halts the collapse. The new- other into deep space. born star settles into a dynamic equilibrium —The Editors that can last millions to trillions of years. The theory is self-consistent and matches a COURTESY OF NASA, ESA AND THE HUBBLE HERITAGE TEAM (STSCI/AURA) 34 SCIENTIFIC AMERICAN FRENETIC STAR FORMATION near the core of the galaxy M83 was captured last year by the Hubble Space Telescope’s new Wide Field Camera 3. Standard theories fail to account for the emergence of the massive bluish stars or the way they return energy to the gaseous clouds out of which they form . [STANDARD THEORY] A Star Is Born—With Diffi culty The standard theory of star formation neatly explains isolated low- to medium-mass stars but leaves many conceptual gaps. Star formation begins with a giant molecular Within the cloud, an especially dense subcloud of gas and The core fragments into multiple stellar embryos. In cloud, a cold, nebulous mass of gas and dust. dust—known as a core—collapses under its own weight. each, a protostar nucleates and pulls in gas and dust. Protostar CLOUD Core PROBLEM #1: Where does the cloud come from? PROBLEM #2: Why does the core collapse? PROBLEM #3: How do the embryos affect one another? A mixture of material produced in the big bang The model does not specify how the balance The standard theory of star formation treats or ejected from stars must somehow coagulate. of forces that stabilizes the cloud is disrupted. stars in isolation. growing body of observations. Yet it is far from massive stars blast their surroundings with ul- complete. Every sentence of the above para- traviolet radiation, high-velocity outfl ows and [THE AUTHOR] graph cries out for explanation. Four questions, supersonic shock waves. This energy feedback in particular, trouble astronomers. First, if the disrupts the cloud, yet the standard theory does dense cores are the eggs of stars, where are the not take it into account. cosmic chickens? The clouds must themselves The need to address these shortcomings has come from somewhere, and their formation is become increasingly pressing. Star formation not well understood. Second, what causes the underlies almost everything else in astronomy, core to begin collapsing? Whatever the initia- from the rise of galaxies to the genesis of plan- tion mechanism is, it determines the rate of star ets. Without understanding it, astronomers formation and the fi nal masses of stars. cannot hope to dissect distant galaxies or make Third, how do embryonic stars affect one sense of the planets being discovered beyond Erick T. Young got his start in another? The standard theory describes individ- our solar system. Although fi nal answers re- astronomy at age 10 by building ual stars in isolation; it does not say what hap- main elusive, a common theme is emerging: a a telescope out of a cardboard pens when they form in close proximity, as most more sophisticated theory of star formation tube. He is now director of Science stars do. Recent fi ndings suggest that our own must consider the environment of a fl edgling Mission Operations for the Strato- sun was born in a cluster, which has since dis- star. The fi nal state of the new star depends not ) spheric Observatory for Infrared Astronomy (SOFIA). Young was an persed [see “The Long-Lost Siblings of the only on initial conditions in the core but also on astronomer at Steward Observato- Sun,” by Simon F. Portegies Zwart; Scientific the subsequent infl uences of its surroundings star formation star ry of the University of Arizona American, November 2009]. How does grow- and its stellar neighbors. It is nature versus nur- from 1978 until 2009. He has been ing up in a crowded nursery differ from being ture on a cosmic scale. on the science teams for nearly an only child? ); DON DIXON ( every major infrared space facility, Fourth, how do very massive stars manage to Swaddled in Dust including the Infrared Astronomi- Young cal Satellite, the Infrared Space form at all? The standard theory works well for If you look at the sky from a dark site, far from Observatory, the NICMOS camera building up stars of as much as 20 times the city lights, you can see the Milky Way arching and the Wide Field Camera 3 mass of the sun but breaks down for bigger over you, its diffuse stream of light interrupted on the Hubble Space Telescope, ones, whose tremendous luminosity should by dark patches. These are interstellar clouds. the Spitzer Space Telescope and the upcoming James Webb blow away the cloud before the nascent star can The dust particles in them block starlight and Space Telescope. accumulate the requisite mass. What is more, make them opaque to visible light. ( YOUNG T. ERICK OF COURTESY 36 SCIENTIFIC AMERICAN February 2010 The protostar shrinks in size, increases in density and offi cially becomes a star when nuclear fusion begins in its core. Planets emerge from the leftover material swirling around it. Protostar Sunlike star Planet PROBLEM #4: How do massive stars form? Nascent stars above 20 solar masses are so luminous that they would be expected to disrupt their own formation, as well as that of nearby stars. Massive star Consequently, those of us who seek to ob- other elements amount to a few percent. Some serve star formation face a fundamental prob- of this material is primordial matter barely dis- lem: stars cloak their own birth. The material turbed since the fi rst three minutes of the big that goes into creating a star is thick and dark; bang; some is cast off by stars during their life- it needs to become dense enough to initiate nu- times; and some is the debris of exploded stars. clear fusion but has not done so yet. Astrono- Stellar radiation breaks any molecules of hy- mers can see how this process begins and how drogen into their constituent atoms [see “The it ends, but what comes in the middle is inher- Gas between the Stars,” by Ronald J. Reynolds; ently hard to observe, because much of the ra- Scientific American, January 2002]. diation comes out at far-infrared and submilli- Initially the gas is diffuse, with about one hy- meter wavelengths where the astronomer’s tool- drogen atom per cubic centimeter, but as it cools box is rel atively primitive compared with other it coagulates into discrete clouds, much as water parts of the spectrum. vapor condenses into clouds in Earth’s atmo- Astronomers think that stars’ natal clouds sphere. The gas cools by radiating heat, but the arise as a part of the grand cycle of the interstel- process is not straightforward, because there are lar medium, in which gas and dust circulate only a limited number of ways for the heat to es- from clouds to stars and back again. The medi- cape. The most effi cient turns out to be far-in- um consists primarily of hydrogen; helium frared emission from certain chemical elements, makes up about one quarter by mass, and all the such as the radiation emitted by ionized carbon www.ScientificAmerican.com SCIENTIFIC AMERICAN 37 [PROBLEM #1] The Dark Origins of Interstellar Clouds Astronomers have gradually identifi ed the stages by which clouds coalesce from diffuse interstellar gas and become Infrared dark cloud progressively denser. The stage immediately prior to protostar formation is represented by so-called infrared dark clouds. Opaque even to infrared light, they show up as black streaks in this image from the Galactic Legacy Infrared Midplane Survey Extraordinaire (GLIMPSE), performed by the Spitzer Space Telescope . Their size and mass are just right for forming stars.