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The Formation of Single and Multiple Stars
Li-Hsin Chien & Mark A. Pitts
Based on the presentation at the Astronomy 734 Astrobiology Seminar by Bo Reipurth, 2004 Fall

Introduction

From the beginnings of the solar system billions of years ago, life on Earth has been dependent on the Sun as an energy source. The light and heat from the Sun have played a fundamental role in creating the conditions for terrestrial life to expand and flourish beyond its initial geothermal niches. Thus, understanding the formation of stars like the Sun is a key step toward understanding the emergence of life in the Universe. Recent studies of star formation make it increasingly clear that the classic view of stars forming in isolation is atypical. A better understanding of star formation involves multiple systems, where two or more stars form in close proximity and affect each other's early evolution. In using this new approach, a large number of previously unexplained phenomena are a natural outcome of these multiple stars interacting with one another. This article provides a brief overview of star formation as it is classically understood, as well as the current studies of multiple star formation and the unique behaviors that result.

The Classic Stages of Star Formation

Stars are formed by the gravitational contraction of gases that permeate the Galaxy. These gases are known as the interstellar medium (ISM). Specifically, the environments required for starbirth are molecular clouds (Figure 1), which are the coldest, most dense areas of the ISM. When a cloud of gas contracts under its own gravity, the temperature rises. This response acts to raise the pressure, which provides an outward force against gravity. Star formation occurs when the gravitational contraction of the molecular cloud is too strong and can no longer be supported by the pressure. It then fragments into very dense cores of gas because of the constant pull of gravity (Goldreich & Kwan 1974). As a cloud core collapses, the in-falling material starts to rotate around the core center. The closer it gets to the center, the faster it rotates. The first stage of star formation (Figure 2a) is the formation of the slowly rotating cloud cores. The second stage (Figure 2b) , or the so-called "class I object," begins when the cloud core becomes unstable and collapses. Gravity causes the gas to collect at the center of the core, a process known as accretion.

The class I object is characterized by the central, dense core that will become the star (a "protostar" ) and an orbiting disk of gas (a "circumstellar disk" ) deeply embedded within the in-falling gaseous envelope. Convection inside the protostar This results in a third state (Figure 2c), or the "class II object", leads to

Figure 1. Orion Nebula is one of the closest star-forming regions, at a distance of 1,500 light-years away from Earth. It is located in the middle of the "sword" region of the constellation Orion. The stars have formed from collapsing clouds of interstellar gas within the last million years. The most massive clouds have formed the brightest stars near the center, and these are so hot that they illuminate the gas left behind after the period of star formation ended. This image was taken with NASA's Hubble Space Telescope (NASA, and C. R. O'Dell and S. K. Wong, Rice University).

the turbulent motion of gas in the interior that entangles with the emerging magnetic field generated by the protostar, with a stellar wind of charged particles emanating from the protostar. This wind prevents the fast-rotating material around the protostar from falling directly onto the surface. Instead, the gas falls toward the rotational axis of the protostar and is subsequently ejected, creating a bipolar outflow. These stars are also known as T Tauri stars, and are typically a few million years old. As time proceeds, more and more of the rotating inflowing matter will fall onto the circumstellar disk rather than onto the star. When most of the matter in the cloud core has been accreted, the infall terminates, revealing the nearly completed star. The object then enters the fourth evolutionary stage (Figure 2d), a T Tauri star with a remnant circumstellar disk, or a "class III object." When nuclear reactions begin within the interior of the protostar to stabilize it from further collapse, it is recognized as a mature star (also called a main-sequence star).

Figures 2a, 2b,2c & 2d . The four stages of star formation (Shu, Adams, & Lizano 1987).

This scenario of star formation regards the protostar as isolated from any other forming stars within the molecular cloud. In truth, however, it is not unusual for stars to form in closely spaced groups. By turning to computer models, we can get a more accurate picture of how most stars form in their crowded environments.

A Computer Model of Star Formation

Star formation is a dynamic process that changes a large cloud of gas into a host of shining stars over millions of years. Astronomers can observe current star-forming regions in the night sky, but these are only short snapshots that are unable to capture the dramatic motions of the cloud over the millennia. To better understand star formation and speed it up so that the whole process can be seen from start to finish, a team of astronomers led by Matthew Bate conducted a computer simulation at the United Kingdom's Astrophysical Fluid Facility (Bate et al. 2003). An imaginary cloud 50 times as massive as the Sun and just over a light-year in diameter (Figure 3) was allowed to collapse under its own gravity over a simulated period of 266,000 years. The model required 100,000 hours of real computing time, which was 10% of the total supercomputer time available for that year at the facility. As the model evolves in time, the random, turbulent motions of the cloud cause the gas particles to collide with each other, resulting in the cloud radiating away its initial energy. This energy loss allows gravity to pull the gas together more tightly, causing more collisions and releasing more energy, and so on (Figure 4) . This cycle of cloud contraction and energy loss eventually creates very dense cores at the center of the cloud, and it is here that star formation truly begins.

Figure 3.The initial cloud of the Bate et al. (2003) simulation. The 50 solar mass cloud is a light-year in diameter and contains a turbulent gas that will become gravitationally unstable. (Shu, Adams, & Lizano 1987)

Figure 4. As kinetic energy is lost, clumps begin to form within the cloud. (Bate et al. 2003)

Within the dense molecular cloud cores, smaller pockets of gas continue to collapse, forming what astronomers call stellar embryos (Figure 5) . As the first stars are born within these embryos, they feed upon the collapsing gas, growing in mass. Eventually, the stars are large enough that they begin to feel the gravity of their fellow newborn stars nearby. The young stars then begin a cosmic dance, pulling each other and causing their orbits in the cloud to change. Some stars come close together and will form bound groups of two, three, or more stars. Other stars get violently thrown about the cloud, and some can even be ejected from the cloud altogether. This chaotic tug-of-war among the young stars scatters them across the cloud (Figure 6).

Figure 5. Eventually, stars begin to form within the densest part of the cloud. (Bate et al. 2003)

Figure 6. As the young stars gravitationally interact with each other, their orbits change in size and orientation. This results in the stars being spread across the cloud. (Bate et al. 2003)

As the stars travel around the cloud, they retain some of the gas from their initial formation (that is, the remnants of the stellar embryo). This gas continues to collapse under its own gravity, but cannot land directly onto the star because it is spinning too rapidly. Instead, it forms a flattened disk around the young star (see previous section). These circumstellar disks can be seen forming in the model (Figure 7). In examining this star formation model, it is clear that stars will form close together in the dense cores of interstellar gas clouds. This provides a good justification for why the study of binary stars is so fundamental to understanding star formation as a whole.

Binary Formation and Evolution

The study of young binaries began with Joy & van Biesbrock (1944) who found five visual binaries among the newly recognized T Tauri stars. Visual binaries were found first because there is a large physical separation between the two stars, allowing them to be resolved in a telescope as separate objects. Herbig (1962) added another twenty-four T Tauri binaries to the list, demonstrating that binaries among young stars are common. More current studies of young binaries are leading to a paradigm shift in star formation. Reipurth & Zinnecker (1993) imaged 238 young stars and detected thirty-seven binaries, which implies that a fairly high pre-main-sequence (PMS) binary frequency of 16% in the projected separation range of 150 to 1800 astronomical units (Figure 8). From more studies of the separations of the PMS binaries, Reipurth & Zinnecker found an excess of PMS binaries at large separations compared to the distribution of main-sequence binaries. This fact indicates that there is orbital evolution in binaries from large PMS orbits to closer main-sequence orbits. This orbit decay serves to produce unique stellar phenomena, an example of which we discuss below (see FUors: Eruptions of Binary Stars).

Figure 7. Large circumstellar disks surround the young stars. (Bate et al. 2003)

The formation of binaries helps to slow down the fast rotating and collapsing cloud after accretion has begun. It is plausible that at least the wider binaries can be formed directly by the fragmentation of rotating cloud cores (Figure 9). Recent simulations show that in a disk of modest mass, fragmentation can occur and lead to the formation of a smaller companion object such as a massive planet or small star (Boss 2001, 2002; Rice et al. 2003). Observed binary systems show a large dispersion in their properties, having no strongly preferred values for parameters such as separation and eccentricity . This broad dispersion suggests a very dynamic and chaotic formation process (Larson 2001; reviewed by Larson 2003).

Figure 8. Examples of mosaic CCD images of pre-main-sequence binaries from Reipurth & Zinnecker survey (1993).

Figure 9. Dust continuum images of IRAS 19410+2336. The largest-scale emission (left) shows two massive gas cores roughly aligned in a north-south direction. An image with more than twice the spatial resolution (center) shows more substructures, with about four sources in the southern core and four in the northern core. At the highest spatial resolution (right), the known gas clumps resolve into even more sub-sources with at least 12 sources per large-scale core. This provides evidence for the hierarchical fragmentation of a high-mass protocluster down to scales of 2000 AU at the earliest evolutionary stages (Beuther & Schilke 2004).

If the orbit of a forming binary system is highly elliptical (as is true for most binaries), the tidal effect of the stars pulling on one another will be time dependent and produce strong disturbances at each time of closest encounter, called periastron (Bonnell & Bastien 1992). This can cause episodes of rapid accretion of mass onto one or both stars (see the following section for more details). The jetlike Herbig-Haro (HH) outflows, which are believed to be powered by rapid accretion onto forming stars at an early stage of evolution, are clearly episodic or pulsed, and this suggests that the accretion process is itself episodic (Reipurth 2000, 2001). The jet sources are frequently found to have close stellar companions: at least 85% of the jet sources are members of binary or triple systems, which is the highest binary frequency yet found among young stellar objects (Reipurth 2000, 2001). A few examples are shown in Figures 10-12; more images taken from the Hubble Space Telescope can be found on the News Center of the Hubble Site.

Detailed analysis by Reipurth (2000) of giant jet energy sources shows that 80-85% appear to be binaries or multiples. Since much fewer stars are binaries and multiples on the main sequence, it follows that some binaries must break up. Figure 13 shows the three stages of motion in the transformation from a nonhierarchical to a hierarchical triple system, which increases the system's long-term stability. In the interplay phase, the three stars are surrounded by substantial circumstellar disks and move around chaotically inside a large envelope. Sooner or later, a close triple approach occurs, which means the stars exchange energy. For the circumstellar disks, it means serious tidal stripping and truncation, possibly even direct disk-disk collisions. An ejection occurs as a result of the exchange of energy, and a new binary is formed. The circumstellar disks are now truncated, and the material culled from the individual disks probably relocates to a circumbinary disk surrounding both stars. The small third star that is ejected may escape, leaving behind the newly formed binary, or it may remain bound in an extended orbit around the two more tightly bound stars.

FUors: Eruptions of Binary Stars

Figure 10. This image of HH 47, taken with NASA's Hubble Space Telescope, reveals a three trillion mile-long jet. The jet's very complicated pattern indicates the star (hidden inside a dust cloud near the left edge of the image) might be wobbling, possibly because of the gravitational pull of a companion star. When the jet collides with interstellar gas, it causes the jet to glow. The white filaments on the left reflect light from the obscured newborn star. The HH 47 system is 1500 light-years away and lies at the edge of the Gum Nebula (J. Morse/ STSci; NASA).

Figure 11, HH 1/HH 2. The HH 1 jet and a neighboring flow, HH 144, emanate from two deeply embedded sources. The jet spans slightly more that a light-year, and the young star lies midway between the jet, hidden from view behind a dark cloud of dust. Optical and infrared images taken from HST have revealed a smaller third jet flow. In other words, the source region contains a triple system. HH 1/HH 2 lies 1500 light-years away in the constellation Orion (J. Hester, Arizona State University; the WFPC2 Investigation Definition Team; NASA).

Figure 12, Haro 6-10 . Near the source, the flow breaks up into two components with different directions. Observations from the Very Large Array of radio telescopes have revealed that there are three sources, forming a not-quite-stable triple system (Reipurth et al. 2004).

Figure 13. A schematic presentation of the three stages of motion in the transformation from a nonhierarchical to a hierarchical triple system (Reipurth 2000).

The interactions within binary star systems have become a popular explanation among astronomers for some exotic stellar behaviors. For example, there are rare stars in the sky that will suddenly become many times brighter in a matter of days, then slowly dim again over many years (Figure 14) . The strange phenomenon was first seen in the star FU Orionis, so these quickly brightening objects in the sky are called FUors. Only a dozen or so of these objects have been observed, and several theories have been put forward over the years to explain what is triggering these stellar "eruptions" (Hartmann & Kenyon 1996; Bell & Lin 1994). Most astronomers agree that the sudden brightness of the star is caused by the rapid accretion of gas from the circumstellar disk. But what would cause the gas to suddenly fall onto the star so fast? Bonnell & Bastien (1992) suggested that FUors may in fact be binary stars, and that it was their periodic close-encounters that would cause material from the circumstellar disk to fall onto the star and cause it to brighten substantially.

As mentioned in the previous section, young stars close together within a cloud core will pull on each other and exchange gravitational energy. As a consequence, some of the stars will be ejected (or at least flung out into very wide orbits) while others will orbit closer to one another. When a pair of young stars is locked into a tight orbit, they may periodically pass through each other's circumstellar disk. The friction upon the passing star caused by the disk will dissipate more energy and cause the stars to spiral toward each other over time. As the orbits of the stars become smaller, their gravitational force upon one another will increase, and their circumstellar disks will become more distorted under the growing tidal forces of the stars. At each periastron, the opposing forces of the stars will cause the disk gas to fall onto the stars, resulting in large outflows and accretion events. The outflows are seen by astronomers as Herbig-Haro jets (see the

Figure 14. Lightcurve (magnitude vs. time) of a FUor object. The x-axis is in days and the y-axis is in magnitudes (the smaller the magnitude, the brighter the object). Note the sudden jump in brightness, followed by a long period of gradual dimming (Scott Kenyon, private communication).

previous section), while the accretion events will trigger eruptions that appear as FUors. Such violent events are believed to end when no more gas remains between the two stars, and the two circumstellar disks have become a more stable circumbinary disk.

As seen in the case of FUors, seemingly odd behaviors in stars can be more easily explained when considering multiple stars in groups instead of thinking of stars as isolated objects.

Sisters of the Sun?

Given that stars appear to form close together and within small groups, it seems reasonable to think that our own Sun might have formed in a very similar environment. Traditional models have always depicted the solar system forming in isolation, involving only the Sun itself and the planets that formed around it. However, it is much more likely that the Sun began its life surrounded by other newly born stars, and there is some evidence that points to this more complex picture of the early solar system. If the Sun had formed in isolation, then its undisturbed circumstellar disk would be in the same plane as the Sun's equator. In other words, both the Sun and the disk of gas that formed the planets would both appear to rotate around the same axis. However, when actual measurements are made today, the mean plane of the planetary orbits is tilted from the Sun's rotation axis by 7 degrees. The close passage of a star could explain this tilt, as the passing star would have pulled on the disk with its enormous gravity. Stars rarely come close to one another in the vastness of interstellar space, but close encounters are probable when the stars are still young and clustered together in star-forming regions (Figure 1). Therefore, a tilt of the planetary orbital plane suggests that the Sun was very close to other stars in its early life. If this theory is correct, then it means that there are other stars out there, spread throughout the Galaxy, that formed from the same cloud as our own parent star: the sisters of the Sun.

Summary

As astronomers gain more insight into the early life of stars, a more dynamic picture of star formation is emerging. What was once thought to be a centralized and isolated process is now seen as decentralized and interactive. From dramatically shifting orbits, to large outflows and eruptions, young stars are proving to be much more active creatures than the older ones such as the Sun. In reflecting on the large frequency of multiple star systems, it raises an important question for the field of astrobiology: How would life develop on a world with two or more suns? The answer to such a question will come only with the continued study of multiple star systems, and their unique properties.

References

Bate, M. R., Bonnell I. A., & Bromm, V. 2003. The Formation of a Star Cluster: Predicting the Properties of Stars and Brown Dwarfs . Monthly Notices of the Royal Astronomical Society (MNRAS), 339, 577-599.

Bell, K. R., & Lin, D. N. C. 1994. Using FU Orionis Outbursts to Constrain Self-Regulated Protostellar Disk Models. Astrophysical Journal (ApJ), 427, 987-1004.

Beuther, H., & Schilke, P. 2004. Fragmentation in Massive Star Formation. Science, 303, 1167-1169.

Bonnell, I. A., & Bastien, P. 1992. A Binary Origin for FU Orionis Stars. ApJ, 401, 31-34.

Boss, A. P. 2001. Gas Giant Protoplanet Formation: Disk Instability Models with Thermodynamics and Radiative Transfer. ApJ, 563, 367-373.

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Goldreich, P., & Kwan, J. 1974. Molecular Clouds. ApJ, 189, 441-454.

Hartmann, L., & Kenyon, S. J. 1996. The FU Orionis Phenomenon. Annual Review of Astronomy and Astrophysics (ARA&A), 34, 207-240.

Herbig, G. H. 1962. Spectral Classification of Faint Members of the Hyades and Pleiades and the Dating Problem in Galactic Clusters. ApJ, 135, 736-747.

Joy, A. H., & van Biesbrock, G. 1944. Five New Double Stars among Variables of the T Tauri Class. Publications of the Astronomical Society of the Pacific (PASP), 56, 330, 123-124.

Larson, R. B. 2001. Implications of Binary Properties for Theories of Star Formation. In: The Formation of Binary Stars (IAU Symposium 200), ed. H. Zinnecker & R. D. Mathieu (San Francisco: Astronomical Society of the Pacific) , 93-106.

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Reipurth, B. 2000. Disintegrating Multiple Systems in Early Stellar Evolution. The Astronomical Journal (AJ), 120, 3177-3191.

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Web Links

Simulation of Bate et al. (2003):

http://www.astro.ex.ac.uk/people/mbate/animations.html

Images of the Herbig-Haro outflows on the Hubble Site:

http://hubblesite.org/newscenter/newsdesk/archive/releases/category/star/protostellar%20jet/

News Center of the Hubble Site:

http://hubblesite.org/newscenter/

Glossary

accretion : The process of gas falling onto the central (proto)star and adding to the object's total mass.

astronomical unit (AU) : The Earth-Sun distance; approximately 150 million kilometers or 93 million miles.

binary : A system of two stars bound together by their mutual gravity.

bipolar outflow : The flow of material in two opposing directions from a central star. Most young stars go through a violent phase of ejecting material. In some, jets are seen moving away from the star at a speed of many hundreds of kilometers per second.

circumbinary disk : The flattened distribution of gas that orbits a pair of stars. It is composed of the remnants of the circumstellar disks that once orbited each individual star.

circumstellar disk : The flattened distribution of gas that orbits the protostar and is depleted of mass over time as a result of accretion and tidal forces.

class I object: A protostar still partly embedded in a gaseous envelope; less than a million years old.

class II object : A T Tauri star with a large circumstellar disk that is a few million years old.

class III object : A T Tauri star with a depleted circumstellar disk that is less than ten million years old

convection : The circulatory motion that occurs in a fluid at a nonuniform temperature because of the variation of its density and the action of gravity.

eccentricity : A measure of how far as orbit diverges from a circle, that is how elliptical it is.

FUor: An object characterized by a sudden jump in brightness, followed by a long period of slow dimming that is caused by large accretion events stimulated by close encounters in a binary star system.

Galaxy (capitalized): The spiral galaxy containing our solar system and all of the stars visible to the naked eye, often referred to as the Milky Way.

Herbig-Haro (HH) outflow : A large outflow of gas from a binary system caused by tidal interactions between the two stars.

interstellar medium (ISM) : The gas spread across the Galaxy that constitutes the raw material of star formation; composed of mostly hydrogen and helium.

infrared image : An image that is taken at infrared wavelengths between about 7 × 10^-7 meters and 1 millimeter, which is just past the red end of the visible spectrum.

light-year : The distance that light travels during one year, approximately 9.5 trillion kilometers or 5.9 million miles.

main sequence : The population of mature stars whose cores are undergoing hydrogen fusion, which stabilizes them against gravitational collapse.

molecular cloud : A cold, dense collection of interstellar gas where stars can form.

molecular cloud core : The densest part of a molecular cloud that fragments under the force of gravity to begin the star formation process.

optical image : An image that is taken at visible wavelengths between about 4 × 10^-7 and 7 × 10^-7 meters.

periastron : The point in an elliptical orbit around a star that is closest to the star's center.

pre-main sequence (PMS) : The population of protostars that have not yet begun nuclear reactions within their cores.

protostar : A dense object at the center of a cloud core that will grow into a star.

stellar embryo : The gaseous envelope that collapses from the cloud core to form a protostar and its surrounding disk.

tidal stripping : The ability of a massive body to raise tides on another body. Tidal forces can bring about the destruction of a satellite orbiting a planet or a comet approaching too close to the Sun or a planet.

T Tauri star : A young star with a circumstellar disk and a bipolar outflow.

visual binary : A binary star in which the two components are sufficiently well separated to be individually detectable visually or photographically.