The Story of Star Formation
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Vast clouds of gas and dust are swirling throughout our Milky Way galaxy. Many of these clouds are stellar nurseries, places where one (in the case of small clouds) to tens of thousands of stars (in the case of the largest and most massive clouds) are being born right now! These clouds range in size from cores that are 100,000 times the size of the Solar System and mass of several Suns (solar masses), to giant clouds more than ten million times the size of our Solar System and many thousands to tens of thousands of solar masses. A typical star-forming cloud might create a very few massive stars (20 solar masses or more), many stars like our Sun, and many more lower-mass stars and brown dwarfs, which are objects of mass well smaller than that needed to produce star fueled by nuclear fusion (0.08 solar masses). An umbrella term for all of these newly- forming objects is young stellar objects (YSOs).
The Spitzer Space Telescope has been incredibly powerful in furthering our understanding of how YSOs form and evolve. There are multi-band Spitzer images of about 300 square degrees of the Galactic plane, with more than 100 million sources. There are maps of about 70 square degrees in nearby (within about 1500 light years) star-forming regions, with about 8 million total sources. Conservatively, I estimate that there are about 20,000 YSOs in this rich data set. We are still learning from Spitzer's data, and we will learn from it for decades to come.
Spitzer's data, alone and combined with data from other telescopes, enable understanding of many things, including the factors that control the efficiency of the star-formation process in different clouds, what determines whether a high- or low-mass star forms in different parts of stellar nurseries, and how the process of star formation across all masses is related to the formation of planetary systems and ultimately life-bearing planets analogous to our own Solar System.
Astronomers have developed guidelines -- reasonable ideas -- regarding the basic processes of star formation for stars like our Sun. This working framework derives both from numerical modeling and observations of forming YSOs, not only with Spitzer, but with other telescopes that are capable of probing the physical, chemical and dynamical state of the gas and dust that ultimately collapses to form stars. Similar processes are likely to operate in much more massive stars (only faster) and brown dwarfs (only slower).
We believe that stars are born in rotating cores, comprised of gas (100 parts) and dust (1 part), which begin to collapse when the force of gravity (dependent on the mass and radius of the core) exceeds the internal pressure (a measure of the internal motions of material within the core -- motions that push back against the inward pull of gravity). The collapsing, rotating core forms a central "stellar seed," surrounded by a geometrically thin disk. The rotating seed-disk system is continually fed from the material in the rotating cloud. The orbiting disk material plays a crucial role in two ways: first, in transporting material from the cloud to the central seed, allowing the seed to eventually (over several hundred thousand years) to reach a mass comparable to that of the Sun; and second, in providing the material from which planets can form. This results in planetary systems like our own, where all the planets are in a plane, rotating in the same direction -- this is a remnant signature of our own protoplanetary disk.
The very young star+disk+cloud system is completely hidden in the optical -- the dusty cocoon of matter obscures the central object in the visible bands. These objects emit the majority of their light in the infrared. Prior to Spitzer, we knew of only about 50 stars at this stage of development. However, with Spitzer, we have now detected several hundred of these protostellar cores, whose presence is inferred from unique patterns of brightness vs. wavelength. At Spitzer's shortest wavelength (3.6 microns), the light comes mainly from the forming star at the heart of the core. At longer wavelengths, the light from the object becomes stronger, a signature that it is not a background star. Also, in the longer wavelengths (8 and 24 microns), you can see the glow from interstellar dust surrounding the YSO, glowing green to red in the Spitzer composite image. This dust consists mainly of a variety of carbon-based organic molecules known collectively as polycyclic aromatic hydrocarbons (PAHs). The red color traces a cooler dust component.
Owing to its tremendous sensitivity, Spitzer is able to probe star-forming clouds more than 20 times further from Earth than had been probed by previous ground- and space- based observations. As a result, we now have a catalog of newly- formed stars spanning nearly the full range of known stellar masses -- a crucial precursor to understanding what kinds of natal protostellar cores give birth to what kinds of stars and why. Follow-up observations with sensitive radio telescopes provide measurements that allow us to assess the masses, internal motions and rotation speeds of these cores -- critical factors to understanding the kinds of conditions conducive to forming stars of different masses, and to determining the characteristics of the disks that ultimately form planetary systems.
The birth of a star is a violent event. Early in its evolutionary history, while the YSO is still embedded and optically invisible, a powerful, highly-collimated, jet- like outflow begins to emerge from the protostellar core. Outflows are a signpost that material from the core has formed a disk surrounding the embedded stellar seed, and that the disk has begun to transport material to the surface of the cocoon. This accretion process results in the launching of jets, or outflows. The precise launching mechanism for jets is unknown, but is likely to be related to rotation, magnetic fields, accretion, and the interaction between the star and the disk. About 10% of the matter that is accreted through the disk is ejected as an outflow or jet. The jets can reach sizes of trillions of miles and velocities of hundreds of thousands miles per hour. The launching of jets through the accretion process may be nature's way of limiting or controlling the ultimate mass of the star that forms -- the jets disrupt the cocoon and remove material that would otherwise ultimately reach the forming star at the center of the cloudlet. With Spitzer, the forming star and its jets of molecular gas appear with clarity; three examples are given above. The 8-micron channel is sensitive to emission from PAHs, which are excited by the surrounding radiation field and become luminescent. The shock fronts from jets of material plowing supersonically into the cold, dense gas nearby are easily visible. In particularly active regions of star formation, as seen in NGC 1333, 100s if not 1000s of jets are found, each (pair) of which can be traced back to a parent YSO.
During the assembly of the star-disk system, both the central star and disk are obscured from observations using conventional optical telescopes by the collective effects of micron-size dust grains in the natal material surrounding the YSO. Ultimately, through the combined effects of incorporation of core material onto the star and the dissipative role of outflows/jets, the cocoon of material surrounding a forming YSO is disrupted, ultimately rendering star-disk systems visible. Superb examples of this phase of star formation are provided by Hubble Space Telescope imaging of nearby star-forming regions such as the Orion Nebula Cluster. Here, the background light from the Orion Nebula (an ionized hydrogen region) allows the emerging star-disk systems to be seen in silhouette, or glowing themselves in response to ultraviolet photons in the region.
Some YSOs are close enough that with Hubble or ground-based interferometers, we can resolve the disk without external illumination as is found in Orion. Edge- on disks provide particularly dramatic views of the disk, where a dark lane slices through the image of the YSO. The initial distribution of dust within a disk mimics the shape of a saucer: flat near the central star, and "flaring" upward at larger distances. This characteristic shape reflects the combined effects of gravity pulling material downward toward the midplane, and thermal pressure of heated gas and dust pushing against the force of gravity.
In more distant star-forming regions, or in regions which lack the background illumination provided by the Orion Nebula, the presence of emerging YSOs can be inferred from Spitzer observations of spectral signatures specifically indicative of systems characterized by orbiting dust, illuminated and heated by a central star. (See figure below.) The brightness as a function of wavelength for a plain star (no dust around it) results in most of the light being produced at shorter wavelengths. In the case of a YSO with a disk of dust and gas around it, the disk is heated by the star, and the warm dust and gas around the star produces its own infrared light, which changes the shape of the spectrum we see from the combined star+disk. The circumstellar material is cooler than the surface of the star, so it emits most of its light at longer infrared wavelengths -- there is an excess of infrared emission, which cannot be coming from the star itself, betraying the presence of the disk. The inner part of the disk can be swept away, perhaps by the formation of a planet via agglomeration of the small dust grains responsible for producing excess emission in the inner regions of the disk. The dust closest to the star is also the hottest, so its absence means that there is less emission from the disk from its inner regions. The only dust producing infrared light is much further away from the star, and much cooler, producing as a result emission only at long wavelengths. This resulting "bump" or inflection in the spectrum indicates a disk with a missing center, and may be the first clue that planets have formed inside the disk.
Using these patterns of brightness, Spitzer observations, combined with optical observations made from the ground, can provide a census of the number of forming stars that in fact are surrounded by disks. Studies of tens of star-forming regions, including the greater Orion complex, reveal 1000s of likely YSOs, and find that more than 80% and probably all stars less than about a million years old -- ranging in mass from 20 solar masses or more, to objects of mass well smaller than that needed to produce star fueled by nuclear fusion (0.08 solar masses) -- are surrounded by disks.
By using Spitzer's power to carry out a virtually complete census of star-disk system in star-forming clouds, it is possible to begin to study under what conditions stars form in relative isolation or in dense clusters, whether the formation of particular kinds of stars can trigger the formation of new generations of stars, and ultimately, how the stars that now populate the sea of stars in our Milky Way galaxies came to be over time.
More than 25 years ago, astronomers noted that regions that form stars with masses well in excess of 10 solar masses -- regions like the Orion nebula -- not only formed signature ionized hydrogen regions (gas fluorescing in response to the strong ultraviolet light emanating from massive stars) but also seemed to be associated with other groups of nearby, newly-formed stars. At the time, a few prescient researchers suggested that the formation of a massive star and its associated nebula of ionized gas could ultimately trigger the formation of another generation of young stars.
Spitzer's ability to pick out newly-formed star-disk systems via their signature colors has provided dramatic proof that triggered star formation occurs, and indeed may play a major role in turning clouds of gas and dust into stars. A newly-formed massive star creates an associated ionized hydrogen region and a plethora of newly-formed stars located at the rim defining the interface between the ionized region and the remainder of the cloud. The propagation of the hot, ionized material into the cold gas and dust in the surrounding star apparently compresses gas and dust at the boundary, forcing clumps of material into volumes small enough so that gravity can overcome internal pressure -- thus beginning the collapse of a YSO. The details of the triggering mechanism, and how the mix of stellar masses formed in triggered regions differs from the mix found in regions which form stars "spontaneously" (e.g., by the passage of a spiral arm through the clouds of gas and dust) is yet another mystery that promises to find ultimate solution by combining Spitzer data and observations from other bands.
Spitzer's census of star-disk systems can provide significant clues regarding the kinds of planetary systems that can form around stars like the Sun (as well as stars of other masses), and ultimately answer a key question: is a Solar System like ours a typical outcome of the star formation process, or is it rare? Follow-up observations suggest that at least half of the disks detected by Spitzer have mass sufficient to form a planetary system similar to our own. More modest systems -- perhaps lacking our gas giant planets -- could well form around the remaining 50%. Hence, most, if not all stars form with disks of mass sufficient to form a planetary system, and at least 1/2 can in principle assemble planetary systems similar to our own (as judged by total available disk mass).
Following the formation of a star-disk system isolated from its parent protostellar core, the micron-size dust embedded within the largely gaseous disk begins to settle toward the midplane -- much as dust raised by a windstorm eventually settles earthward, clearing the air. Dust settling to the midplane results in an increase in the density of solid material, a fact that leads to very frequent collisions among the micron-size grains. Such collisions can quickly lead to merging or fusing of the micron-size grains into larger entities. Simulations -- both in the computer and in the laboratory -- suggest that once micron-size grains settle to the disk midplane, they can quickly form larger grains, and soon thereafter, kilometer-size bodies (within several hundred thousand years).
Collisions among these larger bodies (called planetesimals) can in turn produce even larger entities -- planetary cores of lunar, Earth, and even 10s of Earth-mass size. These collisions are accompanied by a "second generation" of dust that within a few orbits gets smeared out into arcs or rings. In the inner regions of disks, near the orbit of the Earth, buildup from grains to earth-size bodies can take place on timescales of several million years. Terrestrial planets (Mercury, Venus, Earth and Mars) are likely to form following this general picture. The cores of the gas giants probably form similarly, but are able to accumulate more of the lighter elements in a runaway process.
Whether or not Jupiters in other systems are able to form, and remain orbiting their parent suns near the place they formed, depends critically on when they form and how much remaining disk material lies outside their orbits. Extra-solar Jupiters that form early, and in systems with massive outer disks, will, via gravitational interaction with outer disk material, be forced to migrate inward -- perhaps explaining the presence of the large number of extrasolar Jovian-mass planets found orbiting much closer to their parent stars than our own Jupiter. Migration of such Jupiters from their place of origin may have catastrophic effects on already- born terrestrial planets; the powerful gravitational field of the inward moving Jupiter could well eject these planets from their planetary systems. It may be that our Jupiter formed at just the right time -- when there was enough material left in the disk to build a planet of its mass, and late enough, so that material in the outer disk did not force it to migrate inward.
Some collisions that occur late in the planetary formation process can destroy worlds. In one recent Spitzer result, there were spectral signatures of molten rock, suggesting that we are seeing remants of a very recent very large collision.
In the outermost regions of the disk, the material thins out, and even the dust-rich disk midplane is only able to form bodies of much lower mass. These outer disk regions may be the birthplaces of objects such as Pluto, and much smaller icy comets which populate the region exterior to Neptune's orbit in our own Solar System: the Kuiper Belt.
Eventually, the gas and dust in the disk is either assembled into planets or smaller bodies spanning a range of masses and orbital distances, accumulated by the central star or gravitationally ejected from the forming system via gravitational interactions with the more massive orbiting bodies. The basic elements of other planetary systems are likely in place by no more than 10-30 million years after the initial core began its collapse.
At the other end of the scale, stars much more massive than our Sun have enough core temperature and pressure to ignite hydrogen very early in the process described above, quickly blowing away their natal material. Not only will those stars start burning H early on, but eventually they are likely to explode as supernovae and trigger star formation in the surrounding molecular cloud, starting the cycle again.