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The question about the origins of our Solar System has always been part of human speculation going back at least to Kant's and Laplace's hypotheses in the 18th century. The previous chapters have illustrated how stars emerge from molecular cloud cores and demonstrated that the evolving stars are surrounded by what is called an accretion disk, which evolves with the central star. This disk is synonymous with the Solar Nebula, cited in many referrals to the protostellar disk from which the evolution of today's Solar System began. With all the technical advances available at present, the observation of protostellar disks and the detection of evolved planets outside the Solar System is as popular a topic as ever. The following sections attempt to shortly introduce the reader to advances in observations of protostellar disks, their dust properties, as well as some concepts in the discussion of how protoplanets could have formed out of dust disks. [ABSTRACT FROM AUTHOR]

Satisfactory assessments of star-forming regions in the Galaxy are essential for the understanding of early stellar evolution and today a vast amout of astronomical studies are available. Star-forming regions distinguish themselves from other regions in the Galaxy. Many different physical processes are displayed with high levels of variability, a large number of outflow sources, interactions with the circumstellar medium and emissions throughout the entire electromagnetic spectrum. None of the other stages of stellar evolution appear so evidently active in extended, but limited, regions in the sky. Clearly, supernova remnants or planetary nebulae do the same, but they always represent single isolated events. The reason for this may seem obvious. During later stages of stellar evolution, stars with different masses follow different evolutionary paths and only in very rare cases appear clustered. In the early days of stellar formation research these regions were mostly identified as large associations of early-type stars. Even today, OB associations are considered to be synonymous with regions harboring active stellar formation. Although it has been realized that not every star-forming region produces massive stars, those are in the minority. [ABSTRACT FROM AUTHOR]

The discovery of bright X-ray emission from young low-mass stars in the wake of the launch of the EINSTEIN X-ray observatory (see Chap. 2) [235] [278] truly took researchers by surprise. X-rays from Orion had been detected with Uhuru, the first X-ray satellite launched in the early 1970s, but due to the lack of spatial resolving power a connection to young low-mass stars seemed far from likely. At the time, these stars were investigated for their complex emission patterns at long wavelengths. Models for gravitational collapse do not provide many clues about mechanisms for emissions at wavelengths much shorter than a few microns (see Chaps. 5 and 6). High energy emission requires special circumstances and the responsible physical processes are often different from the ones at the origins of the optical and IR emissions. In order to emit at short wavelengths, one either needs very high temperatures or very high magnetic field strengths or some mechanism to produce high-velocity electrons. The previous chapters already demonstrated that gravitational potentials in protostellar systems are by far too small to free enough energy (see (6.7)). Thus the energy has to come from a different pool, which is most likely rotational and magnetic energy inherited from pre-collapse cloud dynamics. Synchrotron radiation from magnetic fields as high as 1011 G can be ruled out on the grounds that superdense degenerate matter, as found in neutron stars is needed to carry such high field densities [18]. The previous chapter also showed that magnetic field strengths in protostellar environments do not exceed 1 kG. [ABSTRACT FROM AUTHOR]

The last decade was full of new developments and concepts leading to the present understanding of physical processes in YSOs. While the early stages prior to collapse were governed by large-scale turbulence, magnetic fields and the presence of dust it seems more and more evident that the processes in the aftermath are once again dominated by these constituents. Dust, magnetic fields, and turbulent processes are within the ingredients that characterize the vicinity of protostars. The main contributing processes relate to accretion, which is a turbulent process, as well as to stellar and disk magnetic fields. Omnipresent again is dust, which is constantly modified by radiative heating and grain-grain collisions [114]. Grain properties change dramatically during the later stages of PMS stellar evolution due to a manifold of physical and chemical processes. For now the discussion of some of these processes will be postponed to appear within the context of protoplanetary disks in the final chapter. [ABSTRACT FROM AUTHOR]

This chapter decribes the evolution of stellar objects after the PMF phase when a young stellar object forms via accretion from its circumstellar cloud. It is difficult, if not impossible, to draw a line between the process of protostellar formation and the evolution of a protostar toward a mature star on the main sequence. The dense central object that forms during the collapse of a cloud core is not yet a star, but it is also not a molecular core anymore. One should define what is meant by the terms protostar, PMS star, and young stellar object as these expressions are common in the literature: Protostar: the optically thick stellar core that forms during the adiabatic contraction phase and grows during the accretion phase.Protostellar system: the entire young stellar system with its infalling envelope and accretion disk. The nomenclature includes binary and multiple systems. The extent of the system is fairly well descibed by its Jeans length.PMS star: the premature star (PMS = pre-main sequence) that becomes visible once the natal envelope has been fully accreted and which contracts towards the main sequence.PMS stellar system: the PMS stars with their accretion disks and their star/disk interaction regions.YSO: the entire stellar system (YSO = young stellar object) throughout all evolutionary phases. However, it is specifically used during the collapse phases and in the case of massive systems, where evolutionary phases cannot easily be distinguished. [ABSTRACT FROM AUTHOR]

One of the most fascinating aspects of stellar formation is the enormously vast change of physical parameters during the process of gravitational cloud collapse. A comparison of the initial density in a molecular cloud core with the average density in mature stars, for example, reveals a difference of twenty orders of magnitude. The typical density of such a cloud is ∼ 10−20 g cm−3, whereas the average density of a star like our Sun is of the order of 1 g cm−3. A change of this magnitude cannot be achieved by external forces but only through the self-gravitational collapse of matter. During this collapse the size of the object reduces from light years across to a few hundred thousand kilometers in radius with internal temperatures rising from a few K to 30 million K. The following sections outline the physics involved in the stellar-formation process from the onset of the collapse to what is generally considered to be a protostar and further describe its first approach towards the main sequence. [ABSTRACT FROM AUTHOR]

Of specific interest for stellar formation are regions where matter in the ISM appears most dense and in the form of clouds. Although there is no particular definition of the term cloud, it describes regions with a density larger than about 10 to 30 atoms per cm3. There are diffuse clouds, which do not reach these densities, and self-gravitating clouds in the range of the above densities. Some also define a third unbound category which are neither pressure nor gravity-supported and are probable clouds in the process of dispersal [225]. Clouds move as entities through the Galaxy with a peculiar velocity on top of the radial velocity due to the general rotation around the Galactic Center. Peculiar motions of such clouds usually have velocities of the order of 5 to 10 km s−1. For a long time it was thought that two forces are at work to keep clouds stable: gravity to contract the cloud and internal (thermal) pressure and turbulence to disperse the cloud. However it has been recognized in recent decades that this view may not be sufficient and magnetic as well as rotational properties of molecular clouds are also important. This chapter describes the global properties of clouds in the Galaxy, outlines relevant physical processes that affect these clouds and then finally discusses initial conditions of cloud cores before they undergo a gravitational collapse. [ABSTRACT FROM AUTHOR]

Observational evidence that the space between the stars in our Galaxy is not empty came as early as the beginning of the 20th century with the discovery of stationary Ca II lines in spectra of the binary star δ Orionis by J. Hartmann in 1904. In 1922 E. Hubble concluded that dust clouds scatter the light of nearby cool stars, which are then detected as reflection nebulae, while hot stars excite the gas in emission nebulae. He concluded that diffuse clouds irradiated by a radiation field have kinetic temperatures of about 10,000 K and frequent collisions would establish Maxwellian velocity distributions for electrons and ions in space. [ABSTRACT FROM AUTHOR]

It is a common perception that astronomy is one of the oldest occupations in the history of mankind. While this is probably true, ancient views contain very little about the origins of stars. Their everlasting presence in the night sky made stars widely used benchmarks for navigation. Though it always was and still is a spectacular event once a new light, a nova, a new star appears in the sky. Such new lights are either illuminated moving bodies within our Solar System, or a supernova and thus the death of a star, or some other phases in the late evolution of stars. Never is a normal star really born in these cases. The birth of a star always happens in the darkness of cosmic dust and is therefore not visible to human eyes (see Plate 1.1). In fact, when a newborn star finally becomes visible, it is already at the stage of kindergarten in terms of human growth. It takes the most modern of observational techniques and the entire accessible bandwidth of the electromagnetic spectrum to peek into the hatcheries of stars. [ABSTRACT FROM AUTHOR]

The study of the formation and early evolution of stars has been an ever growing part of astrophysical research. Traditionally, often in the shadow of its big brothers (i.e., the study of stellar structure, stellar atmospheres and the structure of galaxies), it has become evident that early stellar evolution research contributes essentially to these classical fields. There are a few new items on the list of traditional astrophysical studies, some of which are more related to features known from the extreme late stages of stellar evolution. Examples are accretion, outflows, disks, and large-scale turbulence. The field today requires the most powerful and sensitive instruments and telescopes mankind is able to provide. It utilizes the fastest computers and memory devices to simulate jets, evolving disks and outflows. A network of researchers all over the planet invest resources and time to contribute to the steadily growing knowledge about the origins of stars and planets and ultimately the birth of the Solar System. [ABSTRACT FROM AUTHOR]