Star formation

With star formation is generally described as the developmental stages that are run from a collapsing molecular cloud core in the formation of a main sequence star. These include several collapse phases, the formation of a pre-stellar core of a protostar and finally a pre-main sequence star. While low-mass stars may also occur in isolation, the formation of massive stars takes place in richer clusters mainly. Broader trends in star formation are a key feature of galaxies and a central aspect of galaxy evolution.

2.1 collapse of molecular cloud cores 2.1.1 Stability and collapse
2.1.2 Observation of pre-stellar cores

3.1 second collapse
3.2 circumstellar disk and jets
3.3 The classification of protostars
3.4 Observation of protostars
3.5 Spectral classification

4.2 Observation of pre-main sequence stars
4.3 Spectral classification

Molecular clouds

Prerequisite for the formation of stars is the presence of relatively dense, cool clouds of matter, so-called molecular clouds. The first signs of these clouds arose from observations already in the 18th - 19th Century: Caroline Herschel reported her brother William Herschel had a seemingly starless region corresponding from today's perspective, such a molecular cloud, found in the constellation Scorpio, and with the words "Here is truly a hole in the sky " comments. Only at the beginning of the 20th century were large-scale sky surveys demonstrate by means of photographic plates that these dark regions are caused by interstellar clouds, and thus obscure underlying stars. Bok finally identified these dark clouds as sites of star formation, whereas the composition of which remained a mystery.

Today it is well known that these clouds to around 70 % of molecular hydrogen (H2) exist ( this is also where the name derives molecular cloud ), which is surrounded by a shell of neutral hydrogen HI. In addition to H2 can be found in these clouds still further molecules, such as carbon monoxide (CO). Furthermore, approximately 1 % of the mass is in the form of interstellar dust (ie, silicate or graphite particles of size 0.1 microns ).

In the Milky Way there are molecular clouds, the mean densities of n ( H2) ≈ 102 cm -3 have, mainly in the spiral arms. Some large complexes can reach a diameter of ≈ 50 pc and masses of ≈ 105-6 M ☉, and are therefore mutatis mutandis also known as giant molecular clouds (English: Giant Molecular Cloud or GMC). However, there are also smaller, relatively isolated molecular clouds with masses of ≤ 102 M ☉

Observation of molecular clouds

As in molecular clouds can not be directly observed with temperatures of 10-20 K, molecular hydrogen, it is possible only in an indirect way, to decipher their structure. Such indirect observation methods use either the presence of Deputy molecules or dust. The most common methods are:

Observations of carbon monoxide ( CO): CO is not only the second most abundant molecule in such clouds, but has the characteristic that its rotational transitions - transition from a rotating state to another, in which infrared light is transmitted - may be observed even at low particle densities. From such observations, the distribution on large scales as well as the amount of CO molecules follows. Assuming that the ratio of constant CO -to H2 molecules, a conversion factor between the H2 density and the intensity of certain spectral lines of CO can be determined; assuming this, one can reconstruct from measurements of CO, the overall structure of the molecular cloud and determine its mass. Another empirical relationship exists between the expansion of the cloud and the line width of the CO lines.
Observations of the wavelength dependence of extinction: Blue light is more on the interstellar dust particles scattered than red light. This circumstance can be used to create Extinktionskarten: background stars shining through a molecular cloud, appear systematically redder than the intrinsic color, if there is more dust along the line of sight, and less red with less dust. The redness is directly proportional to the amount of interstellar dust along the line of sight. This permits the assumption of a constant weight ratio of dust to molecular hydrogen conclusions on its distribution, and hence the structure of a molecule cloud. This method is mainly applied to Nahinfrarotwellenlängen. Here for astronomers has the 2MASS ( Two Micron All- Sky Survey ) with observations at 1.2 microns, 1.6 microns and 2.2 microns proved to be real gold mine since it allows them to create so-called Extinktionskarten of the entire sky.
Far-infrared observations: Due to the low temperature of only 10 K, the thermal emission of dust particles lies in the molecular clouds at wavelengths of about 250 microns. In this wavelength range, molecular clouds are mostly optically thin, which allows a direct conclusion on the amount of dust along the line of sight. Since this wavelength range, however, is not in an atmospheric window are observations only via satellite, eg ISO, possible. Launched in 2009 Herschel space telescope provides astronomers unprecedented resolution and sensitivity, and has since been the way astronomers see star formation, revolutionized.

Molecular cloud structure

Molecular clouds are not simple matter clumps or even spherical structures. Instead, they have a strongly pronounced filamentary structure. Along these filaments are like pearls on a chain condensations, which are regarded as the birthplace of stars. The physical background of this structure is still not fully understood. It assumes, however, that a combination of gravity and turbulence is the cause. The turbulence ensures the local condensations from which stars form in the further course. Another cause of a local increase in the gas density can also be the influence of massive stars, which compress together push through stellar winds and the material.

Usually, one defines in a molecular cloud, a hierarchical structure. Although this division probably has no physical background and a molecular cloud probably has more of a fractal dimension, the classification into cloud ( engl. cloud ) Clot (English Clump ) and core (English Core) is widely available and far. As a cloud The entire structure is referred to a clump is a physically contiguous subgroup, and a core is a gravitationally bonded unit, which is typically seen as a direct precursor of a proto star.

Phases

Collapse of molecular cloud cores

Stars form from dense molecular cloud cores that are embedded in a molecular cloud. Within such a molecular cloud affect a variety of forces. Here at this point it should be mentioned especially gravity, which ensures by its attractive effect that they contract cores on. This collapse is mainly effective against the thermal energy, i.e., the self-movement of the molecules which have only those due to their temperature. Important for stability but also magnetic fields and / or turbulence can be.

Stability and collapse

A simple means for stability analysis provides the so-called virial analysis. If a molecular cloud core in equilibrium, then balance (neglecting magnetic fields and turbulence), the kinetic energy of the particles and their potential gravitational energy straight ahead. For the case that the gravitational energy predominates inevitably follows the collapse of this core. The virial analysis is easy to perform for a core with homogeneous density, but only serves as a rough estimate of the stability of a molecular cloud. However, an actual cloud required for stability, and thus a print density gradient, i.e., the pressure inside must be higher than in the more outer layers. Taking this into account in the stability analysis, the criterion for stability is a density ratio between center and shell. In the limiting case for critical stability then one speaks of a Bonnor - Ebert sphere, and the stability criterion can be converted into a so-called Bonnor - Ebert mass, which must exceed the cloud, so that a collapse can begin.

Inevitably exceeds a molecular cloud core critical mass (that is, the thermal motion of the particles, the self-gravity nothing to oppose ), it follows the collapse. The contraction takes place as soon as the limit of the instability is exceeded once, almost in free fall, that is, the falling inward layers only feel the gravitational potential and unbroken fall ( and thus in particular faster than local speed of sound) in the center. The collapse spreads it from the inside out ( "Inside -out Collapse " ): The region that is collapsing around the densest core regions getting bigger, and more and more of the previously static, thin gas is included in the collapse with.

However, as already mentioned, has a core initially an increased density in the center, and therefore in the region of collapse also proceeds faster than in the shell. During this collapse, gravitational energy is converted into thermal energy and radiated in the millimeter wavelength range. However, since the outer shells are permeable to the radiation of these wavelengths, the energy generated is dissipated by the reduction of the gravitational potential fully outward. Therefore, this first stage is isothermal, i.e. the temperature of the first core does not change.

Magnetic fields play an important role, the situation is much more complicated. Plasma can only move along the magnetic field lines, and affects the movement of the surrounding non-ionizing matter. This results in directions in which a collapse is possible without hindrance ( along the field lines ) and those in which the magnetic field of the collapse counteracts (perpendicular to the field lines ) and thus a further time scale brings into play: the rate of matter across the field lines can diffuse into denser areas.

In the further course of the collapse of the density further increases, the envelope is optically thick to the radiation and thus causes heating. Slowly but surely this heating results in the formation of a hydrostatic balance at the center, which slows the collapse and eventually stops.

This so-called first core consisting mainly of hydrogen molecules has a radius, which corresponds approximately to one and a half times the diameter of the orbit of Jupiter. In the center of the collapse is now put on hold, but the areas situated further outside of the envelope rush continues in free fall at this first core. The impact of the material to the hydrostatic core leads to the formation of shock waves that ultimately heat the core even further. This first phase of star formation from the collapse to the formation of a hydrostatic core takes about 10,000 years and is defined by the so-called free-fall time.

Observation of pre-stellar cores

Prestellar cores can be observed on the whole, with the same methods as molecular clouds. Firstly, the dust contained in them swallows the light from background stars, which is why they are seen in the optical and near-infrared as a star- free areas. Secondly, they also radiate through their temperatures of around 10 K at (sub) mm wavelengths, and there can be seen by the thermal emission of the dust.

Just as molecular clouds in general also prestellar cores with the aid of molecular lines are observed and detected. In contrast to the molecular cloud, which is mainly detected by CO, is utilized in the observation of nuclei variety of effects to advantage. First, the center of a core is protected by the casing from the interstellar radiation field. This means that in this region, chemical reactions vonstattengehen that would normally be prevented by this radiation. Thus, in pre-stellar cores molecules occur which are not in the interstellar medium. On the other hand, these nuclei are so dense that higher states are excited in the molecules by collisions with hydrogen molecules, which in turn leads to radiate characteristic lines.

The chemistry within such a pre-stellar core is still the subject of most current research, since in addition to chemical reactions in the gas phase, even the so-called freeze-out of molecules on dust particles and the related chemistry of the dust particles must be taken into account.

Similarly unknown and still not observed at the time, is the transition from a pre-stellar in a protostellar core, that is, the observation of a first core. So far, some candidates have been discovered for such an object, a confirmed observation has failed to materialize today, however.

Protostars

Second collapse

The heating of the so-called first core lasts only until the temperature is sufficient to split the hydrogen molecules into their individual atoms. The energy that is consumed here, however, is no longer stabilization of the core available. This leads to a second collapse, which is only stopped when once again forms a hydrostatic equilibrium. The second core is, however, primarily consisting of hydrogen atoms, and has an extent of about one and a half solar radii. From a pre-stellar core, a protostar is now finally become: A star that still grows in mass and obtains its luminosity primarily from the accretion from the outside to the object falling matter.

Although this protostar already radiates with a temperature of some 1000 K, it is covered from the outside by surrounding it tight envelope. However, its radiation ensures a gradual heating of the molecular cloud from the inside out. In the interior regions, the temperature rises to about 1500 K, so that all the hot dust particles evaporate. There is a largely radiolucent region ( " opacity gap" ) forms inside the dust jacket.

If the temperature in the central regions of the molecular cloud temperatures of around 100 K, the molecules from the ice envelope start to the dust particles auszudampfen and migrate into the gas phase. In this so-called Hot Corino held a variety of chemical reactions by the elevated temperatures and high frequency of molecules in the gas phase. The operations in these regions are therefore unlike those in the cold outer regions of the protostellar cloud that still resemble their pre-stellar conditions produces the seeds.

The further outward shell areas that are still in free fall, rain continues down to the protostar, thus ensuring a steady increase in mass. The majority of the brightness is extracted from this accretion. However, yet is located only about 1 % of the total mass of the molecular cloud core in the central star. The phase in which the star by the incidence of shell material steadily increasing in mass, called Hauptakkretionsphase. In the simplest case this collapse occurs radially symmetrical. In reality, however, the molecular cloud cores have a non-zero angular momentum, so that dust and gas can not fall onto the central star easily.

Circumstellar disk and jets

A collapse requires a redistribution of angular momentum. This often leads to the formation of a double or multiple rating systems, or perpendicular to the axis of rotation a circumstellar disk. Within this disc an effective transport of angular momentum is possible, which leads on one hand to the fact that particles migrate toward the central star, on the other hand also to an expansion of the disc, as the particles absorb the angular momentum continues to drift to the outside. This disc may have an extension of about 100 AU.

In addition to a circumstellar disk, these protostars form from perpendicular to bipolar, highly collimated jets. These are generated by a combination of rotation, magnetic fields and accretion. It is believed that already FHSCs can form weak molecular outflows, while jets are formed in the later phase of evolution. Lined they are by material from the circumstellar disk. They pierce it at supersonic speed into the surrounding shell material, resulting in the formation of shocks. These shocks heat up strongly on, and this allows chemical reactions that can lead to the formation of new molecules. In addition to the jets with velocities of several 100 km / s, there are also slower, less collimated outflows of molecular flow with velocities of up to some 10 km / s This is probably around material that the jet breaks as it flows through the shell with it. The jet thus slowly eats a cavity in the protostellar cloud. This is initially very narrow, with opening angles of only a few degrees, but widens with time progresses ever further and provides the distraction and thinning of the shell in the direction of the effluents.

The protostar itself accreted matter further. This is no longer falls but directly and isotropic on him, but is taken up mainly by the circumstellar disk (which is why this is often called accretion disk ).

The classification of protostars

For a more detailed evolutionary classification so-called spectral energy distributions serve astronomers here (English spectral energy distribution, SED), the color index and in particular the so-called spectral index

Here, the wavelength and the flux density. Typically, the spectral index for near-infrared wavelengths from 2.2 to 10 microns is used for classification. The radiation from protostellar systems is dominated by thermal radiation. In the early phase, with temperatures of a few 10 K the radiation maximum is far far infrared and the radiation intensity thus increases with increasing wavelength at ( α > 0). Upon reaching the main sequence, the SED is dominated by the central star with temperatures of a few 1000 K with the maximum radiation in the optical and a resulting negative spectral index.

Observation of protostars

Observed protostellar cores in the optical and near-infrared wavelength range, so they differ little from pre-stellar cores. The dense shell swallowed the light located behind stars, which is why they are in the sky can also be seen as dark regions. In (sub) mm wavelengths also you can see the thermal radiation emitted by dust in the shell.

Differences can be seen in observations in the intermediate wavelengths, since the shell is transparent at these wavelengths. Since these observations in the mid and far infrared due to the atmosphere were not possible from the surface, this gap could not be closed with the help of satellite missions and progressing detector technology.

The most important instrument for the observation of these early stages of star formation was the IRAS satellite mission that systematically examined the whole sky with broadband filters with central wavelengths of 12 microns, 25 microns, 60 microns and 100 microns. Class 0 protostars were able to usually, depending on the distance of the object to be detected only in the longer wavelengths, since these are still too cold to radiate strongly at wavelengths of only a few 10 microns. With the launch of the Spitzer Space Telescope in 2003 could by its higher sensitivity but also at shorter wavelengths (eg 24 microns ), a number of Class 0 protostars are detected in molecular clouds, which had been held for starless. These objects form the new class of so-called Vellos (English Very Low Luminosity Objects) and are the subject of current research.

In addition to the thermal radiation of the protostellar cloud, it is also possible to observe the bipolar outflows of matter. For this one frequently observed molecular line transitions of CO ( and its isotopes ). These allow inferences about velocities in the effluents, or even over excitation conditions (density, temperature, ...). Other molecules that can be formed, for example, only in the extreme environmental conditions of jets, are also commonly used to explore the nature of jets. The signature of the rotation disk can be seen with the aid of various molecular line transitions. However, the small size of these discs makes it difficult to spatial resolution, which is why often interferometric recordings are necessary.

Spectral classification

Stars are characterized and classified during their formation phase over the spectral energy distribution. The SED of Class 0 protostars is similar in shape to that of a cold black body with a temperature of only about 20-30 K. In the center of a protostar has indeed been formed, whose radiation is, however, completely absorbed by the dense shell and ensure its heating.

The SED of Class I protostars is still dominated by the thermal radiation of cold dust shell. It turns out, however, at shorter wavelengths, the blackbody radiation of the protostar in the center, which already has a temperature of some 1000 K. In addition to thermal radiation are reflected in the SED and spectral characteristics of the shell material. The maximum radiation at 10 microns is due to dust in the form of silicates.

SED of a protostar of Class I

Pre-main sequence

In the early phase of star formation of the protostar derives a large part of the luminosity from the accretion of material from the shell. In the course of evolution, however, takes this set will go down and the luminosity is supplied mainly by the self- contraction of the central star. At this stage, no longer speaks of a protostar, but called such object now as Vorhauptreihenstern.

The astronomical nomenclature for stars at this stage depends on the mass: a mass of less than 2 solar masses is called T Tauri stars in massive stars with up to 8 solar masses of Herbig-Ae/Be-Sternen.

In T Tauri stars, the envelope is already so far thinned that it allows a direct view of the central star and the surrounding disk. It turns out that these young stars are covered due to strong magnetic fields to a large part of " star spots ". Furthermore possess T Tauri stars strong winds, so the more accretion, about 10-7-10-9 solar masses per year, only about the protoplanetary disk happens that makes up initially around 0.5 % of the mass of the central star. In the course of about 2 million years, the disc dissolves by various processes ( accretion, jets, photoevaporation, ...).

Did T- Tauri stars initially strong emission lines, so decreases in intensity during the dissolution of the protoplanetary disk. It speaks so well of Weak T Tauri Stars ( WTTS ) in contrast to the classical T Tauri stars (german classical T Tauri star CTTS ). With decreasing gas mass in the disk and the jet activity decreases.

In the Hertzsprung -Russell diagram (HRD ) dive T Tauri stars on the main sequence and on first walk almost perpendicular to the Hayashi - line down. The focus is increasing the temperature, but goes first only for the energetically significant merger primordial deuterium and lithium. First, the star being optically thick, so that the released inside gravitational energy passes by convection to the outside. Star with a mass greater than 0.5 solar masses sooner or later form a compact core zone, the high acceleration of gravity prevents convection. With the limitation to the radiative transfer, the temperature rises, in the outer part of the convective envelope, and the HRD Vorhauptreihenstern pivots on a nearly horizontal path of evolution. Finally, the nuclear fusion of hydrogen begins and prevents further contraction, the star has reached the main sequence. Stars with a mass less than 0.5 solar masses remain vollkonvektiv to reach the main sequence. Stars with less than 0.08 solar masses ( in about 80 Jupiter masses) do not reach the necessary for the hydrogen burning core temperature. Your contraction ends with the degeneracy of the electrons. As a result, this cool "failed star " as brown dwarfs from.

Observation of pre-main sequence stars

Pre-main sequence stars are observable as protostars using the same methods. But further, it is also possible to observe the proto plate by scattered light. The physics of the scattering processes allows it to draw conclusions about the nature of the scattering dust particles.

With new telescopes (eg ALMA), it will also be possible in the future, planet caused by gaps in protoplanetary disks to observe directly. Indirect indications so far are already found in spectra at infrared wavelengths. In young main sequence stars and pre-main sequence stars of class III, in which the gas has almost completely dissipated in the disc and so-called debris disks are left, it is possible to observe planets directly.

Spectral classification

The spectral energy distribution of T Tauri stars is dominated by the black body radiation of the central star. However, the protoplanetary disk ensures an excess of radiation in the mid and far infrared. By the various components of the disc at different temperatures of this radiation surplus can not be described by a black body at a single temperature.

With the slow thinning of the protoplanetary disk and the radiation component vanishes almost completely and it remains the radiation of the pre-main sequence star. In some systems, you can still find, however, a small excess radiation, which is usually indicative of debris disks.

SED of a protostar of Class III

Star formation in clusters

While stars with low mass may also occur in isolation, to massive stars form only in so-called clusters that arise from larger molecular clouds. In such clusters different processes can lead to modifications from the classic paradigm of star formation. Two proto-stars accrete material from the same molecular cloud core, competing with each other and stop the mass inflow to the respective other protostar; Jets and outflows can penetrate into other protostellar systems, and tidal forces may occur as an additional nuisance. These are just some examples of how the star formation may be different in insulation of development processes in clusters.

Another factor of (. Cluster isolation vs), these two schemes of star formation is different from each other, is the occurrence of more massive stars in clusters. In contrast to the formation of stars like our sun, which is completed after around 10 million years ago, massive stars form with masses greater than 8 solar masses in a much shorter time. Basically doing the same evolutionary stages are run through ( gravitational collapse, forming a disk and jets), but time not so highly differentiated. While in the center already begin nuclear fusion processes, the star is still covered by a thick dust shell. This affects mainly due to the observability of massive pre-main sequence stars that can be observed mainly at infrared and longer wavelengths thus.

Since massive stars have higher surface temperatures of several 10 000 K, is their thermal radiation in large part from the UV and soft X-rays. The radiation pressure may be so great that it prevents further accretion. Further, this radiation is able to ionize neutral hydrogen atoms in the shell. For O stars, these so-called H II region may have a diameter of about 100 light years. The ionization and subsequent recombination lead to the emission of the hydrogen series, the dominant line is the Hα line of the Balmer series with 656.3 nm.

As quickly as these massive stars have emerged, their nuclear fuel is used up; the stars eventually end up as supernovae. Explosively formed by nuclear fusion elements to the interstellar medium are given. Emanating from them pressure waves for local compaction of the surrounding molecular cloud lead to be gravitationally unstable and in turn form new stars.

Stellar populations

The preceding sections describe the best understood of star formation processes: the star formation in the universe today. In the early period of cosmic history, however, were still significantly different conditions, and this requires different models of star formation: Stars reflect on reaching the main sequence its energy almost exclusively from nuclear fusion processes. This eventually leads to the formation of helium, carbon and other heavier elements up to iron. About stellar winds or by powerful supernova explosions these items from ending up in the interstellar medium, and enrich it with metals to (where commonly all elements except hydrogen and helium are referred to as metals in astronomy ). These metals play a quite important role in star formation. Dust particles provide as well as some molecules (eg, CO) for efficient cooling of molecular cloud cores, eventually resulting in the production gravitational instability and collapse.

However, stars in the early universe could form only from the light elements that were formed shortly after the Big Bang. The star formation must therefore differ fundamentally from our understanding of star formation at the present time. One possible mechanism is the massive emergence of hundreds to millions of stars in clusters, where tidal forces and complex interactions between the pile members play an important role. The metal-poor stars that form in the process of so-called Population III stars are likely to be much harder and therefore become even hotter than star nowadays.

The next generation of stars, called stellar population II had been an accumulation of metals in the astronomical sense - elements heavier than helium - which, although not the abundance ratios in stars like our sun reached ( which belongs to the so-called Population I ), but already clearly rapid cooling of the relevant molecular clouds allowed, so that preferably could make stars with lower mass than our sun. While stars of population III were not observed until today, are located in the halo of our Milky Way, an area with a relatively low rate of star formation, metal-poor Population II stars. In the disk of the Milky Way itself there are, however, mainly Population I stars.

Star formation in galaxies

Star formation is a key process in the formation and evolution of galaxies. The central question is where and how efficiently gas is converted into stars in galaxies.

Galaxies can be divided into those that still larger scale form new stars and those in which the star formation activity has largely come to a standstill. This classification corresponds to a characteristic color distribution of galaxies with a group of blueish ( active star formation ) and a group of reddish ( hardly star-forming ) galaxies. The development trends of these two types of galaxies are a key observation of galaxy evolution: the number of star-forming galaxies remains per the considered ( expanding ) cosmic volume largely the same, while the number of "dead" has increased galaxies over the last approximately 10 billion years steadily.

Already in the 1970s it became clear that deformed galaxies - according to current understanding, the results of the interaction of several galaxies with each other - have a more bluish color than ordinary galaxies of the same type in each case. The comparison with models showed that the properties of these galaxies indicate relatively short, ie only tens of millions of year-long period of intense star formation. Such galaxies are called ( in German ) starburst galaxies.

In our home galaxy, the Milky Way, formed around a solar mass of new stars per year.

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