The Lockheed F-104 Starfighter was one of the most famous supersonic single-engined American combat aircraft of the 20th century. The aircraft was designed as a follow-up to the F-100 Super Saber and was operational with the USAF from 1958-1967, during the Cold War. The aircraft was mainly used as a fighter-bomber, but there were also versions that were adapted for a fighter role. The F-104 was used by the US in combat actions in the Vietnam War and until its final phase-out in the US in 1975, the aircraft served with the Air National Guard.


The Starfighter was NATO‘s standard fighter-bomber in the period 1961-1985 after which it was replaced, mostly by the F-16. The aircraft has also served for many years in the Belgian Air Force and the Royal Air Force. The F-104 had the dubious honor of setting the worst safety record. The bribery scandals that came to light later – which had occurred in various cases with European purchases – did not do justice to the name and fame of the device. 26.000 people visit our last flight show.

Star formation

The term star formation identifies the process and the discipline that studies the ways in which a star originates. As a branch of astronomy, star formation also studies the characteristics of the interstellar medium and interstellar clouds as precursors, as well as young stellar objects and the planetary formation process as immediate products.

Although the underlying ideas date back to the time of the scientific revolution, the study of star formation in its current form only came to light between the end of the 19th and the beginning of the 20th century, in conjunction with the numerous advances that the theoretical astrophysics performed at the time. The advent of multi-wavelength observation, especially in the infrared, made the most substantial contributions to understanding the mechanisms underlying the genesis of a new star thanks to this website.

The model currently most accredited by the astronomical community, called the standard model, predicts that a star is born from the gravitational collapse of the densest portions (called “nuclei”) of a molecular cloud and from the subsequent growth of the stellar embryo, originating from the collapse , starting from the materials present in the cloud. This process has a duration that can vary between a few hundred thousand and a few million years, depending on the growth rate and the mass that the nascent star is able to accumulate: it is estimated that a star similar to the Sun takes about a hundred millions of years to fully form, while for the most massive stars the time is considerably shorter, in the order of 100,000 years. The model explains well the modalities that lead to the birth of single stars of small and medium mass (between 0.08 and 10 times the solar mass) and is also reflected in the initial mass function; on the other hand, it is more incomplete as regards the formation of star systems and clusters and of massive stars. For this reason, complementary models have been developed that include the effects of the interactions between stellar embryos and the environment in which they are formed and any other embryos nearby, important for the internal dynamics of the systems themselves and above all for the mass that stars do. unborn babies will be able to reach.

The subsequent phases of the star’s life, starting from the main sequence, are the responsibility of stellar evolution.


The study of star formation, in its modern form, can be dated to between the nineteenth and twentieth centuries, although the underlying ideas have their roots in the Renaissance period when, laid the foundations for the scientific revolution, it was put in place. discussing the geocentric view of the cosmos in favor of the heliocentric one; thanks to the contribution of great personalities such as Copernicus and Kepler and, later, Galileo, the study of the universe became no longer theological but scientific matter of study.

Theories on the formation of stars see their first sketch in the hypotheses formulated to explain the birth of the solar system.

One of the first was Descartes, who in 1644 proposed a “scientific” theory based on the hypothesis of the presence of primordial vortices of contracting matter characterized by different masses and sizes; the Sun originated from one of the largest, while the planets were formed from smaller vortices that, due to the global rotation, placed themselves in orbit around it: it was the sketch of what will be the so-called hypothesis of the nebula, formulated in 1734 by Emanuel Swedenborg, subsequently taken up by Kant (1755) and perfected by Laplace (1796), whose principle is still today, albeit with substantial modifications and improvements, at the basis of what centuries later and despite various events will be defined as the standard model of star formation. This theory suggests that the Sun and the planets that orbit it all originated from the same primordial nebula, the solar nebula. The formation of the system would have started from the contraction of the nebula, which would have determined an increase in its rotation speed, causing it to take on a discoid aspect with a greater densification of matter at its center, from which the proto- Sun. The rest of the circumsolar matter would first condense into rings, from which the planets would later originate. Are you curious? Check this website for more information.

Although it enjoyed great credit in the nineteenth century, the Laplacian hypothesis could not explain some particularities found, first of all the distribution of the angular momentum between the Sun and planets: the planets in fact hold 99% of the angular momentum, while the simple model of the nebula it foresees a more “equitable” distribution of the angular momentum between the Sun and the planets; for this reason this model was largely set aside at the beginning of the twentieth century. The fall of the Laplace model has stimulated astronomers to search for valid alternatives; however, these were often theoretical models that did not find any observational confirmation. The identification then, during the last decades of the twentieth century, of structures similar to the protosolar disk around young stellar objects led to the re-evaluation of the Laplacian idea, in Qatar.

An important contribution to understanding what initiated star formation was made by British astrophysicist James Jeans in the early 20th century. Jeans hypothesized that inside a vast cloud of interstellar gas, gravity was perfectly balanced by the pressure generated by the internal heat of the cloud, but discovered that it was a very unstable equilibrium, so much so that it could easily break in favor of gravity. collapsing the cloud and starting the formation of a star. Jeans’s hypothesis found wide acceptance when, starting from the 1940s, some stars were identified in some dark nebulae of the constellations of Taurus and Auriga that seemed to be related to the clouds within which they had been identified; they were also of a spectral type characteristic of colder and less massive stars, showed emission lines in their spectra and had a considerable variability. The Soviet astronomer Viktor Ambarcumjan suggested, towards the end of the 1940s, that they were very young objects; in the same period Bart Bok studied some small aggregates of dark dust, now known as globules of Bok, and hypothesized that these, together with the larger dark clouds, were the site of active star formation; however, it was necessary to wait for the development of infrared astronomy in the 1960s before these theories were confirmed by observations.

It was precisely the advent of infrared observation that encouraged the study of star formation: Mendoza, in 1966, discovered that some T Tauri stars possessed an important excess of infrared emission, hardly attributable to extinction alone (the absorption of light by the matter placed in front of the light source which manifests itself with a redness of the same) operated by the interstellar medium; this phenomenon was interpreted by hypothesizing the presence of dense dust structures around these stars capable of absorbing the radiation from the central stars and re-emitting it in the form of infrared radiation. The hypothesis was confirmed in the late nineties and early 2000s thanks to observational data obtained through innovative instruments, such as the well-known Hubble Space Telescope, the Spitzer Space Telescope and the Very Large Telescope with its adaptive optics, of dense discs. of matter around forming or newly formed stars; optical interferometry has also made it possible to identify numerous examples and to visualize other structures linked to stars in early stages of their existence, such as jets and molecular fluxes.

Where the stars are born: the star-forming regions

A star is basically a plasma spheroid made up mostly of hydrogen, from whose fusion the star obtains the energy necessary to counter the otherwise inevitable gravitational collapse of the large mass of matter that composes it. Therefore, a necessary condition for a star to form is a source of hydrogen, available in the interstellar medium (ISM, from the English interstellar medium) commonly present within a galaxy.

A typical spiral galaxy, such as the Milky Way, contains large quantities of interstellar medium, which is arranged mainly along the arms that delineate the spiral, where most of the matter that constitutes it, conveyed here due to the rotation motion of the galaxy, it can form diffuse structures. The situation changes as we proceed along the Hubble sequence, up to the smallest quantities of matter present in the interstellar medium of elliptical galaxies; consequently, as the quantity of ISM is reduced, the possibility of diffuse nebular structures is eliminated, unless the deficient galaxy acquires material from other galaxies with which it eventually interacts.

The interstellar medium is initially rather rarefied, with a density between 0.1 and 1 particle per cm³, and is composed of about 70% by mass of hydrogen, while the remaining percentage is mainly helium with traces of heavier elements, generically called metals. The dispersion of energy in the form of radiation in the far infrared (a very efficient mechanism) resulting in a cooling of the cloud, causes the matter of the medium to thicken into distinct clouds, generically called interstellar clouds, appropriately classified according to the state of ionization of hydrogen. Clouds made up predominantly of monoatomic neutral hydrogen are called H I regions (first acca).

As the cooling continues, the clouds become denser and denser; when the density reaches 1000 particles per cm³, the cloud becomes opaque to the galactic ultraviolet radiation. This condition, combined with the intervention of the interstellar dust granules as catalysts, allows the hydrogen atoms to combine into diatomic molecules (H2): thus we have a molecular cloud. The largest examples of these structures, giant molecular clouds, have typical densities of the order of 100 particles per cm³, diameters of over 100 light years, masses greater than 6 million solar masses (M☉) and an average temperature, at interior, of 10 K. It is estimated that about half of the total mass of the interstellar medium of our galaxy is contained in these formations, divided between about 6,000 clouds each with more than 100,000 solar masses of matter inside them. The presence, frequently found, of even very complex organic molecules, such as amino acids and PAHs, within these formations is the result of chemical reactions between some elements (in addition to hydrogen, carbon, oxygen, nitrogen and sulfur) that occur thanks to the energy input provided by the star formation processes that take place within them.

If the quantity of dust inside the molecular cloud is such as to block the visible light radiation coming from the regions behind it, it appears as a dark nebula; among the dark clouds are the aforementioned Bok globules, “small” aggregates of molecular hydrogen and dust that can form independently or in association with the collapse of larger molecular clouds. Bok globules, as well as dark clouds, often appear as dark shapes contrasting with the diffuse glow of the background consisting of an emission nebula or background stars. A typical Bok globule is thought to contain about 10 solar masses of matter in a region of about one light year (a.l.) in diameter, and that double or multiple star systems originate from them. Over half of the known Bok globules contain at least one young stellar object within them.
The eventual achievement of even higher densities (~ 10 000 atoms per cm³) makes the clouds opaque even to infrared, which is normally able to penetrate regions rich in dust. These clouds, called dark infrared clouds, contain important quantities of matter (from 100 to 100 000 M☉) and constitute the evolutionary link between the cloud and the dense nuclei that are formed by the collapse and fragmentation of the cloud. Molecular and dark clouds are the place of choice for the birth of new stars.

The possible presence of young massive stars, which with their intense ultraviolet emission ionize hydrogen at H +, transforms the cloud into a particular type of emission cloud known as the H II region (second acca).

Numerous regions of star formation are known in our Galaxy; the closest to the solar system are the ρ Ophiuchi cloud complex (400-450 al) and the Toro-Auriga cloud (460-470 al), within which formation processes involving low-mass stars are taking place and medium, as well as in the well-known and studied Perseus Cloud, however much more distant than the other two (980 al). Among the H II regions worthy of note are the Carina Nebula, the Eagle Nebula and the famous Orion Nebula, part of an extensive molecular complex, which represents the region closest to the solar system (1300 al) within which verifying the formation of massive stars.

It is hypothesized that the clouds from which the stars are born are part of the cycle of the interstellar medium, that is the matter constituting the interstellar medium (gas and dust) passes from the clouds to the stars and, at the end of their existence, once again becomes part of the ISM, constituting the raw material for a subsequent generation of stars.

Time scales

Two different time scales are considered in the study of the star formation process. The first is the Kelvin-Helmholtz time (thermal time scale, τ ter {\ displaystyle \ tau _ {ter}} \ tau _ {{ter}}), which corresponds to the time required for the gravitational potential energy to be converted into thermal energy and nuclear fusion can begin. It is inversely proportional to mass, since the greater it is, the faster the collapse and warming are. By comparing the values ​​of mass (M), radius (R) and luminosity (L) with the same parameters referring to the Sun (solar mass M☉, solar radius R☉, solar luminosity L☉), its value can be estimated as:

For a star of solar mass it is approximately 20 million years, but for a star with 50 solar masses it is reduced to a hundred thousand years.

The second time scale is represented by the growth time, that is the time necessary for a certain mass to accumulate at a given growth rate; it is directly proportional to the mass itself: it is intuitive, in fact, that more time is needed to collect larger quantities of matter. It is also inversely proportional to the gas temperature, since the kinetic energy, and consequently the pressure, increase as the temperature increases, thus slowing down the accumulation of matter.

Standard model of star formation

The collapse of the cloud

An interstellar cloud remains in a state of dynamic equilibrium until the kinetic energy of the gas, which generates an outward pressure, and the potential energy of gravity, with a centripetal direction, are equal. From a mathematical point of view this condition is expressed through the virial theorem which establishes that, to maintain equilibrium, the gravitational potential energy must be equal to double the internal thermal energy. The breaking of this equilibrium in favor of gravity determines the occurrence of instabilities that trigger the gravitational collapse of the cloud.

The limit mass beyond which the cloud will certainly collapse is called the Jeans mass, which is directly proportional to the temperature and inversely proportional to the density of the cloud: the lower the temperature and the higher the density, the lower the mass necessary for this process to take place. For a density of 100,000 particles per cm³ and a temperature of 10 K the Jeans limit is equal to one solar mass.

The process of condensation of large masses starting from local masses of matter inside the cloud, therefore, can proceed only if the latter already possess a sufficiently large mass. In fact, as the denser regions, on their way to collapse, incorporate matter, locally lower masses of Jeans are reached, which therefore lead to a subdivision of the cloud into hierarchically smaller and smaller portions, until the fragments reach a stellar mass. The fragmentation process is also facilitated by the turbulent motion of the particles and the magnetic fields that are created. The fragments, called dense nuclei, have dimensions between 6000 and 60 000 astronomical units (AU), densities of the order of 105-106 particles per cm³ and contain a variable quantity of matter; the range of masses is very large, but the smaller masses are the most common. This distribution of masses traces the distribution of the masses of future stars, that is the initial mass function, except that the mass of the cloud amounts to about three times the sum of the masses of the stars that will originate from it; this indicates that just one third of the mass of the cloud will actually give rise to stars, while the rest will disperse in the interstellar medium. Dense turbulent nuclei are supercritical, meaning their gravitational energy exceeds thermal and magnetic energy and inexorably initiates them to collapse.

The protostar

Supercritical nuclei continue to contract slowly for a few million years at a constant temperature as long as the gravitational energy is dissipated by the radiation of millimeter radio waves. The occurrence of phenomena of instability causes a sudden collapse of the fragment, which leads to an increase in density in the center up to ~ 3 × 1010 molecules per cm³ and an opacification of the cloud to its own radiation, which causes an increase in temperature ( from 10 to 60-100 K) and a slowdown of the collapse. The heating gives rise to an increase in the frequency of the electromagnetic waves emitted: the cloud now radiates in the far infrared, to which it is transparent; in this way the dust mediates a second collapse of the cloud. At this point, a configuration is created in which a hydrostatic central core gravitationally attracts the matter diffused in the external regions: it is the so-called first hydrostatic core, which continues to increase its temperature according to the virial theorem and shock waves caused by the material at free fall speeds. After this growth phase, starting from the surrounding gas envelope, the nucleus begins a phase of quasi-static contraction.

When the core temperature reaches about 2000 K, thermal energy dissociates the H2 molecules into hydrogen atoms, which soon after are ionized together with the helium atoms. These processes absorb the energy released by the contraction, allowing it to continue for periods of time comparable to the period of the collapse at free fall speed. As soon as the density of the falling material reaches the value of 10−8g cm − 3, the matter becomes sufficiently transparent to allow light to escape. The combination of internal convective motions and the emission of radiation allows the stellar embryo to contract its own beam. This phase continues until the temperature of the gases is sufficient to maintain a pressure high enough to avoid further collapse; a momentary hydrostatic equilibrium is thus achieved. When the object thus formed ceases this first phase of growth takes the name of protostar; the stellar embryo remains in this phase for some tens of thousands of years in cyber space.

Following the collapse, the protostar must increase its mass by accumulating gas; thus begins a second phase of growth which goes on at a rate of about 10-6-10-5 M☉ per year. The accretion of the material towards the protostar is mediated by a discoidal structure, aligned with the equator of the protostar, which is formed when the rotational motion of the falling matter (initially equal to that of the cloud) is amplified due to conservation of angular momentum; this formation also has the task of dissipating the excess angular momentum, which otherwise would cause the dismemberment of the protostar. In this phase, molecular fluxes are also formed, perhaps the result of the interaction of the disk with the lines of force of the stellar magnetic field, which depart from the poles of the protostar, also probably with the function of dispersing the excess angular momentum. . The collision of these jets with the surrounding envelope gas can generate particular emission nebulae known as Herbig-Haro objects.

The addition of mass causes an increase in pressure in the central regions of the protostar, which is reflected in an increase in temperature; when this reaches a value of at least one million kelvins, the fusion of deuterium, an isotope of hydrogen (21H) begins; the resulting radiation pressure slows down (but does not stop) the collapse, while the fall of material from the internal regions of the accretion disk to the surface of the protostar continues. The growth rate is not constant: in fact the future star quickly reaches what will be half of its final mass, while it takes over ten times longer to accumulate the remaining mass.

The accretion phase is the crucial part of the star formation process, since the amount of matter that the rising star manages to accumulate will irreversibly influence its subsequent fate: in fact, if the protostar accumulates a mass between 0, 08 and 8-10 M☉ subsequently evolves into a pre-main sequence star; if, on the other hand, the mass is clearly superior, the protostar immediately reaches the main sequence. Mass also determines the lifespan of a star: less massive stars live much longer than heavier stars: ranging from billions of years for MV stars to a few million years for massive O stars. If, on the other hand, the object is unable to accumulate at least 0.08 M☉, the core temperature allows the deuterium to fuse, but it proves insufficient to trigger the fusion reactions of hydrogen propium, the most common isotope of this element ( 11H); this “missing star”, after a stabilization phase, becomes what astronomers define as a brown dwarf.

The main pre-sequence phase (PMS)

The emission of wind by the protostar at the ignition of the deuterium fusion determines the dispersion of a large part of the envelope of gas and dust that surrounds it; the protostar thus passes to the phase of a pre-main sequence star (or star PMS, from the English pre-main sequence), whose energy source is still the gravitational collapse and not the fusion of hydrogen as in the main sequence stars. Two main classes of PMS stars are recognized: the Orion variables, which have a mass between 0.08 and 2 M☉, and Herbig’s Ae / Be stars, with a mass between 2 and 8 M☉. PMS stars more massive than 8 M☉ are not known, since when very high masses come into play, the stellar embryo reaches the conditions necessary for the triggering of hydrogen fusion extremely quickly and passes directly to the main sequence.

The Orion variables are in turn subdivided into T Tauri stars, EX Lupi stars (EXor) and FU Orionis stars (FUor). They are similar to the Sun in mass and temperature, but some times larger in terms of diameter and, for this reason, brighter. They are characterized by high rotation speeds, typical of young stars, and possess intense magnetic activity, as well as bipolar jets. The FUor and EXor represent particular categories of T Tauri, characterized by sudden and conspicuous changes of their own luminosity and of the spectral type; the two classes differ from each other by spectral type: the FUor are, in a quiescent state, of class F or G; K or M class EXors.

Herbig’s Ae / Be stars, belonging to classes A and B, are characterized by spectra in which the emission lines of hydrogen (Balmer series) and calcium present in the disc residual from the accretion phase dominate.

Hayashi’s trace of a sun-like star.

  1. Collapse of the protostar: totally convective internal.
  2. Increase in effective temperature: initiation of the first nuclear reactions, first sketch of the radiative nucleus (entry into the Henyey trace).
  3. Triggering of the hydrogen fusion: totally radiative core (entry into the ZAMS).

The PMS star follows a characteristic path on the H-R diagram, known as the Hayashi trace, during which it continues to contract. The contraction continues until the Hayashi limit is reached, after which it continues at a constant temperature in a Kelvin-Helmholtz time greater than the growth time; then stars with less than 0.5 solar masses reach the main sequence. The stars from 0.5 to 8 M☉, at the end of the Hayashi trace, instead undergo a slow collapse into a condition close to hydrostatic equilibrium, at this point following a path in the H-R diagram called the Henyey trace.

The collapse phase ends when finally, in the core of the star, the necessary temperature and pressure values ​​are reached for the triggering of the great-uncle hydrogen fusion; when the fusion of hydrogen becomes the predominant energy production process and the excess potential energy accumulated with the contraction is dispersed, the star reaches the standard main sequence of the HR diagram and the intense wind generated as a result of the triggering of the reactions nuclear sweeps away the residual materials, revealing the presence of the newly formed star. Astronomers refer to this stage with the acronym ZAMS, which stands for Zero-Age Main Sequence. The ZAMS curve can be calculated by computer simulations of the properties that stars had at the time of their entry into this phase. In our academy we don’t have special online lessons on how to discover this.

The subsequent transformations of the star are studied by stellar evolution.

Limits of the model

The standard model of star formation is a coherent theory confirmed by observational data; however, it has some limitations. First of all, it does not explain what triggers the collapse of the cloud. Furthermore, it considers the stars in formation only as single entities, not taking into account the interactions that are established between the single stars in formation within a compact group (destined to evolve in a cluster) or the formation of multiple star systems, phenomena that indeed occur in most cases. Finally, it does not explain how extremely massive stars are formed: the standard theory explains the formation of stars up to 10 M☉; higher masses, however, imply the involvement of forces that would further limit the collapse, stopping the growth of the star at masses not exceeding this value.

While clearly explaining the ways in which it occurs, the standard model does not explain what triggers the collapse. The formation of a star does not always start completely spontaneously, due to internal turbulence or due to the decrease in the internal pressure of the gas due to cooling or the dissipation of magnetic fields. Indeed, more often, as countless observational data show, the intervention of some factor is necessary that from the outside perturbs the cloud, causing local instabilities and thus promoting collapse. In this regard, there are numerous examples of stars, mostly belonging to large stellar associations, whose characteristics show that they formed almost simultaneously: since a simultaneous collapse of independent dense nuclei would be an incredible coincidence, it is more reasonable to think that this is the consequence of a force applied from the outside, which acted on the cloud causing the collapse and subsequent star formation. However, examples of spontaneously initiated collapses are not uncommon: some examples of this have been identified by infrared observation in certain relatively quiescent isolated dense nuclei located in clouds close to each other. In some of them, as in the Bok Barnard 355 globule, traces of slow internal centripetal motions were found and infrared sources were also observed, a sign that they could be initiated to the formation of new stars. In our cinema you can see beautiful sharp movies about this phenomenon. In our blog we give more info.

There may be several external events capable of promoting the collapse of a cloud: shock waves generated by the collision of two molecular clouds or by the explosion in the vicinity of a supernova; the tidal forces that arise as a result of the interaction between two galaxies, which trigger a violent star-forming activity called starburst (see also the paragraph Variations in the duration and rate of star formation); the energetic super-flares of another nearby star in a more advanced stage of formation or the wind pressure or intense ultraviolet emission of nearby massive O and B stars, which can regulate star formation processes within of the H II regions (diagram below).

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