Fragmentation of molecular clouds Fragmentation is generally considered to be the initial process that a molecular cloud must undergo before stars can form. Yet its role in determining the final mass spectrum remains obscure. It appears that gravitational fragmentation, considered as a unique process, is unsatisfactory.
Full Answer
What happens when a molecular cloud breaks up?
It is through this complex process that a molecular cloud breaks itself up into the full range of stellar masses. These fragments then cool and collapse under their own gravity to form individual stars.Finally, we zoom back out to see the whole cloud and the retinue of stellar-mass fragments it has produced.
What is a a molecular cloud?
A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit the formation of molecules, most commonly molecular hydrogen (H 2 ). This is in contrast to other areas of the interstellar medium that contain predominantly...
What is fragmentation in mass spectrometry?
(April 2020) ( Learn how and when to remove this template message) Molecular fragmentation (mass spectrometry), or molecular dissociation, occurs both in nature and in experiments. It occurs when a complete molecule is rendered into smaller fragments by some energy source, usually ionizing radiation.
What are the characteristics of a giant molecular cloud?
Giant molecular clouds. Observationally, typical molecular cores are traced with CO and dense molecular cores are traced with ammonia. The concentration of dust within molecular cores is normally sufficient to block light from background stars so that they appear in silhouette as dark nebulae.
What are cloud fragments?
The traditional view of fragmentation is that this collapse generates substructure. As the cloud contracts, it breaks apart, producing several daughter clouds. Each daughter, in turn, collapses and breaks up. After a number of generations, fragments with stellar-type mass are produced – the observed dense cores.
What is the meaning of molecular cloud?
A molecular cloud is an interstellar cloud of gas and dust in which molecules can form, the most common of which is hydrogen (H2). The Hubble Space Telescope has provided us with images of numerous young solar mass stars forming in giant molecular clouds such as the Eagle and Orion Nebulae.
What is the molecular structure of a cloud?
Main. Molecular clouds are primarily composed of molecular hydrogen mixed with trace impurities including interstellar dust grains and rare organic and inorganic molecules.
Why do molecular clouds fragment as they collapse?
We track the collapse and fragmentation of the clouds, showing that under pressure, fragmentation is caused by a combined action of a shockwave driven into the cloud and instabilities behind it, leading to much higher fragmentation rates than in uncompressed cloud.
What causes a molecular cloud?
Molecular clouds are expected to be formed in limited regions where the mean magnetic field is parallel to the direction of shock wave propagation or in regions where an excessive number of shock wave sweepings are experienced. Therefore, molecular clouds can only be found in limited regions in shells.
What is inside a molecular cloud?
Molecular clouds consist mainly of gas and dust but contain many stars as well. The central regions of these clouds are completely hidden from view by dust and would be undetectable except for the far-infrared thermal emission from dust grains and the microwave emissions from the constituent molecules.
Where are molecular clouds located?
spiral galaxiesStar formation takes place exclusively within molecular clouds and observations have shown that they are located primarily in the disk of spiral galaxies and the active regions of irregular galaxies. Since molecular clouds are cold and dark, we cannot observe them directly in visible light.
What's the difference between a molecular cloud and a nebula?
Key Difference: Nebula is a cloud of dust in deep space. Whereas, a molecular cloud is a type of nebula. The formation of molecules is permitted due to its density and size. Nebula is a cloud of gas and dust in space.
How long does a molecular cloud last?
That molecular gas occurs predominantly in the spiral arms suggests that molecular clouds must form and dissociate on a timescale shorter than 10 million years—the time it takes for material to pass through the arm region.
What will happen as the cloud collapses?
A cloud may start with any size or shape, and different clumps of gas within the cloud may be moving in random directions at random speeds. When the cloud collapses, these different clumps collide and merge, resulting in a flattened rotating disk.
What happens when an interstellar cloud fragment contracts?
Stage 2: Fragmentation - Contracting interstellar cloud fragments into smaller pieces due to gravitational instabilities. The pieces continue to collapse and fragment, eventually to form many tens of hundreds of separate stars.
Why did the interstellar gas and dust collapse?
Stars are born within the clouds of dust and scattered throughout most galaxies. A familiar example of such as a dust cloud is the Orion Nebula. Turbulence deep within these clouds gives rise to knots with sufficient mass that the gas and dust can begin to collapse under its own gravitational attraction.
What is a molecular cloud Wikipedia?
A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, H2), and the formation of H II regions.
Where are molecular clouds located?
Interstellar space — the region between stars inside a galaxy — is home to clouds of gas and dust. This interstellar medium contains primordial leftovers from the formation of the galaxy, detritus from stars, and the raw ingredients for future stars and planets.
What's the difference between a molecular cloud and a nebula?
Key Difference: Nebula is a cloud of dust in deep space. Whereas, a molecular cloud is a type of nebula. The formation of molecules is permitted due to its density and size. Nebula is a cloud of gas and dust in space.
Is the Orion Nebula A molecular cloud?
The Orion Nebula is about 1,350 light-years away and more than 10 light-years across. Yet it represents only a small part of the great Orion Molecular Cloud. The cloud contains a mixture of cold hydrogen and dust grains. M42 is known as an emission nebula.
What are some examples of molecular clouds?
An example is the Coalsack in the southern sky. Stars are born within molecular clouds. The Coalsack nebula, photographed from the International Space Station.
What are the physical conditions of a molecular cloud?
The chemistry and physical conditions of the interior of a molecular cloud are quite different from those of the surrounding low-density interstellar medium. In the outer parts of the dark cloud, hydrogen is neutral. Deeper within it, as dust blocks out an increasing amount of stellar ultraviolet radiation, the cloud becomes darker and colder. Approaching the centre, the predominant form of gaseous carbon changes successively from C + on the outside to neutral C (C 0) and finally to the molecule carbon monoxide (CO), which is so stable that it remains the major form of carbon in the gas phase in the darkest regions. At great depths within the cloud, other molecules can be seen from their microwave transitions, and more than 150 chemical species have been identified within the constituent gas. Because of the comparatively low densities and temperatures, the chemistry is very exotic, as judged by terrestrial experiments; some rather unstable species can exist in space because there is not enough energy to convert them to more-stable forms. An example is the near equality of the abundances of the interstellar molecule HNC ( hydroisocyanic acid) and its isomer HCN ( hydrocyanic acid ); in ordinary terrestrial conditions there is plenty of energy to allow the nitrogen and carbon atoms in HNC to exchange positions and produce HCN, by far the preferred species for equilibrium chemistry. In the cold clouds, however, not enough energy exists for the exchange to occur. There is less than one-thousandth as much starlight within a cloud as in the interstellar space outside the cloud, and the heating of the material in the cloud is provided primarily by cosmic rays. Cooling within the cloud occurs chiefly by transitions between low-lying levels of the carbon monoxide molecule.
Why are dark clouds opaque?
Molecular cloud, also called dark nebula, interstellar clump or cloud that is opaque because of its internal dust grains. The form of such dark clouds is very irregular: they have no clearly defined outer boundaries and sometimes take on convoluted serpentine shapes because of turbulence.
How does stellar ultraviolet radiation penetrate the cloud?
Stellar radiation can apparently enter the cloud through channels where the dust (and gas) density is lower than average. The clumpiness of the interstellar material has profound effects on its properties.
How many light years are in a molecule cloud?
They contain much of the mass of the interstellar medium, are some 150 light-years across, and have an average density of 100 to 300 molecules per cubic centimetre and an internal temperature of only 7 to 15 K. Molecular clouds consist mainly of gas and dust but contain many stars as well. The central regions of these clouds are completely hidden ...
How close are the nebulae to the Sun?
The nebulae are remarkably close to the galactic plane; most are within 300 light-years, only 1 percent of the Sun’s distance from the centre. The details of the explanation of why the gas is largely confined to the spiral arms is beyond the scope of this article ( see Milky Way Galaxy: Major components ).
What is the formation of stars?
Formation of stars. In the inner regions of molecular clouds an important event takes place: the formation of stars from the gravitational collapse of dense clumps within the nebula. Initially the cloud consists of a chaotic jumble of smaller clouds, each of which is destined to be an individual stellar system.
How many light years are there in an interstellar cloud?
Clouds of interstellar gas are very turbulent with rapid internal motions. We begin with such a gas cloud, 1.2 light-years across, and containing 50 times the mass of the Sun.
How many directions does a star eject from the cloud?
An unstable system of 5 stars breaks up and ejects stars from the cloud in three different directions (lower right; see animations).
Background
A major objective of theoretical chemistry and computational chemistry is the calculation of the energy and properties of molecules so that chemical reactivity and material properties can be understood from first principles.
Methods
Different methods have been devised to fragment molecules. Among them you can find the following energy-based methods:
Why do molecular clouds need to trap energy?
A molecular cloud needs to trap all the energy released by gravitational contraction in order for its center to become hot enough for fusion.
Why are molecular clouds favorable for star formation?
Molecular clouds are favorable locations for star formation for two reasons: low temperature and high density. Their low temperature keeps their pressures about the same as other interstellar clouds, despite the higher density. But the higher density means that gravity is stronger in molecular clouds, so it is able to overcome the pressure in molecular clouds. This increased gravitational attraction allows collapse, leading to star formation.
How does a protostar form?
It forms when a collapsed cloud fragment can no longer emit its thermal energy via photons so that the core heats up, raising the pressure and resisting the gravitational contraction. Protostars grow in mass because material around the protostar, which feels a weaker pull of gravity and does not collapse as quickly as the protostar, continues to contract. This material rains down on the protostar, gradually increasing the mass.
How are photons generated in clouds?
The photons are generated by the molecules' rotational and vibrational energy levels, which are excited by collisions between molecules. Since the heat can be radiated away effectively, it does not build up and star formation is not stopped.
What is a protostellar disk?
A protostellar disk is a spinning disk of gas around a protostar. Because of friction in the disk, material gradually spirals inward and eventually falls onto the protostar.
Why do stars form in clusters?
Stars tend to form in clusters because more massive clouds are better able to overcome pressure and collapse. As these larger clouds collapse, the density increases and gravity gets an increasingly large advantage over pressure. So smaller and smaller parts of the cloud are able to collapse on their own, leading to fragmentation of the cloud. Each fragment becomes a star or a system of stars
What happens when the cloud's thermal energy can no longer escape in the form of photons?
When the cloud's thermal energy can no longer escape in the form of photons, the cloud heats up and the pressure rises. This resists the pull of gravity and slows the contraction .
What causes interstellar clouds to fragment?
Figure 19.3 As an interstellar cloud contracts, gravitational instabilities cause it to fragment into smaller pieces. The pieces themselves continue to collapse and fragment, eventually to form many tens or hundreds of separate stars.
How does fragmentation stop?
The process of continued fragmentation is eventually stopped by the increasing density within the shrinking cloud. As stage 2 fragments continue to contract, they eventually become so dense that radiation cannot get out easily. The trapped radiation causes the temperature to rise, the pressure to increase, and the fragmentation to cease.
How many stages of evolution are there in interstellar clouds?
Table 19.1 lists seven evolutionary stages that an interstellar cloud goes through prior to becoming a main-sequence star like the Sun. These stages are characterized by varying central temperatures, surface temperatures, central densities, and radii of the prestellar object. They trace its progress from a quiescent interstellar cloud to a genuine star. The numbers given in Table 19.1 and the following discussion are valid onlyfor stars of approximately the same mass as the Sun. In the next section we will relax this restriction and consider the formation of other stars.
What is the protostar in the photosphere?
For the first time, our fragment is beginning to resemble a star. The dense, opaque region at the center is called a protostar—an embryonic object perched at the dawn of star birth. Its mass grows as more and more material rains down on it from outside, although its radius continues to shrink because its pressure is still unable to overcome the relentless pull of gravity. After stage 3, we can distinguish a "surface" on the protostar—its photosphere. Inside the photosphere, the protostellar material is opaque to the radiation it emits. (Note that this is the same definition of "surface" that we used for the Sun in Chapter 16. (Sec. 16.1)) From here on, the surface temperatures listed in Table 19.1 refer to the photosphere and not to the "periphery" of the collapsing fragment, whose temperature remains low.
How does a cloud become unstable?
Perhaps it is squeezed by some external event, such as the pressure wave produced when a nearby O- or B-type star forms and ionizes its surroundings, or perhaps its supporting magnetic field leaks away as charged particles slowly drift across the confining field lines. Whatever the cause, theory suggests that once the collapse begins, fragmentation into smaller and smaller clumps of matter naturally follows, as gravitational instabilities continue to operate in the gas. As illustrated in Figure 19.3, a typical cloud can break up into tens, hundreds, even thousands, of fragments, each imitating the shrinking behavior of the parent cloud and contracting ever faster. The whole process, from a single quiescent cloud to many collapsing fragments, takes a few million years.
How does star formation occur?
Star formation begins when gravity begins to dominate over heat, causing a cloud to lose its equilibrium and start to contract. Only after the cloud has undergone radical changes in its internal structure is equilibrium finally restored.
How to find the luminosity of a protostar?
Knowing the protostar's radius and surface temperature , we can calculate its luminosity. Surprisingly, it turns out to be several thousand times the luminosity of the Sun. Even though the protostar has a surface temperature only about half that of the Sun, it is hundreds of times larger, making its total luminosity very large indeed—in fact, much greater than the luminosity of most main-sequence stars. Because nuclear reactions have not yet begun, this luminosity is due entirely to the release of gravitational energy as the protostar continues to shrink, and material from the surrounding fragment (which we called the solar nebula back in Chapter 15) rains down on its surface.
