An interstellar cloud is an accumulation of gas and dust in our and other galaxies. Put differently, an interstellar cloud is a denser-than-average region of the interstellar medium, the matter and radiation that exists in the space between the star systems in a galaxy. Depending on the density and temperature of a given cloud, its hydrogen can be neutral, making an H I region. Neutral and ionized clouds are sometimes called diffuse clouds. An interstellar cloud is formed by the gas and dust particles from a red giant in its life; the chemical composition of interstellar clouds is determined by studying electromagnetic radiation or EM radiation that they emanate, we receive – from radio waves through visible light, to gamma rays on the electromagnetic spectrum – that we receive from them. Large radio telescopes scan the intensity in the sky of particular frequencies of electromagnetic radiation which are characteristic of certain molecules' spectra; some interstellar clouds tend to give out EM radiation of large wavelengths.
A map of the abundance of these molecules can be made, enabling an understanding of the varying composition of the clouds. In hot clouds, there are ions of many elements, whose spectra can be seen in visible and ultraviolet light. Radio telescopes can scan over the frequencies from one point in the map, recording the intensities of each type of molecule. Peaks of frequencies mean that an abundance of that atom is present in the cloud; the height of the peak is proportional to the relative percentage. Until the rates of reactions in interstellar clouds were expected to be slow, with minimal products being produced due to the low temperature and density of the clouds. However, organic molecules were observed in the spectra that scientists would not have expected to find under these conditions, such as formaldehyde and vinyl alcohol; the reactions needed to create such substances are familiar to scientists only at the much higher temperatures and pressures of earth and earth-based laboratories. The fact that they were found indicates that these chemical reactions in interstellar clouds take place faster than suspected in gas-phase reactions unfamiliar to organic chemistry as observed on earth.
These reactions are studied in the CRESU experiment. Interstellar clouds provide a medium to study the presence and proportions of metals in space; the presence and ratios of these elements may help develop theories on the means of their production when their proportions are inconsistent with those expected to arise from stars as a result of fusion and thereby suggest alternate means, such as cosmic ray spallation. These interstellar clouds possess a velocity higher than can be explained by the rotation of the Milky Way. By definition, these clouds must have a vlsr greater than 90 km s−1, where vlsr is the local standard rest velocity, they are detected in the 21 cm line of neutral hydrogen, have a lower portion of heavy elements than is normal for interstellar clouds in the Milky Way. Theories intended to explain these unusual clouds include materials left over from the formation of the galaxy, or tidally-displaced matter drawn away from other galaxies or members of the Local Group. An example of the latter is the Magellanic Stream.
To narrow down the origin of these clouds, a better understanding of their distances and metallicity is needed. High-velocity clouds are identified with an HVC prefix, as with HVC 127-41-330. List of molecules in interstellar space Nebula Interplanetary medium - interplanetary dust Interstellar medium - interstellar dust Intergalactic medium - Intergalactic dust Local Interstellar Cloud G-Cloud High Velocity Cloud — The Swinburne Astronomy Online encyclopedia
H II region
An H II region or HII region is a region of interstellar atomic hydrogen, ionized. It is a cloud of ionized gas in which star formation has taken place, with a size ranging from one to hundreds of light years, density from a few to about a million particles per cubic cm; the Orion Nebula, now known to be an H II region, was observed in 1610 by Nicolas-Claude Fabri de Peiresc by telescope, the first such object discovered. They may be of any shape; the short-lived blue stars created in these regions emit copious amounts of ultraviolet light that ionize the surrounding gas. H II regions—sometimes several hundred light-years across—are associated with giant molecular clouds, they appear clumpy and filamentary, sometimes showing intricate shapes such as the Horsehead Nebula. H II regions may give birth to thousands of stars over a period of several million years. In the end, supernova explosions and strong stellar winds from the most massive stars in the resulting star cluster will disperse the gases of the H II region, leaving behind a cluster of stars which have formed, such as the Pleiades.
H II regions can be observed at considerable distances in the universe, the study of extragalactic H II regions is important in determining the distance and chemical composition of galaxies. Spiral and irregular galaxies contain many H II regions, while elliptical galaxies are devoid of them. In spiral galaxies, including our Milky Way, H II regions are concentrated in the spiral arms, while in irregular galaxies they are distributed chaotically; some galaxies contain huge H II regions. Examples include the 30 Doradus region in the Large Magellanic Cloud and NGC 604 in the Triangulum Galaxy; the term H II is pronounced "H two" by astronomers. "H" is the chemical symbol for hydrogen, "II" is the Roman numeral for 2. It is customary in astronomy to use the Roman numeral I for neutral atoms, II for singly-ionised—H II is H+ in other sciences—III for doubly-ionised, e.g. O III is O++, etc. H II, or H+, consists of free protons. An H I region being neutral atomic hydrogen, a molecular cloud being molecular hydrogen, H2.
In spoken discussion with non-astronomers there is sometimes confusion between the identical spoken forms of "H II" and "H2". A few of the brightest H II regions are visible to the naked eye. However, none seem to have been noticed before the advent of the telescope in the early 17th century. Galileo did not notice the Orion Nebula when he first observed the star cluster within it; the French observer Nicolas-Claude Fabri de Peiresc is credited with the discovery of the Orion Nebula in 1610. Since that early observation large numbers of H II regions have been discovered in the Milky Way and other galaxies. William Herschel observed the Orion Nebula in 1774, described it as "an unformed fiery mist, the chaotic material of future suns". In early days astronomers distinguished between "diffuse nebulae", which retained their fuzzy appearance under magnification through a large telescope, nebulae that could be resolved into stars, now known to be galaxies external to our own. Confirmation of Herschel's hypothesis of star formation had to wait another hundred years, when William Huggins together with his wife Mary Huggins turned his spectroscope on various nebulae.
Some, such as the Andromeda Nebula, had spectra quite similar to those of stars, but turned out to be galaxies consisting of hundreds of millions of individual stars. Others looked different. Rather than a strong continuum with absorption lines superimposed, the Orion Nebula and other similar objects showed only a small number of emission lines. In planetary nebulae, the brightest of these spectral lines was at a wavelength of 500.7 nanometres, which did not correspond with a line of any known chemical element. At first it was hypothesized that the line might be due to an unknown element, named nebulium—a similar idea had led to the discovery of helium through analysis of the Sun's spectrum in 1868. However, while helium was isolated on earth soon after its discovery in the spectrum of the sun, nebulium was not. In the early 20th century, Henry Norris Russell proposed that rather than being a new element, the line at 500.7 nm was due to a familiar element in unfamiliar conditions. Interstellar matter, considered dense in an astronomical context, is at high vacuum by laboratory standards.
Physicists showed in the 1920s that in gas at low density, electrons can populate excited metastable energy levels in atoms and ions, which at higher densities are de-excited by collisions. Electron transitions from these levels in doubly ionized oxygen give rise to the 500.7 nm line. These spectral lines, which can only be seen in low density gases, are called forbidden lines. Spectroscopic observations thus showed that planetary nebulae consisted of rarefied ionised oxygen gas. During the 20th century, observations showed that H II regions contained hot, bright stars; these stars are many times more massive than the Sun, are the shortest-lived stars, with total lifetimes of only a few million years. Therefore, it was surmised. Over a period of several million years, a cluster of stars will form in an H II region, before radiation pressure from the hot young stars causes the nebula to disperse; the Pleiades are an example of a cluster which has'boiled away' the H II region from which it was formed.
Only a trace of reflection nebulosity remain
The Milky Way is the galaxy that contains our Solar System. The name describes the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye; the term Milky Way is a translation of the Latin via lactea, from the Greek γαλαξίας κύκλος. From Earth, the Milky Way appears as a band. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610; until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies; the Milky Way is a barred spiral galaxy with a diameter between 200,000 light-years. It is estimated to contain 100 -- more than 100 billion planets; the Solar System is located at a radius of 26,490 light-years from the Galactic Center, on the inner edge of the Orion Arm, one of the spiral-shaped concentrations of gas and dust.
The stars in the innermost 10,000 light-years form a bulge and one or more bars that radiate from the bulge. The galactic center is an intense radio source known as Sagittarius A*, assumed to be a supermassive black hole of 4.100 million solar masses. Stars and gases at a wide range of distances from the Galactic Center orbit at 220 kilometers per second; the constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much of the mass of the Milky Way is invisible to telescopes, neither emitting nor absorbing electromagnetic radiation. This conjectural mass has been termed "dark matter"; the rotational period is about 240 million years at the radius of the Sun. The Milky Way as a whole is moving at a velocity of 600 km per second with respect to extragalactic frames of reference; the oldest stars in the Milky Way are nearly as old as the Universe itself and thus formed shortly after the Dark Ages of the Big Bang. The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which form part of the Virgo Supercluster, itself a component of the Laniakea Supercluster.
The Milky Way is visible from Earth as a hazy band of white light, some 30° wide, arching across the night sky. In night sky observing, although all the individual naked-eye stars in the entire sky are part of the Milky Way, the term “Milky Way” is limited to this band of light; the light originates from the accumulation of unresolved stars and other material located in the direction of the galactic plane. Dark regions within the band, such as the Great Rift and the Coalsack, are areas where interstellar dust blocks light from distant stars; the area of sky that the Milky Way obscures is called the Zone of Avoidance. The Milky Way has a low surface brightness, its visibility can be reduced by background light, such as light pollution or moonlight. The sky needs to be darker than about 20.2 magnitude per square arcsecond in order for the Milky Way to be visible. It should be visible if the limiting magnitude is +5.1 or better and shows a great deal of detail at +6.1. This makes the Milky Way difficult to see from brightly lit urban or suburban areas, but prominent when viewed from rural areas when the Moon is below the horizon.
Maps of artificial night sky brightness show that more than one-third of Earth's population cannot see the Milky Way from their homes due to light pollution. As viewed from Earth, the visible region of the Milky Way's galactic plane occupies an area of the sky that includes 30 constellations; the Galactic Center lies in the direction of Sagittarius. From Sagittarius, the hazy band of white light appears to pass around to the galactic anticenter in Auriga; the band continues the rest of the way around the sky, back to Sagittarius, dividing the sky into two equal hemispheres. The galactic plane is inclined by about 60° to the ecliptic. Relative to the celestial equator, it passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux, indicating the high inclination of Earth's equatorial plane and the plane of the ecliptic, relative to the galactic plane; the north galactic pole is situated at right ascension 12h 49m, declination +27.4° near β Comae Berenices, the south galactic pole is near α Sculptoris.
Because of this high inclination, depending on the time of night and year, the arch of the Milky Way may appear low or high in the sky. For observers from latitudes 65° north to 65° south, the Milky Way passes directly overhead twice a day; the Milky Way is the second-largest galaxy in the Local Group, with its stellar disk 100,000 ly in diameter and, on average 1,000 ly thick. The Milky Way is 1.5 trillion times the mass of the Sun. To compare the relative physical scale of the Milky Way, if the Solar System out to Neptune were the size of a US quarter, the Milky Way would be the size of the contiguous United States. There is a ring-like filament of stars rippling above and below the flat galactic plane, wrapping around the Milky Way at a diameter of 150,000–180,000 light-years, which may be part of the Milky Way itself. Estimates of the mass of the Milky Way vary, depending upon the method and data used; the low end of the estimate range is 5.8×1011 solar masses, somewhat less than that of the Andromeda Galaxy.
Measurements using the Very Long Baseline Array in 2009 found
A molecular cloud, sometimes called a stellar nursery, is a type of interstellar cloud, the density and size of which permit the formation of molecules, most molecular hydrogen. This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas. Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most used to determine the presence of H2 is carbon monoxide; the ratio between CO luminosity and H2 mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies. Within molecular clouds are regions with higher density, where lots of dust and gas cores reside, called clumps; these clumps are the beginning of star formation, if gravity can overcome the high density and force the dust and gas to collapse. Within the Milky Way, molecular gas clouds account for less than one percent of the volume of the interstellar medium, yet it is the densest part of the medium, comprising half of the total gas mass interior to the Sun's galactic orbit.
The bulk of the molecular gas is contained in a ring between 3.5 and 7.5 kiloparsecs from the center of the Milky Way. Large scale CO maps of the galaxy show that the position of this gas correlates with the spiral arms of the galaxy; 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. Vertically to the plane of the galaxy, the molecular gas inhabits the narrow midplane of the galactic disc with a characteristic scale height, Z, of 50 to 75 parsecs, much thinner than the warm atomic and warm ionized gaseous components of the ISM; the exception to the ionized-gas distribution are H II regions, which are bubbles of hot ionized gas created in molecular clouds by the intense radiation given off by young massive stars and as such they have the same vertical distribution as the molecular gas. This distribution of molecular gas is averaged out over large distances.
A vast assemblage of molecular gas with a mass of 103 to 107 times the mass of the Sun is called a giant molecular cloud. GMCs are around 15 to 600 light-years in diameter. Whereas the average density in the solar vicinity is one particle per cubic centimetre, the average density of a GMC is a hundred to a thousand times as great. Although the Sun is much more dense than a GMC, the volume of a GMC is so great that it contains much more mass than the Sun; the substructure of a GMC is a complex pattern of filaments, sheets and irregular clumps. The densest parts of the filaments and clumps are called "molecular cores", while the densest molecular cores are called "dense molecular cores" and have densities in excess of 104 to 106 particles per cubic centimeter. Observationally, typical molecular cores are traced with CO and dense molecular cores are traced with ammonia; the concentration of dust within molecular cores is sufficient to block light from background stars so that they appear in silhouette as dark nebulae.
GMCs are so large. These local GMCs are arrayed in a ring in the neighborhood of the Sun coinciding with the Gould Belt; the most massive collection of molecular clouds in the galaxy forms an asymmetrical ring about the galactic center at a radius of 120 parsecs. The Sagittarius region is chemically rich and is used as an exemplar by astronomers searching for new molecules in interstellar space. Isolated gravitationally-bound small molecular clouds with masses less than a few hundred times that of the Sun are called Bok globules; the densest parts of small molecular clouds are equivalent to the molecular cores found in GMCs and are included in the same studies. In 1984 IRAS identified a new type of diffuse molecular cloud; these were diffuse filamentary clouds. These clouds have a typical density of 30 particles per cubic centimeter; the formation of stars occurs within molecular clouds. This is a natural consequence of their low temperatures and high densities, because the gravitational force acting to collapse the cloud must exceed the internal pressures that are acting "outward" to prevent a collapse.
There is observed evidence that the large, star-forming clouds are confined to a large degree by their own gravity rather than by external pressure. The evidence comes from the fact that the "turbulent" velocities inferred from CO linewidth scale in the same manner as the orbital velocity; the physics of molecular clouds is poorly much debated. Their internal motions are governed by turbulence in a cold, magnetized gas, for which the turbulent motions are supersonic but comparable to the speeds of magnetic disturbances; this state is thought to lose energy requiring either an overall collapse or a steady reinjection of energy. At the same time, the clouds are known to be disrupted by some process—most the effects of massive stars—before a significant fraction of their mass has become stars. Molecular clouds, GMCs, are
A supernova remnant is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, consists of ejected material expanding from the explosion, the interstellar material it sweeps up and shocks along the way. There are two common routes to a supernova: either a massive star may run out of fuel, ceasing to generate fusion energy in its core, collapsing inward under the force of its own gravity to form a neutron star or a black hole. In either case, the resulting supernova explosion expels much or all of the stellar material with velocities as much as 10% the speed of light; these ejecta are supersonic: assuming a typical temperature of the interstellar medium of 10,000 K, the Mach number can be > 1000. Therefore, a strong shock wave forms ahead of the ejecta, that heats the upstream plasma up to temperatures well above millions of K; the shock continuously slows down over time as it sweeps up the ambient medium, but it can expand over hundreds or thousands of years and over tens of parsecs before its speed falls below the local sound speed.
One of the best observed young supernova remnants was formed by SN 1987A, a supernova in the Large Magellanic Cloud, observed in February 1987. Other well-known supernova remnants include the Crab Nebula; the youngest known remnant in our galaxy is G1.9+0.3, discovered in the galactic center. An SNR passes through the following stages as it expands: Free expansion of the ejecta, until they sweep up their own weight in circumstellar or interstellar medium; this can last tens to a few hundred years depending on the density of the surrounding gas. Sweeping up of a shell of shocked circumstellar and interstellar gas; this begins the Sedov-Taylor phase. Strong X-ray emission traces the strong shock waves and hot shocked gas. Cooling of the shell, to form a thin, dense shell surrounding the hot interior; this is the pressure-driven snowplow phase. The shell can be seen in optical emission from recombining ionized hydrogen and ionized oxygen atoms. Cooling of the interior; the dense shell continues to expand from its own momentum.
This stage is best seen in the radio emission from neutral hydrogen atoms. Merging with the surrounding interstellar medium; when the supernova remnant slows to the speed of the random velocities in the surrounding medium, after 30,000 years, it will merge into the general turbulent flow, contributing its remaining kinetic energy to the turbulence. There are three types of supernova remnant: Shell-like, such as Cassiopeia A Composite, in which a shell contains a central pulsar wind nebula, such as G11.2-0.3 or G21.5-0.9. Mixed-morphology remnants, in which central thermal X-ray emission is seen, enclosed by a radio shell; the thermal X-rays are from swept-up interstellar material, rather than supernova ejecta. Examples of this class include the SNRs W28 and W44. Supernova remnants are considered the major source of galactic cosmic rays; the connection between cosmic rays and supernovas was first suggested by Walter Baade and Fritz Zwicky in 1934. Vitaly Ginzburg and Sergei Syrovatskii in 1964 remarked that if the efficiency of cosmic ray acceleration in supernova remnants is about 10 percent, the cosmic ray losses of the Milky Way are compensated.
This hypothesis is supported by a specific mechanism called "shock wave acceleration" based on Enrico Fermi's ideas, still under development. Indeed, Enrico Fermi proposed in 1949 a model for the acceleration of cosmic rays through particle collisions with magnetic clouds in the interstellar medium; this process, known as the "Second Order Fermi Mechanism", increases particle energy during head-on collisions, resulting in a steady gain in energy. A model to produce Fermi Acceleration was generated by a powerful shock front moving through space. Particles that cross the front of the shock can gain significant increases in energy; this became known as the "First Order Fermi Mechanism". Supernova remnants can provide the energetic shock fronts required to generate ultra-high energy cosmic rays. Observation of the SN 1006 remnant in the X-ray has shown synchrotron emission consistent with it being a source of cosmic rays. However, for energies higher than about 1018 eV a different mechanism is required as supernova remnants cannot provide sufficient energy.
It is still unclear. The future telescope CTA will help to answer this question. List of All Known Galactic and Extragalactic Supernovae on the Open Supernova Catalog Galactic SNR Catalogue Chandra observations of supernova remnants: catalog, photo album, selected picks 2MASS images of Supernova Remnants NASA: Introduction to Supernova Remnants NASA's Imagine: Supernova Remnants Afterlife of a Supernova on UniverseToday.com Supernova remnant on arxiv.org Supernova Remnants, SEDS
A planetary nebula, abbreviated as PN or plural PNe, is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The term "planetary nebula" is arguably a misnomer because they are unrelated to planets or exoplanets; the true origin of the term was derived from the planet-like round shape of these nebulae as observed by astronomers through early telescopes, although the terminology is inaccurate, it is still used by astronomers today. The first usage may have occurred during the 1780s with the English astronomer William Herschel who described these nebulae as resembling planets, they are a short-lived phenomenon, lasting a few tens of thousands of years, compared to longer phases of stellar evolution. Once all of the red giant's atmosphere has been dissipated, energetic ultraviolet radiation from the exposed hot luminous core, called a planetary nebula nucleus, ionizes the ejected material. Absorbed ultraviolet light energises the shell of nebulous gas around the central star, causing it to appear as a brightly coloured planetary nebula.
Planetary nebulae play a crucial role in the chemical evolution of the Milky Way by expelling elements into the interstellar medium from stars where those elements were created. Planetary nebulae are observed in more distant galaxies, yielding useful information about their chemical abundances. Starting from the 1990s, Hubble Space Telescope images revealed that many planetary nebulae have complex and varied morphologies. About one-fifth are spherical, but the majority are not spherically symmetric; the mechanisms that produce such a wide variety of shapes and features are not yet well understood, but binary central stars, stellar winds and magnetic fields may play a role. The first planetary nebula discovered was the Dumbbell Nebula in the constellation of Vulpecula, it was listed as M27 in his catalogue of nebulous objects. To early observers with low-resolution telescopes, M27 and subsequently discovered planetary nebulae resembled the giant planets like Uranus. William Herschel, discoverer of Uranus coined the term "planetary nebula".
However, in as early as January 1779, the French astronomer Antoine Darquier de Pellepoix described in his observations of the Ring Nebula, "a dull nebula, but outlined. Whatever the true origin of the term, the label "planetary nebula" became ingrained in the terminology used by astronomers to categorize these types of nebulae, is still in use by astronomers today; the true nature of these objects was uncertain, Herschel first thought the objects were stars surrounded by material, condensing into planets rather than what is now known to be evidence of dead stars that have incinerated any orbiting planets. In 1782, William Herschel had discovered the object now known as NGC 7009, upon which he used the term "planetary nebula". In 1785, Herschel wrote to Jerome Lalande: "These are celestial bodies of which as yet we have no clear idea and which are of a type quite different from those that we are familiar with in the heavens. I have found four that have a visible diameter of between 15 and 30 seconds.
These bodies appear to have a disk, rather like a planet, to say, of equal brightness all over, round or somewhat oval, about as well defined in outline as the disk of the planets, of a light strong enough to be visible with an ordinary telescope of only one foot, yet they have only the appearance of a star of about ninth magnitude." Herschel assigned these to Class IV of his catalogue of "nebulae" listing 78 "planetary nebulae", most of which are in fact galaxies. The nature of planetary nebulae remained unknown until the first spectroscopic observations were made in the mid-19th century. Using a prism to disperse their light, William Huggins was one of the earliest astronomers to study the optical spectra of astronomical objects. On August 29, 1864, Huggins was the first to analyze the spectrum of a planetary nebula when he observed Cat's Eye Nebula, his observations of stars had shown that their spectra consisted of a continuum of radiation with many dark lines superimposed. He found that many nebulous objects such as the Andromeda Nebula had spectra that were quite similar.
However, when Huggins looked at the Cat's Eye Nebula, he found a different spectrum. Rather than a strong continuum with absorption lines superimposed, the Cat's Eye Nebula and other similar objects showed a number of emission lines. Brightest of these was at a wavelength of 500.7 nanometres, which did not correspond with a line of any known element. At first, it was hypothesized that the line might be due to an unknown element, named nebulium. A similar idea had led to the discovery of helium through analysis of the Sun's spectrum in 1868. While helium was isolated on Earth soon after its discovery in the spectrum of the Sun, "nebulium" was not. In the early 20th century, Henry Norris Russell proposed that, rather than being a new element, the line at 500.7 nm was due to a familiar element in unfamiliar conditions. Physicists showed in the 1920s that in gas at low densities, electrons can occupy excited metastable energy levels in atoms and ions that would otherwise be de-excited by collisions that would occur
Evaporating gaseous globule
An evaporating gas globule or EGG is a region of hydrogen gas in outer space 100 astronomical units in size, such that gases shaded by it are shielded from ionizing UV rays. Dense areas of gas shielded by an evaporating gas globule can be conducive to the birth of stars. Evaporating gas globules were first conclusively identified via photographs taken by the Hubble Space Telescope in 1995. EGG's are the predecessors of new protostars. Inside an EGG the gas and dust are denser than in the surrounding dust cloud. Gravity pulls the cloud more together as the EGG continues to draw in material from its surroundings; as the cloud density builds up the globule becomes hotter under the weight of the outer layers, a protostar is formed inside the EGG. A protostar may have too little mass to become a star. If so it becomes a brown dwarf. If the protostar has sufficient mass, the density reaches a critical level where the temperature exceeds 10 million kelvin at its center. At this point, a nuclear reaction starts converting hydrogen to helium and releasing large amounts of energy.
The protostar becomes a star and joins the main sequence on the HR diagram. Hubble sees stars and a stripe in celestial fireworks — ESA/NASA Image, July 1, 2008 Embryonic Stars Emerge from Interstellar "Eggs", HubbleSite, Nov. 2, 1995 http://hubblesite.org/newscenter/archive/releases/1995/44/text/ Hubble site http://apod.nasa.gov/apod/image/1207/pillars6_hst_1518.jpg NASA APODly 2012 http://www.msnbc.msn.com/id/17467408/ns/technology_and_science-space/t/suns-baby-twin-spotted-pillars-creation/#. UA0slGGe5uoNBCNews Space, 3/5/2007