|Star Formation:||Readings: Schneider & Arny: Unit 60|
You are watching: A cloud fragment too small to form a star becomes:
How do we know this is happening if it takes so long and is hiddenfrom view in dark clouds? Most of these cloud cores have IR sources,evidence of energy from collapsing protostars (potential energyconverted to kinetic energy). Also, where we do find young stars(see below) we find them surrounded by clouds of gas, the leftoverdark molecular cloud. And they occur in clusters, groups of starsthat form from the same cloud core.Protostars:Once a clump has broken free from the other parts of the cloud core,it has its own unique gravity and identity and we call it aprotostar. As the protostar forms, loose gas falls into its center.The infalling gas releases kinetic energy in the form of heat and thetemperature and pressure in the center of the protostar goes up. Asits temperature approaches thousands of degrees, it becomes a IRsource.Several candidate protostars have been found by the Hubble SpaceTelescope in the Orion Nebula.
During the initial collapse, the clump is transparent to radiationand the collapse proceeds fairly quickly. As the clump becomes moredense, it becomes opaque. Escaping IR radiation is trapped, and thetemperature and pressure in the center begin to increase. At somepoint, the pressure stops the infall of more gas into the core andthe object becomes stable as a protostar.The protostar, at first, only has about 1% of its final mass. Butthe envelope of the star continues to grow as infalling material isaccreted. After a few million years, thermonuclear fusion begins inits core, then a strong stellar wind is produced which stops theinfall of new mass. The protostar is now considered a young starsince its mass is fixed, and its future evolution is now set.T-Tauri Stars:Once a protostar has become a hydrogen-burning star, a strong stellarwind forms, usually along the axis of rotation. Thus, many youngstars have a bipolar outflow, a flow of gas out the poles of thestar. This is a feature which is easily seen by radio telescopes.This early phase in the life of a star is called the T-Tauri phase.
One consequence of this collapse is that young T Tauri stars areusually surrounded by massive, opaque, circumstellar disks. These disksgradually accrete onto the stellar surface, and thereby radiate energyboth from the disk (infrared wavelengths), and from the position wherematerial falls onto the star at (optical and ultraviolet wavelengths).Somehow a fraction of the material accreted onto the star is ejectedperpendicular to the disk plane in a highly collimated stellar jet. Thecircumstellar disk eventually dissipates, probably when planets begin toform. Young stars also have dark spots on their surfaces which areanalogous to sunspots but cover a much larger fraction of the surface areaof the star.The T-Tauri phase is when a star has: vigorous surface activity (flares, eruptions) strong stellar winds variable and irregular light curvesA star in the T-Tauri phase can lose up to 50% of its mass beforesettling down as a main sequence star, thus we call them pre-mainsequence stars. Their location on the HR diagram is shown below:
The arrows indicate how the T-Tauri stars will evolve onto the mainsequence. They begin their lives as slightly cool stars, then heat upand become bluer and slightly fainter, depending on their initialmass. Very massive young stars are born so rapidly that they justappear on the main sequence with such a short T-Tauri phase that theyare never observed.T-Tauri stars are always found embedded in the clouds of gas from whichthey were born. One example is the Trapezium cluster of stars in theOrion Nebula.
The evolution of young stars is from a cluster of protostars deep ina molecular clouds core, to a cluster of T-Tauri stars whose hotsurface and stellar winds heat the surrounding gas to form an HIIregion (HII, pronounced H-two, means ionized hydrogen). Later thecluster breaks out, the gas is blown away, and the stars evolve asshown below.
Often in galaxies we find clusters of young stars near other youngstars. This phenomenon is called supernova induced star formation.The very massive stars form first and explode into supernova. Thismakes shock waves into the molecular cloud, causing nearby gas tocompress and form more stars. This allows a type of stellarcoherence (young stars are found near other young stars) to build up,and is responsible for the pinwheel patterns we see in galaxies.
Brown Dwarfs:If a protostar forms with a mass less than 0.08 solar masses, itsinternal temperature never reaches a value high enough for thermonuclearfusion to begin. This failed star is called a brown dwarf, halfwaybetween a planet (like Jupiter) and a star. A star shines because of thethermonuclear reactions in its core, which release enormous amounts ofenergy by fusing hydrogen into helium. For the fusion reactions to occur,though, the temperature in the star"s core must reach at least threemillion kelvins. And because core temperature rises with gravitationalpressure, the star must have a minimum mass: about 75 times the mass ofthe planet Jupiter, or about 8 percent of the mass of our sun. A browndwarf just misses that mark-it is heavier than a gas-giant planet but notquite massive enough to be a star.For decades, brown dwarfs were the "missing link" of celestial bodies:thought to exist but never observed. In 1963 University of Virginiaastronomer Shiv Kumar theorized that the same process of gravitationalcontraction that creates stars from vast clouds of gas and dust would alsofrequently produce smaller objects. These hypothesized bodies were calledblack stars or infrared stars before the name "brown dwarf" was suggestedin 1975. The name is a bit misleading; a brown dwarf actually appearsred, not brown. In the mid-1980s astronomers began an intensive search for brown dwarfs,but their early efforts were unsuccessful. It was not until 1995 that theyfound the first indisputable evidence of their existence. That discoveryopened the floodgates; since then, researchers have detected dozens of theobjects. Now observers and theorists are tackling a host of intriguingquestions: How many brown dwarfs are there? What is their range of masses?Is there a continuum of objects all the way down to the mass of Jupiter?And did they all originate in the same way?
The halt of the collapse of a brown dwarf during its formation occursbecause the core becomes degenerate before the start of fusion. With theonset of degeneracy, the pressure can not increase to the point ofignition of fusion.Brown dwarfs still emit energy, mostly in the IR, due to the potentialenergy of collapse converted into kinetic energy. There is enoughenergy from the collapse to cause the brown dwarf to shine for over 15million years (called the Kelvin-Helmholtz time). Brown dwarfs areimportant to astronomy since they may be the most common type of starout there and solve the missing mass problem (see cosmology course nextterm). Brown dwarfs eventual fade and cool to become black dwarfs.
Relative sizes and effective surface temperatures of two recentlydiscovered brown dwarfs -- Teide 1 and Gliese 229B -- compared to a yellowdwarf star (our sun), a red dwarf (Gliese 229A) and the planet Jupiter,reveal the transitional qualities of these objects. Brown dwarfs lacksufficient mass (about 80 Jupiters) required to ignite the fusion ofhydrogen in their cores, and thus never become true stars. The smallesttrue stars (red dwarfs) may have cool atmospheric temperatures (less than4,000 degrees Kelvin) making it difficult for astronomers to distinguishthem from brown dwarfs. Giant planets (such as Jupiter) may be much lessmassive than brown dwarfs, but are about the same diameter, and maycontain many of the same molecules in their atmospheres. The challengefor astronomers searching for brown dwarfs is to distinguish between theseobjects at interstellar distances.
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Neither planets nor stars, brown dwarfs share properties with both kindsof objects: They are formed in molecular clouds much as stars are, buttheir atmospheres are reminiscent of the giant gaseous planets.Astronomers are beginning to characterize variations among brown dwarfswith the aim of determining their significance among the Galaxy"sconstituents. In this painting a young brown dwarf is eclipsed by one ofits orbiting planets as seen from the surface of the planet"s moon.