Chapter 19Star Formation19.1 Star-Forming Regions Competition in Star Formation19.2 The Formation of Stars Like the Sun19.3 Stars of Other Masses19.4 Observations of Cloud Fragments and Protostars XXObservations of Brown Dwarfs19.5 Shock Waves and Star Formation19.6 Star Clusters XXEta CarinaeUnits of Chapter 19Star formation is ongoing. Star-forming regions areseen in our galaxy as well as others:19.1 Star-Forming RegionsStar formation happens when part of a dust cloud begins to contract under its owngravitational force; as it collapses, the center becomes hotter and hotter until nuclearfusion begins in the core. That is a basic and simple summary of this chapter.Star formation begins in massive clouds of molecular gas and dustWhen looking at just a few atoms, the gravitational force is far from strongenough to overcome the random thermal motions:But interstellar molecular clouds are enormous, oftencontaining up to a million solar masses of gas (and 1% dust--itis just part of the ISM. That is a lot of self-gravity! The mostmassive clouds have gravity that can overcome the thermalpressure trying to resist gravity, and the cloud must collapse.The cloud shown to the right above is a moderately-sizedcloud with a size of about 10 pc and a mass of about 10thousand solar masses. several hundred young lower-massstars like the sun are forming there.“giant molecular cloud” gragmenting under gravityinto hundreds of stars. Thered parts are densest, and Where the stars are seen. This is a map in the CO molecule.More Precisely 19-1:Competition in Star FormationRotation can also interfere withgravitational collapse, as canmagnetism. Clouds contract in a“distorted way:In fact the clouds are usually muchmore irregular than shown in thistextbook illustration. (Note: all thecolorful emission line nebulaeshown in this chapter are just thissame molecular gas after a massivestars has started pouring outphotons.Stars go through a number of stages in the process of forming from an interstellarcloud: [You don’t have to memorize these numbers, but they do tell an importantstory, and you should understand the names of the objects on the right.]19.2 The Formation of Stars Like the SunNotice that a solar-like star only takes a few tens of millions of years to collapse andContact until they reach the main sequence, but they will spend 10 billion yearsburning hydrogen on the main sequence before running out of nuclear fuel.Stage 1:Interstellar cloud starts to contract, possibly triggered by shock orpressure wave from nearby star. As it contracts, the cloud fragments intosmaller pieces. [Note: this is a cartoon--the process is not nearly as tidyas shown here.19.2 The Formation of Stars Like the SunStage 2:Individual cloud fragments begin to collapse. Once the density is highenough, there is no further fragmentation. Reason: the star hasbecome opaque to its own radiation: It has a photosphere! After this,the ‘trapped radiation heats the interior of the object as it contracts.Stage 3:The interior of the fragment has begun heating and is about 10,000 K.This is hot enough to ionize hydrogen, but not much else. The interiorwill continue to heat as it contracts, until, at several million degrees,something stops it (nuclear fusion will generate the heat to halt thecollapse).19.2 The Formation of Stars Like the SunStage 4:The core of the cloud isnow a protostar andmakes its firstappearance on the H-Rdiagram:19.2 The Formation of Stars Like the SunPlanetary formation has begun, but the protostar is still not inequilibrium—all heating comes from the gravitational collapse.Inspect the picture and understand the different stages represented.19.2 The Formation of Stars Like the SunThe last stages can befollowed as a path in theH-R diagram:The protostar’s luminositydecreases even as itstemperature rises becauseit is becoming morecompact (less surfacearea).19.2 The Formation of Stars Like the SunAt stage 6, the core reaches 10 million K, and nuclear fusionbegins. The protostar has become a star.The star continues to contract and increase in temperatureuntil it is in equilibrium: Internal pressure force outward,balancing the inward force of gravity, at every layer of thestar’s interior. This is stage 7: The star has reached the MainSequence and will remain there as long as it has hydrogen tofuse.19.2 The Formation of Stars Like the SunThis H-R diagramshows the evolution ofstars somewhat moreand somewhat lessmassive than the Sun.The shape of the pathsis similar, but theywind up in differentplaces on the MainSequence.19.3 Stars of Other MassesThe Main Sequence is a band, rather than a line,because stars of the same mass can have differentcompositions.Most important: Stars do not move along the MainSequence! Once they reach it, they are inequilibrium and do not move until their fuelbegins to run out.19.3 Stars of Other MassesSome fragments are too small for fusion ever tobegin. They gradually cool off and simply fade,radiating away whatever heat generated in trying(unsuccessfully) to ignite nuclear fusion.A protostar must have 0.08 the mass of the Sun(which is 80 times the mass of Jupiter) in order tobecome dense and hot enough that fusion canbegin.If the mass of the “failed star” is about 12 Jupitermasses or more, it is luminous when first formed,and is called a brown dwarf.19.3 Stars of Other MassesEmission nebulae areheated by the formation ofstars nearby. In theseimages, we see the parentcloud in stage 1,contracting fragmentsbetween stages 1 and 2,and a new star in stage 6 or7. The new star is the oneheating the nebula.19.4 Observations of Cloud Fragments andProtostarsThe Orion Nebula has many contracting cloudfragments, protostars, and newborn stars:19.4 Observations of Cloud Fragments andProtostarsThese are two protostars in the Orion nebula, ataround stage 5 in their development:19.4 Observations of Cloud Fragments andProtostarsProtostars arebelieved to havevery strong winds,which clear out anarea around the starroughly the size ofthe solar system19.4 Observations of Cloud Fragments andProtostarsThese two jets are matter being expelledfrom around an unseen protostar, stillobscured by dust.19.4 Observations of Cloud Fragments andProtostarsTriggers for star formation:Shock waves generated by:• Death of a sun-like nearby star• Supernova explosion (massive stars only)• Density waves in galactic spiral arms• Galaxy collisions19.5 Shock Waves and Star FormationThis region may very
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