Astronomy - The Milky Way Galaxy - Quick Guides | Astronomy

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Study GuideAstronomy–The Milky Way Galaxy1. Origin and EvoluƟon of the GalaxyAstronomers study theorigin and evolution of the Galaxyby observing where stars are located,how they move, and what they are made of. These observations reveal clues aboutwhen and howdifferent stars formed.Early studies showed that stars in the Galaxy could be grouped based on their age, location, motion,and chemical composition. These groups are calledstellar populations.1.1Stellar PopulaƟons: An OverviewOriginally, astronomers identifiedtwo main stellar populationsbecause their properties were verydifferent. Later, a third population was discovered.The main populations are:•Population I•Population II•Population III(the earliest stars)Each population helps us understand a different stage in the Galaxy’s history.1.2 PopulaƟon I Stars–Young and Metal-RichPopulation I stars are theyoungest stars in the Galaxy.Their key features include:•Found mainly in thegalactic disk•Often located inopen clusters and stellar associations•Containbright, blue O and B stars•Closely associated withgas and dust, from which they recently formed•Havechemical compositions similar to the Sun•Heavy element content ranges fromone-third to twice the solar abundance•Move withsmall velocities relative to the SunThese stars are still forming today in the thin disk of the Galaxy.

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Study Guide1.3 PopulaƟon II Stars–Old and Metal-PoorPopulation II stars aremuch olderthan Population I stars.Their characteristics include:•Found mainly inglobular clusters•Located in aspherical haloaround the galactic center•Containold red giant stars•LackO and B stars•Havelittle or no gas and dust•Possesslow heavy-element (metal) content, ranging from0.001 to 0.5 times the solarabundance•Showlarge velocities relative to the Sun1.4 Stellar PopulaƟons Form a ConƟnuumModern astronomy shows thatstellar populations are not completely separate groups. Instead,they representextremes of a continuous rangeof star properties.•At one extreme aremetal-poor halo starswith random, elongated orbits.•At the other extreme aremetal-rich disk starsconfined to a thin, rotating plane.Between these extremes are stars with intermediate properties.1.5 PopulaƟon III Stars–The First StarsPopulation III stars are theearliest generation of starsin the Galaxy.Their features:•Composed almost entirely ofhydrogen and helium•Containedno heavy elements•Formed directly fromprimordial gas•Were once only theoretical but have now been indirectly identifiedThese stars played a crucial role by producing thefirst heavy elementsthrough nuclear reactionsand supernova explosions.

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Study Guide1.6 Standard Model of GalacƟc FormaƟonAccording to the standard model, the Galaxy began forming very early in the universe.Key stages include:•About10¹² solar massesof hydrogen and helium gas began to collapse under gravity•Thefirst stars (Population III)formed from pure gas•Massive stars evolved quickly and exploded assupernovae•These explosions enriched the surrounding gas withheavy elements1.7 FormaƟon of the Halo and DiskDuring the collapse:•Early stars (including globular clusters) formed while the Galaxy was still collapsing•These stars retainednearly radial orbits, forming thehalo•Gas continued to collapse andflattened into a rotating diskdue to conservation of angularmomentum•Collisions between gas particles reduced random motion•Over time,only circular motion remained, forming a thin diskEach new generation of stars formed fromincreasingly metal-rich gas, creating today’s Population Istars.1.8 Ongoing Star FormaƟonEven today:•Interstellar gas remains in athin disk•This gas is nowrich in heavy elements•New Population I stars continue to formThus, the Galaxy is still evolving.

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Study Guide1.9 AlternaƟve Model: Galaxy MergersSome evidence suggests the Galaxy’s formation wasmore complexthan a single collapse.An alternative theory proposes:•Smaller, flatproto-galaxiesformed first•These systems graduallymergedto form the Milky Way•The Galaxy grew throughcapture and cannibalism of smaller galaxiesThis process likely played amajor rolein shaping the present Galaxy.Key Takeaways•Stellar populations reflect thehistory of star formation•Heavy elements increase witheach generation of stars•The Galaxy formed throughcollapse, rotation, and enrichment•Mergers with smaller galaxies helped shape today’s Milky WayThe stars you see today carry a record of billions of years of galactic evolution.2. Interstellar MaƩer2.1Our Galaxy: The Milky WayTheMilky Wayis a typical galaxy. A galaxy is a huge, independent system made up ofstars, starclusters, and interstellar material(gas and dust found between stars). By studying the Milky Way,astronomers learn how galaxies work in general.The Milky Way is classified as aspiral galaxy. This is because clouds of gas and dust form apinwheel-like spiral patternin the flat plane of the Galaxy.The Galaxy hastwo main parts:•Aflat disk, which contains most of the stars, gas, and dust•Acentral bulge, which is a slightly flattened, spherical region surrounding the disk

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Study Guide2.2What Is Interstellar MaƩer?About3% of the Milky Way’s total massexists asinterstellar matter. This material is spread outbetween stars and is extremely thin.•Average density:about 1 atom per cubic centimeter•Mostly made ofhydrogen atomsTo compare, the air we breathe contains about10²² particles per cubic centimeter, so interstellarmatter is almost empty by everyday standards.2.3Why Interstellar MaƩer Is ImportantInterstellar matter playsthree major rolesin astronomy:1.Star formation–It provides the raw material needed to form new stars.2.Galaxy studies–Observing gas and dust gives information about the Galaxy that stars alonecannot provide.3.Effect on light–Interstellar material can change how light from distant objects appears to us.2.4Components of Interstellar MaƩerInterstellar matter hastwo main components:gasanddust. They behave very differently and affectobservations in different ways.2.5GasGas makes up about98% of interstellar matter.Composition•Mostlyhydrogen and helium•In dense regions,moleculescan form•Over100 types of simple moleculeshave been detected, includingwater, formaldehyde,and alcoholEffect on ObservationsGas hasvery little effecton the brightness or color of stars seen through it. It mainly:

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Study Guide•Addsabsorption linesto a star’s spectrum•Doesnot significantly dim or reddenstarlightThe Importance of the 21-cm LineNeutral hydrogen emits radiation at a wavelength of21 cm, which is extremely important inastronomy.•This radiationpasses through dust easily•It allows astronomers to study parts of the Galaxy that would otherwise be hiddenWithout this 21-cm radiation,most of the Milky Way would be invisibleto us.Gas only produces visible emission lines inhot, dense regions, usually near very hot stars.2.6DustDust makes up the remaining2% of interstellar matter, but it has amuch stronger effect onobservations.Properties of Dust•Made of tiny solid particles•Typical temperature:about 100 K•Emitsthermal radiationdue to its temperatureEffect on LightDust:•Absorbslight passing through it•Scatterslight in different directionsBecause of this, dust:•Dimsdistant stars•Makes objects appearredderIn the plane of the Galaxy, dust prevents astronomers from seeing stars farther than about3kiloparsecs (10,000 light-years)using visible light.

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Study Guide2.7Why Distant Objects Look Redder2.8ExplanaƟon of the Figure: Interstellar AbsorpƟon•Light from adistant objecttravels toward an observer•Blue light(shorter wavelengths) is scattered more easily by dust grains•Red light(longer wavelengths) passes through more easilyAs a result:•Less blue light reaches the observer•The object appearsredder and dimmerthan it really isThis effect is calledinterstellar reddening.Key TakeawayEven though interstellar matter makes up only a small fraction of the Galaxy’s mass, it plays acrucialrolein:•Star formation•Understanding galactic structure•Affecting how we observe distant objectsGas helps us map the Galaxy, while dust limits what we can see—but also teaches us how lightinteracts with matter in space.

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Study Guide3. Interstellar NebulaeInterstellar space is not completely empty. It containsgas and dust, calledinterstellar material.This material is easiest to see where it isdensest, forming clouds known asnebulae.Nebulae are grouped intothree main types, based on how we observe them:1.Nebulae thatemit their own light2.Nebulae thatblock light from stars behind them3.Nebulae thatreflect light from nearby stars3.1Emission Nebulae (H II Regions)Emission nebulaeare clouds of gas thatglow on their own.They are found nearhot, young stars, especially very hotO-type and B-type stars.How They Glow•Hot stars emit strongultraviolet (UV) radiation.•This radiationionizes hydrogen gas, meaning electrons are stripped from hydrogen atoms.•Ionized hydrogen is calledH II(pronounced “H two”), which is why these nebulae are alsocalledH II regions.•When electrons recombine with hydrogen atoms, the gas releasesvisible light.Color and Temperature•The gas temperature is about10,000 K.•Most of the light comes from hydrogen emission lines.•The strongest line is theHαlinein the Balmer series.•This gives emission nebulae ared or pink color.Size of Ionized Regions•Very hot stars can ionize gas overseveral parsecs.•AnO5 star(about 50,000 K) produces a large ionized region.•A cooler star like theSuncan ionize hydrogen only within thesolar system (about 40 AU).

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