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we live in a pretty cool neighborhood: themilky way galaxy. we’re out in the suburbs, sure, but it’s still an interesting place,buzzing with activity. stars, nebulae, stellar clusters of various sorts, the occasionalsupernova. it’s a happening place. in the earliest part of the 20th century,astronomers were just starting to figure this all out. but there were a handful of objectsthat were puzzling: dotting the sky here and

crash diets 2014, there were faint fuzzies displaying a varietyof shapes. some were round, some elongated, and some even seemed to have spiral arms.even with big telescopes they looked smoky, so they were simply called “nebulae.” their existence was puzzling, though. whatwere they? how did they form? were they big,

small, near, far? eventually, astronomershad uncovered the key to these objects, and in one fell swoop our universe got a lot bigger. a lot. in 1920, there were two competing ideas aboutthe universe. one was that our milky way was it, and that everything we saw was in it.the other was that the “spiral nebulae” seen in the sky were also like our milky way;“island universes” in their own right. two astronomers debated this controversy inthat year. harlow shapley argued that the milky way was all there is, while heber curtiswas of the opinion that we were one of many galaxies. it wasn’t a debate as such; moreof a presentation of ideas. and there was

no clear winner; both sides had fragmentarydata and – we now know – some shaky observational evidence that turned out not to be correct. for example, shapley noted that one of thespiral nebulae had been seen to rotate, so it must be small. it turns out that was justwrong, dead wrong. on the other hand, curtis noted that if galaxies were as big as shapleyclaimed – hundreds of thousands of light years across – then other galaxies mustbe impossibly far away. but, hello, galaxies really are that big, and they truly are mind-numbinglydistant. the observation that finally unlocked thismystery was made just a few years later when edwin hubble and milton humason observed thegreat spiral nebula m31 in andromeda, using

what was at the time the largest telescopein the world. they found dozens of pulsating stars in it, literally stars that changedtheir brightness in a regular, periodic fashion. these are called cepheid variables, and theywere critically important, because it was known that the time it took them to pulsewas directly related to their luminosity, how much energy they emitted. that means ifyou can measure their period, you can determine how far away they are simply by measuringtheir apparent brightness. the distance they found to m31 was 900,000light years: clearly outside even the largest estimates of the size of the milky way. theyalso resolved “swarms of faint stars,” cinching the fact that the great andromedanebula was actually…the andromeda galaxy.

at that moment, our understanding of the universeswelled in size, and we’ve never looked back. we’ve learned a lot about galaxies in theensuing century or so. each is a collection of at least hundreds of millions — or eventrillions — of stars, and most contain some amount of gas and dust. they range in sizefrom tens of thousands to hundreds of thousands of light years across, and they come in avariety of shapes. we use these overall shapes to classify them.broadly speaking, there are four major types of galaxies: elliptical, spiral, peculiar,and irregular. spiral galaxies we’ve seen: we live in one!these are characterized by broad, flat rotating

disks of stars, gas, and dust; a central bulgeof older, redder stars, sometimes with a long cylindrical or rectangular bar of stars aswell; and a huge extended halo of older stars. some spirals have large central bulges andsome have much smaller ones. most have bars of stars extending across the central partof the galaxy; our milky way does. the spirals come in a variety of flavors as well. granddesign spirals have magnificent, well-organized spiral arms that extend from the very centerout to the visible edge of the galaxy. others have choppy or patchy arms — these are calledflocculent spirals, resembling tufts of cotton. some spirals have wide-flung arms, while othersare tightly wound. spirals take on wildly different appearancesdepending on their angle to us. some we see

face-on, and these may be the most magnificentobjects in the entire sky. huge and sprawling, their structure is obvious and easy to see.star-forming nebulae are laid out like beads on a string, colored pink by the characteristicglow of warm hydrogen. young, massive, luminous stars blaze blue, tracing the spiral’s form.filigrees of dust clouds align with the arms as well, and the central bulge or bar glowsan eerie reddish yellow, the star formation there long since ceased, the bluer stars allexploded, leaving only the redder stars behind. when spirals are more tilted to our line ofsight some of this structure is hidden. and when they’re edge-on we see them as theflat disks they truly are. dust clouds are tightly restricted to the mid-plane of spiralgalaxies, and we sometimes see them bisecting

the galaxy like a racing stripe right downthe middle. the reason for this is the same reason oursolar system is flat: galaxies probably formed from huge clouds of gas, billions of yearsago. as a cloud collapses, small eddies in the gas would get amplified, and would createan overall spin. this naturally leads to a flattening of the cloud, and by the time thestars in the galaxy were starting to turn on, the overall structure of a flat disk wasin place. elliptical galaxies are, well, elliptical.kinda. some are nearly spherical, gigantic cotton balls of billions of stars. othersare more elongated, shaped like cigars or american footballs. they tend to have no overallstructure as spirals do; ellipticals are puffy.

and they range in size dramatically; someare what are called dwarf ellipticals just a few thousand light years across, to monstersthat hugely outmuscle our own milky way. besides their shape, ellipticals are characterizedby a lack of gas and dust in them, and also are populated with older stars. apparently,star formation in ellipticals came and went eons ago. and all the young, massive starshave long since exploded, and all that’s left are less massive, redder stars. thismakes them similar to the stars in the central bulges of spiral galaxies. it’s not exactly clear how ellipticals form.current thinking is that they are the products of — and this is mind blowing — galactic collisions.yes, you heard me: entire galaxies collide.

when galaxies collide it’s a train wreckon a cosmic scale. galaxies are huge structures, and with hundredsof billions of stars in them, their gravity is pretty strong. if two galaxies get close enoughtogether, they can draw each other in and collide. a galactic collision is a weird event. eventhough the collision speeds can be hundreds of kilometers per second, the event playsout over hundreds of millions of years — remember, we’re talking about distances of tens ofthousands of light years here. in the early stages of the collision, tidaleffects can be strong. stars on the side of the galaxy near the other one get pulled towardit more strongly than the stars farther away, so the galaxies can get stretched, and longtendrils of stars and gas get drawn out.

generally, collisions aren’t head-on, butmore of a sideswipe, so there’s some sideways motion. when that happens, the tidal streamercan become curved; a long, graceful arc. colliding galaxies in this stage form all sorts of bizarreand spectacular shapes. sometimes the galaxies separate,and then together again. when that happens, the main bodies collide. but this isn’t like two cars crashing together.stars are very, very small compared to the space between them, so even though hundredsof billions of stars can be involved, the odds are good that no two stars will everphysically hit each other! space is weird.

gas clouds, though, are huge, and they docollide. they slam into each other, collapse, and form stars at a furious rate. collidinggalaxies can glow pink and blue as stars are born and light up the hydrogen clouds aroundthem. sometimes the two galaxies will collide athigh enough velocity that they pass right through each other! but even then, in mostcases, they’ll slow, stop, then recollide. eventually they merge, their huge energiesof motion absorbed by the orbiting stars, puffing them up into vast, sweeping orbits.the results, so we think, are elliptical galaxies. not all collisions result in galaxies becomingellipticals. if a big spiral collides with a much smaller galaxy, it can tear apart theinterloper and literally absorb it into itself.

sooo, galaxies are cannibals! we think most large galaxies grew to theircurrent bulk by consuming smaller galaxies… including our own milky way. in fact, we haveproof: we’re currently in the process of eating several smaller galaxies right now!terzan 5 is a small knot of stars that may be the leftover core of a galaxy cannibalizedby our own, and two huge, looping streams of stars circling our galaxy are parts ofthe sagittarius and canis majoris dwarf galaxies being torn apart by the milky way. this brings us to the third type of galaxies:peculiars. these are not shapeless, really, but have a shape that’s weird. peculiargalaxies are essentially all due to collisions;

colliding galaxies clearly have structure,but can be all sorts of odd, distorted shapes. sometimes a small galaxy will plunge rightthrough the heart of a much larger galaxy. the gravity of the smaller galaxy draws instars and then flings them away in the bigger one, like ripples in a pond. the result isa ring galaxy, sometimes with the culprit intruder seen nearby. the famous cartwheelgalaxy is a fine example of this. another is hoag’s object, though to be honest it’snot completely clear if this bizarre galaxy is a product of a collision or some otherprocess. it’s peculiar either way! finally, we come to galaxies that truly areshapeless, called irregular galaxies. these tend to be small, and chaotically shaped.bigger ones may be victims of collisions,

their shapes distorted so much that the structurewas lost. smaller ones may simply be too tiny to collect themselves together into a recognizableshape. many galaxies have companion, satellite galaxies.the milky way has a couple of dozen such satellites. the two biggest are the large and small magellanicclouds, visible from the southern hemisphere. both are irregular in shape, though the largemagellanic cloud is barely coherent enough to qualify as a disrupted, though small, barredspiral. both are riddled with gas and dust. the large cloud sports the biggest and mostactive star forming gas cloud in any nearby galaxy: the tarantula nebula. it’s producingso many stars that astronomers think it may be in the process of forming a globular cluster!

there are other types of galaxies as well.some, called active galaxies, pour out far more energy than normal ones. and galaxiesaren’t just scattered evenly across space; they tend to be found in clusters, some containingthousands of galaxies. we’ll get to all that in the next episode. we now know that the universe is far, farlarger than just our milky way galaxy. it’s a recurring theme, isn’t it? we thoughtthe earth was the center of creation, and then the sun, and then the galaxy. but everytime we investigate these situations earnestly, and engage in honest debate, we find thatwe’re smaller and farther removed from the action than we thought -- or than we thoughtwe deserved.

astronomy is really, really good at puttingus in our place. but it’s also really good at showing us just how grand and awe-inspiringthat place is. today you learned that the milky way is agalaxy, one of many. containing gas, dust, and billions of stars or more, galaxies comein four main shapes: elliptical, spiral, peculiar, and irregular. galaxies can collide, and growin size by eating each other. crash course astronomy is produced in associationwith pbs digital studios. head over to their youtube channel to catch even more awesomevideos. this episode was written by me, phil plait. the script was edited by blake de pastino,and our consultant is dr. michelle thaller. it was directed by nicholas jenkins, editedby nicole sweeney, the sound designer is michael

aranda, and the graphics team is thought cafã©.