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superclusters, like the Local Group cluster and the Laniakea supercluster",1,{"id":36,"data":37,"type":38,"maxContentLevel":19,"version":34},"1aa5e668-ce55-41b3-8f51-63ecbbaea827",{"type":38,"intro":39},10,[40,41],"What discovery did Edwin Hubble make about the Andromeda Nebula?","What is the difference between a galaxy cluster and a supercluster?",[43,48,84],{"id":44,"data":45,"type":34,"maxContentLevel":19,"version":25},"4c90a162-9012-4918-a987-cb61f8f6c64d",{"type":34,"markdownContent":46,"audioMediaId":47},"A galaxy is a massive system of stars, planets, gas and dust, all bound together by the force of gravity. When we say 'massive', we really mean it.\n\nOur own galaxy – the **Milky Way** – contains at least 100 billion stars. It also contains at least as many planets, and possibly significantly more.\n\n![Graph](image://38db7fdb-b3d3-4459-987c-17c3833e16d1 \"The Milky Way. Image: NASA/JPL-Caltech/ESO/R. Hurt, Public domain, via Wikimedia Commons\")\n\nIn the early 20th century, there was a lot of debate about galaxies. Was our giant galaxy all that was out there? Or was it just one of many clusters of stars?","b66c1dec-5e7e-47ed-b5e5-4c86735858a9",{"id":49,"data":50,"type":34,"maxContentLevel":19,"version":25,"reviews":53},"48deb9e6-c776-4d0f-89d8-40f1a84bf83f",{"type":34,"markdownContent":51,"audioMediaId":52},"The question of galaxies was finally settled by Edwin Hubble in 1925. He used a powerful telescope to identify a type of pulsing star called a **cepheid variable star**. By observing several of these, he managed to calculate how far away the Great Andromeda Nebula was.\n\nHe found that, in fact, the Andromeda Nebula was not a nebula at all, but another galaxy a vast distance away from our own. The Milky Way wasn't alone in the universe. It had a galactic neighbor.\n\nNot just one, either. Nowadays, astronomers generally believe that the universe is home to between 200 billion and 2 trillion galaxies. These numbers are so large that they stretch beyond the limits of our comprehension.","9791fc30-d76f-4f18-8c27-4a02de182830",[54,75],{"id":55,"data":56,"type":57,"version":34,"maxContentLevel":19},"16217a31-36c9-4c11-9f48-b37f38472f6c",{"type":57,"reviewType":19,"spacingBehaviour":34,"collapsingSiblings":58,"multiChoiceQuestion":62,"multiChoiceCorrect":64,"multiChoiceIncorrect":66,"multiChoiceMultiSelect":6,"multiChoiceRevealAnswerOption":6,"matchPairsQuestion":70,"matchPairsPairs":72},11,[59,60,61],"f65e177d-2312-43f7-83e7-db20121fed48","1d01d0fe-b555-4d38-946b-46dd0059dada","072f61df-f274-48c8-b24d-a4422a448c27",[63],"Who was Edwin Hubble?",[65],"He identified the Andromeda galaxy",[67,68,69],"He asked 'Where is everybody?'","He invented telescopes","He was the first man in space",[71],"Match the pairs below:",[73],{"left":74,"right":65,"direction":19},"Edwin Hubble",{"id":76,"data":77,"type":57,"version":34,"maxContentLevel":19},"690735fd-6b39-4256-8144-3e0277de3e64",{"type":57,"reviewType":25,"spacingBehaviour":34,"binaryQuestion":78,"binaryCorrect":80,"binaryIncorrect":82},[79],"Which pulsing stars, identified by Edwin Hubble, helped him prove there were other galaxies?",[81],"Cepheid variable stars",[83],"White dwarf stars",{"id":85,"data":86,"type":34,"maxContentLevel":19,"version":19,"reviews":89},"6def0ef7-f1a4-4d59-a2f3-520ce759ea10",{"type":34,"markdownContent":87,"audioMediaId":88},"Groups of galaxies can be bound together by gravity into **clusters**. Typically, a galaxy cluster is a few million light-years across.\n\nThe Milky Way is part of the Local Group cluster, which contains less than a hundred galaxies. But our nearest neighbor is the Virgo cluster, which consists of approximately 2000 galaxies.\n\n![Graph](image://593970e4-2650-4f2a-a13b-004d791a48c9 \"The Virgo cluster. Image: Chris Mihos (Case Western Reserve University)/ESO, CC BY 4.0 \u003Chttps://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons\")\n\nThese huge clusters of galaxies can form into even bigger groups, coming together to form **superclusters**. These superclusters can be hundreds of millions of light years across.\n\nTogether, the Local Group and the Virgo cluster form part of the Laniakea supercluster. Around 100,000 galaxies can be found here. However, unlike clusters, a supercluster is not gravitationally bound. Over time, it might drift apart.","24e9f31d-7ed5-43e8-abc8-1da2ae499e81",[90,101,108,116],{"id":91,"data":92,"type":57,"version":34,"maxContentLevel":19},"68a1b54e-9219-44ff-aa2e-05d92a85ae41",{"type":57,"reviewType":19,"spacingBehaviour":34,"multiChoiceQuestion":93,"multiChoiceCorrect":95,"multiChoiceIncorrect":97,"multiChoiceMultiSelect":6,"multiChoiceRevealAnswerOption":6},[94],"The Milky Way is part of which galaxy cluster?",[96],"Local Group",[98,99,100],"Virgo","Centaurus","Magellanic",{"id":102,"data":103,"type":57,"version":25,"maxContentLevel":19},"0bf97590-1f18-4767-a4df-e7e491e25303",{"type":57,"reviewType":19,"spacingBehaviour":34,"multiChoiceQuestion":104,"multiChoiceCorrect":106,"multiChoiceIncorrect":107},[105],"Outside our own galaxy cluster, which other cluster is closest to us?",[98],[96,99,100],{"id":109,"data":110,"type":57,"version":34,"maxContentLevel":19},"8a3ac3a3-d882-48a5-8dfe-68c4b8bdd67e",{"type":57,"reviewType":111,"spacingBehaviour":34,"clozeQuestion":112,"clozeWords":114},4,[113],"Together, the Local Group and the Virgo cluster form part of the Laniakea supercluster.",[115],"Laniakea",{"id":117,"data":118,"type":57,"version":34,"maxContentLevel":19},"2761312e-6f45-43d7-a976-61b39dfd537a",{"type":57,"reviewType":19,"spacingBehaviour":34,"multiChoiceQuestion":119,"multiChoiceCorrect":121,"multiChoiceIncorrect":123,"multiChoiceMultiSelect":6,"multiChoiceRevealAnswerOption":6},[120],"Which of these is gravitationally bound?",[122],"Galaxy clusters",[124,125,126],"Galaxy superclusters","Neither of these","Both of these",{"id":128,"data":129,"type":25,"version":25,"maxContentLevel":19,"summaryPage":131,"introPage":139,"pages":145},"256a2eec-568a-41bf-be26-23b38cfbfd49",{"type":25,"title":130},"Galaxy classification",{"id":132,"data":133,"type":19,"maxContentLevel":19,"version":34},"151aa9b4-9195-4f10-b823-5fa3422dc188",{"type":19,"summary":134},[135,136,137,138],"Spiral galaxies are flat spinning disks with a central bulge","Elliptical galaxies are smooth and featureless, formed by spiral galaxies colliding","Peculiar galaxies are oddly shaped spirals, distorted by interactions with other galaxies","Irregular galaxies are shapeless and chaotic, like the Magellanic clouds",{"id":140,"data":141,"type":38,"maxContentLevel":19,"version":34},"0ebc9dc8-f44d-4e32-b323-62b6d0721385",{"type":38,"intro":142},[143,144],"What are the four types of galaxy?","What type of galaxy is the Milky Way?",[146,180],{"id":147,"data":148,"type":34,"maxContentLevel":19,"version":25,"reviews":151},"b7c44ebc-c920-4f2d-99e6-cbe48ce7f929",{"type":34,"markdownContent":149,"audioMediaId":150},"We can classify galaxies into four different types: spiral, elliptical, peculiar, and irregular.\n\n**Spiral galaxies** include our own Milky Way. They are flat spinning disks of stars, gas, and dust with a central bulge.\n\nOlder, redder stars populate their central bulge, while the arms of the galaxy, spiraling out from the center, are bright with the light of younger, hotter stars.\n\nSpiral galaxies can vary in appearance. The spiraling arms might be well-defined – we call these grand design spiral galaxies – or they might be a bit choppier – we call these flocculent spiral galaxies.\n\n![Graph](image://38db7fdb-b3d3-4459-987c-17c3833e16d1 \"Spiral galaxy. Image: Public domain\")","ba2895b2-00d1-429b-9f34-66bd5d3f716e",[152,163],{"id":153,"data":154,"type":57,"version":34,"maxContentLevel":19},"adfd716e-ca24-4e76-89ae-d5654e5a7fef",{"type":57,"reviewType":19,"spacingBehaviour":34,"multiChoiceQuestion":155,"multiChoiceCorrect":157,"multiChoiceIncorrect":159,"multiChoiceMultiSelect":6,"multiChoiceRevealAnswerOption":6},[156],"What kind of galaxy is the Milky Way?",[158],"Spiral",[160,161,162],"Elliptical","Peculiar","Irregular",{"id":164,"data":165,"type":57,"version":34,"maxContentLevel":19},"dd82a8ec-836f-4f93-8e31-17a5ee1c48b6",{"type":57,"reviewType":20,"spacingBehaviour":34,"matchPairsQuestion":166,"matchPairsPairs":167,"matchPairsShowExamples":6},[71],[168,171,174,177],{"left":169,"right":170,"direction":19},"Grand design spiral galaxies","Spiral galaxies with well-defined arms",{"left":172,"right":173,"direction":19},"Flocculent spiral galaxies","Spiral galaxies with choppier arms",{"left":175,"right":176,"direction":19},"Older, redder stars","Found at the center of spiral galaxies",{"left":178,"right":179,"direction":19},"Younger, brighter stars","Found along the arms of spiral galaxies",{"id":181,"data":182,"type":34,"maxContentLevel":19,"version":25,"reviews":185},"420a8ee9-2374-4920-8a5a-80f7d8c5839a",{"type":34,"markdownContent":183,"audioMediaId":184},"**Elliptical galaxies** can range from nearly spherical to elongated to the shape of a cigar. They are smooth and featureless to look at, characterized by a lack of dust and gas, and populated by older stars. We think they are formed by spiral galaxies colliding. An example is Cygnus A.\n\n![Graph](image://18c0ffc7-da5b-4241-a94c-a661777916b0 \"Elliptical galaxy. Image: Public domain\")\n\n**Peculiar galaxies** are similar to spiral galaxies, but slightly oddly shaped, probably due to interacting with other galaxies. The cartwheel galaxy, with its two distinct rings, is a good example.\n\n![Graph](image://b0f58b72-eaa1-45eb-9df9-0da3b7dce971 \"Peculiar galaxy. Image: Public domain\")\n\nLast but not least, we have **irregular galaxies**, which are utterly shapeless and chaotic. The best known examples are our companion galaxies – the Large and Small Magellanic clouds.\n\n![Graph](image://25b7f943-1e73-42f9-bc6f-1334d29546e0 \"Irregular galaxy. Image: Public domain\")","ea5dce07-7d43-4a4e-902a-b8fe2bde23d7",[186,193,200,207],{"id":187,"data":188,"type":57,"version":34,"maxContentLevel":19},"f82752da-cb93-4915-acbf-02760ed64b57",{"type":57,"reviewType":19,"spacingBehaviour":34,"multiChoiceQuestion":189,"multiChoiceCorrect":191,"multiChoiceIncorrect":192,"multiChoiceMultiSelect":6,"multiChoiceRevealAnswerOption":6},[190],"Which type of galaxy is smooth and featureless, and populated by older stars?",[160],[158,161,162],{"id":194,"data":195,"type":57,"version":34,"maxContentLevel":19},"d532de30-1773-4e63-bde1-3dbf56ac0a53",{"type":57,"reviewType":25,"spacingBehaviour":34,"binaryQuestion":196,"binaryCorrect":198,"binaryIncorrect":199},[197],"Which type of galaxy is similar to a spiral, but slightly more oddly shaped?",[161],[162],{"id":201,"data":202,"type":57,"version":34,"maxContentLevel":19},"046f5d21-ecb5-4a56-8be1-0a19ab7f5696",{"type":57,"reviewType":25,"spacingBehaviour":34,"binaryQuestion":203,"binaryCorrect":205,"binaryIncorrect":206},[204],"Which type of galaxy is shapeless and chaotic?",[162],[161],{"id":208,"data":209,"type":57,"version":34,"maxContentLevel":19},"fe4ef20a-924d-496e-908d-8e7e89d83b13",{"type":57,"reviewType":20,"spacingBehaviour":34,"matchPairsQuestion":210,"matchPairsPairs":211,"matchPairsShowExamples":6},[71],[212,214,216,218],{"left":158,"right":213,"direction":19},"Milky Way",{"left":160,"right":215,"direction":19},"Cygnus A",{"left":161,"right":217,"direction":19},"Cartwheel",{"left":162,"right":219,"direction":19},"Magellanic clouds",{"id":221,"data":222,"type":25,"version":19,"maxContentLevel":19,"summaryPage":224,"introPage":231,"pages":237},"a4632270-2bb1-45cb-8efd-6609949cc1f5",{"type":25,"title":223},"Supermassive black holes",{"id":225,"data":226,"type":19,"maxContentLevel":19,"version":34},"227f42c9-a8a7-4bf6-b559-8c029e138dc2",{"type":19,"summary":227},[228,229,230],"Supermassive black holes are found at the center of most large galaxies","Active galaxy nuclei (AGNs) give off radiation when material falls into the supermassive black hole","Quasars are extreme AGNs that can shine up to a thousand times brighter than the Milky Way",{"id":232,"data":233,"type":38,"maxContentLevel":19,"version":34},"50e5abc5-d3c6-4f19-8718-09fd7ed0a2a0",{"type":38,"intro":234},[235,236],"How did we discover the supermassive black hole at the center of our galaxy?","What is an active galaxy nucleus?",[238,270,296],{"id":239,"data":240,"type":34,"maxContentLevel":19,"version":25,"reviews":243},"9ec8a3c1-fa53-4a6a-9e2b-3506b65982fa",{"type":34,"markdownContent":241,"audioMediaId":242},"In 1931, Bell Telephone Laboratories employed Karl Jansky to help them hunt for the static interference playing havoc with their telephone communications.\n\nTo do so, he built the first radio telescope. While he found local sources of static, he also found something else more puzzling: a powerful radio signal coming from the center of our galaxy. He wanted to investigate this signal – which he called star noise – but he couldn’t find the support to do so.\n\n![Graph](image://dc2f27e6-d4ce-4506-a320-ae1be10acd1d \"Karl Jansky and his telescope. Image: Minimum credit line: Image courtesy of NRAO/AUI (for details, see Image Use Policy)., CC BY 3.0 \u003Chttps://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons\")\n\nYears later, in 1974, Balick and Brown published work identifying the source of these radio waves: a **supermassive black hole**. Named ‘Sagittarius A’ in 1982, this enormous black hole sits at the center of the Milky Way.\n\nVast amounts of energy are released at the edge of the event horizon, where superheated gas gathers in an accretion disk. Currently, 'Sagittarius A' is quiet, but it’s been through active phases. In 2022, an image taken by the Event Horizon Telescope allowed us to see this monster at the heart of our galaxy for the first time: a dark contrast against its bright accretion disk.","fa39194e-315d-46c6-8245-dbb738fa8a03",[244,263],{"id":245,"data":246,"type":57,"version":34,"maxContentLevel":19},"ca59be6a-82fa-48fd-9b9e-e7880554e379",{"type":57,"reviewType":19,"spacingBehaviour":34,"collapsingSiblings":247,"multiChoiceQuestion":251,"multiChoiceCorrect":253,"multiChoiceIncorrect":255,"multiChoiceMultiSelect":6,"multiChoiceRevealAnswerOption":6,"matchPairsQuestion":259,"matchPairsPairs":260},[248,249,250],"f05a6058-6f9b-4e89-a4bd-d9e647bfd26c","2acd1d04-dc50-4248-a467-57cd1c99fa6f","70c68bca-e9d1-421a-ac45-3072e8b63fc8",[252],"What is Sagittarius A?",[254],"Supermassive black hole",[256,257,258],"Largest moon in the solar system","5th largest moon in the solar system","Ancient star",[71],[261],{"left":262,"right":254,"direction":19},"Sagittarius A",{"id":264,"data":265,"type":57,"version":34,"maxContentLevel":19},"db128082-f854-496d-afc0-94602b96ad20",{"type":57,"reviewType":34,"spacingBehaviour":34,"activeRecallQuestion":266,"activeRecallAnswers":268},[267],"Where is Sagittarius A?",[269],"At the center of the Milky Way",{"id":271,"data":272,"type":34,"maxContentLevel":19,"version":25,"reviews":275},"693ec603-0ab6-4b9a-8642-4ff524666d58",{"type":34,"markdownContent":273,"audioMediaId":274},"We now believe that almost all large galaxies have a supermassive black hole (SMBH) at their center. Most, but not quite all. The elliptical galaxy A2261-BCG, for example, is 10 times bigger than the Milky Way, but doesn’t appear to have a SMBH.\n\nWhere they do exist, SMBHs are truly enormous – millions to billions of times the mass of our sun.\n\nOur own galaxy's SMBH is currently a calm, gentle giant. But in the past, it’s gone through feeding frenzies: taking in vast amounts of material and shooting out the leftovers as superheated jets of gas. These created **Fermi bubbles** – huge bubbles of x-rays and gamma-rays ballooning from either side of the galaxy's core.\n\n![Graph](image://a41b518c-761d-4bce-b0a4-33fd81d847bc \"Fermi bubbles. Image: NASA Goddard Space Flight Center from Greenbelt, MD, USA, Public domain, via Wikimedia Commons\")\n\nWe don’t know why so many galaxies have a SMBH at their center. One idea is that they are primordial black holes from the birth of the universe, perhaps formed even before the first stars.","9b4270a5-4f92-4e2b-9e08-448733a257be",[276,285],{"id":277,"data":278,"type":57,"version":34,"maxContentLevel":19},"77baaaa2-d527-48a4-a5d3-e20cf70b98bf",{"type":57,"reviewType":25,"spacingBehaviour":34,"binaryQuestion":279,"binaryCorrect":281,"binaryIncorrect":283},[280],"Do Supermassive Black Holes exist at the heart of all large galaxies?",[282],"No - but they exist at the heart of most",[284],"Yes - galaxies can't form without them",{"id":286,"data":287,"type":57,"version":34,"maxContentLevel":19},"8106090a-528e-46fe-b733-995697de4e9c",{"type":57,"reviewType":19,"spacingBehaviour":34,"multiChoiceQuestion":288,"multiChoiceCorrect":290,"multiChoiceIncorrect":292,"multiChoiceMultiSelect":6,"multiChoiceRevealAnswerOption":6},[289],"What do we call the huge bubbles of x-rays and gamma-rays that balloon from either side of a Supermassive Black Hole?",[291],"Fermi bubbles",[293,294,295],"Hubble bubbles","Lockyer bubbles","Armstrong bubbles",{"id":297,"data":298,"type":34,"maxContentLevel":19,"version":19,"reviews":301},"0d248a89-c44f-4a4b-aaf4-de4b4319b253",{"type":34,"markdownContent":299,"audioMediaId":300},"The SMBHs at the center of most galaxies are often accompanied by something called an **active galaxy nucleus** (AGN).\n\nAGNs are regions of space that give off vast amounts of radiation. This radiation is released when dust and gas starts to fall into the SMBH. As this material is drawn in, particles collide, and generate vast amounts of energy.\n\nAn extreme kind of AGN is something called a quasar. These can shine up to a thousand times brighter than the Milky Way – they're so bright that we're able to see them even when they're incredibly far away.\n\n![Graph](image://b049b60b-1097-4c3c-b9ae-77d3c7e93859 \"Artist’s impression of a quasar. Image: ESO/M. Kornmesser, CC BY 4.0 \u003Chttps://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons\")\n\nOne of the furthest quasars is called J0313-1806. It's about 13.03 billion light years from us – that's right at the edge of the universe.","6c532195-10c2-47bb-9bad-7cdc87ca46ac",[302,309],{"id":303,"data":304,"type":57,"version":34,"maxContentLevel":19},"78bf4359-ec43-467f-9684-9ff66b2508c0",{"type":57,"reviewType":111,"spacingBehaviour":34,"clozeQuestion":305,"clozeWords":307},[306],"When material falls into an SMBH, it gives off vast amounts of radiation. We call this an active galaxy nucleus (AGN).",[308],"active galaxy nucleus",{"id":310,"data":311,"type":57,"version":25,"maxContentLevel":19},"17fffd07-5525-492e-b359-53757b145304",{"type":57,"reviewType":19,"spacingBehaviour":34,"multiChoiceQuestion":312,"multiChoiceCorrect":314,"multiChoiceIncorrect":316},[313],"What do we call extreme examples of active galaxy nuclei, which burn thousands of times brighter than the Milky Way?",[315],"Quasars",[317,318,319],"Pulsars","Radars","Protars",{"id":321,"data":322,"type":25,"version":324,"maxContentLevel":19,"summaryPage":325,"introPage":333,"pages":339},"27075851-92c2-4535-ab14-9a396c4af17e",{"type":25,"title":323},"Galactic motion",5,{"id":326,"data":327,"type":19,"maxContentLevel":19,"version":34},"dfa211e9-71f7-4592-b155-b376c311f407",{"type":19,"summary":328},[329,330,331,332],"When two galaxies come together, stars and planets are unlikely to collide directly","However, gravitational forces will rearrange all the stars and planets into new formations","For some reason, the stars at the center of the galaxy orbit slower than the stars at the edges","Dark matter might explain this unusual motion, but this is only a theory",{"id":334,"data":335,"type":38,"maxContentLevel":19,"version":34},"6105990c-c448-42b2-a9b4-8602a066724d",{"type":38,"intro":336},[337,338],"What happens when two galaxies collide?","What's the greatest mystery about galactic motion, and why might dark matter explain it?",[340,364,379],{"id":341,"data":342,"type":34,"maxContentLevel":19,"version":25,"reviews":345},"f57027b2-bc47-4c22-9bc9-d162e7eb1047",{"type":34,"markdownContent":343,"audioMediaId":344},"Galaxies move. In our own galaxy, the Milky Way, there is movement within the galaxy, as the outer regions orbit around the center. Our sun is carried around the center of the galaxy about once every 200–250 million years.\n\nGalaxies also move relative to each other in local groups – the Milky Way is currently traveling towards the Andromeda Galaxy at about 113 kilometers per second. In about 5 billion years, the two galaxies will probably collide.\n\nOn top of all this, these galaxies are being moved around in clusters and superclusters of galaxies, like our supercluster Laniakea. All of this is happening within a universe that gradually expands.","495339f4-850c-4f84-88fb-e64f97480a8f",[346,353],{"id":347,"data":348,"type":57,"version":34,"maxContentLevel":19},"48cbaa90-9e83-41fe-81be-4c5ec8eeb89b",{"type":57,"reviewType":111,"spacingBehaviour":34,"clozeQuestion":349,"clozeWords":351},[350],"Our sun is carried around the center of the galaxy about once every 200–250 million years.",[352],"200–250 million",{"id":354,"data":355,"type":57,"version":34,"maxContentLevel":19},"5ae8f5e6-9edb-4f12-a228-9f3acf339ca0",{"type":57,"reviewType":19,"spacingBehaviour":34,"multiChoiceQuestion":356,"multiChoiceCorrect":358,"multiChoiceIncorrect":360,"multiChoiceMultiSelect":6,"multiChoiceRevealAnswerOption":6},[357],"Roughly how long do we have before the Milky Way collides with Andromeda?",[359],"5 billion years",[361,362,363],"50 billion years","5 million years","50 million years",{"id":365,"data":366,"type":34,"maxContentLevel":19,"version":111,"reviews":369},"c48b6545-71d3-4a15-a37e-ecc669867c0f",{"type":34,"markdownContent":367,"audioMediaId":368},"Galaxies collide in slow, violent dances. Driven by their combined gravity, they swing around each other, swirling closer until they lose their shapes.\n\nThe likelihood of any two stars colliding in merging galaxies is very low. There is so much space between stars within galaxies that the stars will usually drift past each other, like passing swarms of bees.\n\nBut, even if there aren't any direct collisions, the gravitational forces produced by the two galaxies will rearrange stars and their orbits, changing both of the galaxies dramatically.\n\n![Graph](image://b6957d7d-3d72-4f05-ae1b-10b4e47144cf \"Artist's impression of the Milky Way and the Andromeda galaxy colliding. Image: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger, Public domain, via Wikimedia Commons\")\n\nWhat emerges from a galactic collision depends on what the galaxies looked like before they crashed. If two galaxies of a similar size merge, they might form a new elliptical galaxy. If they have SMBHs at their center, these will eventually merge.","058ab9b7-5042-473a-9199-4f64520d7cc6",[370],{"id":371,"data":372,"type":57,"version":19,"maxContentLevel":19},"122adba9-e7ea-42cf-a304-23c1b83576c3",{"type":57,"reviewType":25,"spacingBehaviour":34,"binaryQuestion":373,"binaryCorrect":375,"binaryIncorrect":377},[374],"When two galaxies collide, what is likely to happen?",[376],"Stars and planets will be rearranged by gravitational forces",[378],"Stars and planets will be destroyed by direct collisions",{"id":380,"data":381,"type":34,"maxContentLevel":19,"version":111,"reviews":384},"1694144c-2a14-4339-a4c5-b71796d42d5e",{"type":34,"markdownContent":382,"audioMediaId":383},"There's one big mystery with galactic motion. In theory, all the stars that orbit around the SMBH should orbit more quickly towards the center of the galaxy, and more slowly at the outer edges.\n\nThis is how gravitational systems are meant to work. In our solar system, the planets orbit more and more slowly the further you get from the sun. But this doesn’t happen in a galaxy. Instead, rotation speed gets slightly faster as you travel further from the center.\n\nThis curious behavior has led us to speculate about the existence of something called **dark matter**. This matter, if it exists, doesn't interact with light, so we cannot see it – but it exerts a gravitational pull.\n\nIf there was a halo of dark matter around every galaxy, its gravity could explain why objects rotate more quickly towards the edge, rather than the center. This is only a theory, but most scientists agree that dark matter exists – otherwise, our understanding of physics must be broken.\n\n![Graph](image://3c17bbe3-4233-4b1b-98ec-8d5505e65487 \"The universe. NASA, ESA, CSA, and STScI, Public domain, via Wikimedia Commons\")","500d79ab-cb69-40ea-a286-7589d5c1c251",[385,394,401],{"id":386,"data":387,"type":57,"version":25,"maxContentLevel":19},"b4a72dd1-f6fc-40f9-a905-ed32babf3373",{"type":57,"reviewType":25,"spacingBehaviour":34,"binaryQuestion":388,"binaryCorrect":390,"binaryIncorrect":392},[389],"Which of these is true in theory (but not in practice)?",[391],"Stars that orbit an SMBH orbit quickly at the center, and slowly at the edges",[393],"Stars that orbit an SMBH orbit slowly at the center, and quickly at the edges",{"id":395,"data":396,"type":57,"version":25,"maxContentLevel":19},"37cdf364-7449-43e0-98a3-da5b80d09321",{"type":57,"reviewType":25,"spacingBehaviour":34,"binaryQuestion":397,"binaryCorrect":399,"binaryIncorrect":400},[398],"Which of these is true in practice (but not in theory)?",[393],[391],{"id":402,"data":403,"type":57,"version":34,"maxContentLevel":19},"a29b67d1-3ac7-42ca-b12f-7080fe25a900",{"type":57,"reviewType":19,"spacingBehaviour":34,"multiChoiceQuestion":404,"multiChoiceCorrect":406,"multiChoiceIncorrect":408,"multiChoiceMultiSelect":6,"multiChoiceRevealAnswerOption":6},[405],"What is thought to explain why stars orbit faster at the edges of galaxies, rather than the center?",[407],"Dark matter",[409,410,411],"Antimatter","Dark energy","Gravitational waves",[413,539,644,771],{"id":23,"data":24,"type":25,"version":19,"maxContentLevel":19,"summaryPage":27,"introPage":35,"pages":414},[415,459,489],{"id":44,"data":45,"type":34,"maxContentLevel":19,"version":25,"parsed":416},{"data":417,"body":420,"toc":457},{"title":418,"description":419},"","A galaxy is a massive system of stars, planets, gas and dust, all bound together by the force of gravity. When we say 'massive', we really mean it.",{"type":421,"children":422},"root",[423,430,442,452],{"type":424,"tag":425,"props":426,"children":427},"element","p",{},[428],{"type":429,"value":419},"text",{"type":424,"tag":425,"props":431,"children":432},{},[433,435,440],{"type":429,"value":434},"Our own galaxy – the ",{"type":424,"tag":436,"props":437,"children":438},"strong",{},[439],{"type":429,"value":213},{"type":429,"value":441}," – contains at least 100 billion stars. It also contains at least as many planets, and possibly significantly more.",{"type":424,"tag":425,"props":443,"children":444},{},[445],{"type":424,"tag":446,"props":447,"children":451},"img",{"alt":448,"src":449,"title":450},"Graph","image://38db7fdb-b3d3-4459-987c-17c3833e16d1","The Milky Way. Image: NASA/JPL-Caltech/ESO/R. Hurt, Public domain, via Wikimedia Commons",[],{"type":424,"tag":425,"props":453,"children":454},{},[455],{"type":429,"value":456},"In the early 20th century, there was a lot of debate about galaxies. Was our giant galaxy all that was out there? Or was it just one of many clusters of stars?",{"title":418,"searchDepth":25,"depth":25,"links":458},[],{"id":49,"data":50,"type":34,"maxContentLevel":19,"version":25,"reviews":53,"parsed":460},{"data":461,"body":463,"toc":487},{"title":418,"description":462},"The question of galaxies was finally settled by Edwin Hubble in 1925. He used a powerful telescope to identify a type of pulsing star called a cepheid variable star. By observing several of these, he managed to calculate how far away the Great Andromeda Nebula was.",{"type":421,"children":464},[465,477,482],{"type":424,"tag":425,"props":466,"children":467},{},[468,470,475],{"type":429,"value":469},"The question of galaxies was finally settled by Edwin Hubble in 1925. He used a powerful telescope to identify a type of pulsing star called a ",{"type":424,"tag":436,"props":471,"children":472},{},[473],{"type":429,"value":474},"cepheid variable star",{"type":429,"value":476},". By observing several of these, he managed to calculate how far away the Great Andromeda Nebula was.",{"type":424,"tag":425,"props":478,"children":479},{},[480],{"type":429,"value":481},"He found that, in fact, the Andromeda Nebula was not a nebula at all, but another galaxy a vast distance away from our own. The Milky Way wasn't alone in the universe. It had a galactic neighbor.",{"type":424,"tag":425,"props":483,"children":484},{},[485],{"type":429,"value":486},"Not just one, either. Nowadays, astronomers generally believe that the universe is home to between 200 billion and 2 trillion galaxies. These numbers are so large that they stretch beyond the limits of our comprehension.",{"title":418,"searchDepth":25,"depth":25,"links":488},[],{"id":85,"data":86,"type":34,"maxContentLevel":19,"version":19,"reviews":89,"parsed":490},{"data":491,"body":493,"toc":537},{"title":418,"description":492},"Groups of galaxies can be bound together by gravity into clusters. Typically, a galaxy cluster is a few million light-years across.",{"type":421,"children":494},[495,507,512,520,532],{"type":424,"tag":425,"props":496,"children":497},{},[498,500,505],{"type":429,"value":499},"Groups of galaxies can be bound together by gravity into ",{"type":424,"tag":436,"props":501,"children":502},{},[503],{"type":429,"value":504},"clusters",{"type":429,"value":506},". Typically, a galaxy cluster is a few million light-years across.",{"type":424,"tag":425,"props":508,"children":509},{},[510],{"type":429,"value":511},"The Milky Way is part of the Local Group cluster, which contains less than a hundred galaxies. But our nearest neighbor is the Virgo cluster, which consists of approximately 2000 galaxies.",{"type":424,"tag":425,"props":513,"children":514},{},[515],{"type":424,"tag":446,"props":516,"children":519},{"alt":448,"src":517,"title":518},"image://593970e4-2650-4f2a-a13b-004d791a48c9","The Virgo cluster. Image: Chris Mihos (Case Western Reserve University)/ESO, CC BY 4.0 \u003Chttps://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons",[],{"type":424,"tag":425,"props":521,"children":522},{},[523,525,530],{"type":429,"value":524},"These huge clusters of galaxies can form into even bigger groups, coming together to form ",{"type":424,"tag":436,"props":526,"children":527},{},[528],{"type":429,"value":529},"superclusters",{"type":429,"value":531},". These superclusters can be hundreds of millions of light years across.",{"type":424,"tag":425,"props":533,"children":534},{},[535],{"type":429,"value":536},"Together, the Local Group and the Virgo cluster form part of the Laniakea supercluster. Around 100,000 galaxies can be found here. However, unlike clusters, a supercluster is not gravitationally bound. Over time, it might drift apart.",{"title":418,"searchDepth":25,"depth":25,"links":538},[],{"id":128,"data":129,"type":25,"version":25,"maxContentLevel":19,"summaryPage":131,"introPage":139,"pages":540},[541,580],{"id":147,"data":148,"type":34,"maxContentLevel":19,"version":25,"reviews":151,"parsed":542},{"data":543,"body":545,"toc":578},{"title":418,"description":544},"We can classify galaxies into four different types: spiral, elliptical, peculiar, and irregular.",{"type":421,"children":546},[547,551,561,566,571],{"type":424,"tag":425,"props":548,"children":549},{},[550],{"type":429,"value":544},{"type":424,"tag":425,"props":552,"children":553},{},[554,559],{"type":424,"tag":436,"props":555,"children":556},{},[557],{"type":429,"value":558},"Spiral galaxies",{"type":429,"value":560}," include our own Milky Way. They are flat spinning disks of stars, gas, and dust with a central bulge.",{"type":424,"tag":425,"props":562,"children":563},{},[564],{"type":429,"value":565},"Older, redder stars populate their central bulge, while the arms of the galaxy, spiraling out from the center, are bright with the light of younger, hotter stars.",{"type":424,"tag":425,"props":567,"children":568},{},[569],{"type":429,"value":570},"Spiral galaxies can vary in appearance. The spiraling arms might be well-defined – we call these grand design spiral galaxies – or they might be a bit choppier – we call these flocculent spiral galaxies.",{"type":424,"tag":425,"props":572,"children":573},{},[574],{"type":424,"tag":446,"props":575,"children":577},{"alt":448,"src":449,"title":576},"Spiral galaxy. Image: Public domain",[],{"title":418,"searchDepth":25,"depth":25,"links":579},[],{"id":181,"data":182,"type":34,"maxContentLevel":19,"version":25,"reviews":185,"parsed":581},{"data":582,"body":584,"toc":642},{"title":418,"description":583},"Elliptical galaxies can range from nearly spherical to elongated to the shape of a cigar. They are smooth and featureless to look at, characterized by a lack of dust and gas, and populated by older stars. We think they are formed by spiral galaxies colliding. An example is Cygnus A.",{"type":421,"children":585},[586,596,604,614,622,634],{"type":424,"tag":425,"props":587,"children":588},{},[589,594],{"type":424,"tag":436,"props":590,"children":591},{},[592],{"type":429,"value":593},"Elliptical galaxies",{"type":429,"value":595}," can range from nearly spherical to elongated to the shape of a cigar. They are smooth and featureless to look at, characterized by a lack of dust and gas, and populated by older stars. We think they are formed by spiral galaxies colliding. An example is Cygnus A.",{"type":424,"tag":425,"props":597,"children":598},{},[599],{"type":424,"tag":446,"props":600,"children":603},{"alt":448,"src":601,"title":602},"image://18c0ffc7-da5b-4241-a94c-a661777916b0","Elliptical galaxy. Image: Public domain",[],{"type":424,"tag":425,"props":605,"children":606},{},[607,612],{"type":424,"tag":436,"props":608,"children":609},{},[610],{"type":429,"value":611},"Peculiar galaxies",{"type":429,"value":613}," are similar to spiral galaxies, but slightly oddly shaped, probably due to interacting with other galaxies. The cartwheel galaxy, with its two distinct rings, is a good example.",{"type":424,"tag":425,"props":615,"children":616},{},[617],{"type":424,"tag":446,"props":618,"children":621},{"alt":448,"src":619,"title":620},"image://b0f58b72-eaa1-45eb-9df9-0da3b7dce971","Peculiar galaxy. Image: Public domain",[],{"type":424,"tag":425,"props":623,"children":624},{},[625,627,632],{"type":429,"value":626},"Last but not least, we have ",{"type":424,"tag":436,"props":628,"children":629},{},[630],{"type":429,"value":631},"irregular galaxies",{"type":429,"value":633},", which are utterly shapeless and chaotic. The best known examples are our companion galaxies – the Large and Small Magellanic clouds.",{"type":424,"tag":425,"props":635,"children":636},{},[637],{"type":424,"tag":446,"props":638,"children":641},{"alt":448,"src":639,"title":640},"image://25b7f943-1e73-42f9-bc6f-1334d29546e0","Irregular galaxy. Image: Public domain",[],{"title":418,"searchDepth":25,"depth":25,"links":643},[],{"id":221,"data":222,"type":25,"version":19,"maxContentLevel":19,"summaryPage":224,"introPage":231,"pages":645},[646,688,729],{"id":239,"data":240,"type":34,"maxContentLevel":19,"version":25,"reviews":243,"parsed":647},{"data":648,"body":650,"toc":686},{"title":418,"description":649},"In 1931, Bell Telephone Laboratories employed Karl Jansky to help them hunt for the static interference playing havoc with their telephone communications.",{"type":421,"children":651},[652,656,661,669,681],{"type":424,"tag":425,"props":653,"children":654},{},[655],{"type":429,"value":649},{"type":424,"tag":425,"props":657,"children":658},{},[659],{"type":429,"value":660},"To do so, he built the first radio telescope. While he found local sources of static, he also found something else more puzzling: a powerful radio signal coming from the center of our galaxy. He wanted to investigate this signal – which he called star noise – but he couldn’t find the support to do so.",{"type":424,"tag":425,"props":662,"children":663},{},[664],{"type":424,"tag":446,"props":665,"children":668},{"alt":448,"src":666,"title":667},"image://dc2f27e6-d4ce-4506-a320-ae1be10acd1d","Karl Jansky and his telescope. Image: Minimum credit line: Image courtesy of NRAO/AUI (for details, see Image Use Policy)., CC BY 3.0 \u003Chttps://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons",[],{"type":424,"tag":425,"props":670,"children":671},{},[672,674,679],{"type":429,"value":673},"Years later, in 1974, Balick and Brown published work identifying the source of these radio waves: a ",{"type":424,"tag":436,"props":675,"children":676},{},[677],{"type":429,"value":678},"supermassive black hole",{"type":429,"value":680},". Named ‘Sagittarius A’ in 1982, this enormous black hole sits at the center of the Milky Way.",{"type":424,"tag":425,"props":682,"children":683},{},[684],{"type":429,"value":685},"Vast amounts of energy are released at the edge of the event horizon, where superheated gas gathers in an accretion disk. Currently, 'Sagittarius A' is quiet, but it’s been through active phases. In 2022, an image taken by the Event Horizon Telescope allowed us to see this monster at the heart of our galaxy for the first time: a dark contrast against its bright accretion disk.",{"title":418,"searchDepth":25,"depth":25,"links":687},[],{"id":271,"data":272,"type":34,"maxContentLevel":19,"version":25,"reviews":275,"parsed":689},{"data":690,"body":692,"toc":727},{"title":418,"description":691},"We now believe that almost all large galaxies have a supermassive black hole (SMBH) at their center. Most, but not quite all. The elliptical galaxy A2261-BCG, for example, is 10 times bigger than the Milky Way, but doesn’t appear to have a SMBH.",{"type":421,"children":693},[694,698,703,714,722],{"type":424,"tag":425,"props":695,"children":696},{},[697],{"type":429,"value":691},{"type":424,"tag":425,"props":699,"children":700},{},[701],{"type":429,"value":702},"Where they do exist, SMBHs are truly enormous – millions to billions of times the mass of our sun.",{"type":424,"tag":425,"props":704,"children":705},{},[706,708,712],{"type":429,"value":707},"Our own galaxy's SMBH is currently a calm, gentle giant. But in the past, it’s gone through feeding frenzies: taking in vast amounts of material and shooting out the leftovers as superheated jets of gas. These created ",{"type":424,"tag":436,"props":709,"children":710},{},[711],{"type":429,"value":291},{"type":429,"value":713}," – huge bubbles of x-rays and gamma-rays ballooning from either side of the galaxy's core.",{"type":424,"tag":425,"props":715,"children":716},{},[717],{"type":424,"tag":446,"props":718,"children":721},{"alt":448,"src":719,"title":720},"image://a41b518c-761d-4bce-b0a4-33fd81d847bc","Fermi bubbles. Image: NASA Goddard Space Flight Center from Greenbelt, MD, USA, Public domain, via Wikimedia Commons",[],{"type":424,"tag":425,"props":723,"children":724},{},[725],{"type":429,"value":726},"We don’t know why so many galaxies have a SMBH at their center. One idea is that they are primordial black holes from the birth of the universe, perhaps formed even before the first stars.",{"title":418,"searchDepth":25,"depth":25,"links":728},[],{"id":297,"data":298,"type":34,"maxContentLevel":19,"version":19,"reviews":301,"parsed":730},{"data":731,"body":733,"toc":769},{"title":418,"description":732},"The SMBHs at the center of most galaxies are often accompanied by something called an active galaxy nucleus (AGN).",{"type":421,"children":734},[735,746,751,756,764],{"type":424,"tag":425,"props":736,"children":737},{},[738,740,744],{"type":429,"value":739},"The SMBHs at the center of most galaxies are often accompanied by something called an ",{"type":424,"tag":436,"props":741,"children":742},{},[743],{"type":429,"value":308},{"type":429,"value":745}," (AGN).",{"type":424,"tag":425,"props":747,"children":748},{},[749],{"type":429,"value":750},"AGNs are regions of space that give off vast amounts of radiation. This radiation is released when dust and gas starts to fall into the SMBH. As this material is drawn in, particles collide, and generate vast amounts of energy.",{"type":424,"tag":425,"props":752,"children":753},{},[754],{"type":429,"value":755},"An extreme kind of AGN is something called a quasar. These can shine up to a thousand times brighter than the Milky Way – they're so bright that we're able to see them even when they're incredibly far away.",{"type":424,"tag":425,"props":757,"children":758},{},[759],{"type":424,"tag":446,"props":760,"children":763},{"alt":448,"src":761,"title":762},"image://b049b60b-1097-4c3c-b9ae-77d3c7e93859","Artist’s impression of a quasar. Image: ESO/M. Kornmesser, CC BY 4.0 \u003Chttps://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons",[],{"type":424,"tag":425,"props":765,"children":766},{},[767],{"type":429,"value":768},"One of the furthest quasars is called J0313-1806. It's about 13.03 billion light years from us – that's right at the edge of the universe.",{"title":418,"searchDepth":25,"depth":25,"links":770},[],{"id":321,"data":322,"type":25,"version":324,"maxContentLevel":19,"summaryPage":325,"introPage":333,"pages":772},[773,795,830],{"id":341,"data":342,"type":34,"maxContentLevel":19,"version":25,"reviews":345,"parsed":774},{"data":775,"body":777,"toc":793},{"title":418,"description":776},"Galaxies move. In our own galaxy, the Milky Way, there is movement within the galaxy, as the outer regions orbit around the center. Our sun is carried around the center of the galaxy about once every 200–250 million years.",{"type":421,"children":778},[779,783,788],{"type":424,"tag":425,"props":780,"children":781},{},[782],{"type":429,"value":776},{"type":424,"tag":425,"props":784,"children":785},{},[786],{"type":429,"value":787},"Galaxies also move relative to each other in local groups – the Milky Way is currently traveling towards the Andromeda Galaxy at about 113 kilometers per second. In about 5 billion years, the two galaxies will probably collide.",{"type":424,"tag":425,"props":789,"children":790},{},[791],{"type":429,"value":792},"On top of all this, these galaxies are being moved around in clusters and superclusters of galaxies, like our supercluster Laniakea. All of this is happening within a universe that gradually expands.",{"title":418,"searchDepth":25,"depth":25,"links":794},[],{"id":365,"data":366,"type":34,"maxContentLevel":19,"version":111,"reviews":369,"parsed":796},{"data":797,"body":799,"toc":828},{"title":418,"description":798},"Galaxies collide in slow, violent dances. Driven by their combined gravity, they swing around each other, swirling closer until they lose their shapes.",{"type":421,"children":800},[801,805,810,815,823],{"type":424,"tag":425,"props":802,"children":803},{},[804],{"type":429,"value":798},{"type":424,"tag":425,"props":806,"children":807},{},[808],{"type":429,"value":809},"The likelihood of any two stars colliding in merging galaxies is very low. There is so much space between stars within galaxies that the stars will usually drift past each other, like passing swarms of bees.",{"type":424,"tag":425,"props":811,"children":812},{},[813],{"type":429,"value":814},"But, even if there aren't any direct collisions, the gravitational forces produced by the two galaxies will rearrange stars and their orbits, changing both of the galaxies dramatically.",{"type":424,"tag":425,"props":816,"children":817},{},[818],{"type":424,"tag":446,"props":819,"children":822},{"alt":448,"src":820,"title":821},"image://b6957d7d-3d72-4f05-ae1b-10b4e47144cf","Artist's impression of the Milky Way and the Andromeda galaxy colliding. Image: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger, Public domain, via Wikimedia Commons",[],{"type":424,"tag":425,"props":824,"children":825},{},[826],{"type":429,"value":827},"What emerges from a galactic collision depends on what the galaxies looked like before they crashed. If two galaxies of a similar size merge, they might form a new elliptical galaxy. If they have SMBHs at their center, these will eventually merge.",{"title":418,"searchDepth":25,"depth":25,"links":829},[],{"id":380,"data":381,"type":34,"maxContentLevel":19,"version":111,"reviews":384,"parsed":831},{"data":832,"body":834,"toc":870},{"title":418,"description":833},"There's one big mystery with galactic motion. In theory, all the stars that orbit around the SMBH should orbit more quickly towards the center of the galaxy, and more slowly at the outer edges.",{"type":421,"children":835},[836,840,845,857,862],{"type":424,"tag":425,"props":837,"children":838},{},[839],{"type":429,"value":833},{"type":424,"tag":425,"props":841,"children":842},{},[843],{"type":429,"value":844},"This is how gravitational systems are meant to work. In our solar system, the planets orbit more and more slowly the further you get from the sun. But this doesn’t happen in a galaxy. Instead, rotation speed gets slightly faster as you travel further from the center.",{"type":424,"tag":425,"props":846,"children":847},{},[848,850,855],{"type":429,"value":849},"This curious behavior has led us to speculate about the existence of something called ",{"type":424,"tag":436,"props":851,"children":852},{},[853],{"type":429,"value":854},"dark matter",{"type":429,"value":856},". This matter, if it exists, doesn't interact with light, so we cannot see it – but it exerts a gravitational pull.",{"type":424,"tag":425,"props":858,"children":859},{},[860],{"type":429,"value":861},"If there was a halo of dark matter around every galaxy, its gravity could explain why objects rotate more quickly towards the edge, rather than the center. This is only a theory, but most scientists agree that dark matter exists – otherwise, our understanding of physics must be broken.",{"type":424,"tag":425,"props":863,"children":864},{},[865],{"type":424,"tag":446,"props":866,"children":869},{"alt":448,"src":867,"title":868},"image://3c17bbe3-4233-4b1b-98ec-8d5505e65487","The universe. NASA, ESA, CSA, and STScI, Public domain, via Wikimedia Commons",[],{"title":418,"searchDepth":25,"depth":25,"links":871},[],{"left":4,"top":4,"width":873,"height":873,"rotate":4,"vFlip":6,"hFlip":6,"body":874},24,"\u003Cpath fill=\"none\" stroke=\"currentColor\" stroke-linecap=\"round\" stroke-linejoin=\"round\" stroke-width=\"2\" d=\"m9 18l6-6l-6-6\"/>",{"left":4,"top":4,"width":873,"height":873,"rotate":4,"vFlip":6,"hFlip":6,"body":876},"\u003Cpath fill=\"none\" stroke=\"currentColor\" stroke-linecap=\"round\" stroke-linejoin=\"round\" stroke-width=\"2\" d=\"M4 5h16M4 12h16M4 19h16\"/>",1778179428040]