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 ====== 2. Galactic age ======
-Socrates: Yesterday, Ravi gave us an overview of the Particle Age and mentioned that his explanation wasn’t nearly enough to truly “understand” it. To really grasp it, there’s no way around the math. Today, Shashi is supposed to start discussing the Galactic age, and this will continue as long as we’re on the banks of the Tsangpo river. So, how do you want to begin?
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+    <title>Galactic Age Table</title>
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-Shashi: Since it’s already late at night, we’re by the Tsangpo’s bank, and the sky is clear, we could start by taking a picture of a galaxy with the telescope.
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-Ravi: Good idea. Shashi, then, why don’t you handle Ashvin-1?
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-Socrates: Ashvin-1? What does that mean?
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-Ravi: We have two telescopes, both named after the twin stars, Ashvin 1 and 2, known as the twin brothers in the Gemini constellation.
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-Shashi: After mounting the telescope, I’ll connect it to the Unistellar app from my phone—see here. Now, I’ll go into the app’s catalog and select a galaxy; once I tap on “GoTo,” Ashvin will start moving. I’ve joined as the operator using my phone, and if you all connect to the same app as observers, you’ll be able to see on your phones what the telescope is viewing.
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-Juno: Yes, I can see it. I think we should target the Whirlpool Galaxy.
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-Shashi: Alright, tapping on it now. Everyone can see Ashvin-1 moving towards the Whirlpool Galaxy. It’s there now. The galaxy isn’t visible yet because we’re in live mode, not accumulating photons. Once I tap on “Enhanced Vision,” Ashvin will start collecting light. Here we go! You can see the exposure time ticking below; it’s already at 7 seconds. The Whirlpool Galaxy is already faintly visible. The more light we accumulate, the clearer the galaxy will become.
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-Socrates: I see—this is actually a merging of two galaxies.
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-Shashi: Up front is the Whirlpool, known as Messier 51, which spans about 75,000 light-years. Just behind it is a small dwarf galaxy, NGC 5195, also called M51b, about 15,000 light-years in size. Both are around 30 million light-years away. The bluish light comes from young stars, while the reddish glow comes from older stars. Our universe now contains roughly a trillion galaxies, all of which formed within the first four billion years of the universe’s 14-billion-year history.
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-Socrates: So, if the universe’s first 300,000 years were the Particle Age, then from then until around four billion years of age was the Galactic Age. But I don’t see any resemblance between this vast structure of gas, stars, and dust and the universe at 300,000 years old. Let me clarify. Yesterday, Ravi showed us an image of the universe at 300,000 years old. He demonstrated that the universe was then a single, boring cloud of gas with almost uniform temperature throughout. There were slight temperature variations, but they averaged only around 300 microkelvin. How did such enormous galaxies emerge from such a bland gas cloud in just four billion years? And not just a few galaxies—around a trillion, or perhaps even more.
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-===== - From Gas to Galaxies =====
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-**Shashi:** Scientists are still trying to understand this. On one hand, observational astronomers are making observations, and on the other, theoretical astrophysicists are doing calculations. Observations and calculations have not yet fully aligned because the massive cosmic "birth event" that began after the universe was 300,000 years old and continued until it was about a billion years old has not yet been directly observed through telescopes. The first 200 million years were the "Dark Ages," followed by the "Cosmic Dawn," a 200-million-year period during which the first stars and galaxies formed. The following 600 million years are known as the "Reionization Epoch," because during this time, the ultraviolet light from the first stars reionized all the primordial neutral hydrogen in the intergalactic medium, stripping electrons from atoms.
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-**Socrates:** At first, you said scientists are still trying to understand, but then you told such a grand story that it sounds like they understand a lot. Which one is correct?
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-**Shashi:** Watch this video and decide for yourself. At the beginning of the 21st century, theoretical astrophysicists created a massive computer simulation called the "Millennium Run." They instructed a supercomputer to use all the fundamental cosmological and physical theories to create a simulation showing the entire history of a small section of the universe from 400,000 years after the Big Bang onward. In other words, they made a time-lapse movie of the universe—not a real movie, but a simulated one.
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-**Socrates:** So, something like the time-lapse we see in Google Earth’s historical imagery?
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-**Shashi:** Good point. Let’s try to understand it using Google Earth. In this map, you can see the "face" of Dhaka in the 2022 image, as the Buriganga River creates a shape on Dhaka's western boundary that people like to see as a human face. But in 1984, this face was not there because, without the gray buildings along the river’s edges, the river’s shape was not visible in satellite images. The closer we get to 2022, the more buildings grow, overtaking the greenery like with gray. You could say that as Dhaka becomes more "human" from 1984 to 2022, greenery is increasingly destroyed.
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-**Socrates:** Are you about to compare this to the entire universe?
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-**Shashi:** Why not? Just as you see buildings overtaking greenery in Dhaka's time-lapse, if we could create a time-lapse of the universe, we’d see how the ultraviolet light from galaxy structures spread out, ionizing and "destroying" the innocent neutral hydrogen gas in the intergalactic medium by stripping electrons from their atoms. If buildings represent galaxies, then the hydrogen gas outside galaxies is like the greenery.
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-**Socrates:** But the Millennium Simulation doesn’t show this.
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+                <div class="col-time">300 ky – 200 My</div>
+                <div class="col-title">The Cosmic Dark Ages</div>
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+                    Following the epoch of recombination and photon decoupling that concluded the Particle Age, the universe transitioned into a prolonged period known as the Cosmic Dark Ages. During this era, spanning hundreds of millions of years, the cosmos was entirely electrically neutral and filled predominantly with a vast, expanding fog of hydrogen and helium atoms. Because the brilliant, initial flash of the Big Bang had faded into the deep infrared and microwave spectrum, and no stars had yet ignited, the universe was plunged into absolute darkness. However, this period was far from stagnant. In the pitch-black void, the invisible hand of gravity was relentlessly at work. It slowly began to pull vast quantities of neutral gas and invisible dark matter into increasingly dense, massive clumps. These hidden concentrations laid the crucial, unseen scaffolding for all future cosmic structures, patiently assembling the raw materials required to eventually spark the universe's first luminous objects and end the long cosmic night.
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-**Shashi:** Correct. We haven’t yet created such a time-lapse movie of the universe. Perhaps in the next few decades, with more advanced telescopes, it may become possible. For now, this simulation is our best resource. In the movie, you’ll see the last 13.5 billion years of history—not of the entire universe, but of a section 600 million light-years across both horizontally and vertically. If we assume an average distance of about 1 million light-years between galaxies, we can imagine around 600 galaxies lined up across the frame, which helps us understand the scale.
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+                <div class="col-time">200 My</div>
+                <div class="col-title">Cosmic Dawn (Reionization)</div>
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+                    The prolonged obscurity of the Cosmic Dark Ages finally shattered roughly 200 million years after the Big Bang with the onset of the Cosmic Dawn. Deep within the densest, most gravitationally compressed pockets of primordial gas, the very first massive stars and nascent protogalaxies suddenly ignited. These pioneering stellar giants were vastly different from modern stars—they were monstrously huge, incredibly hot, and burned their fuel at a ferocious rate. Consequently, they emitted unimaginably intense floods of high-energy ultraviolet radiation into the surrounding cosmos. This fierce, energetic light was so powerful that it began to strike the ubiquitous neutral hydrogen gas, violently tearing electrons away from their atomic nuclei in a process known as cosmic reionization. This pivotal transformation fundamentally altered the physical state of the intergalactic medium. By clearing away the obscuring fog of neutral atoms, the universe was rendered completely transparent to ultraviolet light, allowing the brilliance of these early stars to travel across the cosmos and officially ending the dark ages.
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-**Socrates:** And the "$z$" displayed above—what is that? I see it going from 20 to 0 as the movie plays, similar to how Google Earth moves from 1984 to 2022.
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+                <div class="col-time">500 My – 1 Gy</div>
+                <div class="col-title">Hierarchical Merging</div>
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+                    As the universe continued to expand and evolve between 500 million and 1 billion years after the Big Bang, it was populated not by the grand spiral and elliptical galaxies we see today, but by countless small, irregular "pregalactic blobs" and dwarf galaxies. Guided by the underlying web of dark matter, a violent and chaotic "bottom-up" assembly process known as hierarchical merging began to dominate the cosmos. The immense gravitational pull between these smaller fragments caused them to continuously collide, interact, and amalgamate into progressively larger and more complex galactic structures. This relentless era of cosmic cannibalism and merging built the massive galaxies that now anchor the universe. In our own galactic neighborhood, profound evidence of these early, chaotic mergers remains visible today. The sprawling, spherical galactic halo of the Milky Way, populated by ancient stars and globular clusters locked in highly eccentric and randomly oriented orbits, serves as a permanent, fossilized record of the turbulent collisions that formed our galactic home.
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-**Shashi:** It’s called redshift, but explaining it would take us off track. For now, it’s enough to remember that redshift here is a proxy for time. A redshift of $z=20$ means the universe was 400 million years old, and $z=0$ means the present. If the universe’s current age is 13.9 billion years, then this time-lapse movie actually shows us the entire history of the last 13.5 billion years.
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+                <div class="col-time">1 – 2 Gy</div>
+                <div class="col-title">Rise of Supermassive Black Holes</div>
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+                    As early protogalaxies collided and merged to form increasingly massive structures, tremendous quantities of gas, dust, and stars were driven toward their dense galactic centers. Under the overwhelming force of such extreme matter concentration, these central regions underwent catastrophic gravitational collapse, giving birth to the universe's first supermassive black holes. Boasting masses ranging from millions to billions of times that of our Sun, these gravitational behemoths became the anchors of young galaxies. Their immense pull triggered the violent accretion of surrounding material. As entire star systems and vast clouds of gas spiraled inward, they formed a superheated accretion disk around the black hole's event horizon. The incredible friction and gravitational forces within this swirling disk released staggering amounts of energy before the matter was swallowed entirely. This highly efficient, radiant mechanism powered the first quasars, turning the centers of these nascent galaxies into the most energetic and brilliantly luminous objects in the known universe, outshining the combined light of a trillion normal stars.
+                </div>
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-**Socrates:** But what does it show? The first frame looks familiar, like the 300,000-year-old gas from which the CMB came. But after that, this vast web-like structure that forms from the gas, like a neural network in the human brain—what is that?
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+                <div class="col-time">2 – 3 Gy</div>
+                <div class="col-title">Peak Quasar Epoch</div>
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+                    The period spanning two to three billion years after the Big Bang represents the most explosive and energetic phase in the history of the cosmos, widely known as the Peak Quasar Epoch. During this turbulent era, the universe was significantly smaller and denser, making catastrophic collisions and mergers between gas-rich galaxies incredibly frequent. These constant cosmic pile-ups drove massive, unending torrents of fresh gas and stellar material directly into the cores of young galaxies, providing an abundant and continuous fuel supply to their central supermassive black holes. Consequently, this era saw the maximum activity of Active Galactic Nuclei (AGN), with countless quasars blazing brightly across the universe. However, this period of violent activity was ultimately unsustainable. As these galactic cores eventually consumed, expelled, or exhausted their available reservoirs of gas and dust, the intense feeding frenzy subsided. The once-brilliant quasars gradually faded into darkness, leaving behind the relatively dormant and quiet supermassive black holes that now lurk peacefully at the centers of most modern galaxies, including our own.
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-**Shashi:** Socrates, no one can come up with metaphors like you. The comparison to the brain is indeed interesting. But first, let’s go over the general process of galaxy formation. At the end of the Particle Age, the gas we saw at 300,000 years old had areas where the temperature was slightly lower than average, leading to regions with higher gas density. Even as the universe expanded over time, these dense regions didn’t expand. Instead, due to gravity, they grew denser and denser. When these clumps of gas became dense enough, they started forming stars, marking the beginning of galaxy formation. Initially, galaxies were irregular and unstructured but gradually became more organized. The Millennium Simulation shows us this process. In this movie, each bright dot represents the position of a galaxy. Just as neurons are the structural units of the human brain, galaxies are the structural units of the universe. The cosmic web, consisting of around a trillion galaxies, resembles a web. The part of the cosmic web you see in this simulation gives an idea of the whole. Regions with many galaxies and clusters are called nodes, while the regions connecting these nodes, with fewer galaxies, are called filaments.
+        <!-- Row 6 -->
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+                <div class="col-time">3 Gy</div>
+                <div class="col-title">Large-Scale Structure Formation</div>
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+                    By the time the universe reached three billion years of age, its grand macroscopic architecture had largely taken its final shape, sculpted by the persistent, unyielding influence of dark matter and gravity. Galaxies and massive galaxy clusters, such as our own Local Group, did not drift randomly or remain isolated in the expanding void. Instead, driven by the gravitational pathways laid down during the universe's earliest moments, they organized themselves into a vast, staggeringly complex, interconnected framework known as the cosmic web. This enormous, universe-spanning structure is characterized by sweeping, thread-like filaments and extensive, flat sheets of tightly packed galaxies. These luminous structures intersect at massive, hyper-dense superclusters, creating glowing cosmic nodes. In stark contrast, these densely populated regions surround and enclose immense, utterly unpopulated voids—vast stretches of empty space millions of light-years across. This unique distribution of matter gives the entire cosmos a distinctly "frothy," soap-bubble-like appearance when viewed on the absolute largest macroscopic scales.
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-**Socrates:** You said each bright dot represents a galaxy’s position. Why the position and not the galaxy itself?
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+                <div class="col-time">4 Gy</div>
+                <div class="col-title">Birth of Population I Stars</div>
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+                    Throughout the Galactic Age, a continuous and violent cycle of stellar birth and explosive death slowly but fundamentally transformed the chemical composition of the cosmos. The universe's earliest massive stars—made almost entirely of pure hydrogen and helium—burned through their fuel rapidly and detonated as spectacular supernovae. These colossal explosions blasted newly forged, heavier elements like carbon, oxygen, nitrogen, and iron deep into the surrounding interstellar medium. Over billions of years, this steady chemical enrichment of galactic disks fundamentally altered the raw material available for new stars. By around 4 billion years after the Big Bang, this process had seeded the gas clouds sufficiently to allow for the widespread formation of metal-rich, second- and third-generation stars, formally known as Population I stars. The birth of these chemically complex stars, which includes our own future Sun, marked the universe's full transition into the Stellar Age. Crucially, it established the necessary heavy-element foundation required for the eventual formation of solid, rocky planets and the emergence of biological life.
+                </div>
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-**Shashi:** We can’t call these dots galaxies exactly, as the Millennium Run was created using dark matter.
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-**Socrates:** What’s that?
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-**Shashi:** Scientists believe that of the total energy-matter content in our universe, only about 5% is the ordinary energy-matter we’re familiar with (like what we discussed in the Particle Age), 25% is dark matter, and the remaining 70% is dark energy. Any matter or energy in the universe that we cannot directly detect has been given the prefix “dark.” Dark matter, therefore, is about five times more abundant than visible matter. Since gravity is proportional to mass and dark matter is more massive, galaxy structures should form based on dark matter. By simulating dark matter, we can infer the distribution of visible matter. Thus, the Millennium Simulation models 10 billion “particles” of dark matter, with each particle having a mass equivalent to a billion suns. This means that several dots in the time-lapse movie combine to create a galaxy’s scaffolding, where visible matter might form hundreds of billions of stars.
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-**Socrates:** It seems astronomers have fallen in love with the dark. Perhaps, in the future, theoretical physicists’ theories will all lean toward the dark side. People know far less than they think they do, yet claim to know far more.
-**Mars:** Within the next 20 years, we might begin to see real evidence of the Millennium Run.
-**Socrates:** Good. I have another question. You mentioned all galaxies formed within the first four billion years. Why can’t new galaxies form after that or now or in the future?
-**Shashi:** Because, Socrates, dense gas gradually condenses due to gravity until it becomes dense enough to form a galaxy. But if there isn’t enough dense gas, galaxy formation can’t even begin. By four billion years after the Big Bang, the universe had expanded so much that there wasn’t enough dense gas left to form galaxies. Most of the gas never became galaxies and instead formed the intergalactic medium, the sparse space between galaxies with very low gas density. The cosmic web contains even emptier regions than the intergalactic medium, called cosmic voids, where gas density is even lower. Opposing these voids in the cosmic web, you’ll find clusters and superclusters of galaxies in the densest regions called nodes.
-===== - Milky Way =====
-**Hermes:** Watching your Millennium Run makes me want to run too. How much longer within Earth’s gravity? We’re celestial beings after all. Just looking at the Whirlpool through a telescope doesn’t satisfy me. Let’s go straight to the Whirlpool Galaxy, everyone.
-//[In the Millennium Simulation's stimulation, everyone takes off into the void. After passing the Milky Way, Shashi stops everyone.]//
-**Shashi:** Hold on, hold on. The Andromeda Galaxy is clearly visible, but the Whirlpool is still 30 million light-years away. Just like Voyager 1 once looked back at Earth on its way out of the solar system to capture the famous "Pale Blue Dot" image, let’s also take a moment to look back at the Milky Way.
-//[Everyone gazes back at the Milky Way in contemplation.]//
-**Socrates:** Seeing the face of the Milky Way from this face-on view is truly remarkable. It’s even more beautiful than what we could see in the [[https://eyes.nasa.gov/apps/solar-system/#/home|NASA Eyes animation]]. Explain, Shashi.
-{{:bn:courses:ast100:mw.webp?nolink|}}
-**Shashi:** Our spiral galaxy is lovingly called **Akashganga** in Indian tradition, as if the river Ganges has descended from the sky. Of course, no human could ever see the Milky Way in this form. This galaxy spans 100,000 light-years, which would take 100,000 years to cross at the speed of light. And if we traveled at Voyager’s average speed (17 km/sec), it would take 2 billion years. We’re lucky we’re already dead—otherwise, we’d never experience this fortune.
-**Socrates:** Why is the center of the galaxy so bright?
-**Shashi:** In a spiral galaxy, if the galaxy were a country, the center would be its capital, where the stars are most densely packed. There are so many stars because right at the center, there’s a supermassive black hole, which might remind us of mortality—a giant mass grave sits at the heart of our galaxy.
-**Socrates:** A supermassive black hole is a mass grave?
-**Shashi:** A black hole is the corpse of a massive star, one that was many times larger than the Sun. When these stars die, they become so dense that gravity pulls in everything around them, not even allowing light to escape. Since light cannot escape, they are invisible, hence the name black hole. In the central region of the Milky Way, where stars are densely packed, the concentration of stellar corpses is also high, similar to how urban areas have more cemeteries than villages. These stellar corpses merge over time, growing in size, and eventually forming a supermassive black hole at the galaxy’s core.
-**Socrates:** But why only at the center? Couldn’t a supermassive black hole exist elsewhere?
-**Shashi:** Forget Einstein, if you even understood Newtonian gravity, you wouldn’t be asking this. In any gravitational system, the most massive object naturally resides closest to the center of mass. For instance, in a seesaw, the heavier child sits closer to the fulcrum. Similarly, our black hole, being the most massive object, lies near the galactic center. Extending from either side of it is a bar-shaped region with so many stars that it appears as a single structure from this distance. This bar extends from both ends into the two main spiral arms: from our viewpoint, the Perseus Arm above and the Scutum-Centaurus Arm below. You won’t see the Sun from here, but you can see its orbit in this image. In this orbit around the center, the Sun completes a revolution around our galaxy every 250 million years (1 galactic year).
-**Hermes:** If we take off again, we could see the Milky Way’s edge-on view too, and understand how thin its disk is.
-**Socrates:** Hermes is always eager to take off.
-//[In an instant, everyone moves 500,000 light-years away from the Milky Way, aligned with the disk.]//
-**Shashi:** The thin disk is clearly visible—it’s only about 1,000 light-years thick. One could think of flying out like a drone to photograph it, but that would still take millions of years. On either side of the thin disk lies the thick disk, about 10,000 light-years thick. Within the disk, Population I (younger) stars orbit nearly circularly around the galactic center. Where we previously saw the bar, we now see a bulge—a central, swollen region with a high density of stars. Surrounding this disk is a spherical stellar halo, where Population II (older) stars roam in elliptical orbits, occasionally passing above and below the disk. In the halo, dark matter exerts more influence than visible matter. But the most interesting feature of the halo is the scattered globular clusters, each containing 100,000 to 500,000 stars, all very ancient.
-**Socrates:** How ancient?
-**Shashi:** As old as our galaxy itself—around 13 billion years. Thirteen billion years ago, our galaxy was probably quite irregular, and among the first stars formed, only those smaller than the Sun have survived in the halo or globular clusters above and below the disk. The disk has no place for these older stars; it’s reserved for the young. Nowhere else can you see a better example of how a new generation displaces the old.
-**Socrates:** But how did the galaxy go from an irregular shape to first forming a disk and then spiral arms within that disk?
-**Shashi:** We still don’t fully understand the detailed process. However, when we explain how flat solar systems like ours formed from large, irregular gas clouds during the Stellar Age, this process will become clearer. It’s best to discuss it then, as spiral galaxies are flat like our solar system. For now, I’ll just mention that our galaxy’s thin disk formed 9 billion years ago, and spiral arms appeared only 5.5 billion years ago, meaning truly during the Stellar Age.
-===== - Galaxy Classification =====
-**Hermes:** Then let’s head toward the Whirlpool Galaxy. As we travel, let’s learn about different types of galaxies.
-**Shashi:** Edwin Hubble was the first to conclusively prove that there are many galaxies beyond our own. In 1924, he identified that the fuzzy object known as the Andromeda "Nebula" was actually a separate galaxy outside of the Milky Way. After discovering many more galaxies, he organized them into a “tuning fork” classification, like this image.
-{{:bn:courses:ast100:galclass.webp?nolink|}}
-**Socrates:** It looks like there are three panels here, each representing how different types of galaxies appeared across three eras. The panel on the left is the present. The other two show 4 and 11 billion years ago. It seems like galaxies primarily fall into three categories: elliptical, lenticular, and spiral, with barred spiral as a distinct type within spirals.
-**Shashi:** There’s also an “irregular” category, which isn’t shown here, as it doesn’t conform to any specific shape. You can see elliptical galaxies classified from E0 to E7, with E0 being the most circular and E7 the most elongated. Lenticular galaxies are marked S0, as they’re in between ellipticals and spirals—they have a disk like spirals but lack spiral arms, and they have a large oval bulge like ellipticals around the disk. Spiral galaxies are classified from Sa to Sd based on their bulge and arm types: Sa galaxies have the largest bulge and smoothest arms, while Sd galaxies have the smallest bulge and the most spread-out arms. Barred spirals follow the same classification, but with a bar extending from the bulge.
-**Socrates:** It seems to me that a galaxy begins as an elliptical, slowly becomes lenticular, and eventually turns into a spiral.
-**Shashi:** Hubble himself thought the same. That’s why he called reddish elliptical galaxies "early-type" and bluish spirals "late-type." But in reality, it seems that all these types of galaxies have existed over the past 11 billion years. Only, the further back we go, the more the structure of each type of galaxy appears less defined and less organized. An elliptical galaxy might evolve into a spiral over time, but the reverse can also happen.
-**Socrates:** Why does this transformation happen?
-**Shashi:** Mainly due to interactions with other galaxies, such as merging or environmental effects in galaxy clusters. Speaking of merging, we’re approaching the Whirlpool Galaxy. See it for yourself—this is what a merger looks like, though it can take hundreds of millions of years for two galaxies to completely merge. The Whirlpool appears 10 degrees across in our sky, about 20 times larger than the Sun’s apparent size in Earth’s sky. The smaller, background galaxy is actually irregular or a dwarf galaxy, but this interaction is distorting the shape of the larger galaxy too. A large spiral galaxy can become irregular due to interactions like this. The [[https://durbin.cc/object/ngc4038/|Antennae Galaxies]] are another good example. Let’s take a close look at both of these galaxies.
-{{https://upload.wikimedia.org/wikipedia/commons/thumb/6/67/NGC4038_Large_01.jpg/1280px-NGC4038_Large_01.jpg?nolink&500}}
-**Socrates:** Why are the two tails of the Antennae Galaxies so long? Are they made of gas?
-**Shashi:** No, they’re made of stars. As the two galaxies spiral around each other and merge, their stars don’t actually collide due to the vast distances between them within each galaxy. Instead, the gravitational pull of one galaxy strips stars from the other, ejecting them into space to form the two long tails.
-**Socrates:** The center of the merging galaxies seems brighter than a regular galaxy.
-**Shashi:** That’s because of the collision of interstellar gas. In addition to stars, galaxies contain a lot of interstellar gas. Unlike stars, gas clouds are large, so they collide during a merger. This heats and compresses the gas, triggering intense star formation. This phenomenon is called a **starburst**, and galaxies where it occurs are known as **starburst galaxies**.
-{{https://upload.wikimedia.org/wikipedia/commons/thumb/f/fd/The_VLT_goes_lion_hunting.jpg/1023px-The_VLT_goes_lion_hunting.jpg?nolink&500}}
-**Socrates:** Isn’t there a ring visible at the center of this galaxy?
-**Shashi:** Yes, that’s also due to a merger. In the case of the Antennae pair, the galaxies didn’t collide head-on, but here one galaxy crashed straight into another. This head-on collision in **Messier 95** generates a longitudinal wave that propagates outward from the center. The wave compresses and expands in cycles, creating new stars in the compressed regions. That’s the compression front you’re seeing, Socrates.
-**Socrates:** What happens to the two supermassive black holes of each galaxy during such a merger?
-===== - Active Galaxy =====
-{{:bn:courses:ast100:agn.webp?nolink&600|}}
-===== - From Velocity to Age =====
-{{:bn:courses:ast100:hubble.webp?nolink&750|}}