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--- courses:ast100:2 [2024/11/02 15:59] – [4. Active Galaxy] asad
+++ courses:ast100:2 [2024/11/05 11:31] (current) – [5. From Speed to Age] asad
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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?
+**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?
-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.
+**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.
-Ravi: Good idea. Shashi, then, why don’t you handle Ashvin-1?
+**Ravi:** Good idea. Shashi, then, why don’t you handle Ashvin-1?
-Socrates: Ashvin-1? What does that mean?
+**Socrates:** Ashvin-1? What does that mean?
-Ravi: We have two telescopes, both named after the twin stars, Ashvin 1 and 2, known as the twin brothers in the Gemini constellation.
+**Ravi:** We have two telescopes, both named after the twin stars, Ashvin 1 and 2, known as the twin brothers in the Gemini constellation.
-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.
+**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.
 Juno: Yes, I can see it. I think we should target the Whirlpool Galaxy.
-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.
+**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.
-Socrates: I see—this is actually a merging of two galaxies.
+**Socrates:** I see—this is actually a merging of two galaxies.
-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.
+**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.
-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.
+**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.
 ===== - From Gas to Galaxies =====
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 {{:bn:courses:ast100:agn.webp?nolink&600|}}
-**Shashi:** Anyway, let me clarify AGNs a bit further with this model. All AGNs are essentially the same, but from Earth, people see them at different angles and call them by different names. When viewed from the pole, or jet-aligned, it’s called a blazar; from the equator or disk-aligned, it’s a Seyfert-2 or narrow-line galaxy; and at an intermediate angle, it’s called a quasar, Seyfert-1, or broad-line galaxy. The story is the same in each case: there’s a supermassive black hole at the center with a thin accretion disk around it, where gas and stars are being drawn in, all surrounded by a donut-shaped dusty torus. The gas closer to the accretion disk moves faster, and the gas farther out moves slower; I’ll explain later why that results in broad lines nearby and narrow lines from afar.
+**Shashi:** Anyway, let me clarify AGNs a bit further with this model. All AGNs are essentially the same, but from Earth, people see them at different angles and call them by different names. When viewed from the pole, or jet-aligned, it’s called a blazar; from the equator or disk-aligned, it’s a Seyfert-2 or narrow-line galaxy; and at an intermediate angle, it’s called a quasar, Seyfert-1, or broad-line galaxy. The story is the same in each case: there’s a supermassive black hole at the center with a thin accretion disk around it, where gas and stars are being drawn in, all surrounded by a donut-shaped dusty torus. The gas closer to the accretion disk moves faster, and the gas farther out moves slower; I’ll explain later why that results in broad lines (and what is meant by 'lines') nearby and narrow lines from afar.
 **Socrates:** It looks like the accretion disk is the black hole’s dinner table, and the poles are where the waste is expelled.
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 **Hermes:** Let’s return to Earth now. This time, let’s take a boat and travel along the Tsangpo River.
-===== - From Velocity to Age =====
+===== - Age from Speeds =====
-{{:bn:courses:ast100:hubble.webp?nolink&750|}}
+**Shashi:** A boat is actually the perfect setting to explain what I want to talk about now. At the start of the 20th century, we didn’t even know there were galaxies outside the Milky Way. Everyone thought the Milky Way was the entire universe. In the 18th century, the French astronomer Charles Messier cataloged many nebulae, which were actually galaxies, but back then, they were thought to be gas clouds within our galaxy. However, not everyone dismissed the idea of other galaxies; for example, the German philosopher Immanuel Kant suggested that these known nebulae could be separate "island universes." But without knowing the distances, it was impossible to prove.
+**Socrates:** It’s not just distance; we’d also need to know the size of our galaxy.
+**Shashi:** Right. If we know the size of our galaxy and the distance of a nebula, and we find that the nebula is much farther away than the size of our galaxy, then we’d have to assume that the nebula is actually an independent galaxy. We had a general idea of the Milky Way’s size by the 19th century. But the first solid method to measure distance was introduced by Henrietta Swan Leavitt in 1912. She discovered a relationship between the period of brightness variation in Cepheid variable stars and their actual brightness.
+**Socrates:** What do you mean by “actual” brightness?
+**Shashi:** We never really know the true brightness of a star. For instance, here on the riverbank at night, we see lights inside various houses. Every house might be using the same wattage bulb, but the farther a house is from us, the dimmer its light appears. What we see is the “apparent” brightness. If I somehow know the “true” brightness of one of these bulbs, I can calculate the house’s distance by comparing the apparent brightness with the actual one. For example, if a 20-watt bulb appears to be 10 watts, the house would be at a certain distance; if it appears to be only 5 watts, the house must be farther. But to know the true brightness, we’d need to visit the house and check. Leavitt found an alternative way to determine this—her “house” was the Cepheid variable stars. If we know the true brightness of a Cepheid in another galaxy, we can use it to measure that galaxy's distance.
+**Socrates:** So, did Leavitt calculate the distance to the first galaxy?
+**Shashi:** Leavitt developed the method, but Edwin Hubble was the first to apply it successfully in the 1920s. While measuring the distances and speeds of about thirty galaxies, Hubble noticed a strange phenomenon: the farther a galaxy is from us, the faster it’s moving away. This relationship between distance and speed is shown in the inset of this diagram.
+{{:bn:courses:ast100:hubble.webp?nolink|}}
+**Socrates:** How did he measure the speed?
+**Shashi:** That part is simple—through the Doppler effect. Let me explain. Look over there, a boat is approaching us, and someone on it is playing a dungchen. Do you notice any change in the sound as the boat gets closer?
+**Socrates:** Yes, the sound gets sharper as it approaches.
+**Shashi:** So, the frequency of the sound increases, and the wavelength decreases. Now the boat is passing us and moving away. Listen—the sound becomes less sharp,  the frequency decreases, and the wavelength increases. The diagram illustrates this phenomenon on a cosmic scale. Since the universe is expanding, all galaxies are moving away from each other, just like points drawn on a balloon would move apart as it’s inflated. The galaxies themselves aren’t moving; rather, the space (the balloon) is expanding, increasing the distances between them. If a galaxy is moving away from us, its light frequency will appear lower, similar to the boat’s sound as it moved away. Since red has a lower frequency than blue, this shift in frequency is called **redshift**. We observe that the light from all galaxies is redshifted, moving toward the red end of the spectrum. The degree of this shift indicates the galaxy’s speed of recession. Just as the dungchen on the boat would sound even lower if the boat were moving faster away, the light from a galaxy appears more redshifted the faster it’s moving away. Hubble used this relationship to create his famous diagram, with the speed of recession on the x-axis, distance on the y-axis, and a straight line indicating the relationship.
+**Socrates:** But what does this mean?
+**Shashi:** What do you think?
+**Socrates:** It seems that all galaxies were once closer together, like the balloon was once smaller.
+**Shashi:** Exactly. This idea was something Einstein couldn’t initially accept in the early 1920s. He believed the universe was static. But it turned out that all galaxies were moving away from each other, meaning everything was once in a smaller space. If we go far enough back in time, we reach a point when all the matter and energy in all galaxies were contained in a single point. That’s the Big Bang. So, through studying galaxies in the 20th century, we discovered our cosmic history.
+**Socrates:** Can the speed of galaxies really tell us the age of the universe?
+**Shashi:** Yes, we can estimate the universe’s age using the Hubble constant. The recession speed of galaxies located 1 million light-years (Mly) from us essentially defines the Hubble constant. Based on the best measurements from modern telescopes, its value is about 21 km/s/Mly. This means that for every 1 Mly of distance, galaxies appear to be receding 21 km/s faster. A galaxy 2 Mly away would thus recede at 42 km/s, a galaxy 3 Mly away at 63 km/s, and so on. The inverse of the Hubble constant provides an approximation of the universe’s age. By dividing 1 by 21 km/s/Mly (remembering that 1 Mly equals \(9.5 \times 10^{14}\) km), we obtain an estimated age of around 14 billion years, which is considered the approximate age of our universe.