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--- courses:ast100:3 [2024/11/22 07:30] – [3. Stellar Age] asad
+++ courses:ast100:3 [2024/11/22 07:44] (current) – [5.3 Black Hole] asad
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 **Socrates:** What do you mean by fuel?
-===== - Star Formation =====
+===== - Birth of Stars =====
 {{:bn:courses:ast100:star-formation.webp?nolink|}}
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 **Juno:** At first, it was just a shapeless cloud, but now I see a red sphere in the center surrounded by a flat disk about half a light-year in size. How did the shapeless cloud transform into a spherical core and flat disk over these few million years?
-**Mars:** To understand that, you need to grasp the difference between **gravity** and **rotation**. If I jump into the swift current of the Aungsui River from this rock, I will survive unscathed. However, I would die if I didn’t land in the water because gravity pulls me toward the Earth's center. The closer an astronaut gets to Earth, the stronger Earth's gravity pulls them. This means gravity always attracts, and the closer you are to the center of gravity, the stronger the pull.
+**Mars:** To understand that, you need to grasp the difference between **gravity** and **rotation**. If I jump into the swift current of the Siang River from this rock, I will survive unscathed. However, I would die if I was alive because gravity pulls me toward the Earth's center. The closer an astronaut gets to Earth, the stronger Earth's gravity pulls them. This means gravity always attracts, and the closer you are to the center of gravity, the stronger the pull.
 **Juno:** Is that why you tied a rock to the end of a rope to explain rotation?
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 **Socrates:** Is it gravity that prevents light from escaping?
-**Mars:** Exactly. The stronger the surface gravity, the harder it is to escape. To escape Earth’s gravity, an object must travel at **11 km/s**. For the Sun, the escape velocity is **600 km/s**; for a white dwarf, it’s **5,000 km/s**; and for a neutron star, it’s about **100,000 km/s**. Since light travels at **300,000 km/s**, even it struggles to escape from a neutron star, getting stretched and redshifted by gravity. A black hole is an object where the escape velocity exceeds the speed of light. Since even light cannot escape, we call it a black hole.
+**Mars:** Exactly. The stronger the surface gravity, the harder it is to escape. To escape Earth’s gravity, an object must travel at **11 km/s**. For the Sun, the escape velocity is **600 km/s**; for a white dwarf, it’s **5,000 km/s**; and for a neutron star, it’s about **100,000 km/s**. Since light travels at **300,000 km/s**, even it struggles to escape from a neutron star, getting stretched and redshifted by gravity. A black hole is an object where the escape velocity exceeds the speed of light. Since even light cannot escape, we call it a black hole. While all the energy and matter are concentrated in the **singularity**, the black hole’s gravitational influence extends up to the **event horizon**. Beyond this boundary, nothing can escape—not even light. If something crosses the event horizon, there’s no return. Even approaching the **innermost stable orbit** is dangerous—anything moving closer is destined to cross the event horizon. Just outside this, light bends so much due to gravity that it orbits the black hole like a satellite, forming a spherical **photon sphere**. Like protostars and pulsars, gas falling into a black hole forms a flat **accretion disk** around it. Some of this material is lost forever, but due to the black hole’s rotation, some of it is ejected in jets along the poles at nearly the speed of light.
-While all the energy and matter are concentrated in the **singularity**, the black hole’s gravitational influence extends up to the **event horizon**. Beyond this boundary, nothing can escape—not even light. If something crosses the event horizon, there’s no return. Even approaching the **innermost stable orbit** is dangerous—anything moving closer is destined to cross the event horizon. Just outside this, light bends so much due to gravity that it orbits the black hole like a satellite, forming a spherical **photon sphere**.
+**Socrates:** So, what we saw during the Galactic Age as **supermassive black holes** are essentially combination of many such black holes. But let’s not get lost in the singularity. The Brahmaputra is calling. We must now journey back to Earth, five billion years ago.
-Like protostars and pulsars, gas falling into a black hole forms a flat **accretion disk** around it. Some of this material is lost forever, but due to the black hole’s rotation, some of it is ejected in jets along the poles at nearly the speed of light.
-**Socrates:** So, what we saw during the Galactic Age as **supermassive black holes** are essentially clusters of many such black holes. But let’s not get lost in the singularity. The Brahmaputra is calling. We must now journey back to Earth, five billion years ago.