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--- courses:ast100:3 [2024/11/22 07:44] – [5.3 Black Hole] asad
+++ courses:ast100:3 [2024/11/22 07:44] (current) – [5.3 Black Hole] asad
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 **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.
-**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.
+**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.