Chapter 11 Neutron Stars and Black Holes Discovery

Chapter 11: Neutron Stars and Black Holes 中子星和黑洞

Discovery of Pulsars 脉冲星的发现 Angular momentum conservation 角动量守恒 => 塌缩恒星核自旋周期约为几个毫秒 Magnetic fields are amplified up to B ~ 109 – 1015 G 磁场极强. (up to 1012 times the average magnetic field of the sun) => Rapidly pulsed (optical and radio) emission from some objects interpreted as spin period of neutron stars 有些天体的快速脉冲辐射被解释为中子星的自传周期（脉冲星=>中子星）

The Crab Pulsar 蟹状星云脉冲星 Pulsar wind + jets 脉冲星风＋喷流 Remnant of a supernova observed in A. D. 1054年爆发的一颗超新星的遗迹

The Crab Pulsar 蟹状星云脉冲星 Visual image X-ray image

Light curves of the Crab Pulsar 脉冲星光变曲线

The Lighthouse Model of Pulsars 脉冲星的灯塔模型

Images of Pulsars and other Neutron Stars The Vela pulsar moving through interstellar space The Crab Nebula and pulsar

Black Holes 黑洞 Just like white dwarfs (Chandrasekhar limit: 1. 4 Msun), there is a mass limit for neutron stars: 正如白矮星有质量上限： 1. 4太阳质量 Neutron stars can not exist with masses > 3 Msun 中子星�量不能超� 3个太阳�量 We know of no mechanism to halt the collapse of a compact object with > 3 Msun. 质量大于3个太阳质量，塌缩无法停止 It will collapse into a single point – a singularity: 将塌缩为一个奇点： => A black hole! 黑洞

Escape Velocity 逃逸速度 Velocity needed to escape Earth’s gravity from the surface vesc 脱离地球引力的速度: vesc ≈ 11. 6 km/s. Now, gravitational force decreases with distance (~ 1/d 2) vesc 引力随距离减小 => 离得越远，逃逸速度越小 If you could compress Earth to a smaller radius => higher escape velocity from the surface 质量不变，半径越小，表面的逃逸速度越大。 vesc

The Schwarzschild Radius 施瓦西半径 => There is a limiting radius where the escape velocity reaches the speed of light, c: 极限半径：逃逸速度达到光速时 2 GM Rs = ____ c 2 G = 引力常数 M = 质量 Rs 就是施瓦西半径 (Schwarzschild radius. ) Vesc = c

Schwarzschild Radius and Event Horizon 施瓦西半径和视界 No object can travel faster than the speed of light 没有超光速 => nothing (not even light) can escape from inside the Schwarzschild radius 光也逃不出施瓦西半径 Þ We have no way of finding out what’s happening inside the Schwarzschild radius. 没有办法了解施瓦西半径内发生的事情 Þ “Event horizon” 事界/视界

“Black Holes Have No Hair” 黑洞无毛定理 Matter forming a black hole is losing almost all of its properties 黑洞物质失去几乎所有性质. black holes are completely determined by 3 quantities: 黑洞由 3个量完全决定其性质： Mass 质量 angular momentum 角动量（转动） (electric charge 电荷)

The Gravitational Field of a Black Hole 黑洞的引力场（时空弯曲）引力势 Gravitational Potential 恒星的引力场黑洞的引力场 Distance from central mass 到中心质量的距离 The gravitational potential (and gravitational attraction force) at the Schwarzschild radius of a black hole becomes infinite 黑洞引力在视界处变为无穷大. 引力大，不等于质量大。

General Relativity Effects Near Black Holes 黑洞附近的广义相对论效应 An astronaut descending down towards the event horizon of the black hole will be stretched vertically (tidal effects) and squeezed laterally.

General Relativity Effects Near Black Holes 黑洞附近的广义相对论效应 Time dilation 时间延迟 Clocks starting at 12: 00 at each point. 时钟在每个地方 12点开始，三小时后（观测者远离黑洞）: After 3 hours (for an observer far away from the black hole): Clocks closer to the black hole run more slowly. 黑洞近处时钟变慢。 Time dilation becomes infinite at the event horizon. 时间延迟在视界处变 Event horizon 无穷大。视界

Observing Black Holes 观测黑洞 No light can escape a black hole 光线不能逃出黑洞， => Black holes can not be observed directly. 所以黑洞不能直接观测到 If an invisible compact object is part of a binary, we can estimate its mass from the orbital period and radial velocity. 如果双星系统有一个隐形致密天体，我们可以估计它的轨道周期和径向速度质量 > 3 Msun => 黑洞!