# Astronomy: A Physical Perspective

Cambridge University Press, Jul 31, 2003 - Science - 582 pages
This fully revised and updated text is a comprehensive introduction to astronomical objects and phenomena. By applying some basic physical principles to a variety of situations, students will learn how to relate everyday physics to the astronomical world. The text contains useful equations, chapter summaries, worked examples and end-of-chapter problem sets. It is suitable for undergraduate students taking a first course in astronomy, and assumes a basic knowledge of physics with calculus.

### What people are saying -Write a review

#### User ratings

5 stars
 4
4 stars
 1
3 stars
 0
2 stars
 0
1 star
 0

User Review - Flag as inappropriate

Excellent introduction to astronomy, astrophysics, and cosmology. The books balances theory with the right amount of mathematics.

User Review - Flag as inappropriate

Spaghettification
For the computer programming term, see spaghetti code.
Tidal forces acting on a spherical body in a non-homogeneous gravitational field. The effect originates from a source to the right (or to the left) of the diagram. Longer arrows indicate stronger forces.
In astrophysics, spaghettification (sometimes referred to as the noodle effect[1]) is the vertical stretching and horizontal compression of objects into long thin shapes (rather like spaghetti) in a very strong non-homogeneous gravitational field; it is caused by extreme tidal forces. In the most extreme cases, near black holes, the stretching is so powerful that no object can withstand it, no matter how strong its components. Within a small region the horizontal compression balances the vertical stretching so that small objects being spaghettified experience no net change in volume.
Stephen Hawking[2] describes the flight of a fictional astronaut who, passing within a black hole's event horizon, is "stretched like spaghetti" by the gravitational gradient (difference in strength) from head to toe. The reason this happens would be because the gravity exerted from the singularity would be much stronger at one end of your body from the other. If one were to fall into a black hole feet first, the gravity at his/her feet would be much stronger than at their head, causing the person to be vertically stretched. Along with that, the right side of the body will be pulled to the left, and the left side of the body will be pulled to the right, horizontally compressing the person.[3] However, the term "spaghettification" was established well before this.[4]
Contents [hide]
1 A simple example
2 Examples of weak and strong tidal forces
3 Inside or outside the event horizon
4 Notes
5 References
A simple example
The spaghettification of four objects falling towards a planet
Imagine four separate objects in the space above a planet, positioned in a diamond formation. The four objects follow the lines of the gravitoelectric field,[5] directed towards the celestial body's centre. In accordance with the inverse-square law, the lowest of the four objects experiences the biggest gravitational acceleration, so that the whole formation becomes stretched into a line.
Now imagine these four objects as connected parts of a larger object. A rigid body will resist distortion, and internal elastic forces develop as the body distorts to balance the tidal forces, so attaining mechanical equilibrium. If the tidal forces are too large, the body may yield and flow plastically before the tidal forces can be balanced, or fracture, producing either a filament or a vertical line of broken pieces.
Examples of weak and strong tidal forces
In the gravity field due to a point mass or spherical mass, for a uniform rod oriented in the direction of gravity, the tensile force at the center is found by integration of the tidal force from the center to one of the ends. This gives F =
μ l m
/
4r3
, where μ is the standard gravitational parameter of the massive body, l is the length of the rod, m is rod's mass, and r is the distance to the massive body. For non-uniform objects the tensile force is smaller if more mass is near the center, and up to twice as large if more mass is at the ends. In addition, there is a horizontal compression force toward the center.
For massive bodies with a surface, the tensile force is largest near the surface, and this maximum value is only dependent on the object and the average density of the massive body (as long as the object is small relative to the massive body). For example, for a rod with a mass of 1 kg and a length of 1 m, and a massive body with the average density of the Earth, this maximum tensile force due to the tidal force is only 0.4 μN.
Due to the high density, the tidal force near the surface of a white dwarf is much stronger, causing in the example a maximum tensile force of up to 0.24 N. Near a neutron star, the tidal

### Contents

 III 1 IV 3 V 7 VI 9 VII 10 VIII 12 IX 15 X 17
 LXXX 266 LXXXI 267 LXXXII 270 LXXXIII 272 LXXXIV 274 LXXXV 287 LXXXVI 293 LXXXVII 294

 XI 18 XII 20 XIII 25 XIV 26 XV 27 XVI 32 XVII 36 XVIII 41 XIX 45 XX 46 XXI 53 XXII 58 XXIII 62 XXIV 63 XXV 68 XXVI 75 XXVII 83 XXVIII 84 XXIX 87 XXX 91 XXXI 94 XXXII 96 XXXIII 101 XXXIV 105 XXXV 109 XXXVI 110 XXXVII 113 XXXVIII 123 XXXIX 125 XL 128 XLI 129 XLII 131 XLIII 132 XLIV 139 XLV 141 XLVI 143 XLVII 148 XLVIII 155 XLIX 157 L 159 LI 164 LII 168 LIII 171 LIV 172 LV 177 LVI 179 LVII 183 LVIII 186 LIX 193 LX 197 LXI 199 LXII 205 LXIII 206 LXIV 209 LXV 211 LXVI 213 LXVII 216 LXVIII 218 LXIX 221 LXX 225 LXXI 231 LXXII 232 LXXIII 235 LXXIV 237 LXXV 243 LXXVI 246 LXXVII 251 LXXVIII 258 LXXIX 263
 LXXXVIII 300 LXXXIX 302 XC 304 XCI 306 XCII 313 XCIII 315 XCIV 322 XCV 326 XCVI 330 XCVII 335 XCVIII 339 XCIX 345 C 347 CII 349 CIII 353 CIV 355 CV 361 CVI 362 CVII 368 CVIII 370 CIX 377 CX 378 CXI 381 CXII 384 CXIII 390 CXIV 395 CXV 407 CXVI 410 CXVII 417 CXVIII 419 CXIX 427 CXX 429 CXXI 430 CXXII 435 CXXIII 438 CXXIV 439 CXXV 447 CXXVI 452 CXXVII 454 CXXVIII 465 CXXIX 467 CXXX 469 CXXXI 479 CXXXII 483 CXXXIII 490 CXXXIV 491 CXXXV 494 CXXXVI 497 CXXXVII 500 CXXXVIII 506 CXL 512 CXLI 523 CXLII 524 CXLIII 530 CXLIV 532 CXLV 537 CXLVI 540 CXLVII 541 CXLVIII 543 CXLIX 544 CL 547 CLI 551 CLII 553 CLIII 554 CLIV 555 CLV 558 CLVI 559 CLVII 565 Copyright