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The Nature of Tidal Forces


February 18, 2004 ::

The force of gravity is usually considered a binding force in the universe - holding the planets in orbit around the Sun, keeping galaxies and clusters of galaxies together. However, gravity can also be a destructive force, tearing objects to pieces, especially in the vicinity of a supermassive black hole.

How does this happen? The force of gravity depends on distance, strengthening when you move towards an object. For example, imagine you are in a large spaceship, traveling towards a black hole. As you near the black hole, the tug of gravity on the front of the ship will become much stronger than the tug on the rear. As you might guess, your spaceship would begin to get stretched, and the closer you got to the center of the black hole, the stronger this stretching would be. Eventually, your spaceship would be ripped apart. This difference in gravitational force acting on opposite ends of an object is known as a tidal force.

thumbnail of tidal forces illustration
Illustration of RX J1242-11
Illustration: NASA/CXC/M.Weiss
Tidal forces are thought to be responsible for the huge X-ray flare observed in the galaxy RX J1242-11. A doomed star traveled close to the supermassive black hole in the center of this galaxy and was torn apart by tidal forces. Most of the gaseous debris from the star escaped the black hole, but a small amount of material was captured by its immense gravitational pull, forming a rotating disk of gas. Close to the black hole the heat intensified and the disk of gas began to glow in X-rays.

Although this is the first strong evidence for a supermassive black hole ripping a star apart, this is by no means our first experience with tidal forces in the universe. There are many examples of the destructive work of tidal forces in our solar system.

thumbnail of illustration of Shoemaker-Levy being broken apart by Jupiters gravitational field
Illustration of Comet-Shoemaker-Levy Collision with Jupiter
Illustration: JPL/D.Seal (edited by CXC/M.Weiss)
Recall the Shoemaker-Levy comet which was first noticed in 1993, and then carefully observed over the next year as it drew closer and closer to Jupiter. The tidal forces on the comet were so strong that it broke into at least 21 pieces as it fell in towards the massive planet. The final impacts in the summer of 1994 were huge disturbances in the gaseous surface of Jupiter.

thumbnail of illustration of Saturn's ring
Illustration of Saturn's Rings
Illustration: NASA/CXC/M.Weiss
Another dramatic example of the effects of tidal forces in the solar system are the rings around Saturn. The rings are made up of many small particles. These particles cannot clump together into a large body via gravitational attraction because of the strong tidal forces of Saturn.

Further away from the planet, however, the tidal forces are not strong enough to rip large objects apart and Saturn has a stable retinue of moons outside of the ring system.
thumbnail of image of Pele erupting on Io
Image of Pele erupting on Io
Credit: NASA/USGS
This doesn't mean that life is calm in the planetary suburbs. Even moons that survive tidal forces can still be hugely affected by them. Jupiter's moons provide the most extreme example of this, especially Io which is the most geologically active body in the solar system today. Io is being pulled by massive Jupiter on one side and by the outer moons (Europa, Callisto, Ganymede) on the other. The opposing tidal forces alternately squeeze and stretch its interior, causing the solid surface to rise and fall by about 100 meters. The enormous amount of heat and pressure generated by the resulting friction creates colossal volcanoes and fractures on the surface of this moon.

Jupiter's moon Europa also shows visible signs of tidal forces, with countless cracks in its icy surface. After careful analysis, the cracks and folds in the surface appear to be the accordion like stretching and squeezing lines of opposing tidal forces.

Of course the most familiar example of tidal forces in the universe is right here at home, on Earth. The tides of the oceans are the result of the combined tidal forces of the Moon and the Sun.

While it is fortunate that we can observe tidal effects in our local neighborhood, we certainly would not be able to see the eruption of a moon's volcano at the distance of the galaxy RX J1242-11, or the impact of a comet fragment on a planet or star. These events are simply not bright enough. However, two important effects allow us to observe the central black hole tear a star apart. Firstly, the strong gravitational field of the supermassive black hole makes the gas very hot, so that it glows in X-rays, and secondly the stellar material is swallowed very quickly by the black hole. These effects combine to create a flash of X-rays that is so bright it can easily be detected with Chandra and XMM-Newton in a galaxy like RX J1242-11, 700 million light years away.

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