Table of Contents

Orbital Mechanics

Black Holes

Exotic Objects in Space

Black Holes in Space

Black holes are extremely mysterious objects. Their gravitational field is so strong near their surface that the escape velocity is equal to the speed of light. As a result no radiation of any kind can emerge from a black hole.

If you were to shine a searchlight into a black hole to see what was inside, no light would be reflected back. Gravity would prevent it!

In fact, black holes are absolutely black. Radiation or matter would have a one-way trip when they encountered a black hole...IN! Nothing emerges from a black hole.

It is this fact that make black holes extremely difficult to detect. They seem more or less invisible in the darkness of interstellar space.

Consider the image to the left. The photo shows how a black hole might look through the Hubble Space Telescope.

Examine the star field in the dark-sky image to the left. Can you spot the location of a black hole?

The bright-sky inset is enlarged to help you. Note the distortion of the star field. This effect could also be caused by large massive objects other than black holes. Very massive galaxies also cause this effect when light from more distant galaxies pass near them. These are called gravitational lenses.

A Gravitationally Distorted Image

The only way a black hole can be detected is by noting the unusual behaviour of objects (such as stars), or radiation (such as light), as they pass near a black hole.

The gravitational field very close to a black hole can noticeably bend light rays from stars which lie in the background, as the light rays pass near the black hole. This will cause the optical distortion of images lying close to the line of sight of a black hole.

Even though image distortion occurs, it is difficult to determine if the distorting object is really a black hole, or a dim massive object such as a galaxy.

A Star Orbiting a Black Hole

Just as we can calculate the mass of the Earth from the motion of the our Moon, or the mass of Mars from the orbital motions of its moons, so too can we calculate the mass of objects at the centre of the galaxy from the motion of stars which orbit near the centre of the galaxy.

Something Big (and Dark) at the Centre of the Galaxy

The Sun is one of about 200 billion stars which make up our Galaxy. Since we live "inside" our Galaxy we see it edge-on as the faint night time glow we know as the Milky Way.

We can never get outside and look at our Galaxy, but astronomers have pieced together a fairly accurate picture of what it would look like if we could travel to another galaxy and look back at the Milky Way Galaxy.

The picture below is a spiral galaxy known as M74. It contains a few hundred billion stars and resembles our own galaxy.

All of the stars in our galaxy, which we call the Milky Way Galaxy, travel in huge orbits around the centre of the galaxy. Our Sun (which drags the solar system along with it) takes about 200 million years to complete one orbit.

Since the stars all follow Keplerian orbits around the galactic centre, stars close to the centre orbit faster and have shorter orbital periods than our Sun, and stars further away from the galactic centre orbit more slowly and have much longer orbital periods than our Sun.

The problem is that when we calculate the amount of matter close to the centre of the galaxy it seems much too large for the number of stars that we see near the center of the galaxy. Whatever the massive object is, (or objects are), it (they) seem(s) invisible.

Zooming to the Centre of the Milky Way

If we look towards the constellation Sagittarius we are looking directly towards the centre of our galaxy.

Unfortunately the dark spots are not due to a lack of stars, but rather to huge dust clouds which obscure the stars which cluster near the centre of the Galaxy.

Luckily infrared light can pass through these dust clouds and with specially designed instruments we can get images of the centre of the galaxy.

The image to the left shows a series of six images of increasing magnification, zooming in on the centre of our galaxy. All but the last frame are in the visible part of the spectrum. The final image is an infrared image of the very centre our galaxy...somewhere in there lurks a very strange object.

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