Day 7: Galaxies and Black Holes

Key moments in history: In 1919 a young astronomer named Edwin Hubble was hired to work at Mount Wilson Observatory in Washington. Most astronomers were concerned with the nature of the cloudy patches called nebulae. Most of Hubble’s colleagues at Mount Wilson thought they were all in the Milky Way, but he was not so sure. Hubble went on to take some of the best possible photos of these objects, providing convincing evidence that at least some of them were well beyond the Milky Way. By discovering other galaxies Hubble expanded the known universe 100-fold. By measuring the distances and motions of the galaxies he surprised everyone, including Einstein, by discovering that the universe is expanding.

Key Concepts:

What is a Galaxy?

You’ve probably heard that our Sun is located in the Milky Way galaxy. But what is a galaxy anyway? The simple answer is that a galaxy is a collection of stars held together by their mutual galaxy. In other words, all the stars in a galaxy are kept together by the gravity of all the other stars (as well as the invisible, mysterious dark matter). We know the Milky Way pretty well, so let’s consider it as a good example of a galaxy. The Milky Way is a spiral galaxy. It has a bright central core with a high density of stars, and then a flattened disk surrounding it – like a spinning record. Two spiral arms start just outside the core, and then spiral outward like a pinwheel to the outer edges of the galaxy. The Milky Way measures about 100,000 light-years across, and is thought to contain 200-400 billion stars.

But the stars we can see are just a tiny fraction of the complete galaxy. It’s also surrounded by a vast halo of dark matter. This material is invisible, and doesn’t interact with regular matter or give off any kind of radiation that we can detect. But astronomers can measure its effects because it does exert a gravitational force on other matter. In fact, the Milky Way is made up of mostly dark matter. The stars account for about 580 billion solar masses, and the dark matter could be another 6 trillion solar masses.

Our Milky Way is just an example of a galaxy, though. There is another type of galaxy called elliptical, and they’re even more common. The smallest galaxies in the Universe, the ultra-compact dwarf galaxies are only a little larger than a globular star cluster. But then the largest galaxies in the Universe also have this elliptical (egg-like) shape. A good example is the galaxy M87. It’s thought to have 2.7 trillion stars. Stars are collected together into galaxies. Galaxies are collected together into groups of galaxies, and these groups are collected into clusters. The largest structures in the Universe are galaxy superclusters, which contain millions of galaxies and can measure hundreds of millions of light-years across.

What Is a Black Hole?

A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying. Because no light can get out, people can't see black holes. They are invisible. Space telescopes with special tools can help find black holes. The special tools can see how stars that are very close to black holes act differently than other stars.

How Big Are Black Holes?

Black holes can be big or small. Scientists think the smallest black holes are as small as just one atom. These black holes are very tiny but have the mass of a large mountain. Mass is the amount of matter, or "stuff," in an object. Another kind of black hole is called "stellar." Its mass can be up to 20 times more than the mass of the sun. There may be many, many stellar mass black holes in Earth's galaxy. Earth's galaxy is called the Milky Way.

The largest black holes are called "supermassive." These black holes have masses that are more than 1 million suns together. Scientists have found proof that every large galaxy contains a supermassive black hole at its center. The supermassive black hole at the center of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million suns and would fit inside a very large ball that could hold a few million Earths.

How Do Black Holes Form?

Scientists think the smallest black holes formed when the universe began. A common type of black hole is produced by certain dying stars. A star with a mass greater than about 20 times the mass of our Sun may produce a black hole at the end of its life. In the normal life of a star there is a constant tug of war between gravity pulling in and pressure pushing out. Nuclear reactions in the core of the star produce enough energy and pressure to push outward. For most of a star’s life, gravity and pressure balance each other exactly, and so the star is stable. However, when a star runs out of nuclear fuel, gravity gets the upper hand and the material in the core is compressed even further. The more massive the core of the star, the greater the force of gravity that compresses the material, collapsing it under its own weight.

For small stars, when the nuclear fuel is exhausted and there are no more nuclear reactions to fight gravity, the repulsive forces among electrons within the star eventually create enough pressure to halt further gravitational collapse. The star then cools and dies peacefully. This type of star is called a "white dwarf." When a very massive star exhausts its nuclear fuel it explodes as a supernova. The outer parts of the star are expelled violently into space, while the core completely collapses under its own weight. If the core remaining after the supernova is very massive (more than 2.5 times the mass of the Sun), no known repulsive force inside a star can push back hard enough to prevent gravity from completely collapsing the core into a black hole. From the perspective of the collapsing star, the core compacts into a mathematical point with virtually zero volume, where it is said to have infinite density. This is called a singularity. Where this happens, it would require a velocity greater than the speed of light to escape the object's gravity. Since no object can reach a speed faster than light, no matter or radiation can escape. Anything, including light, that passes within the boundary of the black hole -- called the "event horizon" -- is trapped forever.

Stellar black holes are made when the center of a very big star falls in upon itself, or collapses. When this happens, it causes a supernova. A supernova is an exploding star that blasts part of the star into space. Scientists think supermassive black holes were made at the same time as the galaxy they are in.

If Black Holes Are "Black," How Do Scientists Know They Are There? A black hole can not be seen because strong gravity pulls all of the light into the middle of the black hole. But scientists can see how the strong gravity affects the stars and gas around the black hole. Scientists can study stars to find out if they are flying around, or orbiting, a black hole.

When a black hole and a star are close together, high-energy light is made. This kind of light can not be seen with human eyes. Scientists use satellites and telescopes in space to see the high-energy light.

Could a Black Hole Destroy Earth?

Black holes do not go around in space eating stars, moons and planets. Earth will not fall into a black hole because no black hole is close enough to the solar system for Earth to do that. Even if a black hole the same mass as the sun were to take the place of the sun, Earth still would not fall in. The black hole would have the same gravity as the sun. Earth and the other planets would orbit the black hole as they orbit the sun now.The sun will never turn into a black hole. The sun is not a big enough star to make a black hole.

Video of the Day:

Links: - images of galaxies taken by the Hubble Telescope