Galaxy

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It's a tough galaxy. If you want to survive, you've gotta know...where your towel is. ~ Douglas Adams

A galaxy (Greek: γαλαξίας, galaxias, literally "milky"), is a massive, gravitationally bound system consisting of stars, stellar remnants, planets, moons, an interstellar medium of gas and dust, and dark matter, an important but poorly understood component. The Earth and Solar system are located in the Milky Way galaxy which itself is part of the Laniakea Supercluster. There are probably more than 170 billion galaxies in the observable universe. The majority of these are arranged in galaxy groups and clusters, which, in turn usually form larger superclusters. At the largest scales of our universe, these associations are generally arranged into sheets and filaments, which are surrounded by immense voids.

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Image of galaxy NGC 4945 showing the huge luminosity of the central few star clusters, suggesting there are 10 to 100 supergiant stars in each of these, packed into regions just a few parsecs across.
  • The mass of an object is one of the most fundamental properties of the object. As such, reliable ways to measure mass are exceedingly important. One way is to simply measure the optical emission (luminosity of a galaxy) and to (independently) determine the average mass of a star in the galaxy. This will then lead to an estimate for the mass of the galaxy if all of the luminosity is in fact due to the stars and all material radiates in a similar manner. This method has problems because it is sometimes difficult to know whether all mass produces radiation.
  • All objects are in orbit around the center of our Galaxy. To see how this allows us to infer the mass of our Galaxy, consider the Earth in its orbit in our Solar System.
    • Archive in: "Mass of the Milky Way Galaxy".
  • ...the mass where we can see stars is only 1/3 of the mass of the entire Galaxy. This suggests that a lot of mass in the Milky Way is in some form which does not radiate large amounts of light (Dark Matter).
    • Archive in: "Mass of the Milky Way Galaxy"
Every one of us is, in the cosmic perspective, precious. If a human disagrees with you, let him live. In a hundred billion galaxies, you will not find another. ~ Carl Sagan
The universe was made on purpose, the circle said. In whatever galaxy you happen to find yourself, you take the circumference of a circle, divide it by its diameter, measure closely enough, and uncover a miracle — another circle, drawn kilometers downstream of the decimal point. ~ Carl Sagan
  • Any man that can hitch the length and breadth of the Galaxy, rough it, slum it, struggle against terrible odds, win through and still know where his towel is, is clearly a man to be reckoned with.
  • A long time ago in a galaxy far, far away... It is a period of civil war. Rebel spaceships, striking from a hidden base, have won their first victory against the evil Galactic Empire.
I think that the event which, more than anything else, led me to the search for ways of making more powerful radio telescopes, was the recognition, in 1952, that the intense source in the constellation of Cygnus was a distant galaxy—1000 million light years away. This discovery showed that some galaxies were capable of producing radio emission about a million times more intense than that from our own Galaxy or the Andromeda nebula, and the mechanisms responsible were quite unknown... - Martin Ryle.
  • I think that the event which, more than anything else, led me to the search for ways of making more powerful radio telescopes, was the recognition, in 1952, that the intense source in the constellation of Cygnus was a distant galaxy—1000 million light years away. This discovery showed that some galaxies were capable of producing radio emission about a million times more intense than that from our own Galaxy or the Andromeda nebula, and the mechanisms responsible were quite unknown. … The possibilities were so exciting even in 1952 that my colleagues and I set about the task of designing instruments capable of extending the observations to weaker and weaker sources, and of exploring their internal structure.
  • For as long as there been humans we have searched for our place in the cosmos. Where are we? Who are we? We find that we live on an insignificant planet of a hum-drum star lost in a galaxy tucked away in some forgotten corner of a universe in which there are far more galaxies than people. This perspective is a courageous continuation of our penchant for constructing and testing mental models of the skies; the Sun as a red-hot stone, the stars as a celestial flame, the Galaxy as the backbone of night.
  • Every one of us is, in the cosmic perspective, precious. If a human disagrees with you, let him live. In a hundred billion galaxies, you will not find another.
  • The universe was made on purpose, the circle said. In whatever galaxy you happen to find yourself, you take the circumference of a circle, divide it by its diameter, measure closely enough, and uncover a miracle — another circle, drawn kilometers downstream of the decimal point. There would be richer messages farther in. It doesn't matter what you look like, or what you're made of, or where you come from. As long as you live in this universe, and have a modest talent for mathematics, sooner or later you'll find it. It's already here. It's inside everything. You don't have to leave your planet to find it.

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