Galactic arms

Galaxies

Large galaxies grow by attracting small galaxies as they pass nearby. One can only marvel at the beauty of these bricks of the Universe. The galaxies are hundreds of billions and growing every time our space technology is perfected, pushing back the horizons. The galaxies were born, 3 to 4 billion years after the Big Bang. Gravity is what has transformed diffuse clouds of hydrogen and helium, into embryos of galaxies. These embryos have collapsed as a result of their own gravity. This collapse was compressed and heated gaseous matter, turning it into hundreds of billions of soft balls, a few million degrees, called 'stars'.
When all the gaseous matter has been transformed into stars, these galaxies become elliptical, 30% of galaxies are elliptical. Some galaxies are able to convert 80% of the gaseous mass in stars. The remaining fifth is flattened into a thin disk, continuing to turn into stars, much more slowly and preferably along the spiral arms which are developing.

These are spiral galaxies that dominate the galactic world, 60% of the galaxies are spirals. The galaxy Messier 51, on picture-cons, is one of the finest examples, it looks like a giant hurricane.
Other galaxies are much more time to convert the gaseous matter into stars, because they are dwarfs, they contain only a few billion stars, having no special form, they are called 'irregular galaxies'.
10% of galaxies are irregular. Galaxies live among other galaxies and interact with their environment, especially where the density of galaxies is very high, in the heart of the cluster can be found 1000 to 10 000 galaxies in a cube a few million light years aside. In our local group, there are only 10 galaxies in a cube of equal size. The Hubble Space Telescope shows us here on the picture, the gravitational power NGC 5194 drawing its companion NGC 5195. In fact, this practice is common throughout the universe.

Image: The Whirlpool galaxy NGC 5194 of 60 000 light years in diameter is also known as the M51 or the « whirlpool galaxy. » It absorbs NGC 5195, his companion on the right. Hubble image.

Galactic arms

Each arm of our galaxy describes a kind of logarithmic spiral. There would be four major arms from the center of the Galaxy, the Perseus arm, the arm of Cygnus, the arm of the Centaur and the arm of Sagittarius.
But our journey into the little Sun Orion Arm or Local Arm, shorter than the other 4.
What is the speed of rotation of our galaxy?
The results published in 2009, from tracking the brightest stars in our galaxy by a network of 10 radio telescopes, show that the level of the sun, the galaxy rotates at a speed of 254 km/s.
If the stars revolve around the center, forming a galactic arm, they do not turn in unison. The stars closest to the center buckle their lap faster than those far, like the Sun, located at about 2 / 3 of the galactic radius. This decrease in speed of revolution of the center to the edge of the galactic disk implies that the spirals are not rigid structures, but evolve over time.
How do they keep their arms while their spiral shape?
In the 1960s, Chia-Chiao Lin and Frank Shu of the Massachusetts Institute of Technology (MIT) proposed a complicated theory known as density waves. The spiral arms are "bumps" density waves, which stars accumulate temporarily as a cosmic traffic jam. The orbits of stars in a galaxy are not smooth like those of planets around the Sun. The weight distribution of a galaxy is not a point mass, it forces out to describe ellipses which do not close on themselves.

They form rosettes. This shift of the ellipses is due to galactic density waves.

Image: Orion Arm or Local Arm contains the Sun, small arms in orange in this picture. The Sun is located in the Orion Arm of our galaxy and rotates at a speed of 254 km/s around the galactic center, completing one revolution every 250 million years.