Origin of Structure
Density wave theory is the preferred explanation for the well-defined structure of grand design spirals. According to this theory, the spiral arms are created inside density waves that turn around the galaxy at different speeds from the stars in the galaxy's disk. Stars are clumped in these dense regions due to gravitational attraction towards the dense material, though their location in the spiral arm may not be permanent. When they come close to the spiral arm, they are pulled towards the dense material by the force of gravity; and as they travel through the arm, they are slowed from exiting by the same gravitational pull. This causes material to clump in the dense regions. At the center of these galaxies are super-massive black holes.
Read more about this topic: Grand Design Spiral Galaxy
Famous quotes containing the words origin of, origin and/or structure:
“In the woods in a winter afternoon one will see as readily the origin of the stained glass window, with which Gothic cathedrals are adorned, in the colors of the western sky seen through the bare and crossing branches of the forest.”
—Ralph Waldo Emerson (1803–1882)
“The origin of storms is not in clouds,
our lightning strikes when the earth rises,
spillways free authentic power:
dead John Brown’s body walking from a tunnel
to break the armored and concluded mind.”
—Muriel Rukeyser (1913–1980)
“What is the structure of government that will best guard against the precipitate counsels and factious combinations for unjust purposes, without a sacrifice of the fundamental principle of republicanism?”
—James Madison (1751–1836)