galaxies cities of stars. characteristics of galaxies most stars reside in dense groups known as...
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Galaxies
Cities of Stars
Characteristics of Galaxies• Most stars reside in dense groups known as
galaxies. • These 'island universes' come in many shapes
and sizes, and contain anywhere from a few million stars to a trillion or more.
• astronomers have found that galaxies come in many shapes, sizes, and colors.
• The universe may contain 100 billion galaxies or more.
• Galaxies also contain vast clouds of gas and dust, which are the raw materials for new stars.
• Galaxies also contain vast quantities of "dark matter" -- matter that produces no detectable light or other form of energy, but that reveals its presence through its gravitational pull on the visible stars and gas.
• In theory, if the universe lasts long enough, the galaxies will die. Their stars will burn out. • Some of the stars will drift away, but
some will fall into giant "black holes" that lurk in the hearts of most galaxies. • Eventually, all galaxies will disappear
from sight.
Types of Galaxies• Galaxies can be classified in several
ways. The most common is a system developed by Edwin Hubble, which is based on the shapes of galaxies
Spirals• Spirals are flat disks of stars with
bright bulges in their centers. Spiral arms wrap around these bulges.• In some spirals, a wave organizes the
stars in the center into a bar. The arms of barred galaxies spiral outward from the ends of the bar.
Elliptical• they look like fat, fuzzy footballs. Instead of
spreading out into a thin disk, as they do in spirals, the stars in ellipticals wrap completely around the galaxy's heart in all directions.
• most of them appear to contain "supermassive" black holes at their hearts
• The largest galaxies in the universe are giant ellipticals
Irregulars• The final class of galaxies contains a
hodge-podge of shapes -- anything that looks neither spiral nor elliptical.
• These are the irregulars. • These galaxies have no identifiable form. • Their stars, gas, and dust spread
randomly. • These are the smallest galaxies
Our Galaxy: The Milky Way• The Milky Way consists of a bulge of
stars in the core, a thick bar of stars flanking the core, and bright spiral arms around the core• The Milky Way is a spiral galaxy• Our solar system is about 27,000
light-years from its center.• 100,000 light years in total diameter
The Milky way…
• Milky Way is part of a cluster of galaxies called the local group. • A group of 22 galaxies make up
this cluster• Includes the Andromeda Galaxy
Components….• Our galaxy is full of dust. • Elements like silicon, carbon, and iron are
forged in the cores of stars and released into space late in the stars' lives.
• The dust is concentrated in the Milky Way's disk.
Structure…• Disk contains between 200 billion and 400
billion stars. • A black hole perhaps four million times as
massive as the Sun sits in the middle of the Milky Way.
Where are we? • Our solar system is located in the outer
reaches of one of the spiral arms. • It takes approximately 200-250 million
years for our solar system to orbit once around the Milky Way.
• There are no photos of the Milky Way galaxy taken from outside. That would require a space probe to be able to get beyond the galaxy, but that hasn't yet been done. • What do you think is the most common
type of galaxy?• What do you think is the brightest type
of galaxy?
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