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?