Galaxies

What is a galaxy?

• A galaxy is a very large collection of gas, dust, stars and their solar systems (which include planets, moons, asteroids, comets, etc).

Galaxies

• There are billions of galaxies in our universe.

• Small galaxies have only a few million stars.

• Larger galaxies can have 400 billion stars (or more).

• Galaxies are classified based on their shape.

Types of Galaxies

Galaxy Types

• There are 3 main types of galaxies: • Elliptical

• Spiral

• Irregular

• We can break down the 3 main types even further based on specific characteristics (i.e. shape, spiral arms, etc).

• This system was created by Edwin P. Hubble.

Spiral Galaxies

Spiral Galaxies

• Most common type of galaxy.

• The spirals are large, flat rotating disks of stars, gas, and dust.

• The bright center of a spiral galaxy is called the “galactic bulge”.

• New stars are formed in the arms of the spiral.

• A supermassive black hole may be at the center.

Elliptical Galaxies

Elliptical Galaxies

• Roughly oval shaped and 3-dimensional.

• Often found in galaxy clusters.

• Are usually smaller, but can be very large.

• Stars are often close together, which makes the center appear bright.

• Contain older, low-mass stars. Few new stars are formed in elliptical galaxies.

• May have a supermassive black hole at their center.

Irregular Galaxies

Irregular Galaxies

• Irregular in shape.

• Don’t have structure to be classified as a spiral or elliptical galaxy.

• Some may have started as a spiral or elliptical galaxy, but something (such as a collision with another galaxy) caused it to change shape.

Notable Galaxies

Milky Way

• Barred spiral galaxy

• Diameter is 100,000-120,000 light years across.

• Our solar system is 30,000 light years from the center of the Milky Way.

• Contains 200-400 billion stars

• Contains at least 100 billion planets

10 Facts About the Milky Way

Andromeda

• Spiral galaxy

• Closest major galaxy to the Milky Way

• 2.5 million light years from Earth

• 220,000 light years across

• Largest galaxy in our Local Group

• Visible to the naked eye on moonless nights

• Getting closer to Milky Way at 68 miles per second and is expected to collide in 4 billion years

If Andromeda Were Brighter, This is What You’d See

Canis Major Dwarf

• Irregular dwarf galaxy

• Closest galaxy to the Milky Way

• 25,000 light years away from our solar system

• Contains about 1 billion stars

• The Milky Way’s gravity may be pulling it apart.

Closest Galaxy Discovered

Centaurus A

• Believed to be a giant elliptical galaxy or somewhere between elliptical and spiral (called lenticular)

• Closest giant galaxy to the Milky Way

• About 10-15 million light year away

• 5th brightest galaxy in the sky, but can only be seen in the Southern hemisphere and lower Northern latitudes