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