Galaxy

• Galaxies


  A huge collection of gas, dust, and billions of stars and their solar systems, all held together by gravity
• Galaxies


  • The largest structures in the universe held by gravity
  • Some are dwarf galaxies (with a few billion stars) and giant galaxies (with trillions of stars)
  • Believed to began to form billions of years ago shortly after the Big Bang
  • Constantly evolving with collisions in between shaping their evolution
• Galaxies


  • The Milky Way galaxy containing our solar system (residing in one of its arms, the Orion Arm)
• Milky Way galaxy


  A barred spiral galaxy (due to its central bar) estimated to be 13.6 billion years old with hundreds of billions of stars, along with gas, dust, and dark matter
• Milky Way galaxy


  • Its movement is influenced by its central bar (long, elongated region of stars at its center)
  • A large portion is thought to be dark matter (a mysterious substance that does not interact with light but exerts a gravitational pull)
  • Its size is roughly 100,000 light-years across
  • Located about 25,000 light-years from the galactic center
• Spiral Galaxies


  • Have a flat disk of stars, gas, and dust with spiral arms winding outward from a central bulge where new stars are born and a halo of older stars and mysterious dark matter
• Spiral Galaxies

  • The Milky Way galaxy
  • Andromeda galaxy
• Elliptical Galaxies


  • Shaped like ovals or spheres and have little gas and dust
  • Contain old stars and are not actively forming new ones
  • Thought to form from the collisions of other galaxies
• Lenticular Galaxies


  • Similar to elliptical galaxies but they have a faint disk of stars and dust
  • Mostly contain older stars and don't actively form many new ones
  • Believed to be linked to spiral galaxies, possibly old spirals or the result of mergers
• Lenticular galaxies are less common than spiral or elliptical galaxies
• Violent collision and eventual combinations of celestial object, mostly galaxies


  Mergers
• Irregular Galaxies


  • Does not have a distinct shape
  • Often smaller than other types of galaxies and may be formed by the collision of smaller galaxies
  • Contains a mix of young and old stars and can have a lot of gas and dust which can form new stars
• Active galaxies


  Galaxies with bright centers, powered by supermassive black holes at the galaxy's core, that are much brighter than the light from all their stars
• Active galaxy formation

  Gas and dust falling into the black hole form a glowing disk and jets of particles
• Active galaxies


  • There are different types based on how they appear to us due to the viewing angle
  • Whether we see the dust torus directly or from the side
  • Some are bright in radio waves while some are not
• Seyfert galaxies


  Type of active galaxy that is less luminous than a quasar
• Carl Seyfert first identified Seyfert galaxies


  1940s
• Seyfert galaxies


  • Have bright cores and often show evidence of ongoing star formation
  • Have a supermassive black hole at their center that is accreting or feeding on surrounding gas and dust, releasing a lot of energy
  • Relatively-low energy active galaxies that appear normal in visible light but emit a lot of infrared radiation
  • Some also emit X-rays and radio waves
• Seyfert galaxy types

  • Type I: show signs of rapid motion near the black hole
  • Type II: show signs of slower motion
• Quasars


  Extremely luminous active galaxies believed to be powered by black holes billions of times more massive than the sun
• Quasars


  • Their brightness can outshine entire galaxies, though, as they are found far away, the light we see from them is from billions of years ago
  • Often found at the centers of young galaxies
  • May be fueled by the merging of galaxies, but this active phase is thought to be short-lived
• Blazars


  Type of active galaxy that has a jet pointed directly at Earth and thus appears brighter than other active galaxies
• Blazars


  • Very powerful and can be seen across a wide range of wavelengths
  • Very variable in brightness and can flare up dramatically in a short amount of time
• Pulsars


  Super-dense, rapidly spinning neutron stars that emit beams of radiation like a lighthouse due to strong magnetic fields
• Pulsar formation


  Born in galaxies from leftover cores of massive stars that explode in supernova
• Pulsar locations

  • Found scattered throughout galaxies, wherever massive stars have lived and died
  • Can be by themselves or in binary systems with companion stars
  • Most are found in the Milky Way galaxy but a few were detected in the Magellanic Cloud galaxies