Our Location in the Universe

Our Cosmic Address

• The Sun is the largest object in our solar system; its gravity keeps the solar system in orbit
• Planets in our solar system, in order of distance from the Sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune
• A moon is any object that orbits a planet, Jupiter has more than 70 moons
• Asteroids are made of metal and rock
• Comets contain large amounts of ice
• The Andromeda Galaxy, also called M31, is a spiral galaxy much like our own Milky Way Galaxy
• Earth is a planet in the Solar System, which is one of many star systems in the Milky Way Galaxy
• The Milky Way is just one of countless galaxies in the universe

Structure of the Milky way

• The Milky Way's structure includes the disk, nuclear bulge, halo, and globular clusters
• The shape of the Milky Way was once thought to resemble a grindstone, with the Sun near the center
• Interstellar dust does not absorb radio waves
• Scientists can observe any direction throughout the Milky Way at radio waves

Radio Observations and Galactic Motion

• 21-cm radio observations reveal the distribution of neutral hydrogen throughout the galaxy
• Distances to hydrogen clouds have been determined through radial-velocity measurements using the Doppler effect
• Disk stars move in nearly circular orbits within the disk of the Galaxy
• Halo stars have highly elliptical, randomly oriented orbits; these stars are not slowed by the disk as they plunge through it
• Stars in the disk follow the Galactic Center in the same direction and plane, bobbling up and down due to gravitational pull which gives the disk thickness

Galactic Orbits and Superclusters

• The sun orbits the Galactic center at 220 km/s
• One orbit takes ~240 million years
• Stars closer to galactic center orbit faster
• Stars further out orbit more slowly
• Young, metal-rich stars make up Population I, residing in the spiral arms and disk
• Population II stars are older, metal-poor, and situated in the halo (globular clusters) and nuclear bulge
• Stellar populations are still being studied; stellar ages may pose issues for current traditional theories
• Galactic fountains are produced by multiple supernovae in regions of recent star formation

Stellar Distribution and Galactic Structure

• The sun is located ~28,000 light years from the center, taking 230 million years to orbit
• Sun's distance & speed allow mass determination using Newton's law of gravity to 100 billion solar masses
• While the galaxy rotates, stars in the local solar neighborhood move randomly related to one another
• Within 500,000 ly there are 12 dwarf galaxies, each containing >10 million stars
• Dwarf galaxies orbit the Milky Way, taking billions of years for each orbit
• The local group contains 3 large galaxies: The Milky Way, Andromeda, and Triangulum; this group contains ~ 46 dwarf galaxies
• Our local group is within the Virgo Supercluster dominated by the Virgo cluster

The Universe

• Within 1 billion light years there are filaments ("sheets" and "walls") of clusters with voids in between, containing millions of galaxies
• The traditional theory states that a quasi-spherical gas cloud fragmented to form metal poor stars, which began to rotate and collapse into a disk-like structure
• Superclusters are clusters of clusters of galaxies which show alignments along walls and filaments
• Vast regions of space appears empty "voids"
• The galaxy cluster has the Milky Way, and the Andromeda galaxy and others in the Local Group

Galactic Anomolies and Oddities

• Some local group galaxies behind the center of the Milky Way are hard to observe
• Stars throughout the Milky Way showing varying metallicities, even showing high traces unexpectedly

Mass of the Milky Way

• All mass concentrated in the center shows a modified version of Kepler's 3rd law
• Most mass is not emitting any radiation
• O and B Associations trace out 3 spiral arms near the sun
• Distances to O and B associations determined using cepheid variables
• Hubble Deep Field: 10-day exposure to an apparently empty field shows many distant galaxies
• Vastly different sizes & luminosities range from lower luminosity dwarf galaxies all the way to large galaxies larger than the Milky Way

Galaxy Classifications

• Gas and dust are rich within spirals, with less in other categories.
• Collision of large with small often disrupt and swallow the smaller
• Rich: many galaxies diameter to ~ 3 mega parsec
• Poor: smaller amount with smaller diameter
• Galaxy collisions can produce weird ring shapes

Celestial Sphere and Motion

• The celestial sphere includes stars at different distances which appear on the celestial sphere
• The sun’s celestial movement is known as the ecliptic
• Latitude describes north/south, longitude east/west
• The Ptolemaic model explains the backwards motion of planets
• Stellar parallax is the position appears to shift with more distant stars