Transcript The Milky Way Galaxy

The Milky Way Galaxy
• Contains more than 100 billion stars
• Is one of the two largest among 40 galaxies in
the Local Group
• Our Solar System is located a little more than
half way from the galactic center to the edge
of the galactic disk.
• Only one of roughly 100 billion galaxies in the
Observable Universe.
Galaxies
• Definition: a system that contains millions or
more stars.
• Galaxy clusters: groups of galaxies with more
than a few dozen members
• Supercluster: galaxies and galaxy clusters
tightly packed together.
• Classified using a diagram called “Hubble’s
Tuning Fork”
Our Cosmic Address
1.
2.
3.
4.
5.
6.
Earth (4.5 billion years old)
Solar System (4.6 billion years old)
Milky Way Galaxy (13.2 billion years old)
Local Group
Virgo Supercluster
Observable Universe (13.8 billion years old)
Milky Way Galaxy Structure
• Known to look like flat pancake with a bulge in
the center.
• Named a Spiral Galaxy because of its spiral
arms.
• Approximately 23,000 light years away from
the nuclear bulge.
Parts of the Milky Way Galaxy
• Nuclear Bulge: sphere of stars and other
material at the heart of our galaxy.
– about 4000 parsecs across (about a fifth of the
total spread of the spiral arms of the galaxy.)
– The very center of our galaxy in a super-massive
black hole.
• 3-4 million times as massive as the sun.
• Appears to be powering a bright source of radio
emmision known as Sagittarius A*.
Parts of the Milky Way Galaxy (cont.)
• Disk: Flat, rotating region of stars, dust and gas
called interstellar medium around the Nuclear
Bulge.
– Can obstruct view for observations towards the
Nuclear bulge.
– Approximately 1,000 light years in thickness and
100,000 light years across
Parts of the Milky Way Galaxy (cont.)
• Spiral Arms: well defined bands in the disk
because of brilliantly bright stars.
• Halo: spherical in shape and contains little gas,
dust, or star formation.
• Globular clusters: a large compact spherical
star cluster, typically of old stars in the outer
regions of a galaxy.
Formation of Galaxies
• A Protogalactic cloud contains Hydrogen and
Helium.
• Halo stars begin to form as the protogalactic
cloud starts to collapse.
• Conservation of angular momentum ensures the
remaining gas flattens into a disk.
• Billions of years later, the star-gas-star cycle
supports ongoing star formation within the disk.
The lack of gas in the halo prevents star
formation outside the disk.
http://scienceforkids.kidipede.com/physics/space/momentum.htm
Facts about Galaxies and the Universe
• Galaxies move apart
• Expansion began about 13.8 billion years ago
(the Big Bang)
– This is called cosmological redshift.
– Proven by observing a faint glow of radiation that
is a remnant of heat from the Big Bang.
– Early Universe = hotter and denser
– Today’s Universe = cooler and less dens because of
expansion.
Facts about Galaxies
• If you look at a galaxy that is 7 billion light
years away you are seeing that galaxy as it
appeared 7 billion years ago.
• The number of stars in the observable
universe is equal to the number of grains of
dry sand on all beaches on Earth.
• Large Magellanic Cloud and Small Magellanic
Cloud orbit the Milky Way at 150,000 light
years and 200,000 light years.
http://www.universetoday.com/22828/milky-way-collision/
Nearby Galaxies
• Andromeda Galaxy is comparable in size,
though slightly larger.
• Small galaxies (Sagittarius and Canis Major
Dwarf Galaxies) are colliding with us right
now!
– These small collisions cause ripples in our galaxy
and create new stars at points of collision.
Edwin Hubble (1889-1953)
• Studied stars and galaxies outside the Milky Way
galaxy.
• Designed a period-luminosity relation
• Proved that Andromeda is much too far away to
be part of Milky Way.
• Known for Hubble Law
• The more distant the galaxy, the faster its moving
away = greater redshift
• Galaxies are moving apart in our expanding
universe.