ISP205 Lecture #26, April 10, 2001
- Announcements:
- Review:
- Properties of the Milky Way (example, picture)
- spiral galaxy
- "top" view: spiral arms and bulge
- "side" view: disk, bulge, and halo
- has about 200 billion stars
- diameter: 100,000 Ly
- location of the sun: 26,000 Ly away from the center
in a
secondary spiral arm
- Most of our Galaxy is made of unknown dark matter.
From
the observed motion of stars one can infer that there has to
be more mass than we can see.
Some suggestions:
- Brown dwarfs (found to contribute but less than
half !)
(also called Massive Compact Halo Objects - MACHOS)
- Weakly interacting massive particles (WIMPS)
- Cold neutrinos (if they have some mass)
- The Disk:
- Spiralarms are density waves where stars move
through
- Lots of gas and dust , starformation, young stars,
blue
- The Halo
- No Gas and Dust, old stars, red
- Globular clusters and high velocity stars
- Bulge Mixed: old and young stars star
- Galactic center
-
- Center is in the direction of the constellation
Sagittarius
- Strong radio emission from galactic center
(Sagittarius A)
(pictures)
- Sagittarius A is smaller than Jupiter's orbit
(10 AU) and from
Keplers law one finds it is as massive as 2.3 million solar
masses
Best Theory: black hole
- Classification of Galaxies: (scheme)
- Spirals (barred and unbarred)
- Ellipticals
- Irregulars
- Distances to other Galaxies
- Next neighbours:
- Small and Large Magellanic Clouds (irregular)
(pictures)
~200,000 Ly away
- Andromeda (spiral)
~2.2 million Ly
- Closest galaxy is the newly discovered
"Sagittarius Dwarf", which is located inside the
Milky way galaxy (picture)
- Expansion of the Universe
- 1914 Vesto Slipher discovered that absorption lines of
"nebulae"
(galaxies) are shifted towards the red.
According to Doppler theory this means that they are moving away
from us at great speeds.
- This shift is called redshift
- 1930 E. Hubble and M. Humanson discovered that
redshifts
are proportional to the distance in all directions of the sky.
(picture)
Hubble Law: velocity = H * distance
H: Hubble constant. Current value 20 km/s/Mly
- Redshifts that correspond to galaxies moving away from
us with
90% of the speed of light have been measured.
- Interpretation: Galaxies are not actually moving, but
the Universe
expands, therefore space expands, therefore distances become larger
with time.
(example)
This explains:
- From every point in the universe other galaxies
seem to move away
in all directions - Milky way is not special
- Hubble law
- Redshift: light is streched as it moves through
expanding space
(so it's not the Doppler effect)
- The expanding universe was a result of Einsteins theory
of
relativity, but he could not believe it and introduced a cosmological
constant to keep the universe static.
- In the 90's the cosmological constant has been
revived, but acting
the opposite way and accelerating the universe (more later ...)
- Large scale structure of the Universe
- Galaxy Clusters
- Galaxies come in clusters with 10-10,000 galaxies
- Our cluster: Local Group (picture)
40 members, 3 Mio Ly across
- Other clusters: Virgo, Coma (250 Mio Ly away) (picture)
- Most of the matter in clusters is invisible dark
matter
- Super Clusters
- Galaxy clusters form Superclusters 100 Mio Ly large
- Superclusters are filament structures seperating
great
voids (Swiss cheese model)
- Our supercluster is the "Local Supercluster"
(picture)
- Others: Great Wall's (picture)
- Our Motion in the Universe - or where are we going ?
- Motions can be determined from redshifts when
the Hubble expansion is subtracted
- The earth moves around the sun with 67,000 mph
(18 miles/s)
- The sun moves around center of Milky Way
with 140 miles/s
- Our Milky Way moves with 80 miles/s towards
Andromeda
- Crash: in ~5 billion years (pictures)
- Crash will last billions of years
- The Local Group moves with 150 miles/s towards
the Virgo Cluster
- The Local Supercluster moves together with other
Superclusters towards a mysterious dark region.
called the Great Attractor (pictures)
Speed: 300-400 miles/s
- Absolute speed of sun has been measured against
microwave background: 400 miles/s
- Active Galaxies
- There is a Radio source at the center of our Galaxy
other Galaxies are way more active
- Quasars: (picture sky 26.1)
- 1960 Stars were found that emitted radiowaves and
showed
unknown emission lines.
- 1963 Maarten Schmidt proposed that emission lines
are
tremendously redshifted hydrogen lines.
- Very different from star, therefore: "Quasi
stellar Object" or Quasar
- Redshift implies extreme distances of billions of
Ly
(at the edge of the universe - or in other words very young
objects formed shortly after big bang)
- Distance implies brightness of 1000 times the Milky
Way
- Quasars vary in brightness within a few month - the
energy
source must therefore be only a few light months across.
- Theory: Quasars are galaxies with a supermassive
black hole
in the center that produces energy by sucking up gas, dust, and
stars. Evidence:
- HST sees Quasars in Galaxies
(picture)
- Quasars are typically part of a group of
galaxies
- Only a black hole can create sufficient amount
of
energy on such a small volume.
- Explains large distances naturally:
Shortly after formation of Galaxies there was more gas
and dust in the centers to power quasars - thats why
we see them only far away (back in time)
- Other types:
- Seyfert Galaxies (pictures)
Like Quasars (energy source in center, emission lines)
but fainter and closer
- Radio Galaxies (pictures)
Active galaxies that are especially bright in radio radiation.
Radioemissioncomes mainly from "Lobes" - Jet's that
hit surrounding material
- Quasar with Jet: (picture)
- Gravitational Lensing (picture)
- First discovery 1979: double quasar