ISP205 Lecture #25, April 8, 2001
- Announcements:
- Homework:
- Set 7: Due April 10, 23:59 (TODAY !!!)
- Set 8: Due April 17, 23:59
- Set 9: Not handed out yet; will be due: April 24
- No Set 10 this time (sorry :) )
- New Bonuspoint project:
Find black hole property on the Web
Address on Website
Worth 6 bonus points
- Review:
- Special Relativity
- Basic principle: Speed of light is constant
(no matter how light is moving relative to observer)
- A consequence: Moving Clocks run slower (as seen
from
observer at rest)
- Time travel is possible ! But only into the Future
...
- Special relativity is a theory that works well and
is used
everyday
by scientists.
(This does not mean it will maybe replaced with a
better theory sometimes, but the better theory has
to explain all the weird phenomena relativity
is able to explain.)
- General Relativity
- Basic principle: Equivalence principle
Acceleration generates gravity in opposite direction.
- Consequence 1: Gravity is nothing else then curved
space
- Consequence 2: The stronger the gravity the slower
run clocks
If you are watching from far away (at low
gravity), then at a
place with high gravity:
- Motion seems slower
- Light emitted seems redder
- This is most extreme as one approaches the
event horizon
of a black hole
- General relativity is a theory that works well and
is used
everyday by
scientists and rocket engineers
- Black holes
- A point of mass in space with zero size and
infinite density
- Inside the event horizon light cannot escape
- Clocks at the event horizon seem to stand still
if seen from outside (frozen astronaut)
- Do black holes exist ?
- There is no direct proof yet
- There are compact objects orbiting stars that
are more massive than 3 solar masses and can
therefore not be neutron stars (determined from orbit)
- Matter in the center of many galaxies swirls
around
a compact object of several million solar masses |
generating X-rays.
Best explanation: supermassive black hole
- Our home Galaxy
- The Milky Way (pictures)
- History of Milky Way discovery
- 1610: Galileo discoveres that it's made up
individual stars
- 1785: William Herschel discovers that our sun is
part
of a disk shaped star cloud - Milky Way is seen when
we look into the disk. (picture)
But wrong conclusion that sun is in the center.
- 1917: Harlow Shapley shows that sun is 26,000 Ly
off center
by mapping distances to 97 globular clusters
- 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
- Motion of Stars in the Galaxy
- Every object follows Keplers Law and orbits around
the
center of the Galaxy
Mass of stars within orbit = semi major axis ^ 3 / Period ^ 2
- The sun orbits around the galactic center in 225
million years
- Stars at the edge of the galaxy should move slower
the further
away they are from the center (like planets around the sun)
But they don't ! (picture)
- Conclusion: The Galaxy is much bigger than what we
see. 90%
of our Galaxy is made of some unknown, invisible dark matter
Dark matter is invisible but does not block the view to distant
objects
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)
- Spiral Structure
- Differential rotation is NOT the explanation
- it would wind
up the spiral tighter and tighter until it disappears
- Density Wave Theory: Stars and dust pass through
spiral arms on
their Kepler orbits but slow down because of higher density
- Region of high density has lots of star formation -
spirals are bright
- Spirals rotate but slower than individual stars
- Things in different parts of the Galaxy
- Disk (spiral arms)
- Lots of dust and star formation (blue)
- OB associations (young and massive stars)
- Lots of gas: HI and HII regions
- Halo
- Old Stars and globular clusters (more red)
- No gas and dust
- Halo stars seem to move fast with respect to
earth because
they are not orbiting along with us in the disk
"high velocity stars"
- Bulge
- Mixed: old and young stars star
- Center
- Center is in the direction of the constellation
Sagittarius (picture)
- 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
- Formation of the galaxy
- Contracting gas and dust cloud
- Stars from in dense regions - globular clusters
these objects from spherical halo
- Remaining dust and gas flattens into disk
ends star formation in halo (~10 billion years ago)
- Star formation continues until today in disk
- Other Galaxies
- There are billions of galaxies in the universe
(Hubble
deep field view)
- Classification: (scheme)
- Spirals (examples)
- open spirals (picture)
- S0 (mostly bulge, some spirals)
- Sa (large bulge, small spirals)
- Sb
- Sc (small bulge, large spirals)
- barred spirals
- SBa (long spirals off the bar, tightly
wound)
- SBb
- SBc (short spirals off the bar)
- Ellipticals (picture)
- E0: spherical
- ...
- E7: cigar
- Irregulars
- Not any of the above
- There are many irregulars, for example because
of
galaxy collisions (picture)
- Properties
- Spirals:
- Old and young stars
- Gas, dust, OB stars, ongoing star formation
- Most are similar in luminosity and mass
- Ellipticals:
- Most common
- No gas and dust
- Old stars
- Vary greatly in luminosity and mass
- Distances to other Galaxies
- Next neighbours:
- Small and Large Magellanic Clouds (irregulars)
(pictures)
~200,000 Ly away
- Andromeda (spiral)
~2.2 million Ly
- Size of the universe: ~13 billion Ly's
- The distance ladder:
- Parallax
- HR diagram (spectroscopic parallax)
- Variable stars (with HST up to 65 million Ly)
- Tully-Fisher relation (100 million Ly)
luminosity of spiral galaxy is related to rotational
velocity (Kepler)
- Brightness fluctuations in elliptical
galaxies (100 million Ly)
- Type Ia supernovae (8 billion Ly)
- Brightest galaxy in cluster (13 billion Ly)
- Redshift (13 billion Ly) see next lecture
- The famous Shapley Curtis debates of 1920
- Debate about size of the Universe - and whether
"Galaxies" are
fuzzy nebulae in our Galaxy or similar Galaxies than ours far away
- Harlow Shapley:
(Harvard Astronomer famous for determining
size of the galaxy)
- Thought our Galaxy is ~100,000 ly wide
(correct)
- Thought that is the whole universe and
everything else is in this
one Galaxy (incorrect)
- Heber Curtiz: (From Muskegon, Michigan)
- Thought our Galaxy is much smaller - only
20,000 ly (incorrect)
- Thought that "fuzzy nebulae" are
other Galaxies far away (correct)
- Shapleys arguments:
- Observed Novae in other Galaxies - from typical
Nova brightness
they determined the distance
- Astronomers observed that "fuzzy
nebulae" move over time - so
they have to be close
- Using Cepheid variables he estimated that the
magellanic clouds
(closest galaxies) are only 75,000 Ly away and therefore inside
our Galaxy
- What was wrong with Shapley's arguments ?
- Novae were much brighter supernoave
- Observation of moving galaxies were wrong
(nobody knows why)
- There are 2 classes of Cepheid variables - the
ones Shapley found
belong to the much brighter ones, which he did not know