Transcript November 2008 - Otterbein University

Welcome to
Starry Monday at Otterbein
Astronomy Lecture Series
-every first Monday of the monthNovember 3, 2008
Dr. Uwe Trittmann
Today’s Topics
• Galaxies – Island Universes
• The Night Sky in November
Galaxies – Island Universes
• A historic tour of the discovery of the
dwindling significance of humans in the
universe:
• From the center of the universe towards the
edge of in an average galaxy amongst 100
billion others
How do we know where we are?
• “Obviously” we are living on a flat Earth at
the center of the universe, as a quick look
tells us:
–
–
–
–
The stars, Sun, Moon and planets rotate us
There is no apparent curvature of the ground
The Milky Way is a band that surrounds us
There are no signs for any movement of the
Earth (like wind, or forces throwing us off)
Science to the Rescue
• How do we avoid these wrong conclusions?
– The data were sound
– The interpretation not
Further observations are necessary to decide!
• Do we have to question everything?
– Yes, in principle.
– The signature of genius is to ask the right
question, not necessarily to answer them.
Exploring our own Island Universe:
The Milky Way
• A galaxy is a
huge
collection of
stars, gas,
dust, neutron
stars, and
black holes,
isolated from
others and
held together
by gravity
Our view of the
Milky Way
• Appears as a milky band of
light across the sky
• A small telescope reveals
that it is composed of many
stars (Galileo again!)
• Our knowledge of the Milky
Way comes from a
combination of observation
and comparison to other
galaxies
How do we know?
Obviously a bogus picture of our milky way!
• Question: How can we say anything about
our Milky Way, if we cannot see it from
outside?
Enter the Genius
• William Herschel (XVIII century)
• Simple model:
– Assumed all stars have the same
absolute brightness
– Counts stars as a function of
apparent magnitude
– Brighter stars closer to us; fainter
stars further away
– Cut off in brightness corresponds to
a cut off at a certain distance.
• Conclusion: there are no stars
beyond a certain distance
Herschel’s Findings
• Stars thinned out very fast at right angles to Milky Way
• In the plane of the Milky Way the thinning was slower
and depended upon the direction in which he looked
• Flaws:
– Observations made only in visible spectrum
– Did not take into account absorption by interstellar gas and
dust
Discovering other Island Universes
• Data: Lots of nebulous spots known in the
nightsky
• Questions: What are they? All the same?
Different things?
• Need more observations!
 Build bigger telescopes
Famous Telescopes - Herschel
Herschel detected Uranus (1781)
(Uranus is visible with the unaided eye)
Famous Telescopes – Lord Ross
• 72 inch Reflector
• built during potato famine in Ireland
• Largest Telescope until Mt Wilson
(1917)
The first nebula discovered to have
spiral structure: M51
Lord Rosse, 1845
HST: M51 Spiral Galaxy
M99 is a
spiral, too!
• Q: do we live in a
spiral?
• Q: Are we in the
center of the
spiral?
• Philosophical
answer: probably
not!
Enter: next genius
• Harlow Shapley used variable
stars, e.g. RR Lyrae stars, to
map the distribution of
globular clusters in the galaxy
• Found a spherical distribution
about 30 kpc (30,000 pc)
across
– This is the true size of the
galaxy
• Sun is (naturally!) not at the
center – it’s about 26,000 ly
out
Standing on the shoulders of Giants
• Shapley used methods developed by others
to measure the distance to globulars
• Cepheid variables show luminosity-period
correlations discovered by Henrietta Leavitt
• Shapley single-handedly increase the size of
the universe tenfold!
Cepheids
• Henrietta Leavitt (1908) discovers the
period-luminosity relationship for
Cepheid variables
• Period thus tells us luminosity, which
then tells us the distance
• Since Cepheids are
brighter than RR Lyrae,
they can be used to
measure out to further
distances
Properties of Cepheids or: How do I
know if this star is a Cepheid?
• Period of pulsation: a few days
• Luminosity: 200-20000 suns
• Radius: 10-100 solar radii
Properties of RR Lyrae Stars or: How do
I know if this star is of RR Lyrae type?
• Period of pulsation: less than a day
• Luminosity: 100 suns
• Radius: 5 solar radii
Cepheids and RR Lyrae: Yard-Sticks
• Normal stars undergoing a
phase of instability
• Cepheids are more massive
and brighter than RR Lyrae
• Note: all RR Lyrae have the
same luminosity
• Apparent brightness thus
tells us the distance to
them!
– Recall: B  L/d2
Scientists are humans - Humans err
• Shapley was so convinced by his own findings
that he held the opinion that our galaxy IS the
universe
• He even won the great debate (1920) against
Heber Curtis who correctly held the opposing
view
• See: http://www.aip.org/history/cosmology/ideas/island.htm
Hubble supersedes Shapley
• Edwin Hubble identified single stars in the
Andromeda nebula (“turning” it into a
galaxy)
• Measured the distance to Andromeda to be
1 million Ly (modern value: 2.2 mill. Ly)
• Conclusion: it is 20 times more distant than
the milky way’s radius  Extragalacticity!
 Shapley’s theory falsified!
New Tools –
New Discoveries
Hubble in prime focus of
Mt Palomar.
Einstein visits Mt Wilson
Hubble detected the Expansion of the Universe
 “Proof” of Einstein’s General Relativity Theory
Structure of the Galaxy
Q: How many galaxies are there?
• Hubble Deep Field
Project
– 100 hour exposures
over 10 days
– Covered an area of
the sky about 1/100
the size of the full
moon
• Probably about 100
billion galaxies
visible to us!
• About
1,500
galaxies in
this patch
alone
• Angular
size ~ 2
minutes of
arc
Other Galaxies
• there are ~ 100 billion galaxies in the observable
Universe
• measure distances to other galaxies using the periodluminosity relationship for Cepheid variables
• Type I supernovae also used to measure distances
– Predictable luminosity – a standard candle
• Other galaxies are quite distant
– Andromeda (M31), a nearby (spiral) galaxy, is 2 million
light-years away and comparable in size to Milky Way
• “Island universes” in their own right
Q: How does our galaxy look like
from the outside?
• Probably like others, so observe them!
Hubble Classification Scheme
• Edwin Hubble (~1924) grouped galaxies
into four basic types:
–
–
–
–
Spiral
Barred spiral
Elliptical
Irregular
• There are sub-categories as well
Spirals
(S)
• All have disks, bulges, and halos
• Type Sa: large bulge, tightly wrapped, almost circular
spiral arms
• Type Sb: smaller bulge, more open spiral arms
• Type Sc: smallest bulge, loose, poorly defined spiral arms
Barred Spirals (SB)
• Possess an elongated “bar” of stars and
interstellar mater passing through the center
Elliptical (E)
•
•
•
•
No spiral arms or clear internal structure
Essentially all halo
Vary in size from “giant” to “dwarf”
Further classified according to how circular
they are (E0–E7)
S0/SB0
• Intermediate between E7 and Sa
• Ellipticals with a bulge and thin disk, but no
spiral arms
Q: How do we know we live in a
Spiral Galaxy?
• After correcting for absorption by dust, it is possible
to plot location of O- and B- (hot young stars) which
tend to be concentrated in the spiral arms
• Radio frequency observations reveal the distribution
of hydrogen (atomic) and molecular clouds
• Evidence for
– galactic bulge
– spiral arms
Rotation of the Galaxy
• Stars near the center
rotate faster; those near
the edges rotate slower
(Kepler)
• The Sun revolves at
about 250 km/sec
around the center
• Takes 200-250 million
years to orbit the
galaxy – a “galactic
year”
How do spiral arms persist?
 Why don’t the “curl up”?
“Spiral Density Waves”
• A spiral
compression wave
(a shock wave)
moves through the
Galaxy
• Triggers star
formation in the
spiral arms
• Explains why we
see many young hot
stars in the spiral
arms
Shock Waves
The Mass of the Galaxy
• Can be determined using Kepler’s 3rd Law
– Solar System: the orbital velocities of planets determined by
mass of Sun
– Galaxy: orbital velocities of stars are determined by total
mass of the galaxy contained within that star’s orbit
• Two key results:
– large mass contained in a very small volume at center of our
Galaxy
– Much of the mass of the Galaxy is not observed
• consists neither of stars, nor of gas or dust
• extends far beyond visible part of our galaxy (“dark
halo”)
The Missing Mass Problem
• Dark Matter is dark at all wavelengths, not just visible
light
• The Universe as a whole consists of up to 25% of Dark
Matter!  Strange!
• What is it?
–
–
–
–
–
Brown dwarfs?
Black dwarfs?
Black holes?
Neutrinos?
Other exotic subatomic particles?
• Actually: Most of the universe (70%) consists of Dark
Energy  Even stranger!
Missing Mass Problem
Galaxy Masses
• Rotation
curves of
spiral galaxies
comparable to
milky way
• Masses vary
greatly
The Night Sky in November
• The sun is past autumn equinox -> longer nights!
• Autumn constellations are coming up: Cassiopeia,
Pegasus, Perseus, Andromeda, Pisces
 lots of open star clusters!
Moon Phases
• Today (Waxing Crescent)
• 11 / 5 (First Quarter Moon)
• 11 / 13 (Full Moon)
• 11/19 (Last Quarter Moon)
• 11/ 27 (New Moon)
Today
at
Noon
Sun at
meridian,
i.e.
exactly
south
10 PM
Typical
observing
hour, early
November
Uranus
Neptune
Star
Maps
Celestial
North Pole –
everything
turns around
this point
Zenith – the
point right
above you &
the middle of
the map
40º
90º
West
The summer
triangle
lingers on
…
Due
North
Big Dipper
points to the
north pole
High up – the
Autumn
Constellations
• W of
Cassiopeia
• Big Square
of Pegasus
• Andromeda
Galaxy
Andromeda
Galaxy
• “PR” Foto
• Actual look
SouthEast
Perseus,
Auriga &
Taurus
with Plejades
and the
Double
Cluster
SouthWest –
2006
• Planets
– Uranus
– Neptune
• Zodiac:
– Capricorn
– Aquarius
SouthWest –
2007
• Planets
– Uranus
– Neptune
• Zodiac:
– Capricorn
– Aquarius
South –
2008
• Planets
– Uranus
– Neptune
• Zodiac:
– Capricorn
– Aquarius
Mark your Calendars!
• Next Starry Monday: February 2, 2009, 7 pm
(this is a Monday
• Observing at Prairie Oaks Metro Park:
– Friday, January 30, 7:00 pm
"The Journey to Palomar" airs on PBS on November 10, 2008
Web pages:
– http://www.otterbein.edu/dept/PHYS/weitkamp.asp (Obs.)
– http://www.otterbein.edu/dept/PHYS/ (Physics Dept.)
)
Mark your Calendars II
•
•
•
•
Physics Coffee is every Wednesday, 3:30 pm
Open to the public, everyone welcome!
Location: across the hall, Science 244
Free coffee, cookies, etc.
Famous Telescopes – Mt Palomar
• 5 Meter Telescope – Huge and heavy mirror
• On Mt. Palomar in California
Visiting
Mount
Palomar
• November 2005
Model of the 200” Telescope
Transport of the
Mirror ~ 1945