Transcript astrocoursespring2012lec4

IV Spring 2012 Astronomy Course
Mississippi Valley Night Sky Conservation
The Sky Around Us
Program developed by
Mississippi Valley Conservation Authority
Royal Astronomical Society of Canada
Ottawa Astronomy Friends
Instructors:
Pat Browne
Stephen Collie
Rick Scholes
Course Assistant:
Amy Booth
Special Treat:
AstroPhotography
Presentation,Sanjeev Sivalvurasa
…
Agenda –
Nightsky Around us = Moonlight
Lecture/Observations
Field trip to FLO – weather permitting
Earth Centered Universe software
for illustrations –
courtesy David Lane
Review Last Week
III Sky Around Us
The Sky around us … stars
Stellar evolution and distances via
Absolute magnitude and stellar spectra
on the main sequence
Lunar Study – Courtesy Stephen Collie
Followed by Lunar and Double Star
Observing – ‘Running a telescope’
using the red dot, the finder and the
main eyepieces
(at different magnifications)
Doubles – Polaris, Castor, ,Algeib
Alkaid
Follow the Arc to Arcturus
and Speed on to Spica…
(diagram courtesy Helen
Sawyer Hogg).
Lunar Exercise – How To…
Lunar Exercise How to
Lunar Certificate
program provides
a
handle on how
to study the moon
Observing out the
Summer Kitchen
… or on the
Balcony
http://millstone.typepad.com/files/lunarsampleexercises.doc
First Experience really observing the moon.
Location: ‘Summer Kitchen’ – peering out at the first quarter moon… just
peak
Equipment: 6” F8 Dobsonian using a 24 to 8mm zoom eyepiece providin
60 to 120 power
Camera and sketch activity.
I was really taken by the southern pole at first quarter, the deep brooding
shadows with little detail and all shadow. This will be hard to ident
So I did look at the overall view – for this I needed to switch to the 40 mm
(30 x) eyepiece so that the exit pupil of light going into the camera
made a complete and easy shot.
Hint – Full view with camera – use low power
Points to Ponder 1 On Earth as in Heaven…
Responsible Lighting
is ECONOMIC
Uniformity of lighting in the town  3:1
Illuminance level in urban areas.
Maintain and standardize on
minimum wattage levels to do
the job.
Luminaire Saving:
Pole Spacing standardize and maintain
schedules: from 3:1 to 10:1 based on
usage (traffic, residential,etc)
Economic Saving:
Cost considerations  lower wattage,
well-placed poles:
.043$/kwh*10h*
(.2kw) *365d* 20years =
$628/pole
Reducing wattage from 200W
 100W saves the town
$315/luminaire over 20 years
For 1000 lamps (now)  =
$315000 over 20 years in
reduced wattage
In a nutshell…
Uniformity – work to the minimum difference in lighting level
Lower Wattage – work to the lowest specified lighting level
according to IESNA standards… i.e. 1.2 FootCandles for
road surfaces
Pole Spacing increase on roadways with less traffic – reduces
Pole Count of luminaires
Reduce Wattage and save Energy Costs From 200 -> 100 W
Smart Lighting - extinguish when not being used
Light the Surface not the underbellies of aircraft
Points to Ponder - II Lunar and Planetary
Planetary and Lunar:
Why do we use the temperature units – Degrees Kelvin ?
0 Deg Celsius = 273 .
Handy, units are scaled the same – just a larger offset to
work with
Small satellites and moons can survive inside their planet’s
Roche Limit because their electrochemical bonds are more
significant than their gravitational bonds
To find the critical Radius for a given body orbiting in the
Roche Limit, we compute the Radius of the Sphere which
can be held together by self- gravitation rather than internal
cohesive strength:
Why is the moon a sphere rather than the
more irregular shape of an asteroid ?
Roche Limit:
The critical distance between a planet and its satellite
below
which, the satellite is broken up by tidal forces.
In our case, the moon is not broken up by tidal forces
because it orbits outside of Earths Roche Limit.
L =2.5 Earths Radius ~ 16000km
Moon’s distance 365,447 km .
It is also held together by self-gravitation – that makes it
spherical as gravity and hydrostatic pressure shape the
body
as a sphere. However, When the orbit decays, to within
the
Roche limit, it would have to have enough self-cohesive
strength to resist tidal breakup. As the sample exercise
shows, bodies within the Roche limit (like people) are
typically not spherical, but irregularly shaped.
It is only if self-gravitation dominates over internal
cohesive
forces (tensile strength) that the body is spherical.
This implies a density (kg/m^3) , a tensile strength of
material (Kn/m^2) in order to define a critical radius.
Fgravity = Tensile Strength x Sphere
Surface Area
 Rcritical for spherical object
Points
to Ponder
- III
The Ultimate
Spectral
Distance Ladder
Beyond the solar system and the clusters…
Question Raised: What is cosmological redshift ?
It is the spectral shift in wavelength due to the velocity
of
the space-time fabric between the observer and the
distant object (galaxy).
It is a measurement of the recession velocity – a
velocity
that is not intrinsic to the motion of the object, but due
to the fact that the universe is expanding according to
Hubble’s Law:
Recessional Velocity = Hubble's constant times
distance
V = Ho D
In cosmological redshift, the wavelength at which
the
radiation is originally emitted is (only) lengthened as it
travels through (expanding) space. A Cosmological
redshift results from the expansion of space itself
and not from the motion of the object. So the
recessional
velocity is not the galaxies motion, but the motion of
space-time. This is a very special spectral shift indeed!
Night Sky IV
Clusters – Where…
From Galactic to Globular…
By now we have studied and
observed clusters within the
disk of our galaxy and in our
western sky in the Spring.
These clusters are thousand
or so light years away. They
also are known to be in the
active process of star
formation. Now it is time to
go beyond the disk of the
Milky way to observe
Globular Clusters…
Deep Sky Objects - II
Globular Cluster – What we can
glean
Since most galaxies contain globular clusters and since globular clusters
are so old, the properties of globular clusters can be used to learn about
not only the universe today, but also the universe in the past.
Using modern telescopes and computers, astronomers have studied
numerous properties of globular clusters.
Here are just some of the quantities that we can measure for globular
clusters:
•size (radius),
•mass,
•distance from galactic center,
•distance from Earth,
•brightness,
•age **
•color.
http://www.sciencebuddies.org/science-fair-projects/project_ideas/Astro_p016.shtml
WHEN: HR diagram for Globular Clusters tells us the AGE (billions of years)
Statistical Age
--- still burning
The most massive main-sequence stars will also
have the highest absolute magnitude, and these
will be the first to evolve into the giant star stage.
As the cluster ages, stars of successively lower
masses will also enter the giant star stage.
Thus the age of a single population cluster can be
measured by looking for the stars that are just
beginning to enter the giant star stage.
This forms a "knee" in the HR diagram, bending
to the upper right from the main-sequence line.
The absolute magnitude at this bend is directly
a function of the age of globular cluster, so an
age scale can be plotted on an axis parallel to
the magnitude.
By estimating from energy considerations, how
rapidly the stellar evolution takes place, we
put an age to these clusters.
Already
…
evolved
Observing Spring Globular Clusters M53, M3,
Observing = “Faint Fuzzy”
Globular Clusters: Fuzzy blobs of thousands
ofstars which are resolvable with
telecopes!!!
6. M3 (NGC 5272). Magnitude 6.3, 18.6’x18.6’, Class
VI. M3, yeah, M3. It’s a beauty, of course, but it suffers
by being in the spring sky where it must compete for
our attentions with the hordes of spring galaxies.
http://unclerods.blogspot.ca/2011/12/myfavorite-fuzzies-unks-globularstar.html
7. M53 (NGC 5024). Magnitude 7.7, 14.4’x14.4’, Class
V. If M3 is sometimes ignored, M53 is the forgotten man
of the Messier globs. It has three strikes against it. Like
M3 it is in the spring sky when amateurs tend to be
focused on intergalactic space, it’s a little lackluster at
nearly magnitude 8, and it’s somewhat hard for small
instruments to resolve at Class V. And yet, and yet… It’s
still a Messier, and that means g-o-o-d. If nothing else,
this one provides a welcome break when you tire of
observing yet another faint fuzzie in Coma - Virgo. You
do need 8-inches of aperture before M53 begins to look
like much, but when you have at least that you may be
surprised at how good it is.
Spring Globular Cluster M3 – What
it contains (because its sooo old)
Courtesy
Turn Left at Orion
courtesy astronomy sketch of
the day
asod.info
WHO
Globular
Clusters
– North
Helen
Sawyer
Hogg's
workEast
focused on globular
clusters, in particular the variable stars within
them. She published more than 200 papers,
the Catalogues of Variable Stars in Globular
Clusters, and a number of historical articles,
mostly in the Journal of the Royal Astronomical
Society of Canada, JRASC, and was active
member in a number of professional societies.
She also began categorizing clusters according
to
the degree of concentration the system has
toward the core. The most concentrated cluster
s were identified as Class I, with successively
diminishing concentrations ranging to Class XII.
This
became
known as the Shapley–Sawyer
A Gift
of Stars
Concentration Class.
Personal Introduction to
… personal experiences
of the Night Sky
… Sanjeev will share his
experience next…