Download 0708 - Astronomy

The Dynamics of Visible
Objects in the Universe
How do we measure distance of these objects ?
Distance Measure  a) Laser beam + mirrors
Earth-Moon distance was determined
Distance Measure  b) Parallax
Stellar Parallax
Caused by the heliocentric orbit of Earth
Parallax phenomenon + trigonometry 
stellar distances
This equation can be summarized as:
3.26
𝑑 𝑖𝑛 𝑙𝑦 =
or
𝑝𝑎𝑟𝑎𝑙𝑙𝑎𝑥 (𝑖𝑛 𝑎𝑟𝑐 𝑠𝑒𝑐𝑜𝑛𝑑𝑠)
(a) The geometry of stellar parallax. For
observations made 6 months apart, the baseline
is twice the Earth—Sun distance, or 2 A.U. (b)
The parallactic angle is usually measured
photographically (the shift is greatly
http://lifeng.lamost.org/courses/astrotoday/CHAISSON/AT317/HTML/AT31701.HTM
Distance Measure  b) Parallax
Stellar Parallax
D/d = angle (in radians)
Text
.7”
Distance to star = 4.4 ly
biased for “near-by” objects
D
d(istance)
Nearest Neighbors
A plot of the 30 closest
stars to the Sun,
projected so as to
reveal their threedimensional
relationships. Notice
that many are members
of multiple-star systems.
http://lifeng.lamost.org/courses/astrotoday/CHAISSON/AT317/HTML/AT31701.HTM
All lie within 4 pc (about
How do we know the dynamics of these objects ?
Phenomenon exhibited by a moving source of light:
Doppler effect
Blue shift
Red shift
Which direction is the wave source moving?
Doppler Shift Method:
Limitation:
wavelength shifts
are tiny
Doppler Shift Method:
example:
star wobbles in response to
planet orbiting it
observer
Doppler Shift of light
wavelength stretched  object
moving away the observer
wavelength compacted  object is
moving towards the observer
400 nm
800 nm
Emitted Star Light
(intrinsic)
Red Shift
Receding
Observed Star Light
(detected)
“- v”: TOWARD the observed
“+v”: AWAY from the observed
“Galaxy Crash” can be loaded
from the following website:
http://burro.cwru.edu/JavaLab/GalCrashWeb/main.html
select option “applet” and then you can specify some parameters
of the 2 galaxies whose interactions you would like to model
The Dynamics of Visible Objects
in the Universe
Key Concepts for Week-4, Class-1:
(what You need to know, as You will be tested on this material):
 Term “Proxy”
 Proxy for distance (mirrors & laser light, parallax)
 Proxy for velocity (Doppler Shift Method)
 Doppler Shift Phenomenon / Method
 Blue shift (moving towards) & Red shift (moving away)
 Atomic line spectrum (atomic fingerprint) as an aid
for applying Doppler Shift Method
 Value of Models in Astronomy
The Birth of the
Universe
Lesson 8
Edwin Hubble’s tests reveal “Steady State
Theory” of the Universe is Invalid
Proxy for velocity 
Doppler shift
Distance Measurement 
Parallax or other more
sophisticated methods
Hubble’s Law states galaxies are moving away
from us.
Proxy for velocity 
Doppler shift
Distance Measurement 
Parallax or other more
sophisticated methods
SADLY, HUBBLE DIDN’T REALIZE THE
IMPLICATION OF HIS DISCOVERY…
The Ramifications of Hubble’s Law
Monseigneur Georges Lemaître
Fall 2007
The Cosmic Microwave Background Radiation (CMB) is
Additional Evidence for the Big Bang & Expanding Universe
background temperature
of the Universe
So, there are 3 lines of evidence for the “Big
Bang” Theory
Galaxies are moving away from us
Cosmic Microwave Background Radiation
Observed abundance of Helium matches the
expectations of Helium based on the Big Bang
theory
We Can Visualize this with a Model
What are Models?
help us visualize aspects of
nature that seem fairly
abstract & build intuition
Models provide a conceptual framework for
interpreting the data we collect.
They help us understand what is happening
in the otherwise (to us) invisible world.
Exploring Hubble’s Law
Balloon Activity
Modeling the Big Bang and the Expansion of the
Universe with a Balloon
Tips before you start:
• Don’t place marks too close to the neck or far end of the balloon - it
doesn’t stretch uniformly in these areas
• Blow the balloon up as much as reasonably possible!
• Measure the distance between points along the shortest path
• When done, add your data points to the plot on the board. Each group
use a different type of symbol/color so we can distinguish the data sets.
Modeling the Big Bang and the Expansion of the
Universe with a Balloon
How is it a good model?
How is it a bad model?
Are there better models?
Modeling the Big Bang and the Expansion of the
Universe with a Balloon
How is it a good model?
-it produces an expansion law qualitatively similar to Hubble's
Law, in that the velocity of expansion (with respect to the origin) is
proportional to distance (from the origin)
- it shows that no matter where you draw the origin (i.e., no matter
your location), you will always observe that all points are moving
away from YOU
Modeling the Big Bang and the Expansion of the
Universe with a Balloon
How is it a bad model?
The balloon is finite
The surface of the balloon is only 2-dimensional + curved
The marks drawn on the balloon also expand (they shouldn’t, as
galaxies are held by gravity)
The balloon may not expand uniformly (especially if not inflated
fully)
Modeling the Big Bang and the Expansion of the
Universe with a Balloon
Another model
rising loaf of bread
better, since the raisins
(“galaxies”) don’t
expand as loaf expands
This plot can be thought of as a calibrating relationship for using
velocity as a proxy to measure distance
Velocity is proportional to distance
What does it mean that distance and velocity are proportional?
Can we trace the expansion back to the point where it began?
If the Universe is expanding, does that also mean that
the Galaxy and the Solar system are expanding?
What does it mean that distance and velocity are proportional?
balloon is stretched (multiplicatively) by some factor we need to think of the Universe the same way
Can we trace the expansion back to the point where it began?
that “point” is now the whole balloon/whole Universe... so no.
If the Universe is expanding, does that also mean that
the Galaxy and the Solar system are expanding?
no, they are bound by gravity - they represent parts of
the Universe that have been gravitationally extracted (for
now) from the initial expansion
What is the importance of Hubble’s law?
•
•
Universe expands, changes over time
Universe had a beginning in time: observable Universe is
finite – we can measure its age!
• once relationship is established, it can be used as an easy
way to measure distances to galaxies
Think to the sinusoidal wave drawn on the balloon…
wavelengths get stretched
longer wavelength radiation becomes more red
Expansion competes with gravity:
• galaxies = places where gravity “wins”
• voids = places where expansion “wins”
Galaxy voids and Galaxy Clusters
The Birth of the Universe
Key Concepts for Week-4, Class-2:
(what You need to know, as You will be tested on this material):
 Hubble’s Diagram & Hubble’s Law
 The significance of Hubble’s constant (inverse time)
 Implications behind Hubble’s Law
 Big Bang Theory
 Evidence for Big Bang Theory
 Recall modeling the expansion of the Universe with Balloons
 (recall good & bad aspects of this model).