PHY55 Intro Astronomy

1
PHY55 Intro Astronomy

• Ronen Plesser plesser_at_cgtp.duke.edu
• Shangying Wang sw37_at_phy.duke.edu
• Ariana Minot ariana.minot_at_duke.edu

http//www.cgtp.duke.edu/plesser/phy55-spring08/
2
Astronomy - the Science of the Universe

• Science
• Formulate precise, universal, mathematical laws
  governing phenomena

• Verify by comparing to measurement
• Scientific Understanding depends on computation
  and experiment

The most incomprehensible thing about the
Universe is that it is comprehensible
3
Threads of Ideas

• Universe is uniform same everywhere Physical
  Laws valid at all scales
• Fundamental Laws Astronomical discoveries
• Constructing 3D universe cosmic distances
• New technology New observations
• Historical Pedagogical development

4
What do you see?

• Many, many stars
• Vary in brightness
• Scattered about sky randomly?
• Vary in color
• Vary in size. Mostly, brighter stars are bigger
• Some cloudy haze

5
How Bright?

• Hipparchus/Ptolemy (150-80BC) Brightest stars
  are first magnitude faintest are sixth
  magnitude. Magnitude m is twice as bright as
  magnitude m1
• Pogson (1856) First magnitude is 100 times
  brighter than sixth. What would Hipparchus say?
• Measuring light intensity assign numerical
  magnitude using Pogsons ratio 2.512 and a
  reference star (Vega) of magnitude m0.

• Bellatrix has mB 1.62
• Rigel has mR 0.15
• Which is brighter? By what factor?
• IB 2.512(0.15-1.62) IR 0.26 IR
• Mintaka has intensity 0.56 that of Bellatrix.
  What is its magnitude?
• mM mB - 2.5 log(0.56) 2.3

6
More complications

• Stars are different colors, so how bright they
  look depends on what color you measure. A
  standard eye measures visible intensity. Can
  measure intensity in other colors.
• On different nights, factor separating intensity
  of two stars can change because atmosphere
  becomes more or less transparent to particular
  colors. So magnitude in fact defined to
  eliminate atmosphere altogether.
• Far more complicated than Hipparchuss ideas.
  Also far more information!

7
Where is it?

• Positions in the sky are really directions. We
  do not know (yet) how far things are.
• How to specify a direction?
• In the plane - azimuth

N 0o
W 270o
E 90o
S 180o
8
Altitude/Azimuth
In 3d we need two angles to determine a
direction. A common solution is to add elevation
(altitude) or zenith angle.
9
(No Transcript)
10
Angles and Distances

• The small angle formula will be used extensively.
• If we know the angular separation of two points
  we can relate the distance from observer to their
  physical separation
• Circle of radius R has circumference 2pR
• An arc with angle a includes a fraction of this

• AB (a/360o) 2pR
• (a/57.3o) R

11
Units and Numbers

• Your textbook has the formula
• AB (a/206265) R
• Whos wrong?
• Neither. For convenience, small angles are often
  measured in units other than degrees. In
  particular, we use arcminutes and arcseconds
• 1o 60 3600
• So same angle can be represented by different
  numbers. This makes formulas confusing.

12
An example

• The full Moon has a diameter covering about 0.5o
  in the sky. The Moon is 384,400 km from Earth.
  What is the size of the Moon?
• (0.5/57.3)384400 km 3350 km
• We could also say
• 0.5o 30 1800
• (1800/206265)384400 km 3350 km
• We will introduce other units for distance, mass
  etc as we go. Make sure you remember what units
  you are using when doing a calculation!