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ASTRO 101
Principles of Astronomy
Instructor: Jerome A. Orosz
(rhymes with
“boris”)
Contact:
• Telephone: 594-7118
• E-mail: [email protected]
• WWW:
http://mintaka.sdsu.edu/faculty/orosz/web/
• Office: Physics 241, hours T TH 3:30-5:00
Text:
“Discovering the Essential Universe,
Fifth Edition”
by
Neil F. Comins
Course WWW Page
http://mintaka.sdsu.edu/faculty/orosz/web/ast101_fall2013.html
Note the underline: … ast101_fall2013.html …
Also check out Nick Strobel’s Astronomy Notes:
http://www.astronomynotes.com/
Fall 2013
No appointment needed!
Just drop by!
Where: Room 215, physics-astronomy building (PA-215).
When: All semester long, at the following days and times:
• Monday:
12 – 2 PM; 5 – 6 PM
• Tuesday:
12 – 2 PM; 5 – 6 PM
• Wednesday: 12 – 2 PM; 5 – 6 PM
• Thursday: 1 – 2 PM; 3 – 6 PM
Exam 1:
• N=61 (4 missing)
• Average = 59.4
• low = 27.5, high = 99
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•
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•
•
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A
AB+
B
BC+
C
CD
F
90%--100%
85%--89%
80%--84%
75%--79%
70%--74%
65%--69%
60%--64%
50%--59%
40%--49%
0%--39%
The Celestial Sphere
• The celestial poles and the celestial equator
are the same for everyone.
• The zenith and the horizon depend on where
you stand.
http://www.astronomynotes.com/nakedeye/s4.htm
Venus in the Geocentric View
• Venus is always
close to the Sun on
the sky, so its
epicycle restricts its
position.
• In this view, Venus
always appears as a
crescent.
Venus in the Heliocentric View
• In the heliocentric
view, Venus orbits
the Sun closer than
the Earth does.
• We on Earth can see
a fully lit Venus
when it is on the far
side of its orbit.
Venus in the Heliocentric View
• The correlation between
the phases and the size
is accounted for in the
heliocentric view.
Homework/Announcements
• Homework due Tuesday, October 8: Question
5, Chapter 4 (Describe four methods for
discovering exoplanets)
Coming Up:
• The 4 forces of Nature
• Energy and the conservation of energy
• The nature of light
– Waves and bundles of energy
– Different types of light
• Telescopes and detectors
• Spectra
– Emission spectra
– Absorption spectra
The spectrum
• Definition and types:
– Continuous
– Discrete
• The spectrum and its uses:
– Temperature
– Chemical composition
– Velocity
The spectrum
• A graph of the intensity
of light vs. the color (e.g.
the wavelength,
frequency, or energy) is
called a spectrum.
• A spectrum is probably
the single most useful
diagnostic tool available
in Astronomy.
The spectrum
• A spectrum can tell us about the temperature and
composition of an astronomical object.
• There are two types of spectra of concern here:
 Continuous spectra (the intensity varies smoothly
from one wavelength to the next).
 Line spectra (there are discrete jumps in the intensity
from one wavelength to the next).
The spectrum
• Continuous spectrum.
• Discrete or line spectra.
Images from Nick Strobel (http://www.astronomynotes.com)
Thermal Spectra
• The most common type of continuous spectrum
is a thermal spectrum.
• Any dense body will emit a thermal spectrum of
radiation when its temperature is above “absolute
zero”:
– The “color” depends only the temperature;
– The total intensity depends on the temperature and
the size of the body.
• This type of radiation is often called “black
body” radiation.
Thermal Spectra
• The most common type of continuous spectrum
is a thermal spectrum.
Black body radiation
• Sample spectra from black
bodies of different
temperatures. Note that the
area under the curves is
largest for the hottest
temperature.
• There is always a welldefined peak, which crudely
defines the “color”. The
peak is at bluer wavelengths
for hotter temperatures.
Important points
• The luminosity (energy loss per unit time)
of a black body is proportional the surface
area times the temperature to the 4th power:
• Hotter objects have higher intensities (for a
given area), and larger objects have higher
intensities.
Important points
• The peak of the spectrum is inversely
proportional to the temperature (hotter
objects are bluer):
• Hotter objects are bluer than cooler objects.
How light interacts with matter
and
the line spectrum.
What Things Are Made Of
• Atoms are the basic
blocks of matter.
• The number of protons
determines the element.
For example hydrogen
has 1 proton, helium has
2 protons, uranium has
92 protons, etc.
• In most cases, the
number of electrons
equals the number of
protons.
How Light Interacts with Matter.
• Atoms are the basic
blocks of matter.
• In the 1850s, it was
discovered that an
element, when burned,
gave off a unique
emission line spectrum.
How Light Interacts with Matter.
• An electron will interact with a photon.
• An electron that absorbs a photon will gain
energy.
• An electron that loses energy must emit a
photon.
• The total energy (electron plus photon) remains
constant during this process.
How Light Interacts with Matter.
• Electrons bound to
atoms have discrete
energies (i.e. not all
energies are allowed).
• Thus, only photons of
certain energy can
interact with the
electrons in a given
atom.
How Light Interacts with Matter.
• Electrons bound to
atoms have discrete
energies (i.e. not all
energies are allowed).
• Thus, only photons of
certain energy can
interact with the
Image from Nick Strobel (http://www.astronomynotes.com)
electrons in a given
atom.
How Light Interacts with Matter.
• Electrons bound to
atoms have discrete
energies (i.e. not all
energies are allowed).
• Each element has its
own unique pattern of
energies.
How Light Interacts with Matter.
• Electrons bound to
atoms have discrete
energies (i.e. not all
energies are allowed).
• Each element has its
own unique pattern of
energies, hence its
own distinct line
spectrum.
Image from Nick Strobel (http://www.astronomynotes.com)
How Light Interacts with Matter.
• An electron in free space can have any energy.
– It can absorb a photon of any energy
– It can lose any amount of energy (E) by emitting a photon
with energy equal to E
• An electron in an atom can only have very specific
values of its energy E1, E2, E3, … EN)
– The electron can absorb a photon with an energy equal to (E1E2), (E1-E3), (E2-E3), … and jump to a higher level
– The electron can lose an amount of energy equal to a change
between levels (E1-E2), (E1-E3), (E2-E3), … and move down to
a lower level
How Light Interacts with Matter.
• An electron in an atom can only have very specific values
of its energy E1, E2, E3, … EN)
– The electron can absorb a photon with an energy equal to (E1-E2),
(E1-E3), (E2-E3), … and jump to a higher level
– The electron can lose an amount of energy equal to a change
between levels (E1-E2), (E1-E3), (E2-E3), … and move down to a
lower level
• Since each element has its own unique sequence of energy
levels (E1, E2, E3, … EN), the differences between the
levels are also unique, giving rise to a unique line spectrum
Emission spectra
and
absorption spectra.
Emission and Absorption
• If you view a hot gas against a dark background,
you see emission lines (wavelengths at which
there is an abrupt spike in the brightness).
Emission and Absorption
• If you view a continuous spectrum through cool
gas, you see absorption lines (wavelengths where
there is little light).
Emission and Absorption
Image from Nick Strobel (http://www.astronomynotes.com)
The spectrum
• View a hot, dense
source, get a continuous
spectrum.
• View that hot source
through cool gas, get an
absorption spectrum.
• View that gas against a
dark background, get
emission spectrum.
The spectrum
• View a hot, dense
source, get a continuou
s spectrum.
• View that hot source
through cool gas, get an
absorption spectrum.
• View that gas against a
dark background, get
emission spectrum.
Tying things together:
• The spectrum of a star is approximately a
black body spectrum.
 Hotter stars are bluer, cooler stars are redder.
 For a given temperature, larger stars give off
more energy than smaller stars.
• In the constellation of
Orion, the reddish star
Betelgeuse is a relatively
cool star. The blue star
Rigel is relatively hot.
Tying things together:
• The spectrum of a star is approximately a
black body spectrum.
 Hotter stars are bluer, cooler stars are redder.
 For a given temperature, larger stars give off
more energy than smaller stars.
 However, a closer look reveals details in the
spectra…
The Line Spectrum
• Upon closer
examination, the
spectra of real
stars show fine
detail.
• Dark regions
where there is
relatively little
light are called
lines.
The Line Spectrum
• Today, we rarely
photograph spectra, but
rather plot the intensity vs
the wavelength.
• The “lines” where there is
relatively little light show
up as dips in the curves.
The Line Spectrum
• Today, we rarely
photograph spectra, but
rather plot the intensity vs
the wavelength.
• The “lines” where there is
relatively little light show
up as dips in the curves.
• These dips tell us about
what elements are present
in the star!
Atomic Fingerprints
• Hydrogen has a
specific line spectrum.
• Each atom has its own
specific line spectrum.
Atomic Fingerprints
• These stars have
absorption lines with
the wavelengths
corresponding to
hydrogen!
Atomic Fingerprints
• The Sun (and other stars) have absorption lines
with the wavelengths corresponding to iron.
Atomic Fingerprints.
• One can also look at the
spectra of other objects
besides stars, for example
clouds of hot gas.
• This cloud of gas looks
red since its spectrum is a
line spectrum from
hydrogen gas.
The Doppler Shift: Measuring
Motion
• If a source of waves is
not moving, then the
waves are equally
spaced in all
directions.
The Doppler Shift: Measuring
Motion
• If a source of waves is
moving, then the
spacing of the wave
crests depends on the
direction relative to
the direction of
motion.
The Doppler Shift: Measuring
Motion
• Think of sound waves from a fast-moving car, train,
plane, etc.
 The sound has a higher pitch (higher frequency) when the
car approaches.
 The pitch is lower (lower frequency) as the car passes and
moves further away.
The Doppler Shift: Measuring
Motion
• If a source of light is
moving away, the
wavelengths are
increased, or
“redshifted”.
The Doppler Shift: Measuring
Motion
• If a source of light is
moving closer, the
wavelengths are
shortened, or
“blueshifted”.
The Doppler Shift: Measuring
Motion
• The size of the
wavelength shift
depends on the relative
velocity of the source
and the observer.
The Doppler Shift: Measuring
Motion
• The size of the wavelength
shift depends on the relative
velocity of the source and the
observer.
• The motion of a star towards
you or away from you can be
measured with the Doppler
shift.
Using a Spectrum, we can…
• Measure a star’s temperature by measuring
the overall shape of the spectrum
(essentially its color).
• Measure what chemical elements are in a
star’s atmosphere by measuring the lines.
• Measure the relative velocity of a star by
measuring the Doppler shifts of the lines.
Next:
Comparative Planetology
• Outline and introduction to the Solar System
• Planets around other stars