Transcript Chapter6-7

Chapter 6
Light and Telescopes
Outline
I. Radiation: Information from Space
A. Light as a Wave and a Particle
B. The Electromagnetic Spectrum
II. Optical Telescopes
A. Two Kinds of Telescopes
B. The Powers of a Telescope
C. Buying a Telescope
D. New-Generation Telescopes
E. Interferometry
III. Special Instruments
A. Imaging Systems
B. The Spectrograph
Outline (continued)
IV. Radio Telescopes
A. Operation of a Radio Telescope
B. Limitations of the Radio Telescope
C. Advantages of Radio Telescopes
V. Astronomy from Space
A. The Ends of the Visual Spectrum
B. Telescopes in Space
C. Cosmic Rays
Light and Other Forms of
Radiation
• The Electromagnetic Spectrum
In astronomy, we cannot perform experiments
with our objects (stars, galaxies, …).
The only way to investigate them, is by
analyzing the light (and other radiation) which
we observe from them.
Light as a Wave (1)
l
c = 300,000 km/s =
3*108 m/s
• Light waves are characterized by a
wavelength l and a frequency f.
• f and l are related through
f = c/l
Light as a Wave (2)
• Wavelengths of light are measured in units
of nanometers (nm) or Ångström (Å):
1 nm = 10-9 m
1 Å = 10-10 m = 0.1 nm
Visible light has wavelengths between
4000 Å and 7000 Å (= 400 – 700 nm).
Wavelengths and Colors
Different colors of visible light
correspond to different wavelengths.
Light as Particles
• Light can also appear as particles, called
photons (explains, e.g., photoelectric effect).
• A photon has a specific energy E,
proportional to the frequency f:
E = h*f
h = 6.626x10-34 J*s is the Planck constant.
The energy of a photon does not
depend on the intensity of the light!!!
The Electromagnetic Spectrum
Wavelength
Frequency
Need satellites
to observe
High
flying air
planes or
satellites
Optical Telescopes
Astronomers use
telescopes to gather
more light from
astronomical objects.
The larger the
telescope, the more
light it gathers.
Refracting/Reflecting Telescopes
Focal length
Focal length
Refracting
Telescope:
Lens focuses
light onto the
focal plane
Reflecting
Telescope:
Concave Mirror
focuses light
onto the focal
plane
Almost all modern telescopes are reflecting telescopes.
Secondary Optics
In reflecting
telescopes:
Secondary
mirror, to redirect the light
path towards
the back or side
of the incoming
light path.
Eyepiece: To
view and
enlarge the
small image
produced in
the focal
plane of the
primary
optics.
Seeing
Weather
conditions
and
turbulence in
the
atmosphere
set further
limits to the
quality of
astronomical
images.
Bad seeing
Good seeing
The Best Location for a
Telescope
Far away from civilization – to avoid light pollution
The Best Location for a
Telescope (2)
Paranal Observatory (ESO), Chile
On high mountain-tops – to avoid atmospheric
turbulence ( seeing) and other weather effects
Traditional Telescopes (1)
Secondary mirror
Traditional primary mirror: sturdy,
heavy to avoid distortions.
Traditional Telescopes (2)
The 4-m
Mayall
Telescope at
Kitt Peak
National
Observatory
(Arizona)
Advances in Modern Telescope Design (1)
Modern computer technology has made
possible significant advances in telescope
design:
Segmented mirror
1. Lighter mirrors
with lighter
support
structures, to be
controlled
dynamically by
computers
Floppy mirror
Adaptive Optics
Computer-controlled mirror support adjusts the mirror
surface (many times per second) to compensate for
distortions by atmospheric turbulence
Advances in Modern Telescope Design (2)
2. Simpler, stronger mountings (“Alt-azimuth mountings”)
to be controlled by computers
Examples of Modern
Telescope Design (1)
Design of the
Large Binocular
Telescope (LBT)
The control room of the
4-m Mayall Telescope
on Kitt Peak.
Examples of Modern Telescope
Design (2)
The Very Large Telescope (VLT)
8.1-m mirror of the Gemini Telescopes
The Spectrograph
Using a prism (or a grating), light can
be split up into different wavelengths
(colors!) to produce a spectrum.
Spectral lines in a
spectrum tell us about the
chemical composition and
other properties of the
observed object
Radio Astronomy
Recall: Radio waves of l ~ 1 cm – 1 m also
penetrate the Earth’s atmosphere and can be
observed from the ground.
Radio Telescopes
Large dish focuses
the energy of radio
waves onto a small
receiver (antenna)
Amplified signals are
stored in computers
and converted into
images, spectra, etc.
Radio Maps
Radio maps are
often color coded:
Like different colors in
a seating chart of a
baseball stadium may
indicate different seat
prices, …
colors in a radio
map can
indicate different
intensities of the
radio emission
from different
locations on the
sky.
Radio Interferometry
Just as for optical telescopes, the resolving power of
a radio telescope is amin = 1.22 l/D.
For radio telescopes, this is a big problem: Radio
waves are much longer than visible light
 Use
interferometry to improve resolution!
Radio Interferometry (2)
The Very
Large Array
(VLA): 27
dishes are
combined to
simulate a
large dish of
36 km in
diameter.
Even larger arrays consist of dishes spread out over the
entire U.S. (VLBA = Very Long Baseline Array) or even the
whole Earth (VLBI = Very Long Baseline Interferometry)!
The Largest Radio Telescopes
The 300-m telescope in
Arecibo, Puerto Rico
The 100-m Green Bank Telescope in
Green Bank, WVa.
Science of Radio Astronomy
Radio astronomy reveals several features,
not visible at other wavelengths:
• Neutral hydrogen clouds (which don’t emit any
visible light), containing ~ 90 % of all the atoms
in the Universe.
• Molecules (often located in dense clouds,
where visible light is completely absorbed).
• Radio waves penetrate gas and dust clouds, so
we can observe regions from which visible light
is heavily absorbed.
Infrared Astronomy
Most infrared radiation is absorbed in the lower atmosphere.
NASA infrared
telescope on Mauna
Kea, Hawaii
Infrared cameras need
to be cooled to very low
temperatures, usually
using liquid nitrogen.
However, from high
mountain tops or
high-flying air planes,
some infrared
radiation can still be
observed.
NASA’s Space Infrared
Telescope Facility (SIRTF)
Infrared light with wavelengths much longer
than visible light (“Far Infrared”) can only be
observed from space.
The Hubble Space Telescope
• Launched in 1990; maintained and
upgraded by several space shuttle
service missions throughout the
1990s and early 2000’s
• Avoids turbulence in the Earth’s atmosphere
• Extends imaging and spectroscopy to (invisible)
infrared and ultraviolet
Chapter 7
Starlight and Atoms
Outline
I. Starlight
A. Temperature and Heat
B. The Origin of Starlight
C. Two Radiation Laws
II. Atoms
A. A Model Atom
B. Different Kinds of Atoms
C. Electron Shells
III. The Interaction of Light and Matter
A. The Excitation of Atoms
B. The Formation of a Spectrum
Outline (continued)
IV. Stellar Spectra
A. The Balmer Thermometer
B. Spectral Classification
C. The Composition of the Stars
D. The Doppler Effect
E. How the Doppler Shift Works
F. Calculating the Doppler Velocity
G. The Shapes of Spectral Lines
The Amazing Power of Starlight
Just by analyzing the light received from a
star, astronomers can retrieve information
about a star’s
1. Total energy output
2. Surface temperature
3. Radius
4. Chemical composition
5. Velocity relative to Earth
6. Rotation period
Color and Temperature
Stars appear in
different colors,
from blue (like Rigel)
Orion
Betelgeuse
via green / yellow (like
our sun)
to red (like Betelgeuse).
These colors tell us
about the star’s
temperature.
Rigel
Black Body Radiation (1)
The light from a star is usually
concentrated in a rather
narrow range of wavelengths.
The spectrum of a star’s light
is approximately a thermal
spectrum called a black body
spectrum.
A perfect black body emitter
would not reflect any radiation.
Thus the name “black body”.
Two Laws of Black Body Radiation
1. The hotter an object is, the more energy it emits:
Energy Flux
F = s*T4
where
F = Energy Flux =
= Energy given off in the form of radiation, per
unit time and per unit surface area [J/s/m2];
s = Stefan-Boltzmann constant
Two Laws of Black Body Radiation
2. The peak of the black body spectrum shifts
towards shorter wavelengths when the
temperature increases.
 Wien’s
displacement law:
lmax ≈ 3,000,000 nm / TK
(where TK is the temperature in Kelvin).
The Color Index (1)
The color of a star is
measured by comparing
its brightness in two
different wavelength
bands:
The blue (B) band and the
visual (V) band.
We define B-band and Vband magnitudes just as
we did before for total
magnitudes (remember: a
larger number indicates a
fainter star).
B band
V band
The Color Index (2)
We define the Color Index
B–V
(i.e., B magnitude – V magnitude).
The bluer a star appears, the
smaller the color index B – V.
The hotter a star is, the smaller its
color index B – V.
Light and Matter
Spectra of stars are
more complicated than
pure blackbody spectra.
 characteristic
lines,
called absorption lines.
To understand
those lines, we
need to
understand
atomic structure
and the
interactions
between light
and atoms.
Atomic Structure
• An atom consists of
an atomic nucleus
(protons and
neutrons) and a
cloud of electrons
surrounding it.
• Almost all of the
mass is contained
in the nucleus,
while almost all of
the space is
occupied by the
electron cloud.
Nuclear Density
If you could fill a teaspoon
just with material as dense
as the matter in an atomic
nucleus, it would weigh
~ 2 billion tons!!
Different Kinds of Atoms
• The kind of atom
depends on the
number of protons
in the nucleus.
• Most abundant:
Hydrogen (H),
with one proton
(+ 1 electron).
• Next: Helium (He),
with 2 protons (and
2 neutrons + 2 el.).
Helium 4
Different
numbers of
neutrons ↔
different
isotopes
Electron Orbits
• Electron orbits in the electron cloud are
restricted to very specific radii and energies.
r3, E3
r2, E2
r1, E1
• These characteristic electron energies are
different for each individual element.
Atomic Transitions
• An electron can
be kicked into a
higher orbit
when it absorbs
a photon with
exactly the right
energy.
Eph = E3 – E1
Eph = E4 – E1
Wrong energy
• The photon is
absorbed, and
the electron is in
an excited state.
(Remember that Eph = h*f)
• All other photons pass by the atom unabsorbed.
Kirchhoff’s Laws of Radiation (1)
1. A solid, liquid, or dense gas excited to emit
light will radiate at all wavelengths and thus
produce a continuous spectrum.
Kirchhoff’s Laws of Radiation (2)
2. A low-density gas excited to emit light will
do so at specific wavelengths and thus
produce an emission spectrum.
Light excites electrons in
atoms to higher energy states
Transition back to lower states
emits light at specific frequencies
Kirchhoff’s Laws of Radiation (3)
3. If light comprising a continuous spectrum
passes through a cool, low-density gas,
the result will be an absorption spectrum.
Light excites electrons in
atoms to higher energy states
Frequencies corresponding to the
transition energies are absorbed
from the continuous spectrum.
The Spectra of Stars
Inner, dense layers of a
star produce a continuous
(blackbody) spectrum.
Cooler surface layers absorb light at specific frequencies.
=> Spectra of stars are absorption spectra.
Analyzing Absorption Spectra
• Each element produces a specific set of absorption
(and emission) lines.
• Comparing the relative strengths of these sets of
lines, we can study the composition of gases.
By far the
most
abundant
elements
in the
Universe
Lines of Hydrogen
Most prominent lines
in many astronomical
objects: Balmer
lines of hydrogen
The Balmer Lines
n=1
Transitions
from 2nd to
higher levels
of hydrogen
Ha
Hb
Hg
The only hydrogen
lines in the visible
wavelength range.
2nd to 3rd level = Ha (Balmer alpha line)
2nd to 4th level = Hb (Balmer beta line)
…
Observations of the H-Alpha Line
Emission nebula, dominated
by the red Ha line.
Absorption Spectrum Dominated
by Balmer Lines
Modern spectra are usually
recorded digitally and
represented as plots of intensity
vs. wavelength
The Balmer Thermometer
Balmer line strength is sensitive to temperature:
Most hydrogen
atoms are ionized
=> weak Balmer
lines
Almost all hydrogen atoms in
the ground state (electrons in
the n = 1 orbit) => few
transitions from n = 2 => weak
Balmer lines
Measuring the Temperatures of Stars
Comparing line strengths, we can
measure a star’s surface temperature!
Spectral Classification of Stars (1)
Temperature
Different types of stars show different
characteristic sets of absorption lines.
Spectral Classification of Stars (2)
Mnemonics to
remember the
spectral
sequence:
Oh
Oh
Only
Be
Boy,
Bad
A
An
Astronomers
Fine
F
Forget
Girl/Guy
Grade
Generally
Kiss
Kills
Known
Me
Me
Mnemonics
Stellar Spectra
F
G
K
M
Surface temperature
O
B
A
The Composition of Stars
From the relative strength of absorption lines (carefully
accounting for their temperature dependence), one can
infer the composition of stars.
The Doppler Effect (1)
The light of a
moving source is
blue/red shifted by
Dl/l0 = vr/c
l0 = actual
wavelength
emitted by the
source
Blue Shift (to higher
frequencies)
vr
Red Shift (to lower
frequencies)
Dl = Wavelength
change due to
Doppler effect
vr = radial
velocity
The Doppler Effect (2)
The Doppler effect allows us to
measure the source’s radial velocity.
The Doppler Effect (2)
Example:
The Doppler Effect (4)
Take l0 of the Ha (Balmer alpha) line:
l0 = 656 nm
Assume, we observe a star’s spectrum
with the Ha line at l = 658 nm. Then,
Dl = 2 nm.
We find Dl/l0 = 0.003 = 3*10-3
Thus,
vr/c = 0.003,
or
vr = 0.003*300,000 km/s = 900 km/s.
The line is red shifted, so the star is receding from
us with a radial velocity of 900 km/s.
Doppler Broadening
In principle, line absorption
should only affect a very
unique wavelength.
In reality, also slightly
different wavelengths are
absorbed.
↔ Lines have a finite width;
we say:
Blue shifted
abs.
Red shifted
abs.
vr
vr
Atoms in random thermal motion
they are broadened.
One reason for
broadening:
The Doppler effect!
Observer
Line Broadening
Higher Temperatures
Higher thermal velocities
 broader lines
Doppler Broadening is usually the most
important broadening mechanism.