Download Astronomical Telescopes Light and Other Forms of Radiation Light

Astronomical Telescopes
Often very large to gather
large amounts of light.
Chapter 5
Astronomical Tools
In order to observe
forms of radiation other
than visible light, very
different telescope
designs are needed.
The northern Gemini Telescope on Hawaii
Light as a Wave (I)
Light and Other Forms
of Radiation
λ
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.
c = 300,000 km/s
= 3*108 m/s
•
•
Light waves are characterized by a
wavelength λ and a frequency f.
f and λ are related through
f = c/λ
Wavelengths and Colors
Light as a Wave (II)
• Wavelengths of light are measured in units
of nanometers (nm) or angstrom (Å):
1 nm = 10-9 m
1 Å = 10-10 m = 0.1 nm
Different colors of visible light correspond
to different wavelengths.
Visible light has wavelengths
between 4000 Å and 7000 Å
(= 400 – 700 nm).
1
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:
The Electromagnetic Spectrum
Wavelength
E = h*f
Frequency
h = 6.626x10-34 J*s
is the Planck constant.
The energy of a photon does not
depend on the intensity of the light!!!
Refracting / Reflecting Telescopes
Focal length
Focal length
Need satellites
to observe
High
flying air
planes or
satellites
The 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 light path
towards back or
side of
incoming light
path.
Eyepiece: To
view and
enlarge the
small image
produced in
the focal
plane of the
primary
optics.
Focal length = distance from the center of the lens to
the plane onto which parallel light is focused.
Disadvantages of
Refracting Telescopes
• Chromatic aberration:
Different wavelengths are
focused at different focal
lengths (prism effect).
Can be corrected, but not
eliminated by second lens
out of different material.
• Difficult and expensive
to produce: All surfaces
must be perfectly
shaped; glass must be
flawless; lens can only be
supported at the edges.
2
The Powers of a Telescope:
Size does matter!
1. Light-gathering
power: Depends on
the surface area A
of the primary lens /
mirror, proportional
to diameter squared:
D
The Powers of a Telescope (II)
2. Resolving power: Wave nature of light
=> The telescope aperture produces
fringe rings that set a limit to the
resolution of the telescope.
Astronomers can’t eliminate these
diffraction fringes, but the larger a
telescope is in diameter, the smaller
the diffraction fringes are. Thus the
larger the telescope, the better its
resolving power.
αmin = 1.22 (λ/D)
A=π
(D/2)2
For optical wavelengths, this gives
αmin
αmin = 11.6 arcsec / D[cm]
The Powers of a Telescope (III)
Seeing
3. Magnifying Power = ability of the telescope
to make the image appear bigger.
Weather
conditions and
turbulence in the
atmosphere set
further limits to
the quality of
astronomical
images
A larger magnification does not improve the
resolving power of the telescope!
Bad seeing
Good seeing
The Best Location for a Telescope
The Best Location for a Telescope (II)
Paranal Observatory (ESO), Chile
Far away from civilization – to avoid light pollution
On high mountain-tops – to avoid atmospheric
turbulence (→ seeing) and other weather effects
3
Traditional Telescopes (I)
Secondary mirror
Traditional primary
mirror: sturdy, heavy
to avoid distortions.
Advances in Modern
Telescope Design (I)
Modern computer technology has made possible
significant advances in telescope design:
1. Simpler, stronger mountings (“alt-azimuth
mountings”) to be controlled by computers
Traditional
Telescopes (II)
The 4-m
Mayall
Telescope
at Kitt Peak
National
Observatory
(Arizona)
Advances in Modern Telescope Design (II)
2. Lighter mirrors with lighter support structures,
to be controlled dynamically by computers
Floppy mirror
Segmented mirror
Adaptive Optics
Computer-controlled mirror support adjusts the mirror
surface (many times per second) to compensate for
distortions by atmospheric turbulence
Examples of Modern
Telescope Design (I)
Design of the Large Binocular Telescope (LBT)
4
Examples of Modern
Telescope Design (II)
Interferometry
Recall: Resolving power of a telescope depends on diameter D.
→ Combine the signals from
several smaller telescopes to
simulate one big mirror →
Interferometry
The Very Large Telescope (VLT)
8.1-m mirror of the Gemini Telescopes
CCD Imaging
Negative Images
CCD = Charge-coupled device
The galaxy NGC 891 as it
would look to our eyes (i.e., in
real colors and brightness)
• More sensitive than
photographic plates
• Data can be read directly
into computer memory,
allowing easy electronic
manipulations
Negative images (sky = white;
stars = black) are used to
enhance contrasts.
False-color image to visualize
brightness contours
The Spectrograph
Using a prism (or a grating), light can
be split up into different wavelengths
(colors!) to produce a spectrum.
Radio Astronomy
Recall: Radio waves of λ ~ 1 cm – 1 m also penetrate the
Earth’s atmosphere and can be observed from the ground.
Spectral lines in a
spectrum tell us about the
chemical composition and
other properties of the
observed object
5
Radio Telescopes
Radio Maps
In radio maps, the intensity of the radiation is color-coded:
Large dish focuses
the energy of radio
waves onto a small
receiver (antenna)
Red = high intensity;
Violet = low intensity
Amplified signals are
stored in computers
and converted into
images, spectra, etc.
Radio Interferometry
Just as for optical
telescopes, the
resolving power of a
radio telescope
depends on the
diameter of the
objective lens or mirror
αmin = 1.22 λ/D.
Analogy: Seat prices in a baseball
stadium: Red = expensive; blue = cheap.
The Largest Radio Telescopes
For radio telescopes,
this is a big problem:
Radio waves are much
longer than visible light
→ Use interferometry to
improve resolution!
The Very Large Array (VLA): 27
dishes are combined to simulate a
large dish of 36 km in diameter.
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.
The 100-m Green Bank
Telescope in Green Bank, West
Virginia.
The 300-m telescope in
Arecibo, Puerto Rico
Infrared Astronomy
Most infrared radiation is absorbed in the lower atmosphere.
However, from
high mountain
tops or highflying aircraft,
some infrared
radiation can
still be
observed.
NASA infrared telescope on Mauna Kea, Hawaii
6
Ultraviolet Astronomy
NASA’s Great Observatories in Space (I)
• Ultraviolet radiation with λ < 290 nm is
completely absorbed in the ozone layer
of the atmosphere.
• Ultraviolet astronomy has to be done
from satellites.
• Several successful ultraviolet astronomy
satellites: IRAS, IUE, EUVE, FUSE
• Ultraviolet radiation traces hot (tens of
thousands of degrees), moderately
ionized gas in the universe.
Hubble Space Telescope Images
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 Earth’s
atmosphere
• Extends imaging
and spectroscopy to
(invisible) infrared
and ultraviolet
NASA’s Great Observatories in Space (II)
The Compton Gamma-Ray Observatory
Operated from
1991 to 2000
Mars with its
polar ice cap
A dust-filled galaxy
Nebula around
an aging star
NASA’s Great Observatories in Space
(III)
NASA’s Great Observatories in Space (IV)
The Spitzer Space Telescope
The Chandra X-ray Telescope
Launched in 1999 into a highly
eccentric orbit that takes it
1/3 of the way to the moon!
X-rays trace hot (million
degrees), highly ionized
gas in the universe.
Two colliding
galaxies,
triggering a
burst of star
formation
Observation of
high-energy
gamma-ray
emission, tracing
the most violent
processes in the
universe.
Very hot gas
in a cluster
of galaxies
Launched in 2003
Infrared light traces warm
dust in the universe.
The detector needs to be
cooled to -273 oC (-459 oF).
Saturn
7
Spitzer Space Telescope Images
The Future of Space-Based
Optical/Infrared Astronomy:
A Comet
Warm dust in a
young spiral galaxy
Newborn stars that
would be hidden
from our view in
visible light
The James Webb Space Telescope
8
The Amazing Power of Starlight
Chapter 6
Just by analyzing the light received from a star,
astronomers can retrieve information about a star’s
Starlight and Atoms
1. Total energy output
2. Surface temperature
3. Radius
4. Chemical composition
5. Velocity relative to Earth
6. Rotation period
Light and Matter
Spectra of stars are
more complicated than
pure black body 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.
Different Kinds of Atoms
If you could fill a
teaspoon just with
material as dense as
the matter in an atomic
nucleus, it would weigh
~ 2 billion tons!!
• The kind of atom
depends on the
number of protons
in the nucleus.
• Most abundant:
Hydrogen (H),
with one proton
(+ 1 electron).
Different
numbers of
neutrons ↔
different
isotopes
• Next: Helium (He),
with 2 protons (and
2 neutrons + 2 el.).
Helium 4
1
Electron Orbits
• Electron orbits in the electron cloud are restricted to
very specific radii and energies.
• These characteristic electron energies are different
for each individual element.
r3, E3
Atomic Transitions
• An electron can
be kicked into a
higher orbit
when it absorbs
a photon with
exactly the right
energy.
Eph = E3 – E1
• The photon is
absorbed, and the
electron is in an
excited state.
r2, E2
r1, E1
Eph = E4 – E1
Wrong energy
(Remember that Eph = h*f)
• All other photons pass by the atom unabsorbed.
Color and Temperature
Stars appear in
different colors,
from blue (like Rigel)
Orion
Betelgeuse
via green / yellow (like
our sun)
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 black body
spectrum.
to red (like Betelgeuse).
These colors tell us
about the star’s
temperature.
Black Body Radiation (I)
Rigel
Two Laws of Black Body Radiation
A perfect black body emitter
would not reflect any radiation.
Thus the name ‘black body’.
The Color Index (I)
B band
1. The hotter an object is, the more luminous it is.
2. The peak of the black body spectrum shifts towards
shorter wavelengths when the temperature increases.
→ Wien’s displacement law:
λmax ≈ 3,000,000 nm / TK
(where TK is the temperature in Kelvin).
V band
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 V-band
magnitudes just as we did
before for total magnitudes
(remember: a larger number
indicates a fainter star).
2
The Color Index (II)
Kirchhoff’s Laws of Radiation (I)
We define the Color Index
1. A solid, liquid, or dense gas excited to emit
light will radiate at all wavelengths and thus
produce a continuous spectrum.
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.
Kirchhoff’s Laws of Radiation (II)
2. If light comprising a continuous spectrum
passes through a cool, low-density gas, the
result will be an absorption spectrum.
Kirchhoff’s Laws of Radiation (III)
3. 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
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
(black body) spectrum.
Light excites electrons in
atoms to higher energy states
Transition back to lower states
emits light at specific frequencies
Lines of Hydrogen
Most prominent lines
in many astronomical
objects: Balmer lines
of hydrogen
Cooler surface layers absorb light at specific frequencies.
=> Spectra of stars are absorption spectra.
3
The Balmer Lines
4
n
=
3
n=
2
Hα
Transitions
from 2nd to
higher levels
of hydrogen
Hβ
Absorption spectrum dominated by Balmer lines
n=5
n=
n=1
Hγ
The only hydrogen
lines in the visible
wavelength range.
Modern spectra are usually recorded
digitally and represented as plots of
intensity vs. wavelength
2nd to 3rd level = Hα (Balmer alpha line)
2nd to 4th level = Hβ (Balmer beta line)
…
Emission nebula, dominated
by the red Hα line.
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
Spectral Classification of Stars (I)
Different types of stars show different
characteristic sets of absorption lines.
Temperature
Measuring the Temperatures of Stars
Comparing line strengths, we can
measure a star’s surface temperature!
4
Spectral Classification of Stars (II)
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
The Composition of Stars
Stellar spectra
A
F
G
K
M
Surface temperature
O
B
From the relative strength of absorption lines (carefully
accounting for their temperature dependence), one can
infer the composition of stars.
The Doppler Effect
The light of a moving
source is blue/red
shifted by
∆λ/λ0 = vr/c
Blue Shift (to higher
frequencies)
vr
λ0 = actual
wavelength
emitted by the
source
Red Shift (to lower
frequencies)
∆λ = Wavelength
change due to
Doppler effect
vr = radial
velocity
5
Example (I):
Earth’s orbital motion around the sun causes a
radial velocity towards (or away from) any star.
Example (II):
Take λ0 of the Hα (Balmer alpha) line:
λ0 = 656 nm
Assume, we observe a star’s spectrum
with the Hα line at λ = 658 nm. Then,
∆λ = 2 nm.
We find ∆λ/λ0 = 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.
6