The Electromagnetic Spectrum, Light,

Astronomical Tools

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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.

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Light as a Wave

• Light waves are characterized by a wavelength and a frequency f.

f = c/

c = 300,000 km/s = 3*108 m/s

• f and are related through

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Wavelengths and Colors

Different colors of visible light correspond to different wavelengths.

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Dark Side of the Moon

• “There is no dark side really. It’s all dark.” -- Pink Floyd

Dark Side of the Moon

• What is wrong with this picture?

• Front: Not all primary colors (eg, pink, magenta), also refraction angles inconsistent

• Back: Spectrum is Convergent – I think done for art’s sake

Front cover Back cover

More accurate, from Richard Berg

Light as a Wave

• 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

Visible light has wavelengths between 4000 Å and 7000 Å

(= 400 – 700 nm).

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The Electromagnetic Spectrum

Need satellites to observe

Wavelength

Frequency

High flying air planes or satellites

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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!!!

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Why is energy per photon so important?• Real life example: Ultra-Violet light hitting

your skin (important in Laramie!)– Threshold for chemical damage set by energy

(wavelength) of photons• Below threshold (long wavelengths) energy too

weak to cause chemical changes• Above threshold (short wavelength) energy

photons can break apart DNA molecules

– Number of molecules damaged = number of photons above threshold

– Very unlikely two photons can hit exactly together to cause damage

Temperature and Heat

• Thermal energy is “kinetic energy” of moving atoms and molecules– Hot material energy has more energy

available which can be used for• Chemical reactions• Nuclear reactions (at very high temperature)• Escape of gasses from planetary atmospheres• Creation of light

– Collision bumps electron up to higher energy orbit– It emits extra energy as light when it drops back

down to lower energy orbit– (Reverse can happen in absorption of light)

Temperature Scales• Want temperature scale with energy proportional to T

– Celsius scale is “arbitrary” (Fahrenheit even more so)• 0o C = freezing point of water• 100o C = boiling point of water

– By experiment, available energy = 0 at “Absolute Zero” = –273oC (-459.7oF)

– Define “Kelvin” scale with same step size as Celsius, but 0K = -273oC = Absolute Zero

• Use Kelvin Scale for most astronomy work– Available energy is proportional to T, making equations simple

(really! OK, simpler)– 273K = freezing point of water– 373K = boiling point of water– 300K approximately room temperature

Planck “Black Body Radiation”• Hot objects glow (emit light) as seen in PREDATOR, SSC Video, etc.

– Heat (and collisions) in material causes electrons to jump to high energy orbits, and as electrons drop back down, some of energy is emitted as light.

• Reason for name “Black Body Radiation”– In a “solid” body the close packing of the atoms means than the electron

orbits are complicated, and virtually all energy orbits are allowed. So all wavelengths of light can be emitted or absorbed. A black material is one which readily absorbs all wavelengths of light. These turn out to be the same materials which also readily emit all wavelengths when hot.

• The hotter the material the more energy it emits as light– As you heat up a filament or branding iron, it glows brighter and brighter

• The hotter the material the more readily it emits high energy (blue) photons– As you heat up a filament or branding iron, it first glows dull red, then

bright red, then orange, then if you continue, yellow, and eventually blue

Planck and other Formulae• Planck formula gives intensity of

light at each wavelength– It is complicated. We’ll use two

simpler formulae which can be derived from it.

• Wien’s law tells us what wavelength has maximum intensity

• Stefan-Boltzmann law tells us total radiated energy per unit area

T

Km 000,3

T

Knm 000,000,3Max

) Ks J/(m 105.67 where 42-84 TE

From our text: Horizons, by Seeds

Example of Wien’s law• What is wavelength at which you

glow?– Room T = 300 K so

– This wavelength is about 20 times longer than what your eye can see. Thermal camera operates at 7-14 μm.

• What is temperature of the sun – which has maximum intensity at roughly 0.5 m?

m10 K300

Km 000,3

T

Km 000,3Max

K 000,6m 5.0

K m 000,3K m 000,3

Max

T

From our text: Horizons, by Seeds

Kirchoff’s laws

• Hot solids emit continuous spectra

• Hot gasses try to do this, but can only emit discrete wavelengths

• Cold gasses try to absorb these same discrete wavelengths

Atoms – Electron Configuration• Molecules: Multiple atoms sharing/exchanging electrons (H2O, CH4)

• Ions: Single atoms where one or more electrons have escaped (H+)

• Binding energy: Energy needed to let electron escape

• Permitted “orbits” or energy levels– From quantum mechanics, only certain “orbits” are allowed– Ground State: Atom with electron in lowest energy orbit– Excited State: Atom with at least one atom in a higher energy orbit– Transition: As electron jumps from one energy level orbit to another,

atom must release/absorb energy different, usually as light.

• Because only certain orbits are allowed, only certain energy jumps are allowed, and atoms can absorb or emit only certain energies (wavelengths) of light.

• In complicated molecules or “solids” many transitions are allowed

• Can use energy levels to “fingerprint” elements and estimate temperatures.

From our text: Horizons, by Seeds

Hydrogen Lines• Energy absorbed/emitted depends on upper and lower levels

• Higher energy levels are close together

• Above a certain energy, electron can escape (ionization)

• Series of lines named for bottom level– To get absorption, lower level must be occupied

• Depends upon temperature of atoms

– To get emission, upper level must be occupied• Can get down-ward cascade through many levels

From our text: Horizons, by Seedsn=1

n=2

n=3

Astronomical Telescopes

Often very large to gather large amounts of light.

The northern Gemini Telescope on Hawaii

In order to observe forms of radiation other than visible light, very

different telescope designs are needed.

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Refracting / Reflecting Telescopes

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.

Focal length

Focal length

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Secondary OpticsIn reflecting telescopes: Secondary

mirror, to re-direct 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.

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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.

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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:

A = (D/2)2

D

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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.

min = 1.22 (/D)

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.

For optical wavelengths, this gives

min = 11.6 arcsec / D[cm]

min

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Seeing

Weather conditions and

turbulence in the atmosphere set further limits to the quality of astronomical

images

Bad seeing Good seeing

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The Powers of a Telescope (III)

3. Magnifying Power = ability of the telescope to make the image appear bigger.

A larger magnification does not improve the resolving power of the telescope!

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The Best Location for a Telescope

Far away from civilization – to avoid light pollution

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The Best Location for a Telescope (II)

On high mountain-tops – to avoid atmospheric turbulence ( seeing) and other weather effects

Paranal Observatory (ESO), Chile

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http://en.wikipedia.org/wiki/Paranal_Observatory

Traditional Telescopes (I)

Traditional primary mirror: sturdy, heavy to avoid distortions.

Secondary mirror

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Traditional Telescopes (II)

The 4-m Mayall

Telescope at Kitt Peak

National Observatory

(Arizona)

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Advances in Modern Telescope Design

Lighter mirrors with lighter support structures, to be controlled dynamically by computers

Floppy mirror

Segmented mirror

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Adaptive OpticsComputer-controlled mirror support adjusts the mirror surface (many times per second) to compensate for

distortions by atmospheric turbulence

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Examples of Modern Telescope Design

8.1-m mirror of the Gemini Telescopes

The Very Large Telescope (VLT)

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InterferometryRecall: Resolving power of a telescope depends on diameter D.

Combine the signals from several smaller telescopes to

simulate one big mirror

Interferometry

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CCD ImagingCCD = Charge-coupled device

• More sensitive than photographic plates

• Data can be read directly into computer memory, allowing easy electronic

manipulations

False-color image to visualize brightness contours

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The SpectrographUsing a prism (or a grating), light can Using a prism (or a grating), light can be split up into different wavelengths be split up into different wavelengths

(colors!) to produce a spectrum.(colors!) to produce a spectrum.

Spectral lines in a Spectral lines in a spectrum tell us about the spectrum tell us about the chemical composition and chemical composition and

other properties of the other properties of the observed object observed object

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Radio AstronomyRecall: Radio waves of ~ 1 cm – 1 m also penetrate the Earth’s atmosphere and can be observed from the ground.

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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.

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Radio Interferometry

The Very Large Array (VLA): 27 dishes are combined to simulate a

large dish of 36 km in diameter.

Just as for optical telescopes, the

resolving power of a radio telescope depends

on the diameter of the objective lens or mirror

min = 1.22 /D.

For radio telescopes, this is a big problem:

Radio waves are much longer than visible light

Use interferometry to improve resolution!

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The Largest Radio Telescopes

The 100-m Green Bank Telescope in Green Bank, West

Virginia.The 300-m telescope in

Arecibo, Puerto Rico

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Science of Radio AstronomyRadio 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.

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Infrared AstronomyMost infrared radiation is absorbed in the lower atmosphere.

However, from high mountain tops or high-flying aircraft, some infrared radiation can

still be observed.

NASA infrared telescope on Mauna Kea, Hawaii

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Infrared Telescopes

WIRO 2.3m

Spitzer Space Telescope

Ultraviolet Astronomy• Ultraviolet radiation with < 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: IUE, EUVE, FUSE

• Ultraviolet radiation traces hot (tens of thousands of degrees), moderately ionized gas in the universe.

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NASA’s Great Observatories in Space (I)

• Avoids turbulence in Earth’s

atmosphere

• Extends imaging and spectroscopy to (invisible) infrared

and ultraviolet

• Launched in 1990; maintained and

upgraded by several space shuttle service

missions throughout the 1990s and early 2000’s

The Hubble Space Telescope

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Hubble Space Telescope Images

Mars with its polar ice cap

Nebula around an aging star

A dust-filled galaxy

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NASA’s Great Observatories in Space (II)

The Compton Gamma-Ray Observatory

Operated from 1991 to 2000

Observation of high-energy gamma-ray

emission, tracing the most violent processes in the

universe.

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NASA’s Great Observatories in Space (III)

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

Saturn

Very hot gas in a cluster of galaxies

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Chandra X-ray Observatory

Shuttle launched, highly eccentric orbit.

Grazing incidence mirrors – nested hyperboloids and paraboloids.

The Highest Tech Mirrors Ever!

• Chandra is the first X-ray telescope to have image as sharp as optical telescopes.

NASA’s Great Observatories in Space (IV)

The Spitzer Space Telescope

Launched in 2003

Infrared light traces warm dust in the universe.

The detector needs to be cooled to -273 oC (-459 oF).

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Spitzer Space Telescope Images

A Comet

Newborn stars that would be hidden from our view in

visible light

Warm dust in a young spiral galaxy

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Spitzer Space Telescope

• Discovered by a Wyoming grad student and professor. The “Cowboy Cluster” – an overlooked Globular Cluster.

Kepler’s Supernova with all three of NASA’s Great

Observatories• Just 400 years ago:

(Oct. 9, 1604)• Then a bright, naked eye

object (no telescopes)• It’s still blowing up – now

14 light years wide and expanding at 4 million mph.

• There’s material there at MANY temperatures, so many wavelengths are needed to understand it.

A Multiwavelength Look at Cygnus A

• A merger-product, and powerful radio galaxy.

The Future of Space-Based Optical/Infrared Astronomy:

The James Webb Space Telescope

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Terrestrial Planet Finder

My Bet: Renamed after Carl Sagan. Will use both interferometry and coronagraphs to image Earth-like planets.

A new VLT image of a possible planet around a brown dwarf star.