The Electromagnetic Spectrum, Light,

Astronomical Tools

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

• 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

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).

The Electromagnetic Spectrum

Need satellites to observe

Wavelength

Frequency

High flying air planes or satellites

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!!!

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, 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.

• 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

€

Max =3,000,000 nm K

T=

3,000 μm K

T

€

E =σ T 4 where σ = 5.67 ×10-8 J/(m2 s K4 )

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?

€

Max =3,000 μm K

T=

3,000 μm K

300 K=10μm

€

T =3,000 μm K

λMax

=3,000 μm K

0.5 μm= 6,000 K

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

Continuous Spectrum

The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption.

Emission Line Spectrum

A thin or low-density cloud of gas emits light only at specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines.

Absorption Line Spectrum

A cloud of gas between us and a light bulb can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum.

Atomic (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.

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

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

Seeing

Weather conditions and

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

images

Bad seeing Good seeing

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!

The Best Location for a Telescope

Far away from civilization – to avoid light pollution

The Best Location for a Telescope (II)

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

Paranal Observatory (ESO), Chile

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

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

distortions by atmospheric turbulence

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

Radio AstronomyRecall: Radio waves of ~ 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 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!

The Largest Radio Telescopes

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

Virginia.The 300-m telescope in

Arecibo, Puerto Rico

Infrared Telescopes

WIRO 2.3m

Spitzer Space Telescope

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

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.

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

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).

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.