# The Electromagnetic Spectrum, Light, Astronomical Tools

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The Electromagnetic Spectrum, Light, Astronomical Tools
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 c = 300,000 km/s = 3*108 m/s Light waves are characterized by a wavelength and a frequency f. f and  are related through f = c/

Wavelengths and Colors
Different colors of visible light correspond to different wavelengths.

Dark Side of the Moon “There is no dark side really. It’s all dark.” -- Pink Floyd

More accurate, from Richard Berg
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 = m = 0.1 nm Visible light has wavelengths between 4000 Å and 7000 Å (= 400 – 700 nm).

The Electromagnetic Spectrum
Wavelength Frequency High flying air planes or satellites Need satellites to observe

Light as Particles E = h*f
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!!!

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

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 From our text: Horizons, by Seeds

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? 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

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 n=3 n=2 n=1 From our text: Horizons, by Seeds

Astronomical Telescopes
Often very large to gather large amounts of light. In order to observe forms of radiation other than visible light, very different telescope designs are needed. The northern Gemini Telescope on Hawaii

Refracting / Reflecting Telescopes
Refracting Telescope: Lens focuses light onto the focal plane Focal length Reflecting Telescope: Concave Mirror focuses light onto the focal plane Focal length Almost all modern telescopes are reflecting telescopes.

Secondary Optics In 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.

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.

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 A =  (D/2)2

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. min = 1.22 (/D) min For optical wavelengths, this gives min = 11.6 arcsec / D[cm]

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)
Paranal Observatory (ESO), Chile On high mountain-tops – to avoid atmospheric turbulence ( seeing) and other weather effects

Secondary mirror Traditional primary mirror: sturdy, heavy to avoid distortions.

The 4-m Mayall Telescope at Kitt Peak National Observatory (Arizona)

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
The Very Large Telescope (VLT) 8.1-m mirror of the Gemini Telescopes

Recall: Resolving power of a telescope depends on diameter D.
Interferometry Recall: Resolving power of a telescope depends on diameter D.  Combine the signals from several smaller telescopes to simulate one big mirror  Interferometry

CCD Imaging CCD = Charge-coupled device
CCD = 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

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  ~ 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.

 Use interferometry to improve resolution!
Radio Interferometry Just as for optical telescopes, the resolving power of a radio telescope depends on the diameter of the objective lens or mirror min = /D. For radio telescopes, this is a big problem: Radio waves are much longer than visible light The Very Large Array (VLA): 27 dishes are combined to simulate a large dish of 36 km in diameter.  Use interferometry to improve resolution!

The 100-m Green Bank Telescope in Green Bank, West Virginia. The 300-m telescope in Arecibo, Puerto Rico

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.

NASA infrared telescope on Mauna Kea, Hawaii
Infrared Astronomy Most 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

Spitzer Space Telescope
Infrared Telescopes Spitzer Space Telescope WIRO 2.3m

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.

NASA’s Great Observatories in Space (I)
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

Hubble Space Telescope Images
Mars with its polar ice cap A dust-filled galaxy Nebula around an aging star

NASA’s Great Observatories in Space (II)
The Compton Gamma-Ray Observatory Operated from 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 Very hot gas in a cluster of galaxies Saturn

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).

Spitzer Space Telescope Images
A Comet Warm dust in a young spiral galaxy Newborn stars that would be hidden from our view in visible light

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

Terrestrial Planet Finder
A new VLT image of a possible planet around a brown dwarf star. My Bet: Renamed after Carl Sagan. Will use both interferometry and coronagraphs to image Earth-like planets.

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