Spectroscopy and Atoms

How do we know:

- Physical states of stars, e.g. temperature, density.

- Chemical make-up and ages of stars, galaxies

- Masses and orbits of stars, galaxies, extrasolar planets

- expansion of universe, acceleration of universe.

All rely on taking and understanding spectra: spreading out radiationby wavelength.

Types of Spectra

1. "Continuous" spectrum - radiation over a broad range of wavelengths(light: bright at every color).

3. Continuous spectrum with "absorption lines": bright over a broad range of wavelengths with a few dark lines.

2. "Emission line" spectrum - bright at specific wavelengths only.

The pattern of lines is a fingerprint of the element (e.g. hydrogen, neon) in the gas.

For a given element, emission and absorption lines occur at the same wavelengths.

The Particle Nature of Light

On microscopic scales (scale of atoms), light travels as individual packets of energy, called photons. (Einstein 1905).

cphoton energy is proportional toradiation frequency:

E (or E 1

example: ultraviolet photons are more harmful than visible photons.

The Nature of Atoms

The Bohr model of the Hydrogen atom (1913):

_

+proton

electron

"ground state"

_

+

an "excited state"

Ground state is the lowest energy state. Atom must gain energy to move to an excited state. It must absorb a photon or collide with another atom.

But, only certain energies (or orbits) are allowed:

__

_

+

The atom can only absorb photons with exactly the right energy to boost the electron to one of its higher levels.

(photon energy αfrequency)

a few energy levels of H atom

When an atom absorbs a photon, it moves to a higher energy state briefly.

When it then jumps back to lower energy state, it emits a photon - in a random direction.

Atoms, under normal conditions have equal positive and negative charge. Number of protons (+ charges) defines which element. Protons are in the nucleus along with neutrons (neutral, no charge). Electrons, in the same number are the number of protons, orbit the nucleus.

Different isotopes of element have different number of neutrons, but the same number of protons.

Each element has its own allowed electron energy levels and thus its own spectrum.

Ionization

+

Hydrogen

_

++

Helium

"Ion"

Two atoms colliding can also lead to ionization. The hotter the gas, the more ionized it gets.

_

_

Energetic UV Photon

Atom

Energetic UV Photon

So why do stars have absorption line spectra?

Simple case: let’s say these atoms can only absorb green photons. Get dark absorption line at green part of spectrum.

hot (millions of K), dense interiorhas blackbody spectrum,gas fully ionized

“atmosphere” (thousandsof K) has atoms and ionswith bound electrons

Stellar Spectra

Spectra of stars differ mainly due to atmospheric temperature (composition differences also important).

“hot” star

“cool” star

So why absorption lines?

.

. .

.

.

.

.

..

.

.

cloud of gas

The green photons (say) get absorbed by the atoms. They are emitted again in random directions. Photons of other wavelengths go through. Get dark absorption line at green part of spectrum.

Why emission lines?

.

..

...

hot cloud of gas

- Collisions excite atoms: an electron moves to a higher energy level

- Then electron drops back to lower level

- Photons at specific frequencies emitted.

Molecules

Two or more atoms joined together.

They occur in atmospheres of cooler stars, cold clouds of gas, planets.

Examples

H2 = H + H

CO = C + OCO

2 = C + O + O

NH3 = N + H + H + H (ammonia)

CH4 = C + H + H + H + H (methane)

They have - electron energy levels (like atoms) - rotational energy levels - vibrational energy levels

Rotational and Vibrational energy levels cause molecules to havetheir own unique spectra.

3x1025 1024 1023 1022 1021 1020 1019 1018 1017 1016 1015 1014 1013 1012 1011 1010 109 108 107 106 105 104 103

Frequency [Hz]

Wavelength [m]

10-18 10-17 10-16 10-15 10-14 10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 104 105

Infrared

RadioGamma Rays

UV

X-rays

400 nm[0.4 microns]

4000 Angstroms

700 nm[0.7 microns]

7000 Angstroms

1 10 100Wavelength [microns]

Visible

MWIR3-5 microns

LWIR8-12 microns

VLWIRNIR< 2 microns

Cellphones(AMPS, USDC)

850 MHzBroadcast TV

60 MHz

Shortwave10 MHz

X-band10 GHz

1 eV1.24 microns

e- e+ annihilation0.511 MeV

Broadcast AM0.8 MHz

UV “A” 320-400 nmUV “B” 290-320 nm

Electromagnetic Radiation

Medical x-rays0.1 - 0.5 Angstroms

frequency [Hz]

wavelength [m]

Peak Dayresponse570 nm

PeakDark response

510 nm

micron = m = 10-6 mnanometer = nm = 10-9 mAngstrom = 10-10 m

Photon energy: E = h = hc/

= c

Peak of Cosmic MicrowaveBackground (CMB)

[T =2.736 K]

2900K 290K 29KTemperature of Blackbodywith Peak emission at

Doppler shifts

● Happens for all wave phenomena:

sound => change of pitch

light => change of wavelength (or color)

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V =λ observed − λ emitted

λ emitted

c =Δλ

λ emitted

c

where V is the velocity of the emitting source (m/s), c is the speed of light (m/s).

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V =λ observed − λ emitted

λ emitted

c =Δλ

λ emitted

c

Redshift if receding, blueshift (negative sign) if approaching. Redshift if receding, blueshift (negative sign) if approaching.

Spectral lines are used to measure Doppler shift => gives us information about the motion of an object.

Spectral lines are used to measure Doppler shift => gives us information about the motion of an object.

Star wobbling due to gravity of planet causes small Doppler shift of its absorption lines.

Amount of shift depends on velocity of wobble. Also know period of wobble. This is enough to constrain the mass and orbit of the planet.

We've used spectra to find planets around other stars.

Telescopewithspeectrograph

spectrum

Telescopes

Telescopes

● Basic function of a telescope: extend human vision

– Collect light from celestial object

– Focus light into an image of the object

● Human eye works from 400 – 700 nm or so and uses a lens to form an image on the retina. Astronomical objects emit at much larger range of wavelengths, and can be very faint!

Optical telescopes

Kinds of optical telescopes– Refractor – uses a lens that light passes through, to

concentrate light. Galileo’s telescope was a refractor.

– Reflector – uses a mirror (shape is conic section– typically parabolic). Big, modern research telescopes are reflectors.

Large objective lens at the front of the telescope forms theimage, the eyepiece lens at the back of the telescope magnifiesthe image for the observer.

Focal length, f, is distance from the lens to the focal point.

Objective lens

Eyepiecefocalpoint

focal lengthof objective

focal lengthof eyepiece

Problem with refractors: big diameterobjective lens means huge telescope.

Solution: use concave mirror, not lens, to focus light. Reflecting telescope.

Objectivemirror

Secondarymirror

Barrel (tube)

Reflector advantages● Mirrors can be large, because they can be

supported from behind.● Largest single mirror built: 8.4 m diameter for

the Large Binocular Telescope

● There are 10 m telescopes, but in segments

Reasons for using telescopes● Light gathering power: LGP area, or D2 Main reason for

building large telescopes!

● Magnification: angular diameter as seen through telescope/angular diameter on sky: m=fobj/feyepiece

– Typical magnifications 10 to 100

● Resolution: The ability to distinguish two objects very close together. Angular resolution:

= 2.5 x 105 /D, where is angular resolution of telescope in arcsec, is wavelength of light, D is diameter of telescope objective, both typically in meters.

Two light sources with angular separation much larger thanangular resolution vs. equal to angular resolution

Detectors

Quantum Efficiency = how much light they respond to:– Eye 2%

– Photographic emulsions 1-4%

– CCD (Charge coupled device) 80%● Can be used to obtain images or spectra

Photographic film CCD

Same telescope, same exposure time!

We get spectra of stars, galaxies, etc. using Spectrographs. The various colors of light are spread out by wavelength, using a prism or a reflecting mirror with many finely ruled lines called a “diffraction grating”.

Radio Telescopes

● Problems – low photon energies, long – Remember = 2.5 x 105 /D

● Single Dish: need big diameter to get decent resolution.

● Can also design clever shapes of reflectors, which minimize unwanted radio waves bouncing off feed legs into receiver

● The Green Bank Telescope

● Reflecting surface shouldn’t have irregularities that are larger than 1/16 of wavelength being focussed – are radio or optical telescopes easier to construct in terms of surface accuracy?

● But, wavelength is large – how do we get good

resolution?● Interferometers – e.g., VLA

Use interference of radio waves to mimic the resolution of a telescope whose diameter is equal to the separation of the dishes

Aperture Synthesis

-- Combine signals from multiple apertures.

-- Control the optical (or radio) path length from each aperture so that the signals act as if they were reflected from a single larger, filled aperture.

-- Computers, high accuracy time reference, and specialized signal processing reconstruct an image.

● Our own telescope: the Long Wavelength Array

● Far larger than the VLA. “Stations” of 256 antennas, to be spread across NM

● Square Kilometer Array, currently being designed, will be 50 times collecting area of VLA, with baselines to 1000’s km

Optical-mm Telescope sites

● Site requirements– Dark skies (avoid light pollution)

– Clear skies

– Good “seeing”, stable atmosphere

● High, dry mountain peaks are ideal observatory sites, for optical to cm

Earth at night

Telescopes in spacePros – above the atmospheric opacity so can work

at impossible from ground, above turbulence, weather, lights on Earth

Con – expensive!

● Hubble Space Telescope. 2.4 m mirror, 115nm – 1 micron

● Successor: JWST. 6.5 m mirror, 600 nm – 28 microns

● Spitzer Space Telescope (IR)

The sky at different wavelengths

Visible

Infrared

Gamma ray

Radio(neutralhydrogen)

X-ray

● Adaptive Optics – use a wavefront sensor and a

deformable mirror to compensate for deformations of incoming wave caused by the Earth’s atmosphere.