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 radiation by 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). c photon energy is proportional to  radiation 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 α  frequency) 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 interior has blackbody spectrum, gas fully ionized “atmosphere” (thousands of K) has atoms and ions with 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 H 2 = H + H CO = C + O CO 2 = C + O + O NH 3 = N + H + H + H (ammonia) CH 4 = 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 have their own unique spectra.

3x Frequency [Hz] Wavelength [m] Infrared Radio Gamma Rays UV X-rays 400 nm [0.4 microns] 4000 Angstroms 700 nm [0.7 microns] 7000 Angstroms Wavelength [microns] Visible MWIR 3-5 microns LWIR 8-12 microns VLWIR NIR < 2 microns  Cellphones (AMPS, USDC) 850 MHz Broadcast TV 60 MHz Shortwave 10 MHz X-band 10 GHz 1 eV 1.24 microns e - e + annihilation MeV Broadcast AM 0.8 MHz UV “A” nm UV “B” nm Electromagnetic Radiation Medical x-rays Angstroms frequency [Hz] wavelength [m] Peak Day response 570 nm Peak Dark response 510 nm micron =  m = m nanometer = nm = m Angstrom = m Photon energy: E = h = hc/ = c Peak of Cosmic Microwave Background (CMB) [T =2.736 K] 2900K 290K 29K Temperature of Blackbody with Peak emission at

Doppler shifts ● Happens for all wave phenomena: sound => change of pitch light => change of wavelength (or color) where V is the velocity of the emitting source (m/s), c is the speed of light (m/s).

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.

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. Telescope with speectrograph spectrum

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 the image, the eyepiece lens at the back of the telescope magnifies the image for the observer. Focal length, f, is distance from the lens to the focal point. Objective lens Eyepiece focal point focal length of objective focal length of eyepiece

Problem with refractors: big diameter objective lens means huge telescope. Solution: use concave mirror, not lens, to focus light. Reflecting telescope. Objective mirror Secondary mirror 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 D 2 Main reason for building large telescopes! ● Magnification: angular diameter as seen through telescope/angular diameter on sky: m=f obj /f eyepiece – Typical magnifications 10 to 100 ● Resolution: The ability to distinguish two objects very close together. Angular resolution:  = 2.5 x 10 5 /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 than angular 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 10 5 /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 space Pros – 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 (neutral hydrogen) 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.