light – our tool for understanding the universe
TRANSCRIPT
Light
Our Tool for Understanding the Universe
Dr. Billy Teets
Vanderbilt University Dyer Observatory
October 9, 2020
ISS Flyover Tonight
• Brighter than the planet Jupiter
• Becomes visible at 7:36
• Maximum Altitude ~7:39:30PM
• Ends at 7:40:30PM
Mars at Closest Approach Tonight
OUTLINE
• What is Light? Terminology Refresher
• Determining Compositions of Objects
• Velocities of Objects from Light?
• How Do We Know Temperatures of Stars?
• What are Those Beautiful HST Images Showing Us?
LIGHT IN GENERAL
• Can be thought of as an electromagnetic wave or a particle known as a photon
• Light travels at the speed of light for that medium
• A light wave in empty space travels 299,792.458 km/s (186,000 mi/s) – “c”
LIGHT: WAVELENGTH
• The length of a wave - “λ”
• Determined by measuring distances between points on consecutive waves
LONG WAVELENGTH SHORT WAVELENGTH
λ λ
LIGHT: ENERGY
• Energy is directly related to wavelength, frequency, and color
• Shorter wavelength = more energy
• Higher frequency = more energy
• More energy = “bluer” light
• Less energy = ”redder” light
THE ATOM
• Atoms consist of protons, neutrons, and electrons
• Protons and neutrons form the nucleus
• Electrons orbit the nucleus
Image Source: Physics World
A Spectrum
Image Source: Flickr/ Ryan Wunsch
A Closer Look at the Sun’s Spectrum
Image Source: Stargazing.net
What are those mysterious dark lines?
• First discovered by William Hyde Wollaston in 1802.
• Independently “rediscovered” by Joseph von Fraunhofer in 1814.
• Mapped over 500 lines in the solar spectrum.
Images Source: Wikipedia
The Solar Spectrum
Credit: N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF
Gustav Kirchhoff & Robert Bunsen
• 1860s – Improved upon Fraunhofer’s spectroscopic work.
• Found that elements and compounds have their own unique emission spectra.
• Found the link between emission and absorption spectra.
• Would be about 50 years before it was discovered why materials behave this way.
Images Source: Wikipedia
“Emission Spectra” of Selected Elements
HYDROGEN
HELIUM
OXYGEN
IRON
THORIUM
Kirchhoff’s Laws of Spectroscopy
Image Source: Wikipedia
Atomic Energy Levels
• Atoms have electrons in orbitals
• According to quantum mechanics, orbitalshave specific energies
• Electrons can gain energy and jump to higher energy orbitals
• Electrons can emit energy and jump to lower energy orbitals
• There is no “in-between” – energies are “quantized”
0.7 eV
{
1.9 eV
{
10.2 eV
{
The Hydrogen AtomEnergy Levels
12.1 eV
}} 12.8 eV
0.7 eV
{
1.9 eV
{
10.2 eV
{
The Hydrogen AtomEnergy Levels
12.1 eV
}} 12.8 eV
Ionization – Electrons are Stripped
A Continuous Spectrum
An Emission Spectrum
A Continuous Spectrum Electron orbit energies are distorted, smearing the emission lines.
The Solar Spectrum
Credit: N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF
Image Source: Wikimedia Commons
Helium
Pierre Jules Cesar Janssen – noted a mysterious yellow line found in the Sun’s spectrum during a solar eclipse in 1868.
Sir Norman Lockyer named the element after Helios.
Discovered on Earth in 1895.
Credit: Wikipedia
Coronium
Charles August Young and William Harkins – mysterious green line present in solar corona.
A. Gruenwald – dubbed the new element ``coronium.”
Discovered 60 years later to be due to highly ionized iron.
Credit: Wikipedia
Nebulium
• 1864 – William Huggins obtains first spectrum of a nebula.
• Notes that the appearance is very different from that of a star or “spiral nebula.”
• Two prominent green lines observed.
• Deduced a nebula contains an extra-terrestrial element.
• Over 60 years later – lines proven to be produced by doubly ionized oxygen.
Credit: NASA, ESA, HEIC, and The Hubble Heritage Team (STScI / (AURA)
The Doppler Effect
The Doppler Effect
Velocity
Speed of Light=
Wavelength Shift
Rest WavelengthImage Source: Wikipedia
The Doppler Effect
Image Source: Cornell University
The Doppler Effect
Credit: ESO
Image Source: Lorenzo Comolli
Blackbody (Planck) Spectrum
Wien’s Law
Temperature =2,900,000
lmax (nm)
Temperature =2,900
lmax (mm)
Spectra of Three Stars
Image Source: E. Kyrola et al., 2010
Annie Jump Cannon
Annie Jump Cannon (1863-1941)
• Cannon joined a group of female astronomers in 1896 at the Harvard College Observatory nicknamed "Pickering's Women."
• This team of "human computers" worked to document and empirically classify stars.
• Classified several hundred thousand stars in her career.
• Cannon was noted for her astounding efficiency of classifying up to three stars a minute.
• She is remembered most for the now universal system of spectral division of O, B, A, F, G, K, and M.
Images Source: Wikipedia
O 6.5
B 0
B 6
A 1
A 5
F 0
F 5
G 0
G 5
K 0
K 5
M 0
M 5
F 4
M 4.5
B 1A
STAR TEMPERATURES
TEMPERATURE
HOT
COOL
Image Source: NOAO
Color Index
Observe stars through U, B, V, R, I filters and measure magnitude (brightness) of stars in two or more filters
Calculate difference in magnitudes, such as B-V, to get color index. Note that magnitude system is backwards
More positive color index = redder/coolerMore negative color index = bluer/hotter
Example : M5 star (redder star)Green (V) filter magnitude = 6Blue (B) filter magnitude = 8Color Index: 8-6 = +2
Example : B5 star (bluer star)Green (V) filter magnitude = 4Blue (B) filter magnitude = 3Color Index: 3-6 = -1
Credit: Jodrell Bank Centre for Astrophysics
The Eagle Nebula – “True Color”
Credit: ESO
The Eagle Nebula – Hubble Palette
Credit: Ignacio Diaz Bobillo
The Eagle Nebula – Hubble Palette
Credit: NASA, ESA, The Hubble Heritage Team (STScI/AURA)