Atoms and Spectra

The Sun’s Visible-Light Spectrum

The Amazing Power of Starlight By analyzing light received from a star, astronomers learn about the star’s… Chemical composition Surface temperature Radius Total energy output Density Velocity relative to Earth Rotation period Magnetic Fields

Continuous Spectrum Spectrum of a common (incandescent) light bulb spans all visible wavelengths, without breaks

Kirchhoff’s Laws of Radiation A solid, liquid, or dense gas excited to emit light will radiate at all wavelengths & produce a continuous spectrum.

Emission Line Spectrum Thin or low-density cloud of gas emits light only at specific wavelengths Depends on its composition & temperature, Producing a spectrum with bright emission lines.

Kirchhoff’s Laws of Radiation 2. Low-density gas excited will emit light only at specific wavelengths & produce emission spectrum. Light excites electrons in atoms to higher energy states, Which transition back to lower states, emitting light at specific frequencies.

Emission Line Spectrum LASER Light Amplified Stimulated Emission Radiation

Absorption Line Spectrum A cloud of gas between us & continuous source can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum.

Kirchhoff’s Laws of Radiation 3. Light with continuous spectrum passes through a cool, low-density gas, produces absorption spectrum. Light excites electrons in atoms to higher energy states, The frequencies of light correspond to the transition energies absorbed from the continuous spectrum.

How does light tell us about matter?

What is matter? Matter is protons, neutrons, & electrons Use this figure to define the nucleus; protons, neutrons, electrons; scale of atom and “electron cloud.” Matter is protons, neutrons, & electrons Most of mass in nucleus Most of space in electron “cloud”

What is matter? Use this figure to define the nucleus; protons, neutrons, electrons; scale of atom and “electron cloud.”

Different Kinds of Atoms Different Kinds of Atoms Type of atom depends on # of protons Most abundant: Hydrogen (H) 1 proton, 1 electron 2nd: Helium (He) 2 protons, 2 neutrons, and 2 electrons

Atomic Terminology Atomic Number = # of protons in nucleus Atomic Mass Number = # of protons + neutrons

Chemical Fingerprints Each type of atom has a unique spectral fingerprint. Why?

Chemical Fingerprints Each type of atom has a unique spectral fingerprint.

Electron Orbits Electron orbits restricted to very specific radii and energies. Electron energies different for each individual element. r3, E3 r2, E2 r1, E1

Electron Orbits Electron orbits restricted to very specific radii and energies. Electron energies different for each individual element. Ephoton = E3 – E1 Ephoton = E4 – E1

Electron Orbits Electron orbits restricted to very specific radii and energies. Electron energies different for each individual element. Wrong energy isn‘t absorbed!

Chemical Fingerprints Each type of atom has unique electron energy levels. Each transition corresponds to a unique photon energy & wavelength. Energy levels of hydrogen

Chemical Fingerprints Downward transitions produce a unique pattern of emission lines.

Chemical Fingerprints Upward transitions produce a pattern of absorption lines … at the same wavelengths.

Chemical Fingerprints Observing the fingerprints in a spectrum tells us which kinds of atoms are present.

Example: Solar Spectrum You can use this slide as an example of real astronomical spectra made from a combination of the idealized types. Here we have the continuous (thermal) spectrum from the solar interior; dark absorption lines where the cooler solar atmosphere (photosphere) absorbs specific wavelengths of light.

Three basic types of spectra Continuous Spectrum Emission Line Spectrum Absorption Line Spectrum Spectra of astrophysical objects are usually combinations of these three basic types.

Thought Question Which letter(s) labels absorption lines? D E

Thought Question Which letter(s) labels absorption lines? D E

Thought Question Which letter(s) labels the peak (greatest intensity) of infrared light? A B C D E

Thought Question Which letter(s) labels the peak (greatest intensity) of infrared light? A B C D E

Thought Question Which letter(s) labels emission lines? D E

Thought Question Which letter(s) labels emission lines? D E

Color and Temperature Orion Stars appear in different colors: Stars appear in different colors: Blue (like Rigel), Yellow (like our sun) Red (like Betelgeuse). Colors tell us about star’s temperature. Orion Betelgeuse Rigel

Color and Temperature Read about Annie Cannon!

Author of “Galileo’s Daughter” The Women of Harvard Dava Sobel Author of “Galileo’s Daughter”

The Spectra of Stars Inner, dense layers of a star produce a continuous (blackbody) spectrum. Cooler surface layers absorb light at specific frequencies. => Spectra of stars are absorption spectra.

How does light tell us the temperatures of planets and stars? TWO Laws of Radiation! Hotter objects emit more light at all frequencies per unit area. Hotter = much, Much, MUCH, MUCH brighter Hotter objects emit photons with a higher average energy. Hotter = BLUER

Remind students that the intensity is per area; larger objects can emit more total light even if they are cooler.

How does light tell us the temperatures of planets and stars?

How does light tell us about the motions of planets & stars? DOPPLER effect Motion AWAY “stretches” wavelengths Sounds are lower; light is redder Motion TOWARD “compresses” wavelengths Sounds are higher; light is bluer

Measuring the Shift Stationary

Measuring the Shift Stationary Moving Away

Measuring the Shift Stationary Moving Away Away Faster

Measuring the Shift Stationary Moving Toward Toward Faster

Doppler shift tells us ONLY about part of an object's motion toward or away from us.

Doppler Shift Example: Earth’s orbital motion around the Sun causes a radial velocity towards (or away from) any star.