ASTRONOMY 161Introduction to Solar System Astronomy

Class 10

SpectraWednesday, January 31

“Twinkle, twinkle,little star,How I wonderwhat you are.”

Spectra: Key Concepts

(1) A hot, transparent gas produces an emissionspectrum.

(2) A cool, transparent gas produces an absorptionspectrum.

(3) Every type of atom, ion, and molecule has aunique spectrum.

(4) The most abundant elements in the universe arehydrogen and helium.

(5) The radial velocity of an object is found from itsDoppler shift.

(1) A hot, transparent gasproduces an emission spectrum.

Consider a single,isolated atom:

A nucleus, containingprotons and neutrons,is surrounded by acloud of orbitingelectrons.

Electrons can emit orabsorb photons.

Consider hydrogen (the simplest atom):one proton, one electron.

Behaviour on subatomic scales isgoverned by quantummechanics.

One rule of quantum mechanics:electrons can only exist inorbits of particular energy(energy is quantizied).

When an electron falls from ahigh energy orbit to a lowenergy orbit, the energy lost iscarried away by a photon.

In hydrogen, an electron fallingfrom orbit 3 to orbit 2 emits aphoton withλ = 656.3 nanometers.

Consider a hot, low density cloud of hydrogen.

Light is emitted only at wavelengthscorresponding to energy differences betweenpermitted electron orbits.

Results: an emission line spectrum.

The Carina Nebula

A cloud of hot, lowdensity gas about 7000light years away.

Its reddish color comesfrom the 656.3 nmemission line ofhydrogen.

(2) A cool, transparent gasproduces an absorption spectrum.

Consider a cold, low density cloud of hydrogen infront of a hot blackbody.

Light is absorbed only at wavelengthscorresponding to energy differences betweenpermitted electron orbits.

Result: an absorption line spectrum.

(3) Every type of atom, ion, and moleculehas a unique spectrum.

Ion: an atom with electrons added (negativeion) or taken away (positive ion).

Molecule: two or more atoms bonded together.

The spectrum of each atom, ion,and molecule is a distinctive“fingerprint”.

The morecomplicated theatom, ion or themolecule, themore complexthe spectrum.

electron

neutron

proton

From emission or absorption lines, we know:

1) which elements are present;

2) whether they are ionized;

3) whether they are in molecules.

emission spectrum of the Carina Nebula

(4) The most abundant elements inthe Universe are hydrogen and helium.

It is fairly easy to determine which elements arepresent in a star.

It is much harder to determine how much of eachelement is present.

Strength of emission and absorption lines dependson temperature as well as on the element’sabundance.

Abundance of elementsin the Sun’s atmosphere:

Hydrogen (H): 75%Helium (He): 23%

Everything else: 2%

As discovered in 1920’s, other stars are mostlyhydrogen and helium, too.

Cecilia Payne-Gaposchkin(1900-1979) was a British-American astronomer. Sheleft England in 1922. In1925, she became the firstever Ph.D. in astronomyfrom Harvard. Her thesisestablished that hydrogenwas the overwhelmingconstituent of the stars.

(5) The radial velocity of an objectis found from its Doppler shift.

Radial velocity = how fast an object is movingtoward you or away from you.

If a wave source moves toward you or away fromyou, the wavelength is changed.

The reason for Doppler shifts:

Wave crests are bunched up ahead of the light source,stretched out behind.

If a light source is moving toward you, thewavelength is shorter (called a “blueshift”).

If a light source is moving away from you, thewavelength is longer (called a “redshift”).

Size of Doppler shift isproportional to radial velocity.

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Spectra: Few closing questions:

1) What kind of spectrum will be produced by veryhot, but also very dense hydrogen gas?

2) If you have hot gas in front of a star, what kind ofspectrum will you see?

3) Which spectrum is more complex: that of hydrogenor that of helium?

4) What is the maximum blueshift possible?

5) What is the maximum redshift possible? (tricky)