Lecture 9Stellar Spectra

Announcements Homework 5 is due on Monday.

Quantum states Unlike a planet going around the sun, electrons can

only orbit the nucleus in certain orbits that are fixed distances from the nucleus.

These are called permitted orbits.

Electron Orbits Here’s an example: This is the

nucleus

The solid circles represent permitted orbits. An electron can circle the nucleus in these orbits.

Here’s an example: This is the nucleus

An electron could not orbit in one of these orbits, because it is not a permitted orbit.

Electron Orbits

Here’s How Electrons Move:It takes energy to push an electron from a low orbit …

Electron Orbits

Here’s How Electrons Move:…into a higher orbit. The farther the electron must travel, the more energy is needed.

Electron Orbits

Here’s How Electrons Move:The electron can also “jump” multiple orbits, if you give it enough of a push. For example, it can go from here…

Electron Orbits

Here’s How Electrons Move:…all the way to here (with enough energy for the push).

Electron Orbits

Here’s How Electrons Move:Once in a high orbit, the electron will only stay there for a little while before it automatically jumps into a lower orbit.

Electron Orbits

Here’s How Electrons Move:The electron can jump into any lower orbit, but it must give off energy to do so. A bigger jump gives off more energy.

Electron Orbits

Summary

Electrons may only orbit the nucleus in certain, specific permitted orbits. Electrons can jump from one orbit to the next. An electron has to absorb energy to jump to a

higher orbit. An electron has to give off energy to jump to a

lower orbit.

Atomic Spectra

A very common way to get an electron to jump to a higher orbit is to absorb a photon of light.

But an electron can only absorb photons with just the right energies to put them in the higher orbits.

Atomic Spectra When an electron jumps

from a higher orbit to a lower one, it gives off light.

The light’s wavelength (color) depends on the exact amount of energy the electron needs to give off to make it down to the lower orbit.

E = hf f = frequency (Hz) h = 6.626x10-34 (Js) (Plank’s constant)

Atomic Spectra

Each type of atom (hydrogen, helium, oxygen, etc.) has its own, unique combination of permitted orbits. E.g. two hydrogen atoms

have the same permitted orbits, but the permitted orbits for a hydrogen atom are different than for an oxygen atom.

Atomic Spectra

This means that each type of atom absorbs and emits radiation only at certain, distinct wavelengths (colors)!

Emission Spectra In a hot, thin gas, atomic collisions “pump”

electrons up into high orbits.

Emission Spectra As the electrons jump down into lower orbits, they

give off photons of light at very specific wavelengths (colors) based on the size of the jump.

Emission Spectra The resulting spectrum looks like narrow colored

lines on a dark background, and is called an emission spectrum. The pattern of colored lines is unique to the element.

Absorption Spectra A thin (relatively) cool gas has electrons mostly in

lower orbits.

Absorption Spectra If you shine a light with all colors (i.e. white light)

through the gas, only photons of the correct colors will be used to “jump” the electrons to higher orbits.

Absorption Spectra

The result is a spectrum that looks like a “rainbow” with narrow, dark lines located where the bright lines would be in the atom’s emission spectrum…

Putting it all Together- “Blackbodies” give off light at all

wavelengths, a continuous spectrum

Putting it all Together- A low-density gas excited to emit light will

do so at specific wavelengths and thus produce an emission spectrum.

Putting it all Together- If light comprising a continuous spectrum

passes through a cool, low-density gas, the result will be an absorption spectrum.

Stars Have Absorption Spectra The photosphere of a

star is very hot, but it is much cooler than the inside of the star.

So most stars (except the very, very hot or very, very dense ones) show absorption spectra.

Stellar Spectra The pattern of dark

lines (called spectral lines) in the spectrum tells you about which gasses are in the photosphere of the star.

Spectral Classes Hydrogen is the most common element in

the universe – makes up most (75% by mass) of the material in a star.

The dark lines in the visible light part of the hydrogen spectrum are created by electrons jumping from the 2nd orbit out from the nucleus to higher orbits.

Spectral Classes

In stars with very cool photospheres (about 3,000 K), there are very, very few hydrogen atoms with electrons that have been “bumped” by collisions into the 2nd orbit out.

So the spectra of these stars have very weak (hard to see) hydrogen lines in visible light.

Spectral Classes

In fact, red stars have photospheres so cool, that molecules can actually form!

Molecules make the spectrum look very complicated by adding LOTS of dark lines…

Spectral Classes

The warmer the photosphere, the more collisions “bump” electrons into the 2nd orbit out, and the stronger the hydrogen lines become.

Spectral Classes

BUT, once the photosphere reaches 10,000 K, the collisions become so violent that they bump electrons totally off the hydrogen atoms!

A hydrogen atom without an electron can’t make absorption lines!

Spectral Classes

So the hydrogen lines in the star’s absorption spectrum become stronger and stronger, until you get to stars with photospheres above 10,000 K.

Then, as the photosphere gets hotter, the hydrogen lines rapidly get weaker!

By the time the temperature hits 40,000 K, the hydrogen lines are totally gone!

Spectral Classes

In the 1900s, Annie Cannon, a Harvard Astronomer, developed a classification system for stellar spectra based on how strong their hydrogen lines were.

Spectral Classes

In Cannon’s system, spectra with the strongest hydrogen lines were classed as “Type A”

Weaker lines were “Type B”

Weaker still were “Type C” All the way to “Type O” –

which show no hydrogen lines.

The Spectral Classification System

In 1910, Cannon’s system was adopted by the International Astronomical Union with only a few changes: Several rarely-assigned classes were removed. The remaining classes were each sub-divided

into 10 sub-classes. They were rearranged in order of stars with

decreasing surface temperature.

The Spectral Classification System

The major spectral classes, in order from hottest to coolest were now:

O B A F G K M

Each major spectral class is subdivided into ten numbered sub-classes, going from the hottest (0) to the coolest (9).

For example, class A is sub-divided, from hottest to coolest, into:

A0 A1 A2 A3 A4 A5 A6 A7 A8 A9

The Spectral Classification System

Different classes of stars show different characteristic sets of absorption lines.

The Sun is a G2 star. Mnemonics to

remember the spectral sequence:

Oh Oh Only

Be Boy, Bad

A An Astronomers

Fine F Forget

Girl/Guy, Grade Generally

Kiss Kills Known

Me Me Mnemonics

The Spectral Classification System

Modern spectra are usually recorded digitally and represented as plots of intensity vs. wavelength

Stellar Spectra

OB

A

F

GKM

Surface tem

perature

Spectral Classes

So the spectral class of a star tells you how strong the hydrogen lines are in the star’s spectrum, which relates to the surface temperature of the star.

M2 (3,000 K)

G2 (5,700 K)

A1 (9,800 K)

B8 (11,000 K)

Other Things We Learn From Spectral Lines

If the star (or the star’s surface) is moving toward or away from us, the star’s spectral lines undergo a Doppler shift.

Motion measured in this manner is only that part of the star's motion that is directly toward or away from the observer (radial velocity).

Red ShiftSpectral absorption (or emission) lines are seen

to be shifted toward the red end of the spectrum (red shift) if the motion is away from the observer.

Blue ShiftSpectral absorption (or emission) lines are seen

to be shifted toward the blue end of the spectrum (blue shift) if the motion is toward the observer.

Doppler Shift

Doppler Shift

Here’s the equation for the Doppler shift:

v / c = / In this equation:

v is the speed toward or away from us. It’s positive if away from us. It’s negative if toward us.

c is the speed of light (3×108 m/s) is how far the spectral line is shifted from where it is

supposed to be. It’s positive for a redshift and negative for a blueshift.

is the wavelength in the spectrum where the spectral line is supposed to be.

For Next Time

Read Units 49 and 51 for Monday.