The Spectrum

What it is and what it means

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Light and Sound To understand spectra you need a little

primer on light first It is helpful to think of light and sound

together They are both wave phenomena

Sound is a mechanical wave Light is a electromagnetic wave

You can think of color as pitch and brightness as loudness

If the Rainbow was a Piano…

Red would be on the low side of the scale (bass) and violet would be on the high side (treble)

The range of colors is called the visible spectrum What would be lower than low? What would be higher than high?

A Low Note

A High Note

Here’s what all possible pitches together would sound like:

What would it look like?

That’s Right!

All possible colors added together with the same brightness make white

I know, you’re thinking “but if I mix all colors of paint together, it’s black.”

True, but paint doesn’t make light, it absorbs light. If paint absorbs all colors, what would you have?

Isaac Newton Showed with a

prism* that white light is a continuous band made of all colors

*How does a prism do this? I’ll explain the Physics after class if you wish.

Robert Boyle A contemporary and

adversary of Newton Declared the prism

to be the "usefullest Instrument" for gaining insight into the fleeting array of colors generated when sunlight passes through it.”

William Herschel

100 years after Boyle, Herschel viewed starlight through a prism

Saw differences in the width and intensity of the colors of the spectrum

Also detected “invisible rays”

Used a thermometer on each color and found “temperature” below red

Prisms

Featured on the best selling album of all time

Long before that, though, scientists were interested in improving on the design

One goal was to increase angular dispersion

Why?

The wider the angular dispersion, the more spread out the colors are, and the more detail is apparent

Prism improvers used various kinds of glass, even liquids, and different geometries to improve the resolution

Joseph von Fraunhofer German Optician b. 1787 At age 11, he was apprenticed to a glassmaker,

Philipp Anton Weichelsberger Weichelsberger’s shop collapsed, but both were rescued Maximillian IV, Prince of Bavaria witnessed the rescue and took

him under his wing, providing him with books on physics and mathematics

Maximillian presented him with a sizable contribution to buy his way out of apprenticeship at age 19, so he went to work for lawyer named Joseph von Utzscneider who had entrepreneurial aspirations

In partnership with Utzscneider he strived to make superior optical devices, including better lenses for telescopes and wide dispersion prisms

In 1814, searching for a pure light with which to calibrate his instruments, Fraunhofer turned his prism to the Sun

What are these lines?

Fraunhofer expected a pure, continuous spectrum but was *surprised by the dark lines

Saw similar spectra in the light of Sirius 1821: Developed a superior diffraction grating Invented the spectroscope

The basic design is still used 1822: Became keeper of the museum for the

Royal Academy of Sciences in Munich Died 1826 of tuberculosis, age 39, before

Bunsen and Kirchhoff offered an explanation for the lines later in the century

*William Hyde Wollaston saw the same lines in 1802 but wasn’t impressed

Even though Fraunhofer used his spectroscope on a star*, there was no such thing as stellar spectroscopy at the time

Astronomy was all about position, motion, cataloging, discovery of new objects

Little or no significance was given to the composition of celestial objects

*The Sun was not considered a star until later in the 19th century

[Astronomy] must lay down the rules for determining the motions of the heavenly bodies as they appear to us from the earth…Everything else that can be learned about the heavenly bodies … is not properly of astronomical interest.

Friedrich Wilhelm Bessel(1784-1846)

In astronomy, human ingenuity will, probably, in future, be able to accomplish little more than an improvement in the means of making observations, or in the analysis by which the rules of computation are investigated.

—John Narrien (1833)

Other “expert” voices

We can imagine the possibility of determining the shapes of stars, their distances, their sizes, and their movements;...whereas there is no means by which we will ever be able to examine their chemical composition, their mineralogical structure, or especially, the nature of organisms that live on their surfaces…Our positive knowledge with respect to the stars is necessarily limited to their observed geometrical and mechanical behavior.

—Auguste Comte (1864)

Remember this!

However, the game was afoot Alexander von

Humbolt studies sunspots

42 years of observations revealed that sunspot activity varies on an 11-year cycle

Right: 1845 daguerreotype, perhaps the first photograph of the Sun

Note the sunspots

Earth-Sun connection Sir Edward Sabine in

1852 announces a remarkable coincidence between Earth’s magnetic fluctuations and the sunspot cycle

If the Sun could influence the Earth’s magnetic field, and hence navigational compasses, more intense solar studies were indicated

Physics and Chemistry Unite

Robert Wilhelm Bunsen 1811-1899

Gustav Robert Kirchhoff 1824-1887

In the 1850s Kirchhoff and Bunsen used a prism to examine the light produced when different elements, alone and in combination, are burned

“It is known that several substances have the property of producing certain bright lines when brought into the flame. A method of qualitative analysis can be based on these lines, whereby the field of chemical reactions is greatly widened and hitherto inaccessible problems are solved.”

sample

prism

collimating tube

eyepiece

Burner (yes, a Bunsen burner!)

Spectral Lines…

Fraunhofer’s Lines…

Spectral Lines…

Fraunhofer’s Lines: Hmmmm!

An answer to an old question From this work B&K understood what

the Fraunhofer lines were ‘the vapour of table salt absorbs the

same lines which it also emits. These lines are identical with solar Fraunhofer lines’.

So, there is a connection between emission and absorption! And stars??

Warning! Science Content Follows!

Every element (and molecules too) emits a unique set of spectral lines

It’s like every chord on the piano has a unique set of pitches

But two questions come to mind: Why does each element emit a

unique set of spectral lines? Why did it take so long for scientists

to concertedly look for these lines?

Second Answer First At the time of Newton’s death in 1727,

only 14 of the 114 elements had been identified

Even by 1800, only 32 elements had been isolated

But by 1859, the time of Bunsen and Kirchhoff’s paper, known elements were so numerous that chemists were eager to qualify and quantify them

And, of course, Astronomers weren’t interested at all

Now, the First Answer

This interesting phenomenon, seemingly pertinent only to chemical identification, was one of the motivating forces for a major change in Physics in the 19th and 20th Centuries “Clouds over Physics” “Electromagnetic Catastrophe”

Faraday and Maxwell

It was known through the work of Michael Faraday and James Clark Maxwell that moving electrons radiate energy This is a slightly

more modern picture of an atom

But the (19th C) Problem is:

If an electron is continually buzzing around an atom, it is constantly giving off energy

If it is constantly giving off energy, its path would decay and the electron would spiral into the nucleus The electron would emit the entire spectrum as

it spins down, not discrete colors And atoms and therefore matter

everywhere would collapse

Max Planck’s Kludge

Planck suggested that electrons can travel only in specific paths or orbits about the nucleus

That way they would never lose all their energy and spiral in

He admitted that the atom probably wasn’t like this, but the math worked out as if it did

But Atoms Do Work This Way!

Orbits and Spectra

Every element’s atoms have a unique set of discrete energy levels (orbits)

As the electron drops from a higher to lower orbit it sheds a unique color of light

Small energy jumps: red light Big energy jumps: violet light

The set of colors (a series) gives each atom its distinctiveness

AND, if a photon of just the right energy hits a cool, rarified gas, the gas absorbs that color

Fraunhofer lines!

Without going into great detail…

Planck contributed to a new science called Quantum Mechanics (term dates from 1927)

Quantum Mechanics was a sea change in the way scientists (and non-scientists) saw the cosmos

Newton’s idea of a deterministic universe was supplanted by a probabilistic universe

In other words, there is no absolute certainly (I think)

A Powerful Technique Bunsen and Kirchhoff said something to

the effect that, if Heidelberg was burning they could determine what is was made of

They then metaphorically looked at each other and realized they could tell what the Sun was made of*! And they found it wasn’t burning in the

“normal” sense As an aside, contemporaries Herman von

Helmholtz and Lord Kelvin postulated that the Sun shined from a release of gravitational energy

*Of course, others had this idea as well, but B & K are best know for it.

If we were to go to the sun, and to bring away some portions of it and analyze them in our laboratories, we could not examine them more accurately than we can by this new mode of spectrum analysis.

—Warren De La Rue (1861)

Kirchhoff’s Laws

After further study Kirchhoff postulated three laws: A rarified hot gas gives off emission

spectra A dense cool gas absorbs light at

discrete colors (absorption spectra) A dense hot gas emits a continuous

spectrum

A New Science: Astrospectroscopy

Astronomers break the light from stars, nebulae, and supernovae into its constituent colors

Using Kirchhoff’s Laws they can tell what a hugely distant object is made of, whether it is hot, cold, rarified or dense

Pioneers in Stellar Spectroscopy

Giovanni Battista Donati (1826-1873) Father Pietro Angelo Secchi (1818-

1878) Lewis Morris Rutherfurd (1816-1892)

Comparing stellar spectraDonati (1863)

N.B. These and those that follow are hand-drawn spectragrams

Comparing stellar spectraRutherfurd (1863)

Comparing stellar spectraGreenwich Observatory (1863)

William Huggins1824-1910

English amateur Astronomer Semi-pro might be a better term Amateur at that time did not mean

someone without considerable skills Non-professionals discovered several moons

around Saturn and Uranus Huggins didn’t start observing until he

was 30 He closed the family shop in London and

moved back in with his parents in Upper Tulse Hill

Title page of Huggin’s lab notebook #1

For several years he worked in his observatory on Tulse Hill outside London

But Huggins grew dissatisfied with the positional focus of 19th century Astronomy

Epiphany “It was just about this time … that the

news reached me of Kirchhoff's great discovery of the true nature and the chemical constitution of the sun from his interpretation of the Fraunhofer lines….”

He later wrote: ...This news was to me like the coming upon a spring of water in a dry and thirsty land. Here at last presented itself the very order of work for which in an indefinite way I was looking….

William Allen Miller (1817-1870) Trained as a chemist Founding member of the

Chemical Society Treasurer and VP of the

Royal Society Started collaborating with

Huggins in 1862 Both were members of the

Royal Astronomical Society The two jointly won the

Gold Medal in 1867 for their work on stellar spectra

Worked to improve prismatic analysis with the goal of identification of elements

Induction apparatus to examine metallic spectra

Huggins’star-spectroscope

Huggins’ High Dispersion Spectrascope

Comparing stellar spectraHuggins and Miller, Proc. Roy. Soc. (1863)

Sun

Betelgeuse

Sirius

Aldebaran

The stars were undoubtedly suns after the order of our sun*… —Huggins (1897)

*What was it that Giordano Bruno had conjectured?

Interpreting nebular spectra Huggins and Huggins, Proc. Roy. Soc. (1889)

Remember: these are hand-drawn spectragrams

Other Stellar Mysteries Huggins observed the

planetary nebula 37. H IV Draconis

… after a few moments of hesitation, I put my eye to the spectroscope. Was I not about to look into a secret place of creation?... I looked into the spectroscope. No spectrum such as I expected! A single bright line only!

The bright line was something new in the spectrum Huggins called it nebulium In a planetary nebula, ionized oxygen and other elements

are blasted off a giant star This is the line ionized oxygen makes, not reproducible in

19th century chemistry labs

Margaret Lindsay Huggins (1848-1915)

Married William when he was around 50

Margaret was an accomplished photographer

She designed photographic equipment for use with the spectroscope

Huggins laboratory notebooks also become much more organized and informative due to Margaret

William Huggins in the

Tulse Hill Observatory(ca. 1905)

The Modern Variety of Data

The data plots don’t necessarily look like rainbows

Infrared, gamma-ray, microwave, x-ray spectra are all valuable data

The discovery of helium P.J.C. Janssen (Fr) and J. Norman Lockyer (UK) independently

determined that the solar atmosphere could be studied spectroscopically

Consequently, the bright orange ‘D’ line that was assumed to be sodium turned out to be an indication of a new element

Helium isolated in the laboratory in 1895 by William Ramsay

VVVVVrrrrroooooommmm

Spectra can also tell Astronomers if a distant object is moving towards or away from us And also how fast it is spinning

The technique is to use the Doppler Effect to see how the spectral lines are affected

To sum up that animation (again)

Because light waves travel only at a fixed speed, a light-emitting object like a star will be “redder” if it moves away from us and “bluer” if it moves towards us, meaning that the pattern of spectral lines will shift to the red or blue but maintaining their relative positions to each other

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Armand Hippolyte Louis Fizeau (September 23, 1819 – September 18,

1896

First predicted red shift shortly after Doppler discovered it

Also showed that two telescopes could be combined, forming a single, much larger aperture

Interferometry

Red Shift

Huggins had noted this in 1868

Doppler widening

Spin Astronomers can tell if a distant galaxy is

spinning and how fast The light on one edge is blue (moving towards

us) and the other edge is red (moving away) I p-shopped this up to make a point

What was that about a bad neighborhood?

In the 1920s Edwin Hubble examined the spectra of stars of many galaxies

He did his work at the Wilson Observatory

Here he is with his pipe

You can almost hear his British accent

(He was born in Marshfield, Missouri)

An Expanding Universe Hubble discovered

that the more distant a galaxy was from us, the faster it was moving away

Can only be explained by an expanding Universe A lecture for

another day