Blackbody Radiation

And

Spectra

Light is a form of _______.

Why is this important?

With very few exceptions, the only way we have to study objects in Astronomy is via

the light (energy) they emit.

energy

• The Earth’s atmosphere is __________ to __________, some ________, and the ____. It is opaque to ___, ______, and ______ rays.

X-raysUVgamma

radiovisible light

transparentinfrared

Blackbody Radiation• Ex. Heat an iron rod with a torch.

–1st visible color – “____ Hot”–2nd visible color – Bright _______–3rd visible color – Bright ________–4th visible color – Bright ______ –5th visible color – Bright _____

• As you heat an object it gets _______ and ______ more ______________ _________.radiation

emits electromagneticbrighter

BlueWhite

Yellow

RedOrange

Blackbody Radiation• The dominant color or wavelength of the

emitted radiation changes with temperature.

–_______ objects emit at _______ wavelengths - Red, Infrared

–______ objects emit at _______ wavelengths - Blue, Violet, UV

• Object actually gives off ____ ___________ of electromagnetic spectrum.wavelengths

all

shorterHotter

Cooler longer

Blackbody Radiation

• A blackbody ________ all the electromagnetic radiation that strikes it (none is reflected or scattered).

• Object is heated and then _______ the energy it absorbed at _______ __________ of the EM spectrum.

absorbs

wavelengthsdifferentreemits

BlackbodyCurves

• A body with any temperature radiates _____________, sometimes called blackbody radiation.

• For a given size, ______ objects give off __________ than cooler objects, and are _____.

thermal energy

more energyhotter

bluer

Blackbody Curves

• For a given temperature, larger bodies give off more energy than smaller bodies, in direct proportion to their surface areas.

Visible light

• Most people can perceive ______.

• Different colors correspond to different frequencies (or wavelengths).

• The colors of the rainbow are ROY G BIV: red orange yellow green blue indigo violet.

color

Visible light• In the visible,

____ has the longest __________, the smallest __________, and the lowest _______.

_____ has the shortest _________, the highest __________, and the highest ______.

red wavelengthfrequency

energy

violet wavelengthfrequency

energy

The spectrum• A graph of the

intensity of light vs. the color (e.g. the wavelength, frequency, or energy) is called a ________.

• A spectrum is probably the single most useful diagnostic tool available in Astronomy.

spectrum

Important points• The spectrum of a star is approximately a

black body spectrum.

______ stars are bluer, _____ stars are redder.

For a given temperature, ______ stars give off __________ than smaller stars.

______ the wavelength, the _______ the temperature

Hotter cooler

largermore energy

Shorter greater

• In the constellation of Orion, the reddish star Betelgeuse is a relatively cool star. The blue star Rigel is relatively hot.

The spectrum• A spectrum can tell us about the temperature

and composition of an astronomical object.

• There are _____ major types of spectra of concern here: _________________ - the intensity varies

smoothly from one wavelength to the next. _____________ - there are discrete jumps

in the intensity from one wavelength to the next.

two

Continuous spectra

Line spectra (2)

The Line Spectrum• Upon closer examination, the spectra of

real stars show fine detail.

• Dark regions where there is relatively little light are called lines.

The Line Spectrum

• Today, we rarely photograph spectra, but rather plot the intensity vs the wavelength.

Emission and Absorption

• If you view a gas against a dark background, you see _____________ (wavelengths at which there is an abrupt spike in the brightness).

emission lines

Emission and Absorption

• If you view a continuous spectrum through cool gas, you see __________ ______ (wavelengths where there is little light).lines

absorption

Why is this important?

• The line spectrum tells about what elements are present in the star.

• Why is this?

How Light Interacts with Matter.• Atoms are the basic

blocks of matter.

• They consist of protons and neutrons in the nucleus, surrounded by lighter particles called electrons.

How Light Interacts with Matter.

• An ________ will interact with a photon.

• An electron that _______ a photon will _____ energy.

• An electron that ______ energy must ______ a photon – giving off light.

• The ____________ (electron plus photon) ______________ during this process.

electron

absorbsgain

losesemit

total energyremains constant

Atomic Fingerprints• Hydrogen has a

specific line spectrum.

• Each atom has its own specific line spectrum.

Atomic Fingerprints

• These stars have absorption lines with the wavelengths corresponding to hydrogen!

Atomic Fingerprints.

• This cloud of gas looks red since its spectrum is a line spectrum from hydrogen gas.

The Doppler Shift: Measuring Motion

• If a source of waves is ____ _______, then the waves are _______ _______ in all directions.

notmoving

equallyspaced

The Doppler Shift: Measuring Motion

• If a source of waves ___ _______, then the spacing of the wave crests ________ on the direction relative to the direction of motion.

ismoving

depends

The Doppler Shift: Measuring Motion

• Think of sound waves from a fast-moving car, train, plane, etc. The sound has a ___________

(higher frequency) when the car __________.

The pitch is _____ (lower frequency) as the car ______ and moves further away.

Higher pitch

approaches

lowerpasses

The Doppler Shift: Measuring Motion

• If a source of light is _______ _____, the wavelengths are increased, or “__________”.redshifted

movingaway

The Doppler Shift: Measuring Motion

• If a source of light is _______ _______, the wavelengths are shortened, or “__________”.blueshifted

movingcloser

The Doppler Shift: Measuring Motion

• The size of the wavelength shift depends on the relative velocity of the source and the observer.

Using a Spectrum, we can…

• Measure a ________________ by measuring the overall shape of the spectrum (essentially its color).

• Measure what _______________ are in a star’s atmosphere by measuring the lines.

• Measure the ______________ of a star by measuring the Doppler shifts of the lines.

relative velocity

star’s temperature

chemical elements