the cosmic messenger electric and magnetic fields oscillate together, but perpendicular to each...

The Cosmic Messenger

Electric and magnetic fields oscillate together, but perpendicular to each other and the electromagnetic wave moves in a direction perpendicular to both of the fields.

Light is a electromagnetic wave.

Light as a Wave

• Wavelengths of light are measured in units ofnanometers (nm) 1 nm = 10-9 m orÅngström (Å): 1 Å = 10-10 m = 0.1 nm

Visible light has wavelengths between

4000 Å and 7000 Å or 400 nm and 700 nm.

The abbreviation used for wavelength is the Greek letter lambda  .

Light as a Wave

• Light waves are characterized by a wavelength and a frequency f.

• f and are related through

f = c/

c = 300,000 km/s = 3*108 m/s

Properties of LightProperties of Light

–However, the speed of light is reduced as it However, the speed of light is reduced as it traverses transparent materials and the speed is traverses transparent materials and the speed is also dependent on color also dependent on color

IntroductionIntroductionLight is radiant energy: it does not Light is radiant energy: it does not require a medium for travel require a medium for travel Light is an electromagnetic waveLight is an electromagnetic wave –Light travels at 299,792,458 km/s in a Light travels at 299,792,458 km/s in a

vacuum (fast enough to circle the Earth vacuum (fast enough to circle the Earth 7.5 times in one second)7.5 times in one second)

The Nature of Light

– Newton

•Early discoveries

–Newton discovered that white light passing through a prism is comprised of a spectrum of colors.

Newton said, “Light is made of very small particles”Huygens: “It’s wave-like”

• Visible light is composed of the colors of the rainbow.

• Each color is a different wavelength of light.

• Red is at the long wavelength end and violet is at the short wavelength end.

THE VISIBLE ELECTROMAGNETIC SPECTRUM

Photon energy

Note the trends: bluer light has shorter  , higher f, and more energy. Redder light has longer  , lower f, and less energy.

The electromagnetic spectrum.

4000 A 5000 A 6000 A 7000 A

Which Rays get through our atmosphere?

The Nature of Light– Light travels at a finite speed, c, not

instantaneously.• In 1675 Romer measured the delay in Jupiter’s

moon eclipses

mi/sec 10x86.1

km/sec 10x0.35

5

c

It takes 8 minutes for Sunlight to reach Earth, and 0.13 seconds for light to go around the world

83 10 / secc x m

83 10 / secc x m Or

h = 6.626x10-34 Js h is Planck constant.

The energy of a photon does not depend on the intensity of the light!!!

Energy depends on its frequency (color)

Light can also particles as particles, called photons. A photon has a specific energy E, proportional to the frequency f:

hchE

Light can also behave as a particle and a wave at the same time. An example of light acting as both a particle and a wave is the digital camera---the lens refracts (bends and focuses) waves of light that hit a charge-coupled device (CCD). The photons kick electrons out of the silicon in the CCD.

Young’s Double slit experiment , showing light acts as waves.

–Waves compressed with source moving toward you; sound pitch is higher.

–Waves are stretched with source moving away from you; sound pitch is lower.

The Doppler Effect (Sound)

Doppler effect

0

0

0

c

v

• similar in light and sound•Knowing the rest position and the position due to motion of a spectral line allows us to calculate the speed of the object. The size of the shift gives the speed.

0

0

z

V cz

and

so,

0

Z is the Doppler shift, sometimes called the red shift.

Four Ways in Which Light can Interact with Matter

1. Emission – matter releases energy as light

2. Absorption – matter takes energy from light

3. Transmission – matter allows light to pass through it

4. Reflection – matter repels light in another direction

Light is a form of energy!

Spectra LinesSpectra Lines

There are three laws, known as Kirchhoff's laws, that govern the spectra and allows us interpret the spectra we observe.

1. A hot solid, liquid or gas at high pressure has a continuous spectrum.

Example : the Sun and a light bulb

There is energy at all wavelengths

2. A gas at low pressure and high temperature will produce emission lines.

There is energy only at specific wavelengths.

A closed tube containing a gas heated to a high temperature

3. A gas at low pressure in front of a hot continuum causes absorption lines.

Dark lines appear on the continuum.

As illustrated below, Kirchhoff's laws refer to three types of spectra: continuum, emission line, and absorption line.

Thus when we see a spectrum we can tell what type of source we are seeing.

Low Pressure

Types of spectra: (A) continuous, (B) emission-line, and (C) absorption-line.

Stars have a continuous and absorption spectrum

Gases around the Sun absorb photons

Hydrogen

Helium

Oxygen

Neon

Iron

Emission Spectrum caused by heating up a gas under low pressure to a high temperature.

Astronomers have to find elements among the spectrum of many elements combined.

7000 A4000 A

700 nm400 nm

Red Shift

Compare these spectra.

Spectrum of Hydrogen in Lab

Spectrum a Star

What do these spectra tell us about the star?

Compare these spectra.

Spectrum of Hydrogen in Lab

Spectrum a Star’s spectral

Hydrogen

What do these spectra tell us about the star?

Compare these spectra.

What do these spectra tell us about the object?

Hydrogen

Star

Compare these spectra.

Spectrum of Hydrogen in Lab

Spectrum a Star…..Day 1

What do these spectra tell us about the star?

Spectrum a Star…..Day 2

Spectrum a Star…..Day 3

Spectrum a Star…..Day 4

Stars are different colors due to their temperature.

Composition has no effect on the color because the stars are all made up mostly of the same elements with a small amount of difference.

Blue white are the hottest and red stars are the coolest. The temperature of a star can be determined by the highest point in its trace spectrum.

ORION

Hydrogen

Continuum

Absorption Lines

AA spectrum can be converted to a trace spectrum.

Continuum & LinesContinuum & Lines

Real stars usually have a blackbody-like continuous spectrum, upon which absorption lines are superimposed

Spectral TypesSpectral Types• Annie Cannon classified stars according to the

appearance of their spectra.

• The star were classified according to the strength of the hydrogen absorption lines in the sequence A, B, C….P.

These spectral classes were changed to a temperature ordered sequence and some were discarded. This left the following O-B-A-F-G-K-M.

•Subclasses: …O8, O9, B0, B1, B2…

• Oh, Be A Fine Girl (Guy) Kiss Me

O B A F G K M

50,000 K 3,000 K

Sun(G2)

A5 K7

•Stellar Spectroscopy is the study of the properties of stars by measuring absorption line strengths.

•O B A F G K MHottest Coolest

The Spectral Sequence

Bluest Reddest

Spectral Sequence is a Temperature Sequence

Hottest Coolest50,000K 1300K

O B A F G K M L

O Stars

B Stars

T = 11,000 - 30,000 K; Strong He lines; very weak H lines

A Stars

Hottest Stars: T>30,000 K; Strong He+ lines; no H lines

T = 7500 - 11,000 K; Strongest H lines, Weak Ca+ lines.

F StarsT = 5900 - 7500 K; H grows weaker Ca+ grows stronger, weak metals begin to emerge.

G StarsT = 5200 - 5900 K; Strong Ca+, Fe+ and other metals dominate,

K StarsT = 3900 - 5200 K; Strong metal lines, molecular bands begin to appear

M StarsT = 2500 - 3900 K; strong molecular absorption bands particularly of TiO