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the characteristic wavelengths of electromagnetic radiation that are emitted or absorbed by an object ”


there are 3 types of spectra

continuum spectrum
emission line spectrum
absoprtion line spectrum


Continuous Spectrum

Hot solids/liquids/dense gases emit a continuous rainbow of light

Perfect Blackbody Radiation

The ideal blackbody object will emit a continuous spectrum:
an un-interrupted EM radiation curve over a certain range of wavelengths. Example: cores of stars and light bulbs

Spectra can be displayed in different but ‘equivalent’ ways:


Emission Spectrum

If heated, each chemical element will produce its own unique pattern of bright emission lines at specific wavelengths, against a dark background.

Each element produces its own unique pattern of emission lines – like a ‘bar code’ at the supermarket, identifying each item (‘element’) uniquely.

Fireworks use this same principle
certain elements will display certain colours.
Sr --> red
Ba --> green
Li --> pink
these are heated to a certain temp to show the colour

check ppw for hot gases (slide 20)


Absorption Spectrum

If light with a continuous spectrum shines through a cloud of COOL gas, the gas can
absorb photons

Spectrum graph shows dips (less light) where photons get absorbed

Light (radiation) usually passes through a cooler medium (such as gas in the atmosphere of the Sun, or gas in the atmosphere of Earth) before reaching us.

This cooler gas will absorb some of this light energy, creating dark absorption lines in the original continuous spectrum of the radiation at the exact wavelengths.


Starlight shows an Absorption Spectrum

Light (made in the star’s core) must pass through its own atmosphere before escaping.

Some light gets absorbed


The Doppler Effect

The change in wavelength of radiation (light) due to the relative motion between the source and the observer along the line of sight.”


what happens to the doppler effect when the source is moving towards you with some speed ..

Waves get squeezed in direction of motion (i.e., wavelength decreases).
So, frequency must go up.
Same thing happens for light!


When do you hear the doppler effects

With sound, the Doppler effect is heard when a sound
is moving towards, or away from you.

You hear a high pitch, and then a low pitch.


examples of doppler effect

Car/train/Emergency vehicle zooms by you and you hear a change in pitch (frequency)
Doppler Radar (for weather)
Airplane radar system
Submarine radar system
Ok, anything with radar
Radar gun, used by Law Enforcement Officers…



When the source of light is moving away from the observer the wavelength of the emitted light will appear to increase. We call this a “redshift”.



When the source of light is moving towards the observer the wavelength of the emitted light will appear to decrease. We call this a “blueshift”.


why do astronomers use the doppler effect ?

Astronomers use the Doppler Effect to learn about the radial (along the line of sight) motions of stars, and other astronomical objects


When You Don’t Observe Doppler Effect

“Along the line of sight” means the Doppler Effect happens only if the object which is emitting light is moving towards you or away from you.

An object moving “side to side” or perpendicular, relative to your line of sight, will not experience a Doppler Effect

No Change in Wavelength here


What Can We Learn by Analyzing Starlight?

A star’s temperature
- peak wavelength of the spectral curve
A star’s chemical composition
- dips in the spectral curve or the lines in the absorption spectrum
A star’s motion
- Doppler shift


the universe

The Universe is defined to be the sum total of everything that we know about: stars, galaxies, clusters, superclusters, etc.



is the study of the structure and evolution of the Universe.
There have been many theories about the Universe, but they generally all share one fundamental postulate, the cosmological principle, which has two parts.


Cosmological principle

First, the Universe must be isotropic (the same in every direction).

For example, the Hubble deep field images, extending 12 billion ly away, for two completely different directions, are remarkably similar:

Second, the Universe must be homogeneous (uniform as far as one can travel).
In other words, no matter where you are located in the Universe, it will pretty much look the same as it does here.
Over small distances this is not the case, but over larger distances, however, the Universe does appear to be relatively uniform.


Two implications of the cosmological principle:

A) the Universe can have no center (which would violate isotropy)

B) the Universe can have no edge (which would violate homogeneity).  


The Cosmic Microwave Background Radiation

The spaces between the stars and galaxies is not empty.
Instead there is a very faint glow, almost exactly the same in all directions, not coming from any star or galaxy.

This glow is strongest in the microwave region of the EM spectrum, hence the name
cosmic microwave
background radiation.

The leading theory to explain this radiation holds that it is the left over reverberation from the Big Bang.


Cosmic Microwave Background (CMB)

The universe was once really hot and so its blackbody spectrum would peak at really short wavelengths.
It has now cooled off and its blackbody spectrum peaks in the microwave region
(now at a temperature of ~2.3 K).

Since the CMB is the same everywhere we look, we can conclude that the universe is isotropic.


Edwin Hubble

Demonstrated the existence of other galaxies besides the Milky Way

Hubble wanted to know the distance to the other galaxies

Hubble found that all distant galaxies are rushing away from us!


Hubble's Law

Hubble measured the distance to other galaxies and plotted them versus their speed

He got a direct relationship!
The farther away a galaxy is,
the faster it
is moving away.


Redshift of Galaxies

Most Galaxies are moving away from us!
So, as the Doppler effect tells us, the emission from the galaxies are redshifted.


How can we explain the fact that every distant galaxy is receding away from us?

Certainly we are not at the center of the Universe; there must be some other explanation

Hubble's observation of galactic redshifts demonstrated that the Universe itself must be expanding.
The space between any pair of galaxies increases with time
The further apart they are, the faster they separate.

An important consequence of this is that no galaxy (including our own) is at the "center" of the Universe.


What About the Galaxies Themselves?

On large scales, galaxies are moving apart, with velocity proportional to distance.
It’s not galaxies moving through space.
Space is expanding, carrying the galaxies along!

The galaxies themselves are not expanding

check slide 60 for example


The simplest explanation of the observed expansion

is that the universe received an initial large "push", easily overcoming gravity, which became known as