How do we really know what's out there?

Space is a pretty big place and, after all, we've never been farther from earth than our moon. How do we know what the stars are made of?

This is the electromagnetic spectrum. It consists of waves which vary inlength from very long (radio waves) to incredibly short (gamma rays). Aspecial part of the spectrum consists of waves that we can see. This iscalled the visible spectrum. We see different wavelengths as different colors ranging from red (long wavelengths) to blue/violet (short wave-lengths).

White light consists of all visible wavelengths together. When whitelight passes through a prism, a triangular piece of glass or plastic, thedifferent wavelengths are separated and can be seen individually.

This instrument is a spectroscope. This one is attached to a telescopeto separate the light from stars into a spectrum of different wave-lengths. Scientists can then look for specific patterns of wavelengths.

BASIC WORKING PRINCIPLE OF SPECTROSCOPE

PHYSICS DEPT SPECTROSCOPE

Simple model of the atomElectron absorbs some energy which causes it to “jump” to a higher energy level. This is called excitation.

E1

E2

Energy

Nucleus

De-excitation

E2

E1

Energy

The electron, preferring to be in the lower energy, immediately drops back down to the lower energy level.

In order to conserve energy, a photon (discrete bundle of energy) is emitted.

E2

E1

Photon Energy

Photon Energy = E2 – E1

Because of the behavior of matter and energy at the atomic scale, which is governed by Quantum Mechanics, the energy of the photon is equal to the difference in the energies of the two energy levels and is also proportional to its frequency.

That is: Ephoton = hf where

•h = Planck’s constant. •f = frequency of the photon.

Depending on the element, only certain excitations and de-excitations (or transitions) are allowed for the electrons.

•The cumulative result is that an atom of a particular element will only have light emissions of certain frequencies.

•The combination of all the allowed transitions produces an emission spectrum for that particular element.

•No two elements have the same emission spectrum, so the emission spectrum can be used to identify the element in question.

Emission Spectrum•Emission spectra are emitted by atoms in a gaseous state where the atoms are so far apart that interactions between them are negligible.

i.e. (each atom behaves as an isolated system.)

When any element is heated hot enough it begins to emit light. The pattern of wavelengths emitted by an element are like afingerprint. Each element emits its own unique pattern.

Above you can see the pattern of wavelengths emitted by the element hydrogen. Whenever this pattern is seen in the lightcoming from a star it means that hydrogen is present on thatstar.

This is called the emission spectrum of hydrogen.

This is the pattern of wavelengths emitted by iron. Wheniron is heated until it vaporizes, as in a star, it emits thisunique pattern of wavelengths. This is the emission spectrumof iron.

Below is the emission spectrum of nitrogen. If scientists seethis pattern in the light from a star they know that nitrogen ispresent.

Emission Spectrum of Various Elements

Emission Spectrum and Nebulae

Incandescence Hot Matter in condensed states

(solid or liquid or dense gas) nearly always emits radiation with a continuous spectrum.

•This is called Incandescence.

Continuous Spectrum

•When the atoms are in a condensed state, the electrons can make transitions not only within the energy levels of their own atom, but also between the levels of neighboring atoms.

•This results in a much larger number of possible transitions with corresponding frequencies of radiant energy, producing a continuous spectrum.

Continuous Spectrum

(Chaisson/McMillan) Astronomy

The continuous spectrum emitted by an ideal surface that is completely nonreflecting is called blackbody radiation.

•Example: A Star.

•The predominant frequency of radiation (the peak frequency) is proportional to the temperature of the emitter:

•f Temperature

Blackbody curve of visible light

Absorption Spectrum Absorption of certain lines when viewed

through a spectroscope, we see a absorption spectrum.

If a gas is placed between the source and the spectroscope, we see an absorption spectrum - the inverse of emission spectrum.

Continous Spectrum

Portion of the Emission Spectrum

Absorption Spectrum

Hot Gas

Cold Gas

Spectrum of Hydrogen, Emission lines

Bohr’s formula:

An approximate classificatio

n of  Spectral colors.Violet (380-435nm)

Blue(435-500 nm)Cyan (500-520 nm)Green (520-565 nm)Yellow (565- 590 nm)Orange (590-625 nm)Red (625-740 nm)

The normal wavelength of H- alpha line is 656.3 nm. The light is observed to be shifted to 670 nm, when coming from galaxy. What is the recessional speed of galaxy?

V= 6300 km/s.

Remember, the greater the amount of shift, the faster the star is moving!

Helium

Helium slightly red shifted. Moving away.

Helium more red shifted. Moving away even faster!

ALL of the light from stars and galaxies that reaches the earthis red shifted. What does that mean?

It means that everything is moving away from us!

How can everything be moving away from us?

The only explanation for that is .............................................

THE UNIVERSE IS EXPANDING!And if the universe is expanding then long ago it must havebeen much smaller. The expansion began about 14.7 billionyears ago with the.................

BIG BANG!

THANK YOU