Light, Spectra, Matter, & Forces

“Look, but don’t touch.”

- Astronomers’ Motto

Visible light is just one form of electromagnetic radiation.

The universe contains electrically charged particles: electrons (-) and protons (+).

Charged particles are surrounded by electric fields and magnetic fields.

Fluctuations in these fields produce electromagnetic radiation.

Visible light is a form of electromagnetic radiation -

- but so are radio waves, microwaves, infrared light, ultraviolet light, X-rays, and gamma rays.

Light can be thought of as a wave.

Wave = a periodic fluctuation travelling through a medium.

Ocean wave = fluctuation in the height of water.

Sound wave = fluctuation in air pressure.

Electromagnetic wave = fluctuation in electric and magnetic fields.

Wave Characteristics: (1) Wavelength, λ (lambda): distance between wave

crests (units = meter).

(2) Frequency, ν (nu): number of crests passing per second (units = 1/sec = Hertz).

(3) Amplitude, a: height of wave crests.

Speed of light:

Speed of wave, c, equals wavelength times frequency (units = meter/sec):

c = λ x ν

The speed of light in a vacuum is always

c = 300,000 km/s

(186,000 miles/sec).

Light can be thought of as a particle.

Light shows some properties of a wave: diffraction and interference.

It shows some properties of a particle: the photoelectric effect.

(In the photoelectric effect, particles of light, called photons, kick electrons out of atoms.)

How sound waves would travel without diffraction:

How sound waves actually travel with diffraction:

Diffraction happens for light, too!

Photons

The energy of a photon is related to the frequency of a wave:

E = hν E = energy of photon

ν = frequency of light

h = Planck’s constant (A Small Number)

Light forms a spectrum from short to long wavelength

Visible light has wavelengths from 400 to 700 nanometers. [1 nanometer (nm) = 10-9 meter]

Color is determined by wavelength: Blue: 480 nm

Green: 530 nm Red: 660 nm

The complete spectrum of light

Gamma rays (λ < 0.01 nanometers) X-rays (0.01 – 10 nm) Ultraviolet (10 – 400 nm) Visible (400 – 700 nm) Infrared (700 nm – 1 mm) Microwaves (1 – 100 mm) Radio (> 100 mm) Energy

Visible light occupies only a tiny sliver of the full spectrum.

Earth’s atmosphere is transparent to visible light and some microwaves and radio waves.

To observe efficiently at other wavelengths, we must go above atmosphere.

NASA's Kuiper Airborne Observatory flew a 91-cm telescope to altitudes as high as 45,000 feet. It operated from 1975 to 1995.

Sky: Optical

Sky: Radio

Sky: Microwaves

Sky: Infrared

Sky: X-ray

Atoms

Ordinary matter is found primarily in the form of atoms.

Range of ordinary matter:

–  free subatomic particles (protons & electrons)

–  single atoms (hydrogen, helium, gold, etc.)

–  simple molecules (O2, H2O)

–  macromolecules (DNA, complex polymers)

Atomic Structure

Nucleus of heavy subatomic particles: –  proton: positively charged –  neutron: uncharged (neutral)

Cloud of Electrons orbiting the Nucleus: –  electron: negatively charged. –  mass 1/1860th of proton

Mostly empty space

1 part in 1015 of the volume is occupied.

Simple Atoms

proton electron neutron

1H 4He

Chemical Elements

Distinguish atoms into Elements by the total number of protons in the nucleus. 1 proton = Hydrogen 2 protons = Helium 3 protons = Lithium ... and so on

Number of electrons = Number of protons (at least in conditions here on earth) Elements are Chemically Distinct

Isotopes

Distinguish elements into Isotopes by the number of neutrons in the nucleus.

Example: 12C has 6 protons and 6 neutrons 13C has 6 protons and 7 neutrons 14C has 6 protons and 8 neutrons

same # of protons & electrons, but different # of neutrons

Hydrogen 1 proton

Helium 2 protons

Lithium 3 protons

Proton: Neutron:

1H

3He

2H 3H

4He

6Li 7Li

Radioactivity If too many or too few neutrons in a nucleus, it is

unstable against radioactive decay. Examples:

3H (1p+2n) → 3He (2p+1n) + e- + νe 14C (6p+8n) → 14N (7p+7n) + e- + νe

(basis of radioactive carbon dating)

Free neutrons are unstable: n → p + e- + νe

Fundamental Forces of Nature

All interactions in nature are governed by 4 “fundamental” forces:

●  Gravitational Force

●  Electromagnetic Force

●  Strong Nuclear Force

●  Weak Nuclear Force

Gravitational Force

Gravitation binds masses over long distances

●  Long-range attractive force

●  Weakest force of nature

●  Obeys the Inverse Square Law of distance:

Electromagnetic Force

Acts between charged particles: •  like charges repel each other •  opposite charges attract each other

Long-range, inverse-square law force.

Binds: •  electrons to protons in atoms •  atoms to atoms in molecules

Very strong: 1040 times stronger than Gravity.

Strong & Weak Nuclear Forces

Short-range forces (<10-15 m) in atomic nuclei Strong Force:

–  binds protons & neutrons into nuclei. –  strongest force of nature.

Weak Force: –  responsible for radioactivity (turns neutron into a

proton) –  second weakest force.

Interplay of Forces

Gravity rules on the largest scales.

Electromagnetism rules on intermediate scales (atomic scales up to people)

Strong & Weak Forces rule on nuclear scales.

We will explore the different roles of each in our study of stars, galaxies & the Universe.

Spectra

“Twinkle, twinkle, little star, How I wonder what you are.”

Kirchoff’s Laws of Spectroscopy

1) A hot solid or hot, dense gas produces a continuous spectrum.

2) A hot, low-density gas produces an emission-line spectrum.

3) A continuous spectrum source viewed through a cool, low-density gas produces an absorption-line spectrum.

Continuum Source Cloud

A hot, transparent gas produces an emission spectrum.

Consider a single, isolated atom:

A nucleus, containing protons and neutrons, is surrounded by a cloud of orbiting electrons.

Electrons can emit or absorb photons.

Consider hydrogen (the simplest atom): one proton, one electron.

Behaviour on subatomic scales is governed by quantum mechanics.

One rule of quantum mechanics: electrons can only exist in orbits of particular energy (energy is quantizied).

Line Spectra

●  Electrons can only orbit in discrete Energy Levels.

●  Atoms & molecules can only emit or absorb photons at particular wavelengths. –  a unique “line spectrum” for each type of atom or

molecule.

–  what lines you see depends on the state of excitation and ionization of the system.

Emission & Absorption Lines

●  Emission Lines

Photons emitted at particular wavelengths when an electron jumps from a higher to a lower energy orbit.

●  Absorption Lines

Photons absorbed at particular wavelengths if their energy is exactly enough to make an electron jump up to a higher energy orbit.

Excitation

Start out in the Ground State:

All electrons are in their lowest energy orbits.

To excite an electron into a higher energy orbit, you need to absorb exactly the energy difference between orbits:

–  absorb a photon of exactly that energy

–  collide with an atom or electron and get the energy from the motion of the collider.

Absorb a Photon

Collide with an electron

photon

De-Excitation

Excited states are unstable, and electrons will decay back into their ground states.

To de-excite, an electron must rid itself of exactly the amount of excess energy: –  emit a photon of the exact energy. –  give up the energy to a colliding atom or electron (no

photons are emitted).

Emit a Photon

Collide with an electron

photon

Ionization

If an atom or molecule absorbs enough energy from a photon or a collision, an electron can be ejected.

Ion: positively charged atom or molecule.

–  Changes the spectral line signature

–  Changes the chemical properties

Distinguish ions by the number of electrons removed.

Absorb a Photon

Collide with an electron

ion photon

ion

When an electron falls from a high energy orbit to a low energy orbit, the energy lost is carried away by a photon.

In hydrogen, an electron falling from orbit 3 to orbit 2 emits a photon with λ = 656.3 nanometers.

Consider a hot, low density cloud of hydrogen.

Light is emitted only at wavelengths corresponding to energy differences between permitted electron orbits.

Results: an emission line spectrum.

The Carina Nebula

A cloud of hot, low density gas about 7000 light years away.

Its reddish color comes from the 656.3 nm emission line of hydrogen.

A cool, transparent gas produces an absorption spectrum.

Consider a cold, low density cloud of hydrogen in front of a hot blackbody.

Light is absorbed only at wavelengths corresponding to energy differences between permitted electron orbits.

Result: an absorption line spectrum.

Every type of atom, ion, and molecule has a unique spectrum.

Ion: an atom with electrons added (negative ion) or taken away (positive ion).

Molecule: two or more atoms bonded together.

The spectrum of each atom, ion, and molecule is a distinctive “fingerprint”.

The more complicated the atom, ion or the molecule, the more complex the spectrum.

From emission or absorption lines, we know:

1) which elements are present;

2) whether they are ionized;

3) whether they are in molecules.

emission spectrum of the Carina Nebula

The most abundant elements in the Universe are hydrogen and helium.

It is fairly easy to determine which elements are present in a star.

It is much harder to determine how much of each element is present.

Strength of emission and absorption lines depends on temperature as well as on the element’s abundance.

Abundance of elements in the Sun’s atmosphere:

Hydrogen (H): 75% Helium (He): 23%

Everything else: 2%

As discovered in 1920’s, other stars are mostly hydrogen and helium, too.

Cecilia Payne-Gaposchkin (1900-1979) was a British-American astronomer. She left England in 1922. In 1925, she became the first ever Ph.D. in astronomy from Harvard. Her thesis established that hydrogen was the overwhelming constituent of the stars.

A hot, opaque object produces a continuous blackbody spectrum of light.

The universe is full of light of all different wavelengths. How is light made?

One way to make objects emit light is to heat them up.

An object is hot when the atoms of which it is made are in rapid random motion.

The temperature is a measure of the average speed of the atoms.

Random motions stop at absolute zero temperature.

Temperature Scale:

In physics and astronomy, we use the Kelvin scale, which has a zero at absolute zero.

Kelvin = Celsius + 273

Water boils: 373 Kelvin

Water freezes: 273 Kelvin

Absolute zero: 0 Kelvin

Black Body Radiation

A Blackbody is an object that absorbs all light.

•  Absorbs at all wavelengths

•  Characterized by its Temperature

It is also the perfect radiator:

•  Emits at all wavelengths (continuous spectrum)

•  Total Energy emitted depends on Temperature

•  Peak wavelength also depends on Temperature

Wavelength of maximum emission is inversely related to temperature.

Stefan-Boltzmann Law Energy emitted per second per area by a blackbody

with Temperature (T):

σ is Boltzmann's constant (a number).

In Words: “Hotter objects are Brighter at All Wavelengths”

Blackbody curves:

Solar spectrum:

Betelgeuse: a reddish star (cooler).

Rigel: a bluish stars (hotter).

Taking the temperature of

stars

Stellar spectra in order from the hottest (top) to coolest (bottom).

Few closing questions:

1) Which one is hotter – a red-hot piece of metal, or white-hot piece of metal?

2) Could we have radio eyes?

3) If we double the temperature of a blackbody, how will the energy per unit area it produces change?

4) If we double the temperature of a blackbody, how will its maximum intensity shift in wavelength?

Few closing questions:

1) What kind of spectrum will be produced by very hot, but also very dense hydrogen gas?

2) If you have hot gas in front of a star, what kind of spectrum will you see?

3) Which spectrum is more complex: that of hydrogen or that of helium?

4) If you double the diameter of a telescope, how much more light will it collect?

Extra slides

●  Doppler shift

●  Telescopes

The radial velocity of an object is found from its Doppler shift.

Radial velocity = how fast an object is moving toward you or away from you.

If a wave source moves toward you or away from you, the wavelength is changed.

The reason for Doppler shifts:

Wave crests are bunched up ahead of the light source, stretched out behind.

Lecture 2: Light

The Doppler Effect in Light

Amount of Shift depends upon the emitted wavelength (λem) and the relative speed v:

●  If the motion is away from observer Wavelength gets longer = REDSHIFT

●  If the motion is towards the observer Wavelength gets shorter = BLUESHIFT

If a light source is moving toward you, the wavelength is shorter (called a “blueshift”).

If a light source is moving away from you, the wavelength is longer (called a “redshift”).

Size of Doppler shift is proportional to radial velocity.

Example:

Way to Measure Speeds

Observe the wavelength (λobs) of a source with a known emitted wavelength (λem)

The difference is directly proportional to the speed of the source, v:

The main purposes of a telescope are to gather light and resolve detail.

A telescope is sometimes called a “light bucket”.

Number of photons collected per second is proportional to the area of the lens/mirror:

Area = π/4 x D2

where D = diameter of the lens/mirror.

A convex lens (thicker in the middle) focuses light to a point:

Light from a large area is funneled into a small area.

Light with a short wavelength is bent through a larger angle than light with a long wavelength.

(This is why prisms spread light into a spectrum.)

The world’s biggest telescopes are reflectors (mirrors), not refractors (lenses).

The problem with lenses: 1) Lenses absorb light. 2) Lenses sag. 3) Lenses have chromatic aberration:

colors do not focus at same point.

The world’s largest refracting telescope:

Yerkes Observatory, Wisconsin

1 meter diameter

Completed 1897

A mirror shaped like a parabola focuses light to a point:

Light from a large area is funneled into a small area.

Lenses and mirrors (if shaped correctly) produce an accurate image of an object.

Soon to be the world’s largest reflecting telescope (2x8.4 m):

BIGGER IS BETTER

Larger diameter for your lens or mirror means more light, higher resolution.

Texas A&M is a founding partner in the “Giant Magellan Telescope” project

www.gmto.org