Just by analyzing the light received from a star, astronomers can retrieve information about a star’s:

1. Total energy output

2. Surface temperature

3. Radius

4. Chemical composition

5. Velocity relative to Earth

6. Rotation period

An atom has an atomic nucleus at its center consisting of 2 subatomic particles:› Protons – carry a positive

charge› Neutrons – carry no charge

Thus, the nucleus of an atom has a net positive charge.

The nucleus is surrounded by a cloud of orbiting low-mass particles:› Electrons – carry a negative

charge

Hydrogen atom magnified by 1012

Nucleus grape seed

Diameter of electron cloud 4.5 times larger than football field

Atoms with the same number of protons, but a different number of neutrons are called isotopes.

An atom that has lost or gained one or more electrons is said to be ionized and is called an ion.

Two or more atoms bonded together form a molecule.

› 7 diatomic molecules include: I, H, N, Br, O, Cl, F

Electrons are bound to the atom by the attraction between their negative charge and the positive charge on the nucleus.

› This attraction is known as the Coulomb Force.

To ionize an atom, you need a certain amount of energy to pull an electron completely away from the nucleus.

› This energy is the electron’s binding energy, the energy holding it to the atom.

The size of an electron’s orbit is related to the energy that binds it to the atoms.› If it orbits close large binding energy› If it orbits far small binding energy

The electron in an atom may occupy only certain permitted orbits.

(Step example)

The arrangement of these orbits depends primarily on the charge of the nucleus.

Nature permits atoms only certain amounts (quanta) of binding energy, and the laws that describe how atoms behave are called the laws of quantum mechanics.

› Set of rules describing how atoms and subatomic particles behave.

› You cannot know simultaneously the exact location and the motion of a particle. Scientists prefer to think of a cloud

of electrons, rather than orbits.

Each electron orbit in an atom represents a specific amount of binding energy, so physicists commonly refer to these orbits as energy levels.

› Using this, you can say an electron in its smallest and most tightly bound orbit is in its lowest permitted energy level, which is called the atom’s ground state.

› If you move an electron from a low energy level to a higher energy level, the atom moves to its excited state.

A neon sign glows when atoms of neon gas in a glass tube are excited by electricity flowing through the tube.› As the electrons in the electric

current flow through the gas, they collide with the neon atoms and excite them.

› The photons emitted by excited neon blend to produce a reddish-orange glow.

Neon signs are simple, but stars are more complex … what gives them their color?

Stars appear different in colors, from blue like Rigel, to green/yellow like our Sun and red like Betelgeuse.

› These colors tell us about a star’s temperature.

Betelgeuse

Rigel

ORION CONSTELLATION

The molecules and atoms in any object are in constant motion, and in a hot object they are more agitated than in a cooler object.

This can be referred to as thermal energy.› The flow of thermal energy is

called heat.

› In contrast, temperature refers to the average speed of the particles.

When astronomers refer to the temperature of a star, they are referring to the temperature of the gases on its surface photosphere.› Expressed on the Kelvin scale.

› 0 K = absolute zero (-459.7°F), the temperature at which an object contains no thermal energy that can be extracted.

› Water freezes at 273 K and boils at 373 K.

The radiation emitted by a heated object is known as blackbody radiation, a name translated from a German term referring to the way a perfectly opaque object would behave.› Perfect absorber and emitter of radiation

reflects no radiation, thus appearing black.

› At higher temperatures glow at wavelengths visible to the human eye.

› Blackbody radiation is present in cold objects as well since they are above absolute zero. Humans emit mainly infrared radiation.

The wavelength of maximum intensity (λmax) is the wavelength at which an object emits the most intense radiation and occurs at some intermediate wavelength.› Hotter object ~ shorter λmax

Two features of blackbody radiation are commonly considered radiation laws:› Stefan-Boltzmann Law

Concerning energy› Wien’s Law

Concerning color

The hotter an object, the more energy it emits.

› The total radiation given off by 1 square meter of the surface of an object equals a constant number, represented by Greek lower-case letter sigma,, times the temperature raised to the fourth power.

Stefan-Boltzmann Stefan-Boltzmann LawLaw

E = E = TT44

E = Energy (J/s/m2)

Stefan-Boltzmann constant ~ 5.67 x 10-8

J/s/m2K4

T = temperature, in K

Suppose a star the same size as the Sun had a surface temperature that was twice as hot as the Sun’s surface. How much more energy would this star radiate than the Sun?

› You can see a small difference in temperature can produce a very large difference in the amount of energy a star’s surface emits.

The peak of the blackbody spectrum shifts towards shorter wavelengths when the temperature increases.

› The wavelength at which a star radiates the most energy, its wavelength of maximum intensity (λmax), depends only on the star’s temperature.

Wien’s LawWien’s Law

λλmax max = 2.9 x = 2.9 x 101066/T/Tλmax = wavelength of maximum intensity, in

nm.

T = temperature, in K

What is the wavelength of maximum intensity of a star with a surface temperature of 2900 K?

The stellar spectra of stars are more complicated than pure blackbody spectra.

They contain characteristic lines, called absorption lines.

1. THE CONTINUOUS SPECTRUM

› A solid, liquid, or dense gas excited to emit light will radiate at all wavelengths and produce a continuous spectrum.

2. THE EMISSION SPECTRUM› A low-density gas excited to emit

light will do so at specific wavelengths and thus produce and emission spectrum.

Light excites electrons in atoms to higher

energy states.

Transition back to lower states emits

light at specific frequencies.

3. THE ABSORPTION SPECTRUM

› If light comprising a continuous spectrum passes through a cool, low-density gas, the result will be an absorption spectrum.

Light excites electrons in atoms to higher

energy states.

Frequencies corresponding to

transition energies are absorbed.

The inner, dense layers of a star produce a

continuous (blackbody) spectrum.

Cooler surface layers of the star absorb

light at specific frequencies.

Spectra of stars are absorption spectra.

Each element produces a specific set of absorption (and emission) lines.

Comparing the relative strengths of these sets of lines, we can study the composition of gases.

By far, the two most abundant elements in the universe are Hydrogen (H) and Helium

(He).

When an electron makes a transition from one orbit to another, it changes the energy stored in the atom.

Transitions in the Hydrogen (H) atom can be grouped into series

› Lyman Series UV

› Balmer Series Visible and UV

› Paschen Series Infrared

You can use the Balmer absorption lines as a thermometer to find the temperatures of stars.

› The strength of the Balmer lines of Hydrogen depends on the temperature of the star’s surface layers.

Hot and cool stars have weak Balmer lines.

Medium-temperature stars have strong Balmer lines.

25,000

25,000

Using the Balmer Thermometer, find the temperature of the stars with the following spectral lines:

› Medium-strength Balmer (H) lines; Strong Helium (He) lines.

› Weak Balmer (H) lines; Strong lines of ionized iron (Fe).

Different types of stars show different characteristic sets of absorption lines.

Mnemonics to remember the spectral sequence:

Oh Oh Only

Be Boy, Bad

A An Astronomers

Fine F Forget

Girl/Guy Grade Generally

Kiss Kills Known

Me Me Mnemonics

Modern digital spectra are represented digitally as graphs of intensity versus wavelength with dark absorption lines appears as sharp dips in the curves.

Astronomers measure the wavelengths of the lines in a star’s spectrum and find the velocity of a star.

The Doppler Effect is the apparent change in the wavelength of radiation caused by the motion of the source.› When a moving source of light, such as a star, moves

closer to an observer, they will see a shorter wavelength of light shifted towards the blue-end of the spectrum. This is known as a blue-shift.

› When a moving source of light, such as a star, moves away from an observer, they will see a longer wavelength of light shifted towards the red-end of the spectrum. This is known as a red-shift.

The Doppler Effect is sensitive only to the part of the velocity directed away from you or toward you known as the radial velocity.

You cannot use it for perpendicular velocity.

Radial VelocityRadial Velocity

VVrr/c = /c = ΔλΔλ//λλooVr = radial velocity

c = speed of light

λo = un-shifted wavelength

You observe a line in a star’s spectrum with a wavelength of 600.1 nm. Laboratory measurements show the line should have a wavelength of 600 nm. What is the star’s radial velocity? Is it approaching us or receding from us?