ATOMIC STRUCTURE To understand a star and how it works you need to understand the structure of an atom, and how energy is produced by an atom and its subatomic.

ATOMIC STRUCTURE

To understand a star and how it works you need to understand the

structure of an atom, and how energy is produced by an atom and

its subatomic components.

SUBATOMIC PARTICLES

There are three basic components of an atom,called subatomic particles.

Proton (p+)Electron (e-)Neutron (n0)

Actually there are sub-subatomic particles (i.e., components of theproton, electron, and neutron). One of these is called the neutrino (ν0). There are three kinds of neutrinos. The electron variety will play a pivotal role in understanding the energy production in a star.

The proton (p+) and neutron (n0) have the same mass. Whilecompared to the electron (e-), the proton and neutron have 1836times the mass. In other words, the mass of an atom is concentratedprimarily in the protons and neutrons that make up its structure.

Mass of p+ = Mass of n0 = 1.67 x 10-27 kg

Mass of e- = 3127 1010.91066.11836

1 xx kg

The proton (p+) and electron (e-) possess an electric charge. They have the same size charge, only oppositely directed.

The proton has a positive charge and the electron a negative charge, which means the two subatomic particles attract one another.

Two protons repel one another as do two electrons.

p+ e-

Oppositely charged particles attract.

p+ p+

e-e-

Particles with the same charge repel.

HYDROGEN ATOM

• Combinations of the subatomic particles form the atoms.

• The most basic combination is one proton (p+) and one electron (e-). This is the most common structure in the universe accounting for over 99% of the atoms.

p+

e-

Electron OribtalAtomic Nucleus

• The core of the atom is called the nucleus.

• The nucleus contains the majority of the atom’s mass. In the nucleus are the protons and neutrons.

• The electrons move about the nucleus in a quantuum mechanical cloud called orbitals.

HELIUM ATOM

In the orbitals around the nucleus, there is the same number of electrons as protons in the nucleus.

Under normal temperature and pressure conditions found on Earth’s surface, the atoms are electrically neutral (i.e., the number of protons and electrons are the same).

The nucleus of the helium atom has two protons and two neutrons.

Helium atoms constitute about 1% of the atoms in the universe.

p+

p+

n0

n0

e-

e-

LITHIUM ATOM

The nucleus of a typical lithium atom contains 3 p+ and 4 n0.

ATOMIC NUMBER & ATOMIC MASS NUMBER

• The number of protons in the nucleus of an atom is called its atomic number.

• The combined number of protons and neutrons in the nucleus of an atom is called its atomic mass number.

FACT:

The atoms are identified by their atomic number (i.e., the number of protons in their nucleus).

Hydrogen atoms always have 1 p+, helium atoms have 2 p+, lithium atoms have 3 p+, beryllium atoms have 4 p+, boron atoms 5 p+ and so on.

PERIODIC TABLE OF ELEMENTS

Group**

Period 1 IA 1A

18

VIIIA 8A

1 1 H

1.008

2

IIA 2A

13

IIIA 3A

14

IVA 4A

15

VA 5A

16

VIA 6A

17

VIIA 7A

2 He 4.003

2 3

Li 6.941

4 Be 9.012

5 B

10.81

6 C

12.01

7 N

14.01

8 O

16.00

9 F

19.00

10 Ne 20.18

8 9 10 3

11 Na 22.99

12 Mg 24.31

3

IIIB 3B

4

IVB 4B

5

VB 5B

6

VIB 6B

7

VIIB 7B

------- VIII -------

------- 8 -------

11

IB 1B

12

IIB 2B

13 Al

26.98

14 Si

28.09

15 P

30.97

16 S

32.07

17 Cl

35.45

18 Ar 39.95

4 19 K

39.10

20 Ca 40.08

21 Sc

44.96

22 Ti

47.88

23 V

50.94

24 Cr

52.00

25 Mn 54.94

26 Fe

55.85

27 Co 58.47

28 Ni 58.69

29 Cu 63.55

30 Zn 65.39

31 Ga 69.72

32 Ge 72.59

33 As 74.92

34 Se

78.96

35 Br

79.90

36 Kr 83.80

5 37

Rb 85.47

38 Sr

87.62

39 Y

88.91

40 Zr

91.22

41 Nb 92.91

42 Mo 95.94

43 Tc (98)

44 Ru 101.1

45 Rh 102.9

46 Pd 106.4

47 Ag 107.9

48 Cd 112.4

49 In

114.8

50 Sn 118.7

51 Sb 121.8

52 Te 127.6

53 I

126.9

54 Xe 131.3

6 55 Cs 132.9

56 Ba 137.3

57 La* 138.9

72 Hf 178.5

73 Ta 180.9

74 W

183.9

75 Re 186.2

76 Os 190.2

77 Ir

190.2

78 Pt

195.1

79 Au 197.0

80 Hg 200.5

81 Tl

204.4

82 Pb 207.2

83 Bi

209.0

84 Po (210)

85 At (210)

86 Rn (222)

7 87 Fr

(223)

88 Ra (226)

89 Ac~ (227)

104 Rf (257)

105 Db (260)

106 Sg (263)

107 Bh (262)

108 Hs (265)

109 Mt (266)

110 ---

()

111 ---

()

112 ---

()

114 ---

()

116 ---

()

118 ---

()

Lanthanide Series*

58 Ce 140.1

59 Pr

140.9

60 Nd 144.2

61 Pm (147)

62 Sm 150.4

63 Eu 152.0

64 Gd 157.3

65 Tb 158.9

66 Dy 162.5

67 Ho 164.9

68 Er

167.3

69 Tm 168.9

70 Yb 173.0

71 Lu 175.0

Actinide Series~ 90

Th 232.0

91 Pa (231)

92 U

(238)

93 Np (237)

94 Pu (242)

95 Am (243)

96 Cm (247)

97 Bk (247)

98 Cf (249)

99 Es (254)

100 Fm (253)

101 Md (256)

102 No (254)

103 Lr

(257)

** Groups are noted by 3 notation conventions.

Abbreviation for the atom.

Atomic Number

Atomic Mass Number

The Periodic Table shows that the oxygen atom has an atomic number of 8 and an atomic mass number of 16. In other words, the nucleus of the oxygen atom contains 8 protons and 8 neutrons.

16O8

(# of p+) + (# of n0)

# of p+

The Periodic Table shows that the aluminum atom has an atomic number of 13 and an atomic mass number of 27. In other words, the nucleus of the oxygen atom contains 13 protons and 14 neutrons.

27Al13

(# of p+) + (# of n0)

# of p+

ISOTOPES

• The number of protons in the nucleus (i.e., the atomic number) defines the type of atom. A hydrogen atom always has one proton in its nucleus, a helium always has two, and so on.

• The number of neutrons in a nucleus can vary, however. An atom with different numbers of neutrons in the nucleus is called an isotope.

ISOTOPES OF HYDROGEN

There are three different isotopes of hydrogen.

1H1

2H1

3H1

Ordinary Hydrogen

Deuterium – “Heavy” Hydrogen

Tritium - Radioactive Hydrogen

ISOTOPES OF HELIUM

4He2

Ordinary Helium

3He2

STABILITY OF ISOTOPES• The role of the neutron in the nucleus of an atom is to

keep the positively-charged protons from repelling each other (i.e., to keep the nucleus from flying apart).

• At the lower end of the Periodic Table for an isotope to be stable, the number of neutrons in the nucleus is approximately equal to the number of protons.

• At the higher end of the Periodic Table, the number of neutrons is much greater than the number of protons in order to maintain stability.

TOO FEW NEUTRONS

12C6

The most stable isotope of carbon,# of n0 = # of p+

10C6

Unstable isotope of carbon,# of n0 < # of p+

10C68Be4 2 1H1

+

TOO MANY NEUTRONS

12C6

The most stable isotope of carbon,# of n0 = # of p+

14C6

Unstable isotope of carbon,# of n0 > # of p+

14C611B5

+ 3H1

IONS

• Under conditions found at Earth’s surface, most atoms have equal numbers of protons and electrons (i.e., they are electrically neutral).

• It is possible for atoms to lose or gain electrons giving them a net electrical charge.

• Atoms that have have more or fewer electrons than protons are called ions.

IONS OF HYDROGEN

Hydrogen with no electrons. It is positively charged.

Hydrogen with two electrons. It is negatively charged.

Unionized hydrogen. It is electrically neutral.

H II

H I

H-

IONS OF HELIUM

Unionized helium.

Singly-ionized helium.

Doubly-ionized helium.

He I

He II

He III

ELECTROMAGNETIC RADIATION (EMR)

• Electromagnetic Radiation occurs in the universe in a variety of different forms.

• One common form is visible light. The human eye can perceive visible light.

• Another form is infrared radiation (IR). The human eye cannot perceive IR but we feel its effects and sense it as warmth coming, for example, from a campfire.

• Ultraviolet Radiation is not perceivable to the human eye, either. However, when exposed at length to UV the skin experiences sunburn.

• Radio Waves are a form of EMR. These waves are modulated and used for communication purposes.

• X-Rays are a form of EMR. This form of EMR is used in medicine to take shadow photographs of the inside of the human body.

• Finally, γ-Rays are a form of EMR that comes directly from the nucleus of an atom or the hot thermonuclear core of a star like the Sun.

ENERGY SCALE FOR EMR

The various forms of EMR can be put along scale based upon their energy. This scale is called

the electromagnetic spectrum.

IR Visible LightRadio Waves UV Radiation X-Rays γ-Rays

Direction of increasing energy

Low Energy EMR High Energy EMR

TWO MODELS FOR EMR

• Physicists have two models for EMR. The first is the wave model, in which EMR is thought of as a wave traveling through space. This model was utilized in the 19th Century until the start of the 20th Century when the model was not sufficient to explain some of the observed characteristics of EMR.

• The second model is the particle model, in which EMR is thought of as a particle called a photon.

DUALITY OF EMR• No one truly knows what EMR is. They can describe it,

predict its behavior, and use it as a tool but no one truly knows its fundamental nature.

• To complicate matters further, sometimes the wave model accurately predicts the behavior of EMR while the particle model fails to do so. Other times it is just the opposite.

• To date no behavior of EMR has ever been observed that cannot be explain by either model or both models. This is called the dual nature or duality of EMR.

WAVE MODEL OF EMR

To describe a wave you need to measure two parameters:

Wavelength (λ) Frequency (ν)

λ

The wavelength of EMR is the distance from one wave crest to the other.

Direction of motion of the wave.

The frequency of EMR is the number of wave crests that pass by each second.

cSpeed of Lightc = 300,000 km/s

Wavelength (λ)

Frequency (ν)

LOOKING AGAIN AT THE ELECTROMAGNETIC SPECTRUM

Low Energy EMR High Energy EMR

Radio Waves IR Visible Light UV Radiation X-Rays γ-Rays

Long Wavelength EMR Short Wavelength EMR

Low Frequency EMR High Frequency EMR

WAVELENGTHS FOR VISIBLE LIGHT

• EMR perceptible to the human eye is called visible light.

• Visible light has a wavelength in the range of 4 x 10-7 m to 7 x 10-7 m.

• Astronomers define the Angstrom (Å) to be 10-10 m, and use this unit for the wavelength measurement of visible light.

• The wavelength of visible light varies between 4000 Å and 7000 Å.

THE HUMAN EYE PERCEIVES WAVELENGTH AS COLOR

Color Wavelength

Violet 4000 Å

Blue 4500 Å

Green 4800 Å

Yellow 5000 Å

Orange 5500 Å

Red-Orange 6000 Å

Red 7000 Å

Infrared radiation (IR) has a wavelength longer than 7000 Å. Ultraviolet radiation (UV) has a wavelength shorter than 4000 Å.

UV

IR

WHITE LIGHT IS THE MIXTURE OF ALL COLORS…

IN EQUAL AMOUNTS

Pass white light through a prism.

A prism refracts light by its color (i.e., its wavelength).

The white light is refracted into its component wavelengths.

FORMATION OF ELECTROMAGNETIC RADIATION

Electromagnetic Radiation (EMR) is formed when charged subatomic particles (protons or electrons)

are accelerated (i.e., they change their velocity).

• Collisions between atoms or between subatomic particles.

• Charged subatomic particles passing through an electric field or a magnetic field.

• Electron dropping from a high energy orbital to a low energy orbital.

• Protons dropping from a high energy state in the nucleus of an atom to a low energy state.

FORMATION OF 6563 Å HYDROGEN LINE (Hα)

When the electron drops from the high energy orbital #3 to the low energy orbital #2, it emits a red photon with a wavelength of 6563Å.This is sometimes call the Hα line, and is very prominent in the types

of photons emitted by the electron in the hydrogen atom.

e-

p+

6563 Å red photon

n = 3

n = 2n = 1

COLLISIONS OF SUBATOMIC PARTICLES OR ATOMS

PRODUCE EMR

e- e-

e-

e-

Prior to the collision

After the collision

The energy (i.e., wavelength) of the photon depends upon on the exchange of energy between the two subatomic particles.

Ephoton = (E1,prior + E2,prior )– (E1,after + E2,after )

Photon

ALL MATTER IN THE UNIVERSE EMITS EMR

It is a fundamental property of matter that any object with atemperature above absolute zero (T > 0 K) emits EMR.

This EMR is often referred to as thermal radiation.

Thermal radiation is produced by the collisions of the atoms that make up the object.

THE STATE OF THE MATTER DETERMINES THE NATURE OF

ITS THERMAL RADIATION

• A solid, liquid, or high density gas produces EMR at all energies (i.e., wavelengths)…but not in equal amounts at each of the wavelengths.

• A low density gas does not produce EMR at all wavelengths. Instead, it produces EMR at specific energies (i.e., wavelengths) that are unique to the types of atoms that make up the gas.

Continuous Spectrum

Emission Line Spectrum

THERMAL RADIATION FROM A SOLID, LIQUID, OR

HIGH DENSITY GAS PRODUCES A

CONTINUOUS SPECTRUM

Wavelength

Inte

nsi

ty o

f E

MR

Planck’s Curve

λmax

Maximum Intensity of EMR

WIEN’S LAW

λmax is inversely proportional to the temperature of the solid, liquid, or high density gas.

Imagine putting a fireplace poker into a roaring fire.

Before heating, the poker is at room temperature. The thermal radiation is not seen. The reason is that most of the EMR is in infrared (10,000 Å), which the human eye cannot detect

As the poker increases its temperature, the first color the eye detects is red (7000 Å).

At the temperature increases even more the poker glows as yellow (5000 Å).

At even higher temperatures it glows blue-white hot (4500 Å).

As the temperature of the fireplace poker increases, the wavelength of its thermal radiation decreases.

T

x 7

max

10898.2

Wien’s Law

Maximum Wavelength in ÅTemperature in K

max

710898.2

x

T

APPLICATIONS OF WIEN’S LAW

The human body is approximately 100o F. What is the maximum wavelength

of its thermal radiation?

31127338273

38689

532100

9

532

9

5

CK

FC

o

oo

o

Ax

T

x180,93

311

10898.210898.2 77

max

The star Sirius appears blue-white. This color is the maximum wavelength of the

thermal radiation from its photosphere. What is the photospheric temperature of Sirius?

Kxx

T 440,64500

10898.210898.2 7

max

7

Photosphere

Interior

Core

The photosphere of a star is a high density gas. Its thermal radiation produces a continuous spectrum.

The star Antares appears red. This color is the maximum wavelength of the

thermal radiation from its photosphere. What is the photospheric temperature of Antares?

Kxx

T 140,47000

10898.210898.2 7

max

7

THE FRINGE COLOR OF THE STAR DETERMINES THE TEMPERATURE

OF ITS PHOTOSPHERE

Fringe Color Wavelength Temperature

Violet 4000 Å 7,245 K

Blue 4500 Å 6,440 K

Green 4800 Å 6,037 K

Yellow 5000 Å 5,796 K

Orange 5500 Å 5,269 K

Red-Orange 6000 Å 4,830 K

Red 7000 Å 4,140 K

Wien’s Law

FRINGE COLOR OF A STAR GIVES λmaxIn

ten

sity

of

EM

R

Wavelength 7000 Å λmax

White Light

White Light with a red fringe

THERMAL RADIATION FROM A LOW DENSITY GAS PRODUCES AN

EMISSION LINE SPECTRUMIn

ten

sity

of

EM

R

Wavelength

4800 Å 5000 Å 7000 Å

THE PATTERN OF COLORS (i.e., WAVELENGTHS) IN THE

EMISSION LINE SPECTRUM IS UNIQUE TO THE TYPE OF ATOM

THAT MAKES UP THE LOW DENSITY GAS

In other words the pattern of colors or wavelengths can be usedto identify the types of atoms (and molecules) in the low density outer atmospheres of stars or in the low density clouds of gas found within the equatorial plane of the Milky Way Galaxy.

THE EMISSION LINE SPECTRUM

IS LIKE AN ATOM’S FINGERPRINT

4300 Å 4900 Å 6700 Å 4000 Å 4300 Å 5100 Å 5900 Å 6800 Å

Inte

nsi

ty

Inte

nsi

ty

Emission Line SpectrumHydrogen

Emission Line SpectrumHelium

ABSORPTION LINE SPECTRUM

When a continuous spectrum passes through a cool, low density gas, an absorption line spectrum is produced.

The atoms in the gas absorb EMR at the same wavelengths that they emit it.

Absorption Line Spectrum

Continuous Spectrum

Cool, Low Density Gas

AN ABSORPTION LINE SPECTRUM IS LIKE A REVERSE VIDEO IMAGE OF

THE EMISSION LINE SPECTRUM

Inte

nsi

ty

4300 Å 4900 Å 6700 Å

Emission Line SpectrumHydrogen

4300 Å 4900 Å 6700 Å

Absorption Line SpectrumHydrogen

Inte

nsi

ty

KIRCHOFF’S RADIATION LAWS

1. A hot solid, liquid, or high density gas produces a continuous spectrum

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

3. A continuous spectrum passing through a cool, low density gas produces an absorption line spectrum