Scientific Method

• The principles & empirical processes of discovery &

demonstration considered characteristic of or necessary

for scientific investigation, generally involving

• The method employed in exact science & consisting of

a) The observation of phenomena

b) The formation of a hypothesis concerning the phenomena

c) Experimentation to demonstrate the truth or falseness of

the hypothesis

d) & a conclusion that validates or modifies the hypothesis

a) Careful & abundant observation & experimentation

b) Generalization of the results into formulated “Laws” &

statements

The Sun

• Nearest Star

• Importance

• Outline of Discussion of Sun

1) Heating source (solar constant = 1370 Watts/ m2)

2) Source of illumination

1) General Properties

2) Distance to Nearest Stars

3) How is the Sun powered?

4) Spectroscopy

5) Structure of Sun

6) Comparison of Sun to Other Stars

Properties of Sun

• Mass = 2.0x1030 kg (333,000 Earth masses)

• Diameter = 1.4x109 m (109 Earth Diameters)

• Average Density = (Mass/Volume) = 1.4 g / cm3

• Luminosity (i.e., total power output) = 4x1026

Watts

(Distance to the Sun from the Earth = 1 AU = 1.5x1011 m)

How are these quantities measured

• The distance to the Sun is measured from

Kepler’s 3rd Law, i.e., (Period)2 = (Distance)3.

This allows a determination of the diameter.

• Luminosity = 4 ! (Solar Constant)(Distance)2.

• The mass can be determined using Newton’s law

of gravity,

• Density = Mass / Volume

(G Msun mearth) / D2 = (mearth [vearth]

2) / D

Msun = [vearth]2 D / G

How do we know others stars are like

the Sun?

• By measuring the

distances to them, then

calculating their

luminosities

• Method: Parallax – the

apparent displacement

of an object caused by

the motion of the

observer

Earth-Sun Distance

Distance to Star" =

Parallax – Another Example

Matter

• Element: a substance that cannot be broken

down by chemical means into simpler

substances

• Atom: The smallest particle of an element that

has the properties that characterize that element

How is the Sun Powered?

• Atoms have nuclei comprised of positively charged protons& neutrally charged neutrons, as well as negatively chargedelectrons that orbit the nucleus

• Isotope: Any of several forms for the same element whosenuclei all have the same number of protons but differentnumbers of neutrons

The Sun is powered by thermonuclear

fusion

• Einstein: the equivalence of Mass

& Energy

E = mc2

• Thermonuclear Fusion: The

joining of atomic nuclei at high

temperatures to create a new,

more massive atom with the

simultaneous release of energy

• Why is energy released?

Mass of 4 hydrogen = 4mproton

Mass of helium = 3.97mproton

Converted to energy = 0.03 mproton

Another look at the book-keeping• The thermonuclear reactions occurring in the core of the

sun

• However, the masses don’t add up

• The missing link, the release of energy

• Thus the efficiency of converting mass to energy is

4 (hydrogen nuclei mass) = 1 (helium nucleus mass)

4 (proton mass) = 3.97 (proton mass)

0.03 / 4 = 0.0075, or 0.75% -> E = 0.0075 mp+ c2

A more detailed look at the process

Two important points about Fusion

1) Fusion is the way by which elements heavier

than hydrogen are built

• As stars evolve, they fuse different forms of light

nuclei into heavier nuclei (such a Carbon & Iron)

• Thus, without fusion, there would be no planets like

the earth

Two important points about Fusion,

cont.

2) The balance between the force of the (outward)

radiation pressure from fusion reactions & the

(inward) force of gravity is what keeps stars

stable

• Such stability is important for life on planets

• The Sun will stay in its present state for

• The Sun is already about 5 billion years old, so it has

5 billion more years to go in its present state

Lifetime =0.0075 Masssun c

2

Luminositysun

(0.1) ~ 1010 years.

How do we determine the composition

of astronomical objects?

• Answer – Spectroscopy

• Our eyes are sensitive to optical light, but we can buildinstruments sensitive to other forms of “light” (orradiation)

Photon – discrete unit of electromagnetic

energy• Massless

• Travels at 3x108 m / s (speed of light)

• Has specific frequency & wavelength

• Energy = h x (frequency), h = 6.63x10-34 J.s

• Speed of wave = (frequency) x (wavelength)

Wavelength & Frequencies – some

examples

Different kinds of atoms emit & absorb

different kinds of photons

Emission & Absorption

• Ionization: the process by which an atom loses electrons

• Ion: an atom that has become electrically charged due tothe loss of one or more electrons. Note that isolatedatoms are electronically neutral – i.e, they have thesame number of protons & neutrons – unless they areionized.

Emission & Absorption – more

examples

Spectrum of the Sun

• We can then use a

spectrometer to obtain a

spectrum of the Sun &

determine what elements

are present

• This process can be used

for all astronomical

objects

Emission

vs.

Absorption

Lines

Cosmic Abundances of Major Elements

• The Sun is primarily

Hydrogen & Helium

• The abundance of the

Earth & Life (on Earth) is

different from that of the

Sun

• The Earth’s crust is

primarily Oxygen,

Aluminum, & Calcium

• Life is primarily

Hydrogen, Oxygen,

Carbon, & Nitrogen

I.e.,

Interior of the Sun

• Core: center of Sun (15x106 K)

• Radiative zone: region of sun where energy istransported via radiation

• Convective zone: region of the sun where energy istransported to the photosphere via blobs of warm, risinggas

• Time required to move energy from the core to thesurface ~ million years

General features of

the Sun

• Photosphere: The regionin the solar atmospherefrom which most of thevisible light escapes intospace (5800 K)

• Sunspots: A region of thesolar photosphere that iscooler than itssurroundings & thereforeappears dark (~4800 K)

• Sunspots can be used todetermine the sun’srotation period ~ 24-27days

• Sunspots were discoveredby Galileo

Sunspots

Close-up of Sunspot

The

Photosphere

(Video)

Close-up of Photosphere

• Granulation: Caused by convective cells

X-ray image of the Sun

• These fields prevent

convection from carrying as

much heat into the sunspots

Corona

• Corona: The outeratmosphere of the Sun. Ithas temperatures inexcess of a milliondegrees & extends formillions of kilometers intospace

• Coronal gas expands &flows away from the Sunand forms the Solar Wind

• Note that a solar eclipseis the best time to see thecorona directly

Corona in

Visible Light

(Video)

H# Emission

(Video)

Magnetic Fields• Much like gravity affects

anything with mass,magnetic fields affectanything with an electriccharge. Charged particlesspin around magnetic fieldlines

• For the Sun, chargedparticles get trapped inmagnetic fields, spiralingalong then from one sunsportto another.

• Convective material is veryhot (and thus comprised ofion & free electrons). Thismaterial cannot cross thefield lines without beingswept into magnetic fields

The Nature of Sunspots

• The Sun rotates faster at its equator than its pole

• The magnetic field lines winds up as a result of differential

rotation

• “Sunspots” occur when the magnetic fields poke through

the photosphere

• The 22-year cycle in which the solar magnetic fieldreverses direction, consisting of two 11-year sunspotcycles

• The Aurora (i.e., dancing light in the earth’s sky causedby charged particles entering our atmosphere) are moreintense during the solar maxima.

• Cause: Winding of magnetic fields?

Solar Cycle

Some unanswered questions

• What causes the solar wind

• How is the corona heated

• How is the solar wind accelerated