Download Scientific Method

Scientific Method
• The principles & empirical processes of discovery &
demonstration considered characteristic of or necessary
for scientific investigation, generally involving
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
• The method employed in exact science & consisting of
a) Careful & abundant observation & experimentation
b) Generalization of the results into formulated “Laws” &
statements
The Sun
• Nearest Star
• Importance
1) Heating source (solar constant = 1370 Watts/ m2)
2) Source of illumination
• Outline of Discussion of Sun
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,
(G Msun mearth) / D2 = (mearth [vearth]2) / D
Msun = [vearth]2 D / G
• Density = Mass / Volume
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 charged
electrons that orbit the nucleus
• Isotope: Any of several forms for the same element whose
nuclei all have the same number of protons but different
numbers 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
4 (hydrogen nuclei mass) = 1 (helium nucleus mass)
4 (proton mass) = 3.97 (proton mass)
• The missing link, the release of energy
• Thus the efficiency of converting mass to energy is
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
0.0075 Masssun c2
Lifetime =
(0.1) ~ 1010 years.
Luminositysun
• The Sun is already about 5 billion years old, so it has
5 billion more years to go in its present state
How do we determine the composition
of astronomical objects?
• Answer – Spectroscopy
• Our eyes are sensitive to optical light, but we can build
instruments sensitive to other forms of “light” (or
radiation)
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 to
the loss of one or more electrons. Note that isolated
atoms are electronically neutral – i.e, they have the
same number of protons & neutrons – unless they are
ionized.
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
I.e.,
• The Earth’s crust is
primarily Oxygen,
Aluminum, & Calcium
• Life is primarily
Hydrogen, Oxygen,
Carbon, & Nitrogen
Interior of the Sun
• Core: center of Sun (15x106 K)
• Radiative zone: region of sun where energy is
transported via radiation
• Convective zone: region of the sun where energy is
transported to the photosphere via blobs of warm, rising
gas
• Time required to move energy from the core to the
surface ~ million years
General features of
the Sun
Sunspots
• Photosphere: The region
in the solar atmosphere
from which most of the
visible light escapes into
space (5800 K)
• Sunspots: A region of the
solar photosphere that is
cooler than its
surroundings & therefore
appears dark (~4800 K)
• Sunspots can be used to
determine the sun’s
rotation period ~ 24-27
days
• Sunspots were discovered
by Galileo
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 outer
atmosphere of the Sun. It
has temperatures in
excess of a million
degrees & extends for
millions of kilometers into
space
• Coronal gas expands &
flows away from the Sun
and forms the Solar Wind
• Note that a solar eclipse
is the best time to see the
corona directly
Corona in
Visible Light
(Video)
H# Emission
(Video)
• Much like gravity affects
anything with mass,
magnetic fields affect
anything with an electric
charge. Charged particles
spin around magnetic field
lines
• For the Sun, charged
particles get trapped in
magnetic fields, spiraling
along then from one sunsport
to another.
• Convective material is very
hot (and thus comprised of
ion & free electrons). This
material cannot cross the
field lines without being
swept into magnetic fields
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
Solar Cycle
• The 22-year cycle in which the solar magnetic field
reverses direction, consisting of two 11-year sunspot
cycles
• The Aurora (i.e., dancing light in the earth’s sky caused
by charged particles entering our atmosphere) are more
intense during the solar maxima.
• Cause: Winding of magnetic fields?
Some unanswered questions
• What causes the solar wind
• How is the corona heated
• How is the solar wind accelerated