Scientific Method - astro.sunysb.edu
Transcript of Scientific Method - astro.sunysb.edu
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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
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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
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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)
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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
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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" =
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Parallax – Another Example
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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
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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
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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
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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
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A more detailed look at the process
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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
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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.
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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)
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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)
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Wavelength & Frequencies – some
examples
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Different kinds of atoms emit & absorb
different kinds of photons
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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.
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Emission & Absorption – more
examples
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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
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Emission
vs.
Absorption
Lines
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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.,
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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
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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
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Close-up of Sunspot
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The
Photosphere
(Video)
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Close-up of Photosphere
• Granulation: Caused by convective cells
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X-ray image of the Sun
• These fields prevent
convection from carrying as
much heat into the sunspots
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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
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Corona in
Visible Light
(Video)
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H# Emission
(Video)
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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
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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
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• 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
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Some unanswered questions
• What causes the solar wind
• How is the corona heated
• How is the solar wind accelerated