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Lecture 7

Lecture 7 and 8 in class notes

Course Code
Rick Halpern

of 3
Lecture 7
273.15K = 0oC
Blackbody Curves, Ideal vs Reality
oA blackbody absorbs all radiation falling on it but then
reemits otherwise temperature would not change
oBlackbody curve represents intensity of radiation emitted by
any object over all possible frequencies
oThe peak emission wavelength decreases with temperature
oObject with the suns surface temperature peaks in the
visible spectrum
oThe total amount of energy emitted per unit area increases
temperature as fourth power
oComparison of blackbody curves of four cosmic objects. The
frequencies and
owavelengths corresponding to peak emission are marked.
oA cool, invisible galactic gas cloud called Rho Ophiuchi. At a
oof 60 K, it emits mostly low-frequency radio radiation.
oA dim, young star (shown red in the inset photograph) near
the center of
othe Orion Nebula. The stars atmosphere, at 600 K, radiates
primarily in the
oinfrared, here falsely coloured to represent differences in
oThe Suns surface, at approximately 6000 K, is brightest in
the visible
oregion of the electromagnetic spectrum.
oSome very hot, bright stars in a cluster called Omega
Centauri, as
oobserved by a telescope aboard a space shuttle. At a
temperature of
o60,000 K, these stars radiate strongly in the ultraviolet.
(Harvard College
oObservatory; J. Moran; AURA; NASA)
Wiens Law
oT in kelvins
oStefan Blotzman Law
oWhere sigma=5.67x10-8W/m2K4
oBlackbody curves can be used as thermometers to determine
the temperature ob distant objects
oThe Suns surface temperature is about 5800K. What is the
wavelength of the peak radiation of a blackbody with this
From Wiens Law we find that
Lambdamax=0.29cm/5800=500nm which is yellow
oWhat is the energy output (luminosity) of the Sun?
By Stef Law the solar energy flux per unit time is
Surface area of a sphere with Sun radius is
r=6.96x105 km.
So Area = 4pi*r2 = 6.09x1012 km2
Luminosity = Flux*Area
Now we know about several dwarf planets (Pluto, Ceres, Eris and
Eris 27% more massive than Pluto
Pluto has high orbital inclination 17 degree relative to ecliptic
All planets move anticlockwise if look down at Suns north pole
Lecture 8: Planetary Properties
oTerrestrial Planets: Mercury, Venus, Earth and Mars (All have atmosphere
except for mercury)
oEarth and Mars rotation is almost the same
oVenus and Mercury rotate much slower
Venus rotates in opposite direction
oEarth and Mars have moons
oVenus and Mercury
oEarth and Venus have magnetic fields but venus and mars does not
oJovian Planets: Jupiter, Saturn, Uranus, Neptune (All have atmosphere)
oMost asteroids are between 2.2 and 3.3 AU from the sun
oKirkwood gaps –associated with resonance
oSpecial ratio between the asteroid Jupiter so that two meet
regularly in the same location
o1/3 means asteroid with this semimajor axis will orbit the sun 3
times for every one orbit by Jupiter which means the gravity of
Jupiter changes the asteroids trajectory overtime
oInterplanetary orbits
oComets: Icy with some rock parts
o1-10km in size and made of truly primordial material
oComets are from outer reaches of solar system