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ASTRO 101
Principles of Astronomy
Instructor: Jerome A. Orosz
(rhymes with
“boris”)
Contact:
• Telephone: 594-7118
• E-mail: [email protected]
• WWW:
http://mintaka.sdsu.edu/faculty/orosz/web/
• Office: Physics 241, hours T TH 3:30-5:00
Exam 1:
• N=55 (4 missing)
• Average = 59.8
• low = 26, high = 86.5
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A
AB+
B
BC+
C
CD
F
90%--100%
85%--89%
80%--84%
75%--79%
70%--74%
65%--69%
60%--64%
50%--59%
40%--49%
0%--39%
Venus in the Geocentric View
• Venus is always
close to the Sun on
the sky, so its
epicycle restricts its
position.
• In this view, Venus
always appears as a
crescent.
Venus in the Heliocentric View
• In the heliocentric
view, Venus orbits
the Sun closer than
the Earth does.
• We on Earth can see
a fully lit Venus
when it is on the far
side of its orbit.
Venus in the Heliocentric View
• The correlation between
the phases and the size
is accounted for in the
heliocentric view.
Homework/Announcements
• Homework due Tuesday, March 5: Question 5,
Chapter 4 (Describe four methods for
discovering exoplanets)
Next:
Comparative Planetology
• Outline and introduction to the Solar System
• Planets around other stars
Quick Concept Review
• Some useful concepts:
– Density
– Albedo
Density and Albedo
• The concepts of density and albedo are
useful in planetary studies.
• Density = mass/volume
– The density of water is 1 gram per cubic cm.
– The density of rock is 3 grams per cubic cm.
– The density of lead is 8 grams per cubic cm.
• The density of an object can give an
indication of its composition.
Density and Albedo
• The concepts of density and albedo are
useful in planetary studies.
• Albedo = % of incident light that is
reflected.
– A perfect mirror has an albedo of 100%
– A black surface has an albedo of 0%.
• The albedo of an object is an indication of
the surface composition.
The Planets
• Why solar system planets are special:
The Planets
• Why solar system planets are special:
 Planets are resolved when seen through
telescopes (i.e. you can see the disk, surface
features, etc.).
The Planets
• Why solar system planets are special:
 Planets are resolved when seen through
telescopes (i.e. you can see the disk, surface
features, etc.).
 You can also send spacecraft to visit them.
The Planets
• Why solar system planets are special:
 Planets are resolved when seen through
telescopes (i.e. you can see the disk, surface
features, etc.).
 You can also send spacecraft to visit them.
 Stars always appear pointlike, even in the
largest telescopes.
The Planets
• Why solar system planets are special:
 Planets are resolved when seen through
telescopes (i.e. you can see the disk, surface
features, etc.).
 You can also send spacecraft to visit them.
 Stars always appear pointlike, even in the
largest telescopes. Also, they are so far away
that we cannot send probes to study them.
The Solar System
• The Solar System refers to the Sun and the
surrounding planets, asteroids, comets, etc.
The Solar System
• The Solar System refers to the Sun and the
surrounding planets, asteroids, comets, etc.
• Do not confuse “solar system” with “galaxy”:
– The solar system is the local collection of planets
around the Sun.
– A galaxy is a vast collection of stars, typically a
hundred thousand light years across.
The Solar System Census:
• There were 5 planets known since antiquity:
–
–
–
–
–
Mercury
Venus
Mars
Jupiter
Saturn
The Solar System Census:
• There were 5 planets known since antiquity:
–
–
–
–
–
Mercury
Venus
Mars
Jupiter
Saturn
• Since the 1600s (Kepler, Galileo, Newton),
the Earth was considered a planet as well.
New Members
• Uranus: discovered in 1781 by William Herschel.
New Members
• Uranus: discovered in 1781 by William Herschel.
• Neptune: discovered in 1846 by Johann Galle
(based on the predictions of John C. Adams and
Urbain Leverrier).
New Members
• Uranus: discovered in 1781 by William Herschel.
• Neptune: discovered in 1846 by Johann Galle
(based on the predictions of John C. Adams and
Urbain Leverrier).
• Pluto: discovered in 1930 by Clyde Tombaugh.
New Members
• Uranus: discovered in 1781 by William Herschel.
• Neptune: discovered in 1846 by Johann Galle
(based on the predictions of John C. Adams and
Urbain Leverrier).
• Pluto: discovered in 1930 by Clyde Tombaugh.
• Asteroids: thousands, starting in 1801.
New Members
• Uranus: discovered in 1781 by William Herschel.
• Neptune: discovered in 1846 by Johann Galle
(based on the predictions of John C. Adams and
Urbain Leverrier).
• Pluto: discovered in 1930 by Clyde Tombaugh.
• Asteroids: thousands, starting in 1801.
• Kuiper Belt Objects: Dozens, starting in the
1980s.
Pluto “Demoted”!
• The definition of a “planet” was changed
recently:
– Planets: The eight worlds from Mercury to
Neptune.
– Dwarf Planets: Pluto and any other round object
that "has not cleared the neighborhood around
its orbit, and is not a satellite."•
– Small Solar System Bodies: All other objects
orbiting the Sun.
http://www.space.com/scienceastronomy/060824_planet_definition.html
The Solar System
• The planets orbit more or less in the same plane in
space. Note the orbit of Pluto.
• This view is a nearly edge-on view.
Classifications of Solar System
Objects
The Solar System
• The Solar System refers to the Sun and the
surrounding planets, asteroids, comets, etc.
• The scale of things:
– It takes light about 11 hours to travel across the Solar
system.
The Solar System
• The Solar System refers to the Sun and the
surrounding planets, asteroids, comets, etc.
• The scale of things:
– It takes light about 11 hours to travel across the Solar
system. This is 0.001265 years.
The Solar System
• The Solar System refers to the Sun and the
surrounding planets, asteroids, comets, etc.
• The scale of things:
– It takes light about 11 hours to travel across the Solar
system. This is 0.001265 years.
– It takes light about 4.3 years to travel from the Sun to
the nearest star.
The Solar System
• The Solar System refers to the Sun and the
surrounding planets, asteroids, comets, etc.
• The scale of things:
– It takes light about 11 hours to travel across the Solar
system. This is 0.001265 years.
– It takes light about 4.3 years to travel from the Sun to
the nearest star.
– It takes light about 25,000 years to travel from the
Sun to the center of the Galaxy.
Scale Model Solar System
• Most illustrations of the
solar system are not to
scale.
Scale Model Solar System
• Most illustrations of the
solar system are not to
scale.
• Usually, the size of the
planets shown is too large.
Scale Model Solar System
• Build your own scale model of the solar
system:
http://www.exploratorium.edu/ronh/solar_system/
http://pages.umpi.edu/~nmms/solar/index.htm
Scale Model Solar System
• Build your own scale model of the solar
system:
http://www.exploratorium.edu/ronh/solar_system/
http://pages.umpi.edu/~nmms/solar/index.htm
• Conclusion: The solar system is pretty
empty
Scale Model Solar System
• Most depictions of asteroids in the movies
are wrong…
The Scale Model Solar System
• Most depictions of asteroid fields are also not to
scale. Image from the official Star Wars pages
The Scale Model Solar System
• Most depictions of asteroid fields are also not to
scale. Image from Star Trek Voyager.
Two Types of Planets
• Planets come in two
types:
– Small and rocky.
– Large and gaseous.
Or
– Terrestrial
– Jovian
The Terrestrial Planets
• The terrestrial planets are
Mercury, Venus, Earth
(and Moon), and Mars.
• Their densities range
from about 3 grams/cc to
5.5 grams/cc, indicating
their composition is a
combination of metals
and rocky material.
The Terrestrial Planets
• The terrestrial planets are Mercury, Venus, Earth (and Moon), and Mars.
The Giant Planets
• The giant planets are Jupiter, Saturn, Uranus, and
Neptune.
The Giant Planets
• The radii are between about 4 and 11 times
that of Earth.
• The masses are between 14 and 318 times
that of Earth.
The Giant Planets
• The radii are between about 4 and 11 times
that of Earth.
• The masses are between 14 and 318 times
that of Earth.
• However, the densities are between 0.7 and
1.8 grams/cc, and the albedos are high.
The Giant Planets
• The radii are between about 4 and 11 times
that of Earth.
• The masses are between 14 and 318 times
that of Earth.
• However, the densities are between 0.7 and
1.8 grams/cc, and the albedos are high.
• The planets are composed of light elements,
mostly hydrogen and helium.
The Gas Giants
• The composition of the giant planets,
especially Jupiter, is close to that of the Sun.
• The internal structures of these planets is
completely different from that of the Earth.
In particular, there is no hard surface.
• These planets are relatively far from the Sun
(more than 5 times the Earth-Sun distance),
so heating by the Sun is not a big factor.
Next:
• The formation of the Solar System
Star Formation
• The starting point is a giant molecular cloud.
The gas is relatively dense and cool, and usually
contains dust.
• A typical cloud is several light years across, and
can contain up to one million solar masses of
material.
• Thousands of clouds are known.
Side Bar: Observing Clouds
• Ways to see gas:
 By “reflection” of a nearby light source. Blue light
reflects better than red light, so “reflection nebulae”
tend to look blue.
 By “emission” at discrete wavelengths. A common
example is emission in the Balmer-alpha line of
hydrogen, which appears red.
Side Bar: Observing Clouds
• Ways to see dust:
 If the dust is “warm” (a few hundred degrees K)
then it will emit light in the long-wavelength
infrared region or in the short-wavelength radio.
 Dust will absorb light: blue visible light is highly
absorbed; red visible light is less absorbed, and
infrared light suffers from relatively little
absorption. Dust causes “reddening”.
Giant Molecular Clouds
• This nebula is in the belt of Orion. Dark dust lanes
and also glowing gas are evident.
Giant Molecular Clouds
• Interstellar
dust makes
stars appear
redder.
Giant Molecular Clouds
• This images
shows dust
obscuration, an
emission nebula,
and a reflection
nebula.
Giant Molecular Clouds
• Inside many
nebula one finds
very dense cores
called Bok
globules that are
ready to
collapse…
Gravity and Angular Momentum
• There are two important concepts to keep in
mind when considering the fate of giant
molecular clouds:
– Gravity: pulls things together
– Angular momentum: a measure of the spin of
an object or a collection of objects.
Gravity
• There are giant clouds of gas and dust in the
galaxy. They are roughly in equilibrium,
where gas pressure balances gravity.
Gravity
• There are giant clouds of gas and dust in the
galaxy. They are roughly in equilibrium,
where gas pressure balances gravity.
• Sometimes, an external disturbance can
cause parts of the cloud to move closer
together. In this case, the gravitational
force may be stronger than the pressure
force.
Gravity
• Sometimes, an external disturbance can
cause parts of the cloud to move closer
together. In this case, the gravitational
force may be stronger than the pressure
force.
• As more matter is pulled in, the
gravitational force increases, resulting in a
runaway collapse.
Angular Momentum
• Angular momentum is a measure of the spin
of an object. It depends on the mass that is
spinning, on the distance from the rotation
axis, and on the rate of spin.
• I = (mass).(radius).(spin rate)
• The angular momentum in a system stays
fixed, unless acted on by an outside force.
Conservation of Angular Momentum
• An ice skater demonstrates
the conservation of angular
momentum:
Conservation of Angular Momentum
• An ice skater demonstrates
the conservation of angular
momentum:
• Arms held in: high rate of
spin.
• Arms extended: low rate of
spin.
• I = (mass).(radius).(spin rate)
(angular momentum and
mass are fixed here)
Conservation of Angular Momentum
• If an interstellar cloud has some net
rotation, then it cannot collapse to a point.
Conservation of Angular Momentum
• If an interstellar cloud has some net
rotation, then it cannot collapse to a point.
Instead, the cloud collapses into a disk that
is perpendicular to the rotation axis.
Next
• The Condensation Theory
• Planets around other stars