ASTRO 101
Principles of Astronomy
Instructor: Jerome A. Orosz
(rhymes with
“boris”)
Contact:
• Telephone: 594-7118
• E-mail: orosz@sciences.sdsu.edu
• WWW:
http://mintaka.sdsu.edu/faculty/orosz/web/
• Office: Physics 241, hours T TH 3:30-5:00
Homework/Announcements
• Homework due Tuesday, March 19: Question
6, Chapter 7 (What role did Charon play in
enabling astronomers to determine Pluto’s
mass?).
• Tuesday, March 26: Wrap-up, review
• Thursday, March 28: Exam #2
Next:
Chapter 7
(Dwarf Planets and Small
Solar System Bodies)
Pluto
• Essentially nothing was known about Pluto
until the 1970s.
 It is far from the Sun (more than 40 times the
Earth-Sun distance), hence it receives little
sunlight. In turn, we see very little light
reflected back.
 The planet looks nearly pointlike owing to its
large distance from us, so it was not possible to
see surface features.
Pluto’s Orbit
• Pluto’s orbit is highly inclined relative to the orbits
of the other planets.
Pluto’s Orbit
• Pluto’s orbit is also more eccentric than the orbits of
the other planets. The period is 247.7 years
Pluto’s First Moon
• Pluto’s first moon was
discovered in 1978.
• The orientation of the
moon’s orbit is such that
during certain times, Pluto
and its moon appear to pass
in front of each other every
3.2 days as seen from Earth.
Pluto’s First Moon
• Pluto and Charon passed in front of each other in the
late 1980s.
Pluto’s First Moon
Images from Courtney Seligman (http://www.cseligman.com)
• Pluto and Charon passed in front of each other in the
late 1980s.
• Studies of these events allowed astronomers to measure
the masses and radii of Pluto and Charon.
Pluto’s First Moon
• The masses can be
deduced from
Kepler’s Laws.
• The radii can be
deduced by looking at
the total brightness
over time.
Pluto’s First Moon
• Pluto’s mass is 1/500 of the Earth’s mass.
• Pluto’s diameter is 2300 km.
Pluto’s First Moon
• Pluto’s mass is 1/500 of the Earth’s mass.
• Pluto’s diameter is 2300 km. This is smaller than
Jupiter’s large moons, and also Earth’s moon.
More Moons for Pluto
• Two additional moons were found using the
Hubble Space Telescope in 2005.
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
New Classification Scheme for the
Solar System
• Many catagories overlap.
For example, some asteroids
are dwarf planets.
• In most cases, the
distinctions between the
catagories are not important
for us.
Dwarf Planets
• Ceres was discovered
in 1801 by Piazzi.
• Its orbit lies between
Mars and Jupiter.
• Thousands of
asteroids are now
known to be in the
“asteroid belt”.
• Ceres was
reclassified as a
dwarf planet in 2006.
Dwarf Planets
• Since 2003, three
additional dwarf
planets have been
discovered beyond
the orbit of Pluto.
• Eris, the largest one,
is apparently
composed largely of
water ice.
• Haumea appears to
be slightly oval
shaped.
Dwarf Planets
• Since 2003, three
additional dwarf
planets have been
discovered beyond
the orbit of Pluto.
• Eris has a very
eccentric and inclined
orbit. The period is
557 years.
Next: Small Solar System Bodies
• Asteroids
• Kuiper Belt Objects
• Comets
Small Solar System Bodies
• The first asteroid, Ceres, was discovered in
1800 using visual observations through a
telescope.
• By the late 1800s, a total of about 300
asteroids were found.
• The big breakthrough came in 1891 when
photography was used...
Small Solar System Bodies
Small Solar System Bodies
• As of September 2008, roughly 190,000
asteroids have been cataloged.
• Only three asteroids have diameters larger
than 300 km. The smaller you go in
diameter, the more asteroids there are.
Small Solar System Bodies
• Most of the asteroids orbit between Mars and Jupiter.
• Note the deceptive scale: the average spacing is
1,000,000 km between objects!
Small Solar System Bodies
• Most of the asteroids orbit between Mars and Jupiter.
• Jupiter’s gravity causes the asteroids to gather into
certain orbits.
Small Solar System Bodies
• Most of the asteroids orbit between Mars and Jupiter.
• Some cross the orbit of the Earth. Asteroid 1994
XM1 was closer than the Moon on its nearest
approach.
Small Solar System Bodies
• Most of the asteroids orbit between Mars and Jupiter.
• Some have the same orbit as Jupiter, but either lead or
lag the planet by 60 degrees.
What Asteroids Look Like
• Asteroids have irregular shapes, and typically have
craters and other features.
What Asteroids Look Like
• Asteroids have irregular shapes, and typically have
craters and other features.
What Asteroids Look Like
• A probe crashed into Eros on February 12, 2001.
What Asteroids Look Like
• A probe crashed into Eros on February 12, 2001.
• The chemical composition of Eros is similar to that of
old meteorites, indicating Eros contains “primitive”
material.
Comets and Kuiper Belt Objects
• There are two main
regions where the
comets live: the Kuiper
belt and the Oort Cloud.
• The Kuiper belt extends
roughly 30 to 50 AU.
• The Oort Cloud may
extend as far as 50,000
AU.
Comets and Kuiper Belt Objects
• There are two main
regions where the
comets live: the Kuiper
belt and the Oort Cloud.
• There are roughly 1500
known KBOs and
counting. Most are a
few tens of km in
diameter.
Comets and Kuiper Belt Objects
• KBOs are extremely faint and must be observed
with the largest telescopes.
Comets and Kuiper Belt Objects
• A few KBOs have diameters approaching 1500
km. Most are much smaller.
Comets and Kuiper Belt Objects
• Sedna has an orbital period of about 10,500
years. It is one of the most distant known solar
system objects.
Comets
• When a comet comes close to the Sun,
things get interesting…
Comets
• Comets are bright, fuzzy
objects that appear
relatively suddenly.
• Most usually have a
rather long, diffuse “tail”.
Comets
• Historically, comets were considered to be “bad omens.”
• This tapestry depicts the appearance of Halley’s comet
during the coronation of Harold as King of Britain on
January 6, 1066.
Comets
• Historically, comets were considered to be “bad omens.”
• This tapestry depicts the appearance of Halley’s comet
during the coronation of Harold as King of Britain on
January 6, 1066. The Norman invasion came later that
year.
• See http://www.bayeuxtapestry.org.uk/Index.htm
Comets as Omens
•
The English word “disaster” has two
roots:
Comets as Omens
•
The English word “disaster” has two
roots:
1) “dis”, a Latin term for “bad”.
Comets as Omens
•
The English word “disaster” has two
roots:
1) “dis”, a Latin term for “bad”.
2) “astro”, the Latin term for “star”.
The Nature of Comets
• In 1705, Edmund Halley
applied Newton’s new
theories of gravity and
motion to the orbits of
bright comets. He
suggested that the bright
comets that appeared in
1531, 1607, and 1682 were
the same object. Also, the
comet of 1066 is now
known to be the same
object.
The Nature of Comets
• Comets are objects orbiting
the Sun.
The Nature of Comets
• Comets are gravitationally bound to the
Sun.
 The orbits can be highly elliptical (recall the
orbits of the planets are nearly circular).
 The orbital planes of the comets can be highly
inclined with respect to the Earth’s orbital
plane (i.e. the ecliptic).
The Nature of Comets
• The 8 planets orbit in nearly the same plane in space
The Nature of Comets
• Comets are gravitationally bound to the
Sun.
 The orbits can be highly elliptical (recall the
orbits of the planets are nearly circular).
 The orbital planes of the comets can be highly
inclined with respect to the Earth’s orbital
plane (i.e. the ecliptic).
The Nature of Comets
• Comets are gravitationally bound to the
Sun.
 The orbits can be highly elliptical (recall the
orbits of the planets are nearly circular).
 The orbital planes of the comets can be highly
inclined with respect to the Earth’s orbital
plane (i.e. the ecliptic).
 The orbital periods range from a few years to
thousands of years.
The Nature of Comets
The Nature of Comets
• Observationally, comets have
 A bright head, which consists of a small
nucleus (a few km across) and a coma made up
of glowing diffuse gas, which may be 100,000
km across.
 A dust tail, blown off by the radiation from the
Sun.
 A gas tail, blown off by particles from the Sun.
 The tails always point away from the Sun.
The Nature of Comets
• Sunlight and the solar
wind blow the
comet’s dust particles
away from the Sun.
• As a result, the tails
always point away
from the Sun.
The Nature of Comets
• The dust tail is seen by
reflected sunlight.
• The gas tail is seen by
its own emission (i.e.
it is a cloud of hot
gas).
• The dust tail can be up
to 150,000,000 km
long.
The Nature of Comets
• A summary of the main parts. Note the difference
in the size of the actual nucleus and the visible
coma.
The Nature of Comets
• The most popular model for a comet is the “dirty
snowball” model.
• The nucleus is made up of icy material, including water,
CO2, NH3, and CH4. The total mass is typically about
10-9 of the Earth’s mass.
The Nature of Comets
• In 2005, NASA
sent the Deep
Impact probe to the
Comet Temple 1.
• The probe crashed
into the nucleus at a
velocity of 22,000
mph.
The Nature of Comets
• In 2005, NASA
sent the Deep
Impact probe to the
Comet Temple 1.
• The probe crashed
into the nucleus at a
velocity of 22,000
mph.
The Nature of Comets
• In 2005, NASA
sent the Deep
Impact probe to the
Comet Temple 1.
• The probe crashed
into the nucleus at a
velocity of 22,000
mph.
• Seconds after
impact, hot debris
was spotted.
The Nature of Comets
• In 2005, NASA
sent the Deep
Impact probe to the
Comet Temple 1.
• The probe crashed
into the nucleus at a
velocity of 22,000
mph.
• Seconds after
impact, hot debris
was spotted.
The Nature of Comets
• This experiment
provided evidence
that the nucleus is
composed of
weakly bound
fragments of rock
and ice.
• Many organic
molecules were
observed in the
debris.
The Nature of Comets
• This experiment
provided evidence
that the nucleus is
composed of
weakly bound
fragments of rock
and ice.
• Other comets have
been observed to
fragment when they
pass too close to the
Sun or Jupiter.
The Nature of Comets
• A comet is a very small, dirty snowball.
• When it is far from the Sun, it is very faint, and
usually impossible to see from Earth.
• When a comet comes close to the Sun (and
hence relatively close to the Earth), the Sun
vaporizes the material, giving rise to the coma
and the tail(s). Comets generally disappear after
many trips near the Sun.
Next…
Meteors: Things That Fall From
the Sky
Meteors: Things That Fall From
the Sky
• Large impacts have been observed
recently…
Comet Shoemaker-Levy 9
• This comet was discovered in 1993, subsequent
photographs showed a “string of pearls”.
Comet Shoemaker-Levy 9
• This comet was discovered in 1993, subsequent
photographs showed a “string of pearls”.
• It was found to be orbiting Jupiter rather than the Sun.
Comet Shoemaker-Levy 9
• This comet was discovered in 1993, subsequent
photographs showed a “string of pearls”.
• It was found to be orbiting Jupiter rather than the Sun.
• It was on its final pass around Jupiter, and would hit
Jupiter in July of 1994.
Comet Shoemaker-Levy 9
• Comet Shoemaker Levy 9 gave us a chance to study a
comet in detail:
Comet Shoemaker-Levy 9
• Comet Shoemaker Levy 9 gave us a chance to study a
comet in detail:
• The collision tells us about the structure of the comet (e.g.
tightly or loosely packed).
Comet Shoemaker-Levy 9
• Comet Shoemaker Levy 9 gave us a chance to study a
comet in detail:
• The collision tells us about the structure of the comet (e.g.
tightly or loosely packed).
• We can see material from deep in Jupiter’s atmosphere that
has been ejected.
Comet Shoemaker-Levy 9
• Comet Shoemaker Levy 9 gave us a chance to study a
comet in detail:
• The collision tells us about the structure of the comet (e.g.
tightly or loosely packed).
• We can see material from deep in Jupiter’s atmosphere that
has been ejected.
• We can see cometary material that has been ejected.
Comet Shoemaker-Levy 9
• Comet Shoemaker Levy 9 gave us a chance to study a
comet in detail:
• The collision tells us about the structure of the comet (e.g.
tightly or loosely packed).
• We can see material from deep in Jupiter’s atmosphere that
has been ejected.
• We can see cometary material that has been ejected.
• The collision tells about the structure of Jupiter’s atmosphere.
Comet Shoemaker-Levy 9
• The colliding fragements made dark spots that were
easily seen from Earth.
• The typical collision released 100,000 times more
energy than an H-bomb.
Meteors
• Looking closer to home…
Meteors
• There are many small chunks of matter
orbiting the Sun.
 A piece that is in space is a meteoroid.
 A piece that burns up in the Earth’s atmosphere
is a meteor (a bright streak of light).
 A piece that lands on Earth is a meteorite.
Meteors
• Many “meteor showers” are associated with
comets.
Dust from Comets
• The dust tail contains
small particles
evaporated from the
comet.
• These particles remain
in orbit about the Sun.
• If the Earth passes
through the “dust
cloud”, then several
meteors may be seen.
Dust from Comets
• The dust tail contains
small particles
evaporated from the
comet.
• These particles remain
in orbit about the Sun.
• If the Earth passes
through the “dust
cloud”, then several
meteors may be seen.
Meteor Showers
• During periods of
high meteor activity,
most of the events
appear to come from
one spot on the sky.
Meteor Showers
• During periods of
high meteor activity,
most of the events
appear to come from
one spot on the sky.
• This point is roughly
where the comet’s
tail was.
Meteor Showers
• During periods of
high meteor activity,
most of the events
appear to come from
one spot on the sky.
• This point is roughly
where the comet’s
tail was.
•
Dust particles enter the atmosphere and burn up, causing a
streak of light.
Rocks from Space
• Some early cultures were aware that rocks
sometimes fell from the sky. These items
had great religious value, e.g. the Black
Stone of Ka’aba.
Rocks from Space
• Some early cultures were aware that rocks
sometimes fell from the sky. These items
had great religious value, e.g. the Black
Stone of Ka’aba.
• “Enlightened” scientists in the 18th and 19th
centuries declared that stones cannot
possibly fall from space. It was all
primitive superstition.
Rocks from Space
• Thomas Jefferson said: “It is easier to
believe that two Yankee professors [Profs.
Silliman and Kingsley of Yale] would lie
than that stones would fall from the sky.”
Rocks from Space
• Thomas Jefferson said: “It is easier to
believe that two Yankee professors [Profs.
Silliman and Kingsley of Yale] would lie
than that stones would fall from the sky.”
• Jefferson was wrong: stones do fall from the
sky.
Rocks from Space
• Evidence that rocks fall from space:
Rocks from Space
• Evidence that rocks fall from space:
 There have been eyewitness accounts of
impacts.
Rocks from Space
• Evidence that rocks fall from space:
 There have been eyewitness accounts of
impacts.
 In many cases, the mineral composition of
samples indicates the material cannot be native
to Earth.
Rocks from Space
• Evidence that rocks fall from space:
 There have been eyewitness accounts of
impacts.
 In many cases, the mineral composition of
samples indicates the material cannot be native
to Earth.
 Most older samples are iron, most “fresh”
samples are stony material.
Rocks from Space
Rocks from Space
• Occasionally rocks from space do damage…
• http://www.newsinhistory.com/blog/meteorite-smashes-through-roof-hitsnapping-woman
• http://news.discovery.com/space/asteroids-meteors-meteorites/russian-meteorblast-bigger-nasa-130217.htm
Where to Find Meteorites
• Antarctica is one of the
best places to find
meteorites on Earth,
owing to the high
contrast (black rocks on
white snow).
http://www-curator.jsc.nasa.gov/curator/antmet/program.htm
Where to Find Meteorites
• Over time, meteorites
tend to get
concentrated in certain
areas because of largescale ice flows.
http://www-curator.jsc.nasa.gov/curator/antmet/program.htm
Meteorites
• Most older samples are iron.
 Iron is dense and not easily weathered.
• Most “fresh” samples are composed of
stony materials.
 This material is easily weathered and does not
last long on the Earth’s surface.
Rocks from Space
• Why is are meteorites useful?
• They are material samples from outside the
Earth that can be analyzed in the laboratory.
• We can measure the age of the solar
system by studying meteorites.
Next
• Meteorites and the age of the solar system