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 5: Question 5,
Chapter 4 (Describe four methods for
discovering exoplanets)
Homework/Announcements
• Homework due Tuesday, March 12: Question
4, Chapter 5 (Why is Earth’s surface not
riddled with craters as is that of the Moon?).
Comets!
•
http://earthsky.org/space/comet-panstarrs-possibly-visible-to-eye-in-march-2013
• Comet PANSTARRS should be viewable in the
western skies starting March 7. It will be close to
the crescent Moon on March 12.
•
http://earthsky.org/space/big-sun-diving-comet-ison-might-be-spectacular-in-2013
• Comet ISON might be very spectacular in
December, 2013, provided it survives its close
encounter with the Sun.
Coming up:
• Chapter 5 (The Earth)
• Chapter 6 (Other Planets and Moons)
Quick Concept Review
• Some useful concepts:
–
–
–
–
Density
Albedo
Gravity
Atmospheres
• Phases of matter
• Surface gravities
• Escape velocities
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.
Surface Gravities
• The surface gravity of a planet is an
indication of the force of gravity at the
surface.
– The surface gravity of a planet depends on its
mass and on its radius.
http://www.exploratorium.edu/ronh/weight/index.html
Surface Gravities and Escape
Velocities
• The surface gravity of a planet is an
indication of the force of gravity at the
surface.
– The surface gravity of a planet depends on its
mass and on its radius.
• The escape velocity of a planet is how fast
you have to go to escape the planet’s gravity.
– The escape velocity depends on the surface
gravity.
Atmospheres
• An atmosphere is a thin layer of gas bound
to a planet by gravity.
• The existence (or lack thereof) an atmosphere
has a profound effect on the surface features
of a planet:
– The nature of a planetary atmosphere (if any)
depends on the type of gas and on the size of the
planet.
Phases of Matter
•
Matter has three “phases”
Phases of Matter
•
Matter has three “phases”
1. Solid. Constant volume and constant shape.
2. Liquid. Constant volume but variable shape.
3. Gas. Variable volume and variable shape.
Phases of Matter
• Material can change its phase under certain
conditions:
 Water turns into ice by lowering the temperature.
 Water turns into steam by heating it.
 Ice can melt under pressure. This is how an ice
skater glides across the ice.
 CO2 gas can be made into a solid by cooling it.
The Gas Phase
• In a gas, the atoms and/or molecules are widely
separated and are moving at high velocities:
– Relatively heavy molecules such as CO2 move
relatively slowly.
– Relatively light molecules like H2 move relatively
quickly.
– The average velocities of the gas particles depend
on the temperature of the gas.
Heating a Gas
• The velocity of a gas particle depends on the mass of
the particle and its temperature.
Image from Nick Strobel (http://www.astronomynotes.com)
Atmospheres
• In general, a gas will expand to fill its container.
In the case of a planetary atmosphere, gravity is
the “container”:
– The gas particles will have a characteristic velocity
depending on the temperature at the surface of the
planet and on the nature of the gas.
– Depending on the planet, there is a limit on how fast
something can go before it escapes the gravitational
pull: remember the escape velocity!
Escape Velocities and
Atmospheres
• An atmosphere is a thin layer of gas bound
to a planet by gravity. The gas particles
will have some typical velocity depending
on the temperature and type of gas.
• The velocity of gas particles can exceed the
escape velocity of the smaller planets.
Escape Velocities and
Atmospheres
• An atmosphere is a thin layer of gas bound
to a planet by gravity. The gas particles
will have some typical velocity depending
on the temperature and type of gas.
• The velocity of gas particles can exceed the
escape velocity of the smaller planets. This
means the planet will not have an
atmosphere.
Temperature vs. Gravity
Image from Nick Strobel (http://www.astronomynotes.com)
• Heavier gasses tend to stay closer to the surface than
lighter gasses.
• If the gravity is weak, even the heavier gasses may
escape.
Planetology
•
Some things we want to know about a planet:
1)
2)
3)
4)
5)
What are the surface features like?
What is the interior like?
What is the atmosphere like (if any)?
How did it form?
Is there (or was there) life?
Planetology
•
Some things we want to know about a planet:
1)
2)
3)
4)
5)
•
What are the surface features like?
What is the interior like?
What is the atmosphere like (if any)?
How did it form?
Is there (or was there) life?
An understanding of other planets may lead to
a better understanding of our own Earth.
Next:
The Terrestrial Planets
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 Earth
• The place where we keep
all of our stuff.
The Earth
The Earth
The Earth
The Earth
The Earth
The Earth
The Earth
The Earth
The Earth
• The density is 5.5 g/cc, indicating a
substantial amount of dense elements,
probably iron.
The Earth
• The density is 5.5 g/cc, indicating a
substantial amount of dense elements,
probably iron.
• There is a substantial atmosphere (mostly
nitrogen), and water in its three phases
(liquid, solid, and gas).
The Earth
• The density is 5.5 g/cc, indicating a
substantial amount of dense elements,
probably iron.
• There is a substantial atmosphere (mostly
nitrogen), and water in its three phases
(liquid, solid, and gas).
• Owing to erosion, the surface features are
relatively young.
The Earth’s Atmosphere
•
The Earth’s atmosphere is useful in at
least three ways:
1) It keeps the Earth warmer than it would
otherwise be.
2) It keeps most of the harmful UV and X-ray
radiation from reaching the ground.
3) It allows us to breathe.
The Earth’s Atmosphere
• The Earth’s atmosphere today consists of
mostly nitrogen (N2) and oxygen (O2), plus
water vapor (H2O) and carbon dioxide (CO2).
• There has been considerable evolution of the
atmosphere
– The first atmosphere was mostly H and He
– The second came from the Earth in the form of
CO2, H2O, and N2
– The oceans absorbed much of the CO2, and early
life converted it into O2.
The Earth’s Atmosphere
• The temperature of the atmosphere has a
complex dependence on the height:
The Greenhouse Effect
Image from Nick Strobel (http://www.astronomynotes.com)
The Greenhouse Effect
• The Sun heats the Earth. Some of the energy is
scattered, and some heats the ground and water.
• The Earth tries to cool at night, but the atmosphere
traps much of the radiation.
The Greenhouse Effect
• Because of the Atmosphere, the Earth is
about 30 degrees Celsius warmer than it
would otherwise be.
The Greenhouse Effect
• Because of the Atmosphere, the Earth is
about 30 degrees Celsius warmer than it
would otherwise be.
• During the last 50 years, the concentration
of CO2 and other gasses has risen
dramatically owing to human activity.
The Greenhouse Effect
• The buildup of CO2 is leading to a general warming
trend, known as “global warming.”
Global Warming
• The concentration of CO2 in the atmosphere
has risen steeply in recent years as a result of
human activity.
• There is substantial evidence of climate change
taking place as a result of this increased level
of CO2.
• The future consequences of this climate
change could be dramatic, e.g. a major shift in
the ocean currents, melting of polar ice leading
to rising sea levels, etc.
The Interior of the Earth
• The Earth has a radius of about 6400 km. With
current technology, we can drill only a few km
deep. Therefore we must use indirect
techniques to study the deep interior of the
Earth…
The Interior of the Earth
• We can use earthquakes to
study the interior of the
Earth. There are two types
of waves:
– P-waves, which travel
through solids and liquids.
– S-waves, which only go
through solids.
Image from Nick Strobel (http://www.astronomynotes.com)
The Interior of the Earth
• We can use earthquakes to
study the interior of the
Earth. There are two types
of waves:
– P-waves, which travel
through solids and liquids.
– S-waves, which only go
through solids.
Image from Nick Strobel (http://www.astronomynotes.com)
The Interior of the Earth
• There is a solid inner core of iron, surrounded by a
liquid iron core, surrounded by the mantle composed
of silicates. A thin crust is on the outside.
The Interior of the Earth
• Natural radioactivity provides the energy
source that heats the Earth’s interior.
• The thin crustal “plates” float on top of the
liquid mantle.
• The motion of the crustal plates (a few cm
per century) causes earthquakes and
volcanoes.
The Interior of the Earth
• Convection causes motions of the crustal plates.
The Interior of the Earth
• Convection causes motions of the crustal plates.
• The spreading apart of two plates created this
mountain range under the Atlantic.
The Interior of the Earth
• Convection causes motions
of the crustal plates.
• The continental land
masses constantly move
relative to each other.
The Interior of the Earth
• Areas near plate boundaries are prone to earthquakes.
The Magnetic Field of the Earth
• The Earth has a “dipole” magnetic field, much like a
bar magnet.
The Magnetic Field of the Earth
• The Earth has a “dipole” magnetic field, much like a
bar magnet.
• This field helps protect us from the “solar wind”.
The Magnetic Field of the Earth
• The Earth has a “dipole” magnetic field, much like a
bar magnet.
• This field helps protect us from the “solar wind”.
• The interaction between the solar wind and the field
can produce the northern lights.
Next:
The Moon