Transcript Lecture 4 - Twin Cities - University of Minnesota

Lecture 4
Ast 1001
6/6/07
Energy in Atoms
• Electrons can only exist in specific states
called Energy Levels
• Electron volts (eV) are often used to
measure the energy of the levels
• Ground state is the lowest energy level
• Excited states are energy levels with more
energy than the ground state
Transitions
• Electrons can
change energy
levels by gaining
energy
• If electrons gain too
much energy, then
the atom will
become ionized
Spectroscopy Basics
• Reading the information
from a spectrum is
called Spectroscopy
• Usually done graphically
• Amount of radiation, or
Intensity on y axis
• Energy, wavelength, or
frequency of light on the
x axis.
Types of Spectra
• Continuous spectra are those that show all
of the colors in the rainbow
– Incandescent lightbulb is the primary example
• Emission Spectrum only has discrete bright
lines
– Fluorescent lightbulbs
• Absorption Spectrum has most of the
rainbow, but dark lines have been removed
Emission
Absorption
How Do You Get the Spectra?
• Continuous spectra come from “blackbody”
sources
• Emission spectra come from something like
a cloud of gas that is getting energy from
somewhere
• Absorption spectra is when light from a
continuous source is absorbed
Why Spectroscopy is Important
• Each atom (or molecule) has
unique lines
– Thus, we can figure out what
things are made out of by
analyzing the light
• If the source is continuous (or
mostly so) we can tell what
temperature it is
– Hotter things peak at higher
energies
Some Math
• Stefan-Boltzmann law tells you how much
energy per unit area is coming off of a
source
– Emitted power = σ*T4
• Wien’s Law is the relationship between
temperature and spectroscopic peak
– λmax = 2,900,000/Temperature
The Doppler Shift
• Common example: a siren
moving towards and then
away from you
• If its moving away from
you, light is redshifted
• If its moving towards you,
light is blueshifted
• Can also be used to
determine rotation rates
Detectors
• Astronomical detectors
are usually CCDs
• Consist of a number
(millions?) of pixels
– When light hits the pixel,
electrical charge builds up
• Far superior to film for a
number of reasons
– Much more sensitive to
light
– Can work at many
different wavelengths
Why Use Telescopes?
• To collect light
– Ability to collect light depends on the area of the aperture
• To increase angular resolution
– Angular resolution is the ability to tell that two nearby
dots are distinct
– Telescopes can be 60x better at resolving angles than the
human eye
The Kinds of Telescopes
• Refracting Telescopes
– Uses lenses
– The earliest telescopes were
refracting
– Largest refractor is 1 meter
in size
• Reflecting Telescopes
– Uses mirrors
– Professional telescopes are
reflecting
– Largest reflectors are 10
meters in size
What Astronomers Do
• Imaging
– Basically taking pictures
– Usually the images are in black and white
– Filters are heavily used
• Spectroscopy
– Use a diffraction grating to split light into parts
– Spectral resolution is how much information
we can get from the spectral lines
What Astronomers Do cont.
• Timing
– Many objects vary with time
– Measure brightness over time
• Getting time to observe is difficult
– Most astronomers only observe a couple of times a year
– Time on telescopes is very competitive
The Atmosphere
• Light pollution
• Twinkling
(turbulence)
– Air in the
atmosphere
moves, modifies
light
– Can put
telescopes above
the atmosphere
– Use adaptive
optics
Non Visible Light
• Radio telescopes
– Basically reflecting
telescopes
– Can be very large
• Infrared telescopes
– Similar to visible light
– Atmospheric problems
are greater for IR than
visible light
More Non Visible Light
• Ultraviolet Telescopes
– Must be above
atmosphere
– Currently somewhat
unpopular
• X-Ray/Gamma Ray
astronomy
– Must be above
atmosphere
– Fairly recent kind of
astronomy
Arrays
• Interferometry greatly increases angular results
– Not nearly as efficient for increasing light collecting
ability
Solar System Properties
• Patterns of motion already discussed
– Planets revolves, orbits Sun
• Two kinds of planets
– Small, rocky, terrestrial planets
– Large, gassy, jovian planets
• Lots of little rocky objects
– Asteroid belt, Kuiper belt, Oort cloud
Nebular Theory
• First proposed by Kant
(1755), Laplace (1795)
• Involves gravitational
collapse of a cloud of
gas
• Most of the gas formed
the Sun, leftover gas
formed the planets
Where Did the Gas Come From?
• Stars die, spew out their
guts
– Supernovae, novae
• Our Sun is a second
generation star
• We can see other gas
clouds forming
– Can also see stars forming
within gas clouds
– Best example: the Orion
Nebula
From Gas to the Solar System
• Initially gas was spread out over several
light-years
• Gas starts to collapse
– Temperature rises
– Cloud begins to rotate more and more quickly
– Flattens
Planet Formation
• Problem: solar nebula was 98%
hydrogen/helium, planets aren’t
• Planets formed via condensation
• Early solar system breakdown:
–
–
–
–
98% hydrogen/helium
Hydrogen compounds (1.4%)
Rock (.4%)
Metals (.2%)
• Frost Line dictates where hydrogen
compound things form (jovian planets) and
where rocky things form (terrestrial planets)
More Planet Formation
• Accretion is the primary mechanism for
planet growth
– Small particles build up planetesimals
– Planetesimals combine to form planets
• Jovian planets got big enough that their
gravity was great enough to capture
hydrogen and helium gas
The End of Planet Formation
• Eventually the solar wind
pushed all of the gas out
into interstellar space
• Sun was spinning much
more quickly
• Eventually the Sun’s
magnetic field dispersed
its angular momentum
After Planet Formation
• Lots of planetesimals left
over
– Became comets, asteroids
• Bombardment Phase
– Planets knocked around
– Water (probably) brought to
Earth
– Moon formed
• Moons captured by big
planets
The Age of the Solar System
• The Solar System is about 4.6 billion years
old. How do we know this?
• Atoms are identical: young atoms are
indistinguishable from old atoms
• Atoms can undergo spontaneous radioactive
decay and turn into other elements or
isotopes
Radioactive Dating
• Example: potassium-40 can decay into argon-40
– Potassium is the parent isotope, argon is the daughter
isotope
• Rate at which atoms decay is characterized by the
half-life
• If you start out with a given amount of potassium40, and no argon-40, you can look at how much
argon you have now and figure out how old the
material is
Group Work
• Lets say that you know a rock is 3 billion
years old and measure that it currently has
.75 units of potassium-40. How much
argon-40 would you predict that the rock
should have? (Hint: follow the example on
the bottom of page 241)