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Electromagnetic Spectrum
Making Waves
• Waves begin with vibrations and transfer
energy
• There are two types of waves:
– Compressional--such as sound waves,
require a medium to transfer energy
– Transverse--such as light waves, can travel
through a vacuum (space)
Electromagnetic Wave
• A transverse wave that transfers electric &
magnetic waves.
• It consists of vibrating electric & magnetic
fields that move through space at the speed of
light (300,000 km per second).
• Produced by charged particles
Electromagnetic Waves
Electromagnetic waves cont.
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Electromagnetic radiation: the energy
transferred through space by EM waves
• Travels through empty space
• Doesn’t require a medium
• This is why we can see the sun & stars
 All electromagnetic waves travel at the same
speed in a vacuum, but have DIFFERENT
wavelengths & DIFFERENT frequencies.
Wavelength & Frequency
• Wavelength: the distance
between the crest of one
wave & the crest of the next
wave
• Frequency: number of
wavelengths repeating during
a unit of time
• Wavelength & Frequency
• As wavelength decreases, the frequency INCREASES.
• Waves with longest wavelengths = lowest frequency AND
have less energy
• Waves with shortest wavelengths=highest frequency AND
more energy
Electromagnetic Spectrum
 TgThe complete range of electromagnetic
waves placed in order of increasing frequency
 Made up of: radio waves, infrared rays, visible
light, ultraviolet rays, X-rays & gamma rays
Radio Waves
• the longest wavelengths in the
electromagnetic spectrum
• used by radios, TV, cell phones
Microwaves
• shorter than radio waves but longer than infrared
• used by microwave ovens, cell phones, tv, weather radar
• transmits information over long distances
Radar
• Stands for “radio detection and ranging”
• uses reflected radio waves to detect objects &
measure their distance & speed
• Used in police radar guns
Infrared
• between microwaves & visible light on the EMS
• we experience this type of infrared radiation every day in the
form of heat
• Uses: remote controls, infrared imaging
Visible Light
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the only electromagnetic waves we can see
appear as the colors of the rainbow (ROY G BIV)
Have shorter wavelengths, higher frequency than infrared
Things that appear red = longest wavelength
Things that appear violet = shortest wavelength.
As the wavelengths decrease, other colors are visible
Visible light that appears white is a mixture of the other colors
Ultraviolet
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shorter wavelengths than visible light (carry more energy)
produced by the sun, blocked by the ozone
causes sunburns, skin cancer, damage eyes
Good use: vitamin D (for bones/teeth)
X-rays
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smaller wavelengths & higher energy than ultraviolet waves
the ozone blocks all x-rays from space
X-rays can go through most matter
Used by doctors to diagnose & treat health problems
Gamma Ray
• the smallest wavelengths & the most energy of any other
wave (highest frequency)
• generated by radioactive atoms & in nuclear explosions
• Most penetrating of all EM waves
• used to kill cancerous cells (radiation)
Stars
Classifying Stars
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Color and Temperature: color indicates surface temperature
Cooler stars appear RED.
Hot stars are BLUE & about 20,000 degrees C
Our sun is yellow & about 5,500 degrees C
Size of stars
 SStars can be small, medium or large.
 Our Sun is a medium sized star.
 Large stars are called super giants or giant stars. Ex.
Betelgeuse
Classifying Stars
• Chemical composition: most are about 73% H, 25% He, & 2%
other
• Spectrograph: a device that breaks light into colors and
produces an image of the resulting spectrum.
• Since different gases absorb different wavelengths, we can
identify what gases are in a star.
Measurement
• Light year: the distance that light travels in one year, about
9.5 trillion kilometers
• Parallax: the apparent change in position of an object when
you look at it from different places
How a star is born
• Nebula: a large cloud of gas and dust spread out in an
immense volume
• Protostar: a contracting cloud of gas & dust with enough
mass to form a star. Earliest stage of a star’s life.
Life Spans
• Low mass star: 200 billion years
• Medium mass star: 10 billion years
– Our sun is estimated to be 4.6 billion years
• High mass star: 10 million years
Life Cycle of a medium sized star
Low/medium mass nebulaL/Med. Mass star
Red GiantPlanetary NebulaWhite
DwarfBlack dwarf
Life cycle of a high mass star
High mass nebulaHigh mass
starsupergiant
Supernovaneutron star OR black hole
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Phases of Death
• When stars begin to run out of fuel, the core shrinks and the
outer portion expands
• Eventually, the outer parts grow larger and drift out into space
creating a planetary nebula.
Phases
• White dwarf: the blue-white core of the star
that is left behind
• Supernova: results from an exploding
supergiant and can form a nebula which has
the potential for new stars
Phases
• Neutron stars: the remains of a supergiant
and can have a mass three times that of the
sun but the size of a city
• Pulsars: radio waves emitted by neutron stars
that are spinning
Phases
• Black hole: has gravity so strong that not even
light can escape. These can be detected by
other objects’reaction when near a black hole.
Galaxies and the Universe
• Clusters of stars:
• All the stars in a cluster formed from the same nebula at
about the same time & the same distance from Earth.
Clusters
• 2 Types of clusters:
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1. Open: disorganized, loose appearance contains no more
than 1000 stars; a lot of bright super giants & lots of gas &
dust
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• 2. Globular: large groupings of older stars;
round & densely packed with stars; some may
contain more than a million stars
• Galaxies: enormous swarms of stars, gas, dust & dark matter
held together by gravity
• Our solar system is in the _________________ Galaxy.
• The MWG is one of a billion galaxies in the universe (that have
been observed)!!
Types of Galaxies
1. spiral: pinwheel shaped, young stars are
found on the arms, older stars found in the
nucleus Ex. Milky Way
• 2. Elliptical: oldest & largest galaxies, smooth & oval shaped
• 3. Irregular: don’t have a distinct shape, may be young
galaxies that haven’t formed or 2 gal. colliding
Milky Way Galaxy
• Contains more than 200 billion stars
• About 100,000 light years wide
• Sun orbits the galaxy’s core every 240 million
years.
Origins of the Universe
• 3 common theories about the origin:
• 1. Steady state theory: the universe has always been the
same & will NEVER change
• Not a readily accepted theory based on scientific evidence
• 2. Oscillating Model: expansion of the Universe began &
everything moved outward, but over time, expansion slowed
& the matter moved back in (think of a slinky), then it
repeated over and over
• 3. BIG BANG THEORY: everything started with
a BANG……. Most widely accepted theory,
matter clumped together to form H & He
Our Expanding Universe
• The Doppler Shift explains how we are expanding:
• If a star is moving towards Earth, the wavelengths are
compressed & you see the spectrum BLUE.
• If a star is moving away from Earth, the light
wavelengths are stretched out & you see the
spectrum in RED.
The Red Shift-Hubble Law
• In 1929, Edwin Hubble published a paper about light from
other galaxies.
• All galaxies beyond the Local Group (our group of galaxies)
show a red shift in their spectrum, this shows they are moving
away from us.
• The universe is EXPANDING!!!!
Dark Energy
• The universe is accelerating due to dark energy.
• Dark energy: force causing the galaxies to be moving far apart
at a faster rate
How long does it take for the light from stars to be visible here
on Earth?
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Other stars are so much farther away that it is convenient to express the distance
to them in units of the distance traveled by light in one year. This unit is called a
light year.
The next closest star to us is Proxima Centauri. This star is 4.3 light years away
which means that light from it takes 4.3 years to reach us.
Our galaxy is about 100,000 light years across. This means that it can take tens of
thousands of years for light from some stars in our galaxy to reach us.
For stars that we can see in nearby galaxies it can take millions of years. The
farthest objects we can see are quasars. They are so distant that the light we see
from them today left billions of years ago.
• So when we look up at the stars we are
looking back in time. This is useful for
astronomers because when we look at very
distant objects we can see what the Universe
was like a long time ago.
Future of the Universe
3 possibilities:
• 1. all stars will run out of fuel & burn out, the universe will be
cold & dark
• 2. force of gravity will pull the galaxies back together (“Big
Crunch”), all matter would be crushed into a black hole
• 3. universe will keep on EXPANDING
Age of our Universe
• About 13.7 billion years old
• Scientists know this from measurements of
how fast the galaxies are moving away from us
AND cosmic background radiation (thermal
energy leftover from the big bang)