Transcript Light Intro and Inverse Sq Law 2012

Chapter 5
Light
Astronomers can explore most of the
light
Universe by studying the ________
that comes to Earth from distant
objects.
Properties of Light
•Light behaves as both a ________
particle and
a _____
wave
•Light particles are called __________,
photons
which can be counted individually.
•Light is also an ___________________
electromagnetic wave
Light is an electromagnetic
wave
Wavelength
=
Electromagnetic Radiation
Electromagnetic radiation can be
• ______________________
photons
described in terms of a stream of _______,
each traveling in a wave-like pattern, moving
at the _______________
and carrying some
speed of light
amount of energy.
• The only difference between radio waves,
visible light, and gamma-rays is the energy
of the photons. ______
Radio waves have photons
with low energies, ___________
microwaves have a little
infrared
more energy than radio waves, ________
visible ___________,
ultraviolet
has still more, then ______,
________,
X-rays and ___________.
gamma-rays
Examples from Space!
Light is Fast!
• 186,282 miles per second
• 670,616,629 miles per hour
• 300,000 kilometer per second
Sun
Earth
At this speed it can go around the
world ________
8 times in one second.
Radio
• Emitted by
– Astronomical Objects
– Radio Station
transmitters
• Detected by
– Ground based radio
telescopes
– Radios
Microwave
• Emitted by:
– Gas clouds collapsing
into stars
– Microwave Ovens
– Radar Stations
– Cell Phones
• Detected by
–
–
–
–
Microwave Telescopes
Food (heated)
Cell phones
Radar systems
Infrared
• Emitted by
– Sun and stars (Near)
– TV Remote Controls
– Food Warming Lights
(Thermal)
– Everything at room
temp or above
• Detected by
– Infrared Cameras
– TVs, VCRs,
– Your skin
Visible
• Emitted by
– The sun and other
astronomical objects
– Laser pointers
– Light bulbs
• Detected by
– Cameras (film or
digital)
– Human eyes
– Plants (red light)
– Telescopes
Ultraviolet
• Emitted by
–
–
–
–
Tanning booths (A)
The Sun (A)
Black light bulbs (B)
UV lamps
• Detected by
– Space based UV
detectors
– UV Cameras
– Flying insects (flies)
X-ray
• Emitted by
– Astronomical objects
– X-ray machines
– CAT scan machines
– Older televisions
– Radioactive minerals
– Airport luggage
scanners
• Detected by
– Space based X-ray
detectors
– X-ray film
– CCD detectors
Chandra X-ray Observatory
Chandra X-ray
Observatory
• Chandra is designed to
observe X-rays from high
energy regions of the
universe, such as the
remnants of exploded stars.
• One of the most
sophisticated observatory
built.
• Deployed by the Space
Shuttle Columbia on July
23, 1999.
Gamma Ray
• Emitted by
– Radioactive materials
– Exploding nuclear
weapons
– Gamma-ray bursts
– Solar flares
• Detected by
– Gamma detectors and
astronomical satellites
– Medical imaging
detectors
X-rays and Gamma Rays
• Black holes
• Active Galaxies
• Pulsars
• Diffuse emission
• Supernovae
• Gamma-ray bursts
• Unidentified
• Apparent Brightness
• Inverse Square Law
Stellar Luminosity
Luminosity is the total amount of power the
__________
star radiates into space.
It is measured in power units (Watts).
The __________________
apparent brightness is the amount of
light reaching us per unit area.
__________of
Brightness a star in the sky depends on
the distance to a star and its luminosity.
Magnitude system for brightness
Smaller
brighter stars.
• ________numbers
imply ________
• “Apparent magnitude” is a measure of
apparent brightness. Antares has mag. 1;
Polaris has mag. 2; naked eye limit is
about ___.
Sirius has mag. –1.5.
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• The faintest stars observed with HST are
of ~ _____
30th magnitudes.
2.5 times
• A star of magnitude 1.00 is ____
brighter than a star of magnitude 2.00.
The Nearest Stars
Name
Distance
Apparent
Luminosity
(light years) magnitude (compared to
sun)
–26.7
Sun
1
Alpha Centauri A
4.4
0
1.5
Alpha Centauri B
4.4
1.4
0.44
Alpha Centauri C
4.3
11
0.00006
Barnard’s star
5.9
9.5
0.00042
Wolf 359
7.6
13.5
0.00002
HD 05735
8.3
7.5
0.0055
Luyten 726-8A
8.4
12.5
0.00006
Luyten 726-8B
8.4
13.0
0.00004
Sirius A
8.6
–1.4
21.8
Sirius B
8.6
8.3
0.003
Ross 154
9.4
10.5
0.00048
Ross 248
10.3
12.3
0.00011
The Brightest Stars
(as viewed from earth)
Name
Distance
Apparent
Luminosity
(light years) magnitude (compared to
sun)
–26.7
Sun
1
Sirius A
8.6
-1.4
21.8
Canopus
310
-0.6
14,000
Arcturus
37
-0.1
110
Alpha Centauri A
4.4
0.0
1.5
Vega
25
0.0
48
Capella
42
0.1
130
Rigel
770
0.2
40,000
Procyon
11.4
0.4
7.0
Betelgeuse
430
0.5
9400
Achernar
144
0.5
1070
Hadar (Beta Cen)
525
0.6
12,000
Brightness of Stars
• True brightness (“_________”)
luminosity is a
star’s actual rate of energy output,
measured (for example) in watts. The
Sun’s luminosity is about 4 x 1026 watts.
• Apparent brightness is determined by
the “________”
intensity of starlight striking a
detector. It is measured (for example) in
watts per square meter. The Sun’s
apparent brightness from Earth’s
location is about 1400 watts per square
meter.
True
brightness
Formula: Apparent brightness =
4π(distance)2
Brightness of Stars
Formula:
Apparent brightness =
True brightness
4π(distance)2
• The inverse
_______________
square law
is due to the dilution of
the light. At each radius
same total
you have the _____
amount of light going
through the surface of an
imaginary sphere.
Surface area of a sphere
increases by R2.
• The light/area therefore
1/R2
decreases by ____.
Inverse Square Law
Light has 1/4 the
intensity compared
to a distance of R
R
2R
Light more concentrated
Apparent Brightness
Apparent brightness obeys an inverse
square law with distance.
At the distance of
Jupiter (__
5 A.U.),
the Sun is
_______
25 times
dimmer than on
Earth.
Alpha Centauri
• How does the apparent brightness of
Alpha Centauri compare to the Sun?
– Alpha Centauri radiates almost the
same amount of light as the Sun.
– Distance to Sun = 1.5 x 108 km (1 A.U.)
– Distance to Star = 3.8 x 1013 km (4 ly)
1ly = 63,000 A.U.
3.8 x 1013 km
1.5 x 108 km
=
252,000 times
further
Alpha Centauri
• Alpha Centauri is located 252,000 times
Earth’s distance from the Sun.
• Inverse Square Law 1/distance2
1/(252,000)2
• Thus, its apparent brightness is __________
62.5 billion
times less than that of the Sun.
• Suppose we move the Sun to three
times its current distance. How much
fainter will the Sun appear?
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1
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3
QUESTION
• QUESTION: Two stars have the same
luminosity, but one is 2X further away
from the earth. It will appear
– 1/4 as bright
– 1/2 as bright
– 2X brighter
– 4X brighter
• Repeat for the case that the second star
is 2X closer. Use choices from above.
QUESTION
• QUESTION: Two stars have the same
luminosity, but one is 2X further away
from the earth. It will appear
– 1/4 as bright
– 1/2 as bright
– 2X brighter
– 4X brighter
• Repeat for the case that the second star
is 2X closer. Use choices from above.
QUESTION
• QUESTION: Two stars have the same
luminosity, but one is 2X further away
from the earth. It will appear
– 1/4 as bright
– 1/2 as bright
– 2X brighter
– 4X brighter
• Repeat for the case that the second star
is 2X closer. Use choices from above.
QUESTION
• QUESTION: Two stars have the same
luminosity, but one is 2X further away
from the earth. It will appear
– 1/4 as bright
– 1/2 as bright
– 2X brighter
– 4X brighter
• Repeat for the case that the second star
is 2X closer. Use choices from above.