Download Thursday October 1 - Montana State University

Agenda
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Review EM radiation
How far away are stars?
What can starlight tell us?
NASA
Sunlight carefully separated by
wavelength
Absorption and emission
Absorption spectrum
Emission spectrum
(glowing material)
Blackbody spectrum
U. Tennessee
Consider the following photo
Compare and contrast the
stars in the photo
NASA
What if all stars had the same
luminosity?
• Luminosity means total
power output as light.
• Determining their distance
from us would be easy if they
all were the same luminosity!
Given only 25 Watt bulbs
The brightness depends only on their distance from us
Stars aren’t all 25 Watt bulbs!
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A star’s distance can be determined:
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If we know how luminous it really is
Compared to how bright it looks to us
Apparent Magnitude scale
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Classifies objects by how bright they
appear in the sky
Larger number means less bright
1 magnitude = difference in brightness
by a factor of 2.5
Apparent Magnitude scale
Sirius
Full moon
Venus
Sun
-25
-20
-15
-10
-5
Faintest object Faintest object
With binoculars 4 m telescope
Naked
eye
0Polaris5
10
15
20
25
Absolute magnitude
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Luminosity = how much light a star is
giving off
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(like wattage for light bulbs)
Compared to the Sun (Lsun)
Absolute magnitude = how bright (what
magnitude) a star would appear at 10
parsecs (32.6 light years)
Example: Antares versus
Sirius
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Apparent magnitudes:
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Antares -4,
Sirius -1.5
Absolute magnitudes:
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Antares +2, Sirius +1.4
Which appears brighter in the sky?
A. Antares
B. Sirius
C. same
D. Cannot conclude
Example: Antares versus
Sirius
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Apparent magnitudes:
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Antares -4,
Sirius -1.5
Absolute magnitudes:
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Antares +2, Sirius +1.4
Which is more luminous?
A. Antares
B. Sirius
C. same
D. Cannot conclude
So how do we determine
the distance to other stars?
Compare absolute and
apparent magnitudes
• If we know the luminosity (or
absolute magnitude) of a star,
we can find its distance.
• A star of known luminosity
is called a standard candle.
• More on this later...
So how do we determine
the distance to other stars?
There is another, more direct way...
Triangulation (parallax)
Measure the two angles (a, b) to find how wide the Missouri river is!
Trigonometric parallax
Earth
Known
Distance
(2 AU)
Some star
p
Sun
Unknown distance
Earth
What makes parallax
measurement difficult?
A.
B.
C.
D.
It’s hard to read your protractor when
it is dark outside.
The parallax angle is very small
because the stars are so far away.
We can’t see any of the same stars
six months apart.
It actually is not difficult!
Parsec (a unit of distance)
1 arcsecond = 1/60 arcminute = 1/3600 degree
1 AU
If p = 1 arcsecond
This distance is 1 parsec (3.26 light years)
Unfortunately, there are no stars this close to us.
Limits with this method
Earth
Known
Distance
(1 AU)
Sun
Some star
p
Unknown distance
Beyond 300 light years, p is too small for us to measure.
(Our galaxy is about 100,000 light years across)
What can we learn by looking
at starlight?
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Distance to star
Ingredients
Temperature
Exam one is Tuesday at 6 pm
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Tuesday – optional review during class
Individual portion
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Group portion
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Multiple choice
2 essay questions
Multiple choice
Grab a review sheet!
Challenge: write one exam question
(multiple choice), and email it to me!
Team activity # 4
Trigonometric parallax
Include last names and team
number on all activities!
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Part I: Do NOT place any measuring
device beyond the line!! You must
estimate the angles!
Part II: You may want to go into the
hallway.
Grab your graded activities on the way
out!
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Nearby stars
appear to move
(Not apparent to
the naked eye)
Wikipedia