Download 9 spectroscopic parallax

Agenda
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Review magnitude scale and HR
diagrams
Another way to measure distance:
Spectroscopic Parallax
Please pick up essays after class
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

Luminosity = how much light a star is
giving off



(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)
Standard Candles
The Sun has an apparent magnitude
of –26. What might its absolute
magnitude be?
A. –26
B. –23
C. 4.85
D. –33
E. Cannot determine
Recall from last time…
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We know temperature

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OBAFGKM
We can determine luminosity (if we
know distance)
Temperature and luminosity are
related!
-10
-5

main sequence
0
5
1
01
5
The HR Diagram!!
Absolute magnitude
Bright-10
Dim
Main Sequence Stars
-5
Red
Giant
Stars
0
5
Sun
White
Dwarf
Stars
10
15
O
HOT
B
A
0123456789
F
Spectral Class
G
K
M
COOL
Given two stars, which one
looks brighter in the night sky?
B.
C.
D.
Star A
Star B
Look the same
Cannot
conclude
-10
Absolute magnitude
A.
-5
0
B
5
10
A
15
O
B
A
F
Spectral Class
G
K
M
Given two stars, which is
larger (more surface area)?
B.
C.
D.
Star A
Star B
Look the same
Cannot
conclude
-10
Absolute magnitude
A.
-5
0
B
5
10
A
15
O
B
A
F
Spectral Class
G
K
M
Spectroscopic “parallax”
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
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Another way to measure distance
It’s not parallax!
How does it work?
1.
2.
3.
Measure star’s spectral type
Get luminosity (absolute magnitude)
from spectral type (how?)*
Use the “standard candle” method to get
distance.
*Temperature => Luminosity
Absolute magnitude
Bright-10
Dim
Main Sequence Stars
-5
Red
Giant
Stars
0
5
Luminosity
Sun
White
Dwarf
Stars
10
15
O
HOT
B
0123456789
A
F
Spectral Class
Works only for main sequence!
G
K
M
COOL
Summary
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We can measure apparent magnitude
and spectral type.
The main sequence lets us infer
absolute magnitude from spectral type.
Comparing apparent magnitude to
absolute magnitude gives us distance.
Something to ponder...

Astronomy, as an observational science,
has limitations:



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Can’t build stars in our laboratory
Can only observe what happens to be available
Can’t go back in time to watch stars form, or go
forward in time to see them die.
How, then, can we learn about the life
cycles of stars?
Sample stars that exist now in various
stages of development.
Next time
Star Clusters — and how they hint at
Stellar Life Cycles
Activity #7
Spectroscopic Parallax