Download OBAFGKM

ASTR 1120: Stars & Galaxies
Topics for Today
• Why are spectral classes O, B, A … linked
to star’s surface temperature ?
• How do we measure brightness of stars:
and why apparent vs absolute magnitudes?
Cat’s Eye
Planetary
Nebula
Prof. Juri Toomre TA: Licia Ray
Lecture 15
Mon 16 Feb 04
• Mid-Term Exam 1 (graded) should be
returned in class on Wed
• Homework Set 3 still available: due in class
this Friday (we have office hours -- we are
happy to help with anything)
zeus.colorado.edu/astr1120-toomre
Brightness / Distance
Clicker Q
•
B.
Leonardo and Guinevere are two stars
that have the same apparent brightness.
Leonardo has a larger parallax angle
than Guinevere. Which star is more
luminous?
• A. Leonardo
• B. Guinevere
• C. Cannot determine from data given
Spectra help classify stars
Brightness / Distance
• Leonardo has a larger parallax angle -thus he is closer to us
• They both have the same APPARENT
brightness, but Leo is closer
• B.
Guinevere must be more luminous
OBAFGKM
• Spectral (color)
classification
O= bluest, hottest
G = yellow (Sun)
M = reddest, coolest
1
RECALL the
refinements:
DECIMAL
SUBDIVISION
LUMINOSITY
CLASSES
Sun is:
G2 V
COLOR CLASS
Why temperature
and spectral lines
are linked?
SAHA gives
the answer:
Cecelia figured out WHY stellar spectra
are so different: TEMPERATURE
• She showed that SURFACE
TEMPERATURE is the big
factor
• She used the newly-devised
SAHA EQUATION that could
estimate how many electrons
remain attached to atoms as
temperature is changed (or the
level of ionization)
Cecelia Payne-Gaposchkin
(Harvard PhD thesis 1925)
O B A F G K M Æ decreasing temperature
SAHA predicts
(spectral)
line-strengths
with temperature
can estimate
“population of
different energy
levels” in
H, He …
and ionization
Spectral Classification: O B A F G K M
Stars and their spectral classification
Hottest stars: O B
ionized helium only
Hot stars: A F helium,
hydrogen
Cooler stars: G
hydrogen, heavier
atoms
Coolest stars: M
molecules, (complex
absorption bands)
2
Oh to describe a star !
Surface
Temperature
• Which is a red supergiant ?
•
•
•
•
Magnitudes:
Apparent vs Absolute
• Giving measures
to stellar
luminosities
• Built on choices
made by Greeks!
Measuring
BRIGHTNESS
B.
A.
B.
C.
D.
Spectral type G2, luminosity class V
Spectral type M2, luminosity class I
Spectral type O9, luminosity class I
Spectral type M1, luminosity class V
Stellar
MAGNITUDES
Weird system:
brighter is smaller
magnitude, even
negative!
Of cultural
importance,
even if a bit
confusing
(secret society)
(Slightly) screwy
world of
MAGNITUDES
magnitudes
m
apparent mag:
what looks like in sky
M
absolute mag:
what would look like
if at 10pc distance
( LUMINOSITY )
IF can estimate
distance, then
can determine M
given m
M = m if at
distance 10pc
3
DISTANCE
MODULUS
Color Index:
B-V
measure of
distance to
the star
magnitudes
assigned using
photometric
FILTERS
(quicker than
taking spectra to
classify stars)
H-R Namesakes
HertzsprungRussell (H-R)
Diagram
Luminosity
(magnitude)
vs
Spectral class
(temperature)
Ejnar Hertzsprung
Henry Norris Russell
For the Next Lecture
• Finish reading all of Chap 16, Properties of
Stars, pretty carefully
• We shall use binary stars to help determine
stellar MASSES
• We shall see how to determine stellar
RADII, needed to make sense of the H-R
diagram
• Work on Homework Set 3, both on the H-R
Diagram web tutorial, and in deciding which
nearby stars are the winners for Part B.
Luminosity
Å Temperature
4