Download Lecture 10: Stars

2/13/14 ASTR 1040: Stars & Galaxies
Eskimo
Planetary
Nebula
Topics for Today & Tues
&  What can we measure in other stars?
&  How do we begin to classify other stars?
&  Vital work by Annie Jump Cannon in devising
a sensible “spectral sequence” for stars
&  Why temperature and spectral lines are
closely linked in classifying stars O B A…M
Prof. Juri Toomre TA: Ryan Orvedahl
Lecture 10
Thur 13 Feb 2014
zeus.colorado.edu/astr1040-toomre
Logistics
Chap 15 – SURVEYING THE STARS
&  Read Chap 15.1: Properties of Stars
&  Overview read for recitation Chap S3:
Spacetime & Gravity (Black Holes)
&  First Mid-Term Exam in class today
(9:50am) -- 50 minutes
&  Observatory #3 was last night
&  Homework #4 due on in class on Tues
&  Fiske Planetarium next Thur (20 Feb) on
“Black Holes: Other Side of Inifinity”
Often only seeing a point of light
&  Stars are so small
compared to their distance
that we almost never have
the resolution to see their
sizes and details directly –
“point sources”
&  We deduce everything by
measuring the amount of
light (brightness) at
different wavelengths
(color, spectra)
&  Measuring stellar
luminosities
&  Measuring
distances
&  Measuring
temperatures
So what can
we find out about
other stars?
APPARENT
BRIGHTNESS
POSITION
SPECTRUM
1 2/13/14 Most Basic Problem in Astronomy
Inverse Square Law of Brightness
Apparent
Brightness
≈
Lo / (distance)2
Star of given APPARENT BRIGHTNESS
could be either
A. very luminous star far away
B. low luminosity star closer by
Need to know the DISTANCE to the star
Stellar Luminosity
&  What we measure:
APPARENT BRIGHTNESS
or how bright it appears to us
here on Earth
&  What we want to know:
(absolute) LUMINOSITY
or how much energy is
emitted (joules/sec or watts)
Parallax – to determine distance
o  Measure the apparent
movement of stars over a
year
o  Movement is caused by
Earth’s movement
around the Sun
o  Closer objects will move
more than farther objects
&  Need to know DISTANCE to
the star
How Stellar Parallax Works
Class self-demo of parallax
&  Your nose is the Sun
&  Your left eye is the Earth in January
&  Your right eye is the Earth in June
Watch the apparent motion of your thumb against
a distant reference point (repeat at arm’s length)
Which “move” more -- closer or farther objects?
2 2/13/14 Stellar Parallax:
measuring nearby
distances
Best parallax measurers:
Hipparcos satellite 1989-1993
GAIA satellite Dec 2013 à
&  Space measurements not
affected by atmosphere
&  Measurement made many
times until accurate to
~0.001 arcsec (Hipparcos
à1,600 light years)
&  100,000 stars mapped; 2.5
million lesser accuracy
&  GAIA: 10 micro-arcsec,
billion stars; 10,000+ ly
Spectra help classify stars
Measuring Surface
TEMPERATURE
Shape of
spectrum good
… but
spectral lines
much better
OBAFGKM ?!?
&  Spectral (color)
classification
O = bluest, hottest
G = yellow (Sun)
M = reddest, coolest
A bit of history: Classifying Stars
World War I, Harvard
College observatory
Women were hired by
Pickering as
“calculators” to help
with a new survey of
the Milky Way
Most had studied
astronomy, but were
not allowed to work
as scientists
3 2/13/14 Devising the strange temperature code
Which absorption (dark) lines are strongest?
&  Original classification of
spectra (1890) was:
A = strongest hydrogen
feature
B = less strong
hydrogen …C, D, etc.
&  Annie Jump Cannon
realized that a different
sequence made more
sense (~1910)
à O B A F G K M
!!
Spectral Classification: O B A F G K M
Which ABSORPTION
lines are strongest
First Mid-Term Exam
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)
Rules of the Game
&  Closed book, closed notes, can use doublesided handwritten “crib sheet”; 50 minutes
&  Print your name and student ID on top of
pages 1 and 6 of exam sheets
&  Print and encode your name and student ID on
scan sheet (and nothing else)
&  Use # 2 (soft) pencil for marking your answers
on scan sheet ($ 1 buys you a pencil !)
&  Respond carefully to Essay Question 46, with
full and lucid sentences (even a sketch or two)
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