Chapter 19 The Nature of the Stars

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Announcements Friday Feb 24

• Exam 2
• Next Wednesday (March 1) in class
• 25 multiple choice questions
• Covers Chapters 6 (Telescopes), 14 (The Sun), 15
  (Surveying Stars)
• PRS Quiz 1
• Average was 7.4 out of 10 (74)
• PRS quizzes count 25 course pts.
• Grades Posted on course website (PRS button)

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Chapter 15 Topics

• Using parallax to determine far away are the
  stars
• The stellar magnitude scale
• Stellar spectra and colors
• The chemical composition of stars
• The HR diagram the key to understanding stellar
  evolution
• Properties giant, supergiant, and white dwarf
  stars
• Properties of binary star systems
• Estimates of Mass determination
• Spectroscopy evidence for binarity
• Eclipsing binaries precise mass determination

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Equations needed for Chapter 15

• Parallax equation
• Luminosity ratio equation

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Parallax nearby objects appear to shift with
respect to background as observer moves
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Stellar Parallax As Earth moves from one side of
the Sun to the other, a nearby star will seem to
change its position relative to the distant
background stars. d 3.26 / p d distance to
nearby star in light years p parallax angle of
that star in arcseconds
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Example Using parallax to determine distance

• The bright star Vega has a measured parallax of
  0.1 arcsec (p0.1?)
• This means that Vega appears to move from 0.1?
  to -0.1? with respect to distant stars over a
  years observation
• D(pc) 3.26/p(?) 3.26 / 0.1 32.6 l.y.
• Vega is 32.6 light years from Earth
• But (Las) Vegas is only 1,500 miles from Iowa ?

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• Note The brightness of a star is not a good
  indicator of distance.
• e.g., Polaris is closer than Betelgeuse but
  Betelgeuse appears brighter.

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Stellar magnitudes

• The apparent magnitude scale historically ranged
  from 1 (brightest visuallu) to 6 (dimmest
  visually).
• Magnitude scale dates from Ptolemy (150 AD).
• Note that fainter objects have larger magnitudes!
• Today, the apparent magnitude scale extends into
  the negative numbers for very bright objects
  (Sirius, planets) and to 30 for the faintest
  detectable objects (Hubble deep field).
• Absolute magnitude, on the other hand, is how
  bright a star would look if it were 10 pc away.
• Absolute magnitude is a measure of luminosity
  (wattage of the stellar light bulb). It is
  independent of distance.

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Apparent magniude scale
19 faintest object detectable using Iowas
Rigel telescope
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Pleiades
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Little Dipper (Ursa Minor) Guide to naked-eye
magnitudes
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Q. A nearby star has an annual parallax of 0.5
arcsec. What is its distance?

• 0.5 light years
• 1.63 light years
• 2.00 light years
• 3.26 light years
• 6.52 light years

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Q. Star A has a magnitude 10 while star B has a
magnitude -1. Which statement is correct for
visual observing these stars?

• Star A is very bright, Star B is faint
• Star A is faint, Star B is very bright
• Star A and Star B are both too faint to see
• Star A and Star B are both very bright
• Star A is too faint to see, Star B is very bright

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Astronomy Picture of the DaySupernova 1987A
blast wave
Large Magellanic Cloud
Small Magellanic Cloud
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Spectral Lines(fingerprints of elements and
molecules)
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Importance of stellar spectra 1 Lines
determine what are stars made of (like
fingerprints)
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The spectrum of a star-forming cloud
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Importance of stellar spectra 2 Determine speed
of stars (and galaxies)
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Solar Spectrum
Strongest lines Calcium!
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The spectra of stars reveal their chemical
compositions as well as surface temperatures.
Element helium first seen on Sun (helios) then
found on Earth
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Determining stellar surface temperature by
observing spectral lines
Sun
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The spectra of stars reveal their chemical
compositions as well as surface temperatures.

• In the late 19th Century, spectra was obtained
  for hundreds of thousands of stars.
• These stellar spectra were grouped into a
  (mistaken) classification scheme of spectral
  types A through O by a team at Harvard.

• Today we recognize the spectral types O, B, A, F,
  G, K, and M as running from hottest to coolest.

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The Harvard College Observatory Women Astronomers
c.1910(they systematically classified stars
based on their spectra)
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Spectral TypeCodes that indicates surface
temperature

• Sequence is O B A F G K M
• O type is hottest (25,000K), M type is coolest
  (2500K)
• Star Colors O blue to M red
• Sequence subdivided by attaching an integer, for
  example F0, F1, F2, F3 F9 where F1 is hotter
  than F3 . The Sun is a G2 star (5,800 K)
• Useful mnemonics to remember OBAFGKM
• Our Best Astronomers Feel Good Knowing More
• Oh Boy, An F Grade Kills Me
• (Traditional, politically incorrect?!) Oh Be a
  Fine Girl (or Guy), Kiss Me

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Very faint
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Q. Which spectral type has the hottest surface
temperature?

• B
• A
• F
• O
• M