Chapter 19 The Nature of the Stars

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Announcements Monday Feb 27

• Exam 2
• This Wednesday (March 1) in class
• 25 multiple choice questions
• Covers Chapters 6 (Telescopes), 14 (The Sun), 15
  (Surveying Stars)
• Practice exam is posted on course website (exams
  page)
• PRS Survey questions
• Mostly for fun, compare statistically with other
  classes
• Not tracked by student name!
• All survey answers count 1 pt (all answers
  correct)
• PRS quizzes and EC Honor code applies!
• Dishonest behavior Talking (unless sanctioned),
  using multiple PRS (friends), looking at notes or
  neighbors responses - violators will be
  dropped from PRS participation

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Survey question
Q. Suppose you have a young child and could
somehow choose his/her future career. Of the
following, what would be your top choice?

• U.S. Senator
• Poet laureate of the U.S.
• Nobel prize winning research scientist
• Highly successful company CEO
• Supreme Court Justice

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Survey question
Q. Suppose you have a young child and could
somehow choose his/her future career. Of the
following, what would be your bottom choice?

• U.S. Senator
• Poet laureate of the U.S.
• Nobel prize winning research scientist
• Highly successful company CEO
• Supreme Court Justice

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Clear Sky Highlight of Week Leo the Lion
Harbinger of Spring
Rigel
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Gemini
Saturn
Orion
Canis Minor
Leo
Leo the Lion chases the winter hunter Orion of
out the late winter sky
Canis Major
830 pm Looking SSE
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Leo
The sickle
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Ch. 15 Properties of Stars, Continued
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Spectral TypeLetter codes 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)

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Survey Question

• Which Mnemonic do you prefer for the spectral
  sequence OBAFGKM?
• Oh Boy, An F Grade Kills Me
• On Beautiful Afternoons Friends Gather Keg Money
• One Bad Astronomical Formula Gives Killer
  Migraines
• Oh Be a Fine Girl (or Guy) Kiss Me
• Officially, Bill Always Felt Guilty Kissing
  Monica

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A stars color depends on its surface temperature.
Cool star Red
Hot star Blue
Wiens law the hotter the object, the shorter the
wavelength of its maximum emission.
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Example Betelgeuse, Rigel in Orion Constellation
Betelgeuse (T 3100K)
Rigel (T 12,000K)
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Q. Which constellation is Leo the Lion?
B
C
E

• A
• B
• C
• D
• E

D
A
830pm looking SSE
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Q. What spectral types are cooler than spectral
type G?

• K, M
• F,K
• B, F
• A, M
• O, K

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Hertzsprung-Russell (H-R) diagramThe key to
understanding the evolution of stars.
HR DIAGRAM PLOTS Absolute magnitude (or
Luminosity) vs Temperature (or spectral type)
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Hertzsprung-Russell (H-R) diagram

• Main sequence stars
• Stable stars found on a line from the upper left
  to the lower right.
• Hotter is brighter
• Cooler is dimmer
• Red giant stars
• Upper right hand corner (big, bright, and cool)
• White dwarf stars
• Lower left hand corner (small, dim, and hot)

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Determining the Sizes of Stars from an HR Diagram

• Main sequence stars are found in a band from the
  upper left to the lower right.
• Giant and supergiant stars are found in the upper
  right corner.
• Tiny white dwarf stars are found in the lower
  left corner of the HR diagram.

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Q. An H-R stellar diagram plots ___ vs. ___?

• Size, distance
• Luminosity, surface temperature
• Radius, distance
• Size, surface temperature
• Luminosity, size

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Width of a stars spectral lines reveal whether
it is a giant, a white dwarf, or a main-sequence
star.
Both of these stars are spectral class B8.
However, star a is a luminous super giant and
star b is a typical main-sequence star. Notice
how the hydrogen absorption lines for the more
luminous stars are narrower.
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Luminosity Classes

• Class I includes all the supergiants.
• Class V includes the main sequence stars.
• e.g., the Sun is a G2 V

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Q. Where on this H-R diagram is the supergiant
stars (luminosity class Ia)?
D
E

• A
• B
• C
• D
• E

C
A
B
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Stars come in a wide variety of sizes

• Stefan-Boltzmann law relates a stars energy
  output, called LUMINOSITY, to its temperature and
  size.
• LUMINOSITY 4pR2sT4
• LUMINOSITY is measured in joules per square meter
  of a surface per second and s 5.67 X 10-8 W m-2
  K-4
• Small stars will have low luminosities unless
  they are very hot.
• Stars with low surface temperatures must be very
  large in order to have large luminosities.

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Luminosity (Radius Temperature) Law
2
1
R Rsun T T sun
R 2 Rsun T ½ T sun
Which star is more luminous? (Emits the most
energy every second)
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Luminosity Law
2
1
Star 2 (Sun) is four times as luminous as star 1
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Q. Compare the luminosities of Star 1 (R 8
Rsun, T 5,000K) and Star 2 (R 2 Rsun, T
10,000K)

• Star 1 is 4x as luminous as Star 2
• Star 1 is 2x as luminous as Star 2
• Star 1 is as luminous as Star 2
• Star 2 is 2x as luminous as Star 1
• Star 2 is 4x as luminous as Star 1

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Binary star systems Classifications

• Double star a pair of stars located at nearly
  the same position in the night sky.
• Optical double stars stars that appear close
  together, but are not physically connected.
• Binary stars, or binaries stars that are
  gravitationally bound and orbit one another.
• Visual binaries true binaries that can be
  observed as 2 distinct stars
• Spectroscopic binaries
• binaries that can only be detected by seeing two
  sets of lines in their spectra
• They appear as one star in telescopes (so close
  together)
• Eclipsing binaries binaries that cross one in
  front of the other.

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Visual Binary Star Krüger 60 (upper left hand
corner)
About half of the stars visible in the night sky
are part of multiple-star systems.
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Mass- luminosity relation for main-sequence stars.
Bigger is brighter!
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There is a relationship between mass and
luminosity for main-sequence stars.
Mass in units of Suns mass
More massive implies more luminous
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Spectroscopy makes it possible to study binary
systems in which the two stars are very close
together.
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Light curves of eclipsing binaries provide
detailed information about the two stars.
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Light curves of eclipsing binaries provide
detailed information about the two stars.
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Light curves of eclipsing binaries provide
detailed information about the two stars.
Click for Eclipsing binary simulation program
(web-based)
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Light curves of eclipsing binaries provide
detailed information about the two stars.
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Eclipsing Binary EQ Tau
Light curve from Astronomical Laboratory Course
Fall 2003