Chapter 10 Measuring the Stars. Star Cluster NGC 3603 20,000 light-years away.
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Transcript of Chapter 10 Measuring the Stars. Star Cluster NGC 3603 20,000 light-years away.
Chapter 10Measuring the Stars
Chapter 10Measuring the Stars
Star Cluster NGC 360320,000 light-years away
Figure 10.1Stellar Parallax
Stellar parallaxStellar parallax
• Measure distance to nearest stars• Baseline is earth’s orbital diameter of 2 A.U.• Parallactic angle (or parallax) is half of total
angle• Star with parallax of 1” is 1 parsec distant• 1 parsec (1 pc) is about 3.3 light-years• Smaller parallax means more distant
Proxima CentauriProxima Centauri
• Part of Alpha Centauri triple star system• Closest star to earth, largest parallax• 0.76” parallax (difficult to measure)• 1.3 pc or 4.3 light-years away• About 300,000X further away than sun
Figure 10.2Sun’s Neighborhood
Nearest neighbor starsNearest neighbor stars
• Less than 100 stars within 5 pc• Several 1000 stars within 30 pc
Hipparcos satelliteHipparcos satellite
• Can measure out to 200 pc• Nearly a million stars
• Future satellites will measure to 25,000 pc
Stellar motionStellar motion
• Radial velocity - along line of sight• Measure using Doppler shift
• Transverse velocity - perpendicular to line of sight
• Monitor star’s position on sky
Figure 10.3Real Space Motion
a), b) Barnard’s Star - 22 years apart
c) Alpha Centauri
Proper motionProper motion
• Annual movement of star across sky• Corrected for parallax• Barnard’s star moved 227” in 22 years• 10.3” per year proper motion• 1.8 pc distance - transverse velocity is 88
km/s
Brightness and distanceBrightness and distance
• Luminosity or absolute brightness (intrinsic)• Apparent brightness is how bright star looks
from earth• Apparent brightness depends on luminosity
and distance
Figure 10.4Inverse-Square Law
Figure 10.5Luminosity
Apparent brightness proportional to
luminosity/distance2
Apparent brightness proportional to
luminosity/distance2
Magnitude scaleMagnitude scale
• Greek astronomer Hipparchus (2nd century BC)
• Ranked stars into six groups• Brightest stars are 1st magnitude• Next brightest stars are 2nd magnitude• Faintest stars (to naked eye) are 6th
magnitude• Larger magnitude means fainter star
Apparent magnitudeApparent magnitude
• Expanded beyond stars visible to naked eye• One magnitude difference is 2.5X in
brightness• A 1st magnitude star is 2.5X brighter than a
2nd magnitude star• Full moon has an apparent magnitude of -12.5 • Faintest objects visible by Hubble or Keck
telescopes are apparent magnitude 30
Figure 10.6Apparent Magnitude
Absolute magnitudeAbsolute magnitude
• It is the apparent magnitude of a star viewed from a distance of 10 pc
• Measure of absolute brightness or luminosity
• Our sun has absolute magnitude of 4.8
• (If sun were 10 pc from us, its apparent magnitude would be 4.8, which is faint)
More Precisely 10.1More on the Magnitude Scale
Figure 10.7Star Colors
Stellar colorStellar color
• Temperature of star determines blackbody curve
• Measurements at two wavelengths can determine blackbody curve temperature
• Use B (blue) and V (visual - green/yellow) filters
• Determines star’s color index or color
Figure 10.8Blackbody Curves
Table 10-1Stellar Colors and Temperatures
Stellar spectraStellar spectra
• Absorption spectrum• Aborption lines determine elements• Temperature determines strength of lines• Hotter stars have more ionized atoms• Coolest stars can have molecular lines
Figure 10.9Stellar Spectra
Spectral classificationSpectral classification
• In 1800’s letter classification used for stellar spectra
• Later rearranged into order of decreasing temperature
Table 10.2Spectral Classes
O B A F G K MO B A F G K M
• Highest to lowest temperature• Mnemonic:• Oh, Be A Fine Girl, Kiss Me• Oh, Be A Fine Guy, Kiss Me• Oh Beastly And Fearsome Gorilla, Kill Me• (Make up your own)
Spectral class subdivisionsSpectral class subdivisions
• Each letter has 10 subdivisions, 0 - 9• 0 is hottest, 9 is coolest, within letter class• Sun is G2 (cooler than G1, hotter than G3)
Direct size measurementDirect size measurement
• Several stars are large, bright and close enough to measure their size directly
Figure 10.10Betelgeuse
Indirect size measurementIndirect size measurement
• Most stars’ sizes can’t be measured directly• Use luminosity temperature4
• And luminosity area (or radius2)• Indirectly determines radius
Figure 10.11Stellar Sizes
Stellar sizesStellar sizes
• R - radius of sun
• Giants - 10X to 100X R
• Supergiants - up to 1000X R
• Dwarf - comparable to or smaller than R
Hertzsprung-Russell DiagramHertzsprung-Russell Diagram
• H-R diagram• Each point represents a star• Luminosity on vertical scale• Temperature (decreasing) on horizontal scale
Figure 10.12H-R Diagram of Well-Known Stars
Stellar radii and H-R diagramStellar radii and H-R diagram
• Radius-luminosity-temperature relationship gives radius
• Diagonal dashed lines on H-R diagrams represent constant radius
Figure 10.13H-R Diagram of Nearby Stars
Analogy 10.1People along a main sequence
Main SequenceMain Sequence
• Most stars in H-R diagram on main sequence• Runs from top left (High luminosity and temp)• To bottom right (low luminosity and temp)• Runs from blue giants and supergiants to red
dwarfs
Figure 10.14H-R Diagram of Brightest Stars
More Precisely 10.2Estimating Stellar Radii
Non-Main SequenceNon-Main Sequence
• 90% of stars on main sequence• 9% of stars are white dwarfs (bottom left)• 1% of stars are red giants (upper right)
Figure 10.15Hipparcos H-R Diagram
Figure 10.16Stellar Distance
Analogy 10.2Traffic lights further away are fainter
Main sequence or not?Main sequence or not?
• Spectral line widths affected by pressure and density
• Determines if main sequence or not
Table 10-3Stellar Luminosity Classes
Figure 10.17Stellar Luminosities
Table 10.4Variation in Stellar Properties within a Spectral Class
SunSun
• Spectral class G
• Subdivision 2
• Luminosity class V
• G2V
Binary starsBinary stars
• Most stars are members of multiple-star systems - Binary-star systems (2) most common
• Visual binaries (see 2 stars)• Spectroscopic binaries (detect Doppler
shift from one or both orbiting stars)• Eclipsing binaries (one passes in front
of other, varying light output)
Figure 10.18Binary Stars
Determining stellar massesDetermining stellar masses
• Measure binary properties• Use orbital radii and period• Universal law of gravitation
Figure 10.19Stellar Masses
More Precisely 10.3Measuring Stellar Masses in Binary Stars
Figure 10.20Stellar Mass Distribution
Table 10.5Measuring the Stars
Stellar lifetimeStellar lifetime
• Depends on mass (how much fuel) and• Luminosity (how fast fuel is consumed)• A B2V star lives 90 million years• A G2V star lives 10,000 million years (our
sun)• An M5V star lives 16,000,000 million years
Table 10.6Key Properties of Some Well-Known Main-Sequence
Stars
Figure 10.21Stellar Radii and Luminosities