PES 106        Spring 2003 Previous Lecture General Astronomy II Next Lecture

Lecture Notes:

Properties of Stars: Line Spectra and Binary Stars

text: Chapter 12 (sections 12.3 - 12.5)

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Star's spectra

Absorption spectra: most stars have cooler, less dense "photospheres" and so have absorption spectra

These spectra can give us information about

• composition
  • as we discussed before - from "fingerprints" of each element
  • amount of element is represented by how dark the absorption lines are
  • many stars are relatively similar in composition
• temperature
  •  location of electron in orbits of atoms depend on temperature
    • hot atoms can have electrons excited into higher orbits
    • this changes the absorption spectrum
    •  as temperatures change the darkness of absorption lines will change
    • fingerprint does not really change
      • some lines may become very faint
      • other lines that are usually very faint may become dark
  • See Figure 12.7 in text for examples [Link to Figure 12.7]
    • notice the dark line in the red part of the spectrum - easy to see in A, faint in B, gone in C

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Temperature: Spectral Classes (spectral types)

By comparing absorption spectra from many stars:

• divide star spectra into 7 "spectral classes"
• work done in late 1800's and early 1900's by Harvard women astronomers

  O -- B -- A -- F -- G -- K -- M

  Oh Be A Fine Girl/Guy Kiss Me

Originally the order was alphabetical from A - P, but that was before the connection to temperature was understood

CLASS	O	B	A	F	G	K	M
TEMPERATURE	>25,000 K				6000 K		<3500 K
COLOR	blue				yellow		red

Each spectral class has 10 subdivisions: ...A0, A1, A2, ..., A9, F0, F1, F2, ...

Sun is a G2 star

Figure 12.9 in the text shows typical absorption spectra for each spectral class. [Link to Figure 12.9]

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Star Motions

relative motion toward or away from us (radial motion) can be measured with the Doppler Effect

motion side to side (tangential motion) is very hard to measure

rotation of stars can also be measured using the Doppler Effect

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Magnetic Fields

magnetic fields can cause SPLITTING of absorption lines

often this effect can be covered up by broadening of lines from rotation

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Mass - Binary Stars

Hard to measure mass of stars - unless they interact gravitationally with something else

Can observe planets orbiting some stars.

Binary stars - stars which orbit another star

>40% of all stars are in binary systems

Use Kepler's third law to find the Masses

• Measure orbital period and separation distance
• Use Kepler's Third Law to find the TOTAL mass
• use actual orbital paths to find the individual mass of each star

Ways to observe Binary stars:

• visual binaries - telescope images show both stars orbiting one another
• spectroscopic binaries- no separate images - but spectra show 2 stars with Doppler shifts from motion
  • Figure 12.13 show this. [Link to Figure 12.13]
  • This JAVA applet demonstrates the Doppler shift: http://instruct1.cit.cornell.edu/courses/astro101/java/binary/binary.htm
• eclipsing binaries - stars pass directly between the Earth and the companion star causing a dimming in the total brightness of the stars.
  • Figure 12.15 shows this. [Link to Figure 12.15]

[A nice PC-based shareware software program is available to demonstrate eclipsing binary stars. The program, Binary 3.0, shows the light curves for eclipsing binaries. You can change the luminosity, radius and mass of the stars as well as the orbital parameters. Download the file binary30.exe from ftp://ftp.seds.org/pub/software/pc/stars/.]

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Summary

Typical observed ranges of star properties: (already covered in previous lecture !!)

Property	Minimum	Maximum
Temperature	3000K	30,000 K
Mass	0.1 x Sun	30 x Sun
Luminosity	0.001 x Sun	1 million x Sun
Radius	0.01 x Sun	1000 x Sun
Composition	similar to Sun	similar to Sun

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