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ASTRONOMY

ASTRONOMY. Chapter 26 Doubles, Variables, and Clusters. Binary Stars. Most stars that we see are actually contain two or more component stars. The terms binary stars and double stars are the same Binary stars are two (or more) stars held together by their gravitational attraction.

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ASTRONOMY

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  1. ASTRONOMY Chapter 26 Doubles, Variables, and Clusters

  2. Binary Stars • Most stars that we see are actually contain two or more component stars. • The terms binary stars and double stars are the same • Binary stars are two (or more) stars held together by their gravitational attraction. • There are several different ways to find binary star systems.

  3. Double Stars http://astrobob.areavoices.com/astrobob/images/AlbireoHunterWilson1.jpg

  4. Binary Stars • Optical doubles are two stars that are in the same line of sight but are not near each other. • Since these are not connected by gravity, we will not consider these to be binaries. • Visual binaries are stars that can be seen to be doubles through a telescope with sufficient resolution. • About 5-10% of stars are visual binaries.

  5. Spectroscopic Binaries • Spectroscopic binaries are systems that can be detected by a Doppler shift that varies with time. • As one star orbits the other, its Doppler lines change from red to blue and back again in a periodic manner.

  6. Eclipsing Binaries • In a few cases, one star of a binary system will pass in front of the other, causing a periodic change in the light curve of the stars. • Proper orientation of the orbit to Earth is necessary for us to observe this. • To be detectable, the plane of orbit must be close to our line of sight.

  7. Eclipsing Binaries

  8. Astrometric Binaries • In some cases, the second star of a binary system is only detectable by the “wobble” of the visible star. • These are called astrometric binaries.

  9. Astrometric Binaries

  10. The Mass-Luminosity Relation • Binary stars can be used to determine the masses of stars. • When the luminosity of stars is graphed against their masses an interesting relationship arises. • The mass-luminosity relation is only valid for main sequence stars. • Mass is the primary factor in determining where on the main sequence a star will settle.

  11. The Mass-Luminosity Relation

  12. Variable Stars • Variable stars vary in brightness over time. • The period for a variable star is the amount of time it takes to go through one complete cycle. • Eclipsing binaries will do this, but some individual stars do this by themselves. • These are called intrinsic variables.

  13. Types of Variables • Mira Variables • Red giants • Period of 3 months to 2 years • Spectral type M • 700× the diameter of the Sun • Emit mostly infrared • Most numerous type of variable • Sometimes called long-period variables.

  14. Types of Variables • Cepheid • Characteristic shark fin shape • Period of 1 to 100 days • Very regular • Brightness change is due to surface oscillating in and out. • A very close relationship exists between absolute magnitude and period. • Used to measure distances to other galaxies.

  15. Period-Luminosity Relation • For Cepheid variables. • Average brightness is on vertical axis.

  16. Types of Variables • RR Lyrae Variables • Periods are short (less than 1 day) and regular • Distinctive light curve • All have the same absolute magnitude • Many appear in clusters, so they are also called cluster variables. • Can be used to measure inter-galactic distances in the same way as Cepheids.

  17. Clusters • A cluster is a group of stars that are close (relatively) together • Stars of a cluster are all about the same distance from Earth • Stars of a cluster are all about the same age.

  18. Open Clusters • Open clusters are also called galactic clusters. • Found in the galactic plane. • No specific shape. • Best-known example is the Pleiades or Seven Sisters. • Contain hundreds to a few thousands of stars, almost all of which are Population I stars similar to our sun.

  19. Open Clusters

  20. Globular Clusters • Spherical in shape • Contain ten thousand to a million stars • Most are seen in a halo above and below the galactic disk • Have a relatively low abundance of heavy elements.

  21. Globular Clusters http://www.nasa.gov/mission_pages/hubble/science/ancient-stars.html

  22. Ages of Clusters • The ages of clusters can be determined from the Hertzsprung-Russell diagram for the cluster • The length of the main sequence indicates the age

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