Variable Stars Help
Introduction to Variable Stars—Eclipsing Binaries
Some of the fluctuating “stars” are actually binary systems (pairs of stars in mutual orbit) that eclipse each other. If one of the stars is large and dim and the other is smaller and brighter, and if the orbital plane of the stars appears edge-on to us, we will see a decrease in the brilliance of the system when the dimmer star passes in front of the brighter one. These eclipsing binaries are characterized by steady brilliance with periodic sharp dips. The dips always have the same depth; that is, the minimum brilliance is always the same. The dips occur at regular intervals.
Eclipsing binaries can be recognized in another way. The absorption-line spectra of the two stars alternately shift slightly toward the red and blue ends of the spectrum as one star recedes from us and the other approaches. When an object moves away from us, the spectral lines are red-shifted (they move toward longer wavelengths); when an object approaches, the spectral lines are blue-shifted (they move toward shorter wavelengths). This is a result of the well-known Doppler effect , the same phenomenon that makes a car horn sound higher in pitch as the car comes at you and lower in pitch as it passes and then moves away from you. Dual red-blue back-and-forth spectral shifting is a dead giveaway that a variable “star” is an eclipsing binary.
Certain variable stars, known as Cepheid variables or Cepheids , are intriguing because of the clockwork regularity of their changes in brilliance. These stars get their name from the fact that one of the first-discovered and most well-known of them is in the constellation Cepheus .
Individual Cepheids change brightness at a consistent rate. However, two different Cepheids can have different periods (lengths of time from one peak of brilliance to the next). Some Cepheids have periods of less than 1 Earth day; others have periods of several weeks. Some Cepheids vary greatly in brightness, whereas others vary only a little. Polaris, the familiar North Star that has been used by navigators for centuries, is a Cepheid variable. Its brilliance does not fluctuate enough to be noticeable to the casual observer.
After tirelessly searching for coincidences and patterns, astronomers discovered that the periods of Cepheids are correlated with their absolute visual magnitudes. This correlation is so precise that these stars can be used as distance-measuring beacons for determining distances in interstellar and intergalactic space. The longer the period, the brighter the star, averaged over time. Cepheids are massive stars that, when plotted on an H-R diagram, fall in the upper middle region, off the main sequence. These are called yellow supergiants . Not all yellow supergiants are Cepheids, but all Cepheids seem to be yellow supergiants.
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