STANDARD CANDLES
The most reliable method of determining distances to celestial objects is the method of trigonometric parallax , which is similar to the method that earth-bound surveyors use. However, after a certain distance, parallax angles become too small to measure;  even with satellites such as Hipparchos and the Hubble Space Telescope, we can only measure the distances of objects less than 200 parsecs away.  We must turn to other methods to measure distances to farther objects. These methods utilize the distance modulus and objects called standard candles.

Recall that distance modulus takes the mathematical form (m-M), apparent magnitude minus absolute magnitude. We can measure the apparent magnitude m from Earth, but how do we discover the absolute magnitude M? We must find a relationship between M (or, equivalently, the luminosity L) and some distance-independent parameter (DIP), a quantity of the star that can be known accurately without knowing the object's distance. The most often used standard candles are listed below, in order of increasing distance attainable and decreasing reliability. The DIP involved is in boldface in each case.

Spectroscopic Parallax or Main-sequence Fitting

    Utilizes: The main sequence (MS) relationship between Luminosity and Temperature 

Advantages:

  1. MS is well-defined; little error

  2. MS stars are common 

Disadvantages 

  1. Can only use MS stars 

  2. Most MS stars are too dim to be seen in other galaxies

Cepheid Variables

Utilizes: The Cepheid relationship between Luminosity and Period (or speed) of pulsation 

Advantages:

  1. P-L relationship is fairly well-defined

  2. Cepheids are bright

  3. Cepheids are very distinctive and easily found

Disadvantages

  1. There are two types of Cepheids, with different relationships

  2. Relation is not as well-defined as MS

  3. Cepheids are rare, a short-lived phase in a star's life

  4.  Some galaxies are too far away to display any individual stars, including Cepheids 

Supernovae

Utilizes: Stellar Physics Theory to divine how bright a supernova (SN) should get

Advantage: SN are bright and can be seen at great distances

Disadvantages

  1. Our understanding of SN is poor

  2. SN are rare (about one every 50 years in a spiral, even less frequent in ellipticals) and thus are usually surprises

  3. SN are rarely caught exactly at maximum

Tully-Fisher method 

Utilizes: A relationship between rotation velocity and total luminosity;  For spiral galaxies, the faster the spin the brighter (in general), and for elliptical galaxies, the larger the variety of rotation speeds, the brighter (in general)

Advantage: We can get a lot of galaxies this way 

Disadvantages 

  1. Not all matter in a galaxy is luminous, but all of it contributes to rotation

  2. Note the heavy use of "in general" above!
Updated 3/14/00
By James E. Heath
  
 
 

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