How much farther away than the RR Lyrae stars can Cepheids be used as distance-measuring tools?

Question

How much farther away than the RR Lyrae stars can Cepheids be used as distance-measuring tools?

Answer 1

RR Lyrae stars are red stars that are used for measuring distances inside our galaxy.  Red light has the useful feature of penetrating dust more readily than other wavelengths.  

Cepheids are a particular class of variable stars that are measured out to a few tens of millions of light years.  In practical terms, this means that Cepheids can be used to assess nearby galaxies, which would mean the galaxies in the Messier Catalog and some of the closer galaxies in the NGC catalog.   Cepheids are especially useful because, being able to use them at further distances, we occasionally get to measure a Cepheid variable and a Type 1A supernova explosion in the same galaxy (not often, but not never).  This means the two techniques can be compared for accuracy.  Cepheids can also be compared to RR Lyrae stars as another validation test, inside our own galaxy.

Once you get past the Cepheids it's Type 1A all the way.  I'm attaching a cool picture taken in an 8" telescope by an amateur.  The Vixen R200  he used is "very good but not top of the line."   You see a dozen galaxies and a Type 1A explosion marked with two bars set at right angles (lower right quadrant of picture).    For me this helps visualize the process of "candles" since these galaxies are pretty close.  Most of the NGC galaxies (there are 7,840 objects in this 19th century catalog, but not all are galaxies) within 500 million light years...assuming average brightness characteristics, that's how it worked out for 19th century optical capabilities.  By contrast galaxies are now imaged out about 20+ times as far with scopes such as Hubble.  

Hope that helps,

GN

Answer 2

RR Lyrae variables — red giants typically used for measuring distances within the galaxy and in nearby globular clusters.

Eclipsing binaries — In the last decade, measurement of eclipsing binaries' fundamental parameters has become possible with 8 meter class telescopes. This makes it feasible to use them as indicators of distance. Recently, they have been used to give direct distance estimates to the LMC, SMC, Andromeda Galaxy and Triangulum Galaxy. Eclipsing binaries offer a direct method to gauge the distance to galaxies to a new improved 5% level of accuracy which is feasible with current technology up to a distance of around 3 Mpc.

Cepheid variables — the preferred choice in extragalactic astronomy, out to the range of a few tens of Mpc.

Type Ia Supernovae — that have a very well-determined maximum absolute magnitude as a function of the shape of their light curve and are useful in determining extragalactic distances up to a few hundred Mpc.

So we are talking tens of Megaparsecs further away.