Based on the number of galaxies visible in the Hubble Deep Field (Figure 15.1 in your textbook), the estimated number of galaxies in our observable universe is about ______.
Which of the following is not one of the three major categories of galaxies?
A standard candle is _________.
a light source of known luminosity
What is main-sequence fitting?
a method for determining the distance to a star cluster by assuming that its main sequence should line up with the main sequence on a standard H-R diagram. By comparing the apparent brightness of stars along the cluster's main sequence to the known luminosities of standard main sequence stars, we can calculate the distance (from the inverse square law for light).
What is a Cepheid variable?
A type of very luminous star that makes an excellent standard candle
What two observable properties of a Cepheid variable are directly related to one another?
The period between its peaks of brightness and its luminosity. The period-luminosity relation allows us to determine its luminosity from the period between its peaks of brightness.