Question

Use an analogy of a candle in a room to explain the standard candle model to estimate astronomical distances. Explain its context in the Tully-Fisher relation to estimate distances.

Answer 1

The best standard candles have some easily measurable property that is directly related to their luminosity, just like in Cepheid variables where the period of the variability is directly proportional to the luminosity. Some other examples of standard candles are:

The orbital speeds of objects are directly related to the sum of the masses of the orbiting objects similar way of Newton's version of Kepler's third law. So, if we increase the mass of a galaxy, the rotation speeds of stars in that galaxy will increase. So, faster-rotating galaxies must be more massive. More massive galaxies are also more luminous because they contain more stars. So, if we measure a galaxy's rotation curve by using the Doppler-shifted lines in their spectra, you can estimate with some accuracy that galaxy's luminosity. Then, if you measure its flux, you can calculate its distance, just like we do for stars.

• Type I Supernovae
• Spiral Galaxies with measured rotation speeds (The Tully-Fisher Relation relates the rotation speed to the luminosity for these objects).