T Monocerotis is a bright pulsating variable star belonging to the class of Cepheid variables. The Cepheids are variables that undergo self-sustaining, spherically symmetric pulsations with periods of days to weeks. They have long played an important role in astrophysics, and are one of the primary ways that we can measure distances in the universe. Cepheids are known to have a relationship between their periods and their luminosities; the longer the period, the larger the amount of light they emit. If we measure the period of a Cepheid variable, we can compare the apparent magnitude of the star with the absolute magnitude predicted by the period-luminosity relation to derive the distance. This period-luminosity relation was discovered by Henrietta Swan Leavitt early in the 20th Century, and is now known as the Leavitt Law. It has been used to measure distances to Cepheids within our own Milky Way as well as to Cepheids in other galaxies. In fact, measurements of Cepheids in other galaxies were the primary way that astronomers like Edwin Hubble were able to prove that other galaxies were at great distances from us.
T Mon is a bright equatorial variable that's visible to both northern and southern hemisphere observers. It has a period of 27.02 days -- and a distance of over 1500 parsecs!