THE COSMIC DISTANCE SCALE
An important task of modern astronomy is to determine the distance to faraway objects (stars, galaxies, etc..) . Indeed the distance, together with the angular coordinates, is the key step to reconstruct the 3D structure of nearby galaxies and clusters of galaxies, and on a cosmic scale to quantify the expansion law of the Universe (Hubble constant) together with its acceleration and to infer the existence of the cosmological constant. There are many other astrophysical phenomena and yet unsettled problems (e.g. the age of the globular clusters) that much depend on knowing the distance of celestial bodies.
Direct methods to measure distances based on parallaxes can be used only to a few hundreds of pc
(see Hipparcos. GAIA has to come!). They are used to calibrate the mass-luminosity relation for nearby stars and with some effort to determine the distance of galactic clusters and the Magellanic Clouds. The next step of the “so-called distance ladder” is to calibrate the period-luminosity relation of Cepheids and the absolute magnitude of RR Lyrae, two classes of pulsating stars that, thanks to their luminosity can be detected at large distances (up to 20 Mpc for Cepheids), allowing distance measurements for larger volumes of space. These are also used to calibrate some secondary distance indicators such as the luminosity function of Planetary Nebulae and the maximum light emitted by Type Ia supernovae, which add another step to the distance ladder. Planetary nebulae are easy to detect and the fall-off of their number above a certain absolute magnitude seems to be the same in all galaxies thus providing an important distance indicator (up to 30 Mpc). Astronomers have reasons to believe that maximum light output from all type Ia supernova is the same or related to a few known parameters, so that their ability as distance indicators is obvious and they can be used to reach distances above 1000 Mpc. Insofar intrinsic properties of the stars have been used to define the so-called standard candles, objects whose absolute luminosity (magnitude) is thought to be very well known. Distances from Type Ia set the limit for stellar candles. Other methods involve the properties of galaxies or clusters of galaxies. To mention a few: the Tully-Fisher technique relying on the observation that the luminosity of spiral galaxies is related to their total mass, so that measuring the mass from the rotation curve, the luminosity and the distance (up to about 100 Mpc) can be estimated; the surface brightness fluctuations over elliptical galaxies and spiral bulges that are found to correlate with the distance and allow to go up to 70 Mpc; and others.
Are the above distance limits free of uncertainty? Are the primary stellar candles set on a solid ground as far as the stellar physics is concerned? Is the PN luminosity function an univocal indicator of distance or are there other effects (for instance age and / or metallicity of the stellar population of a galaxy) affecting its capability ? Is the luminosity of type Ia supernovae really constant? All these question marks and many others tell us that the cosmic distance ladder is far from being settled down.
The aim of the school is twofold: first to provide an overview of the subject, high-lightning assessed and uncertain aspects of it, and second to stimulate your curiosity on future developments. Certainly the problem of the cosmic distance scale not only is basic to many aspects of modern research but also it is an interdisciplinary school on astrophysics.