Standard stars should normally be constant in the characteristic for which they have been chosen to be standards. Individually they should be capable of testing the instrument used in their measurement, and collectively (usually together with a particular type of instrument) they must define a system of measurement. A star should not be adopted as a standard until several years' observation have demonstrated its constancy. The number of independent observations in that interval is as important as its length. Many of these points are illustrated by particular reference to standard-velocity stars. Even 100 observations of such a star may fail to reveal a detectable variation that is present. A distinction is drawn between “primary” standards, meeting the above criteria, and “reference” or “comparison” stars which need not be so severely tested since their use is more limited. Standard stars must be calibrated in some fundamental way. Since random errors of observation can introduce systematic errors in calibration, the use of the intermediate step of standard stars may decline with increasing internal precision of observations.

Most systems of classification or quantitative measurement depend on standards. It is of the greatest importance for the user to be aware of the mandate of a particular system. If the mandate is not understood by casual users, a system can be either underutilized or abused.

In the particular case of the MK system of spectral classification, types are defined by the standard stars. They can be calibrated, and the calibration may evolve with time, but the types are relatively stable because they are defined by the standards. The autonomy of this powerful system is crucial to its success, but some astronomers do not understand the importance of this distinction. Recent suggestions to change the spectral type of the Sun show an ignorance of the way the system works.

Precautions in the use of standard stars and the frequency of their use depend on the particular system and on its mandate.

The precision obtained in equating a very precise linear dimension to a very small and imprecise angular dimension sets a limit on the precision of the distance scale of the Universe. Within the Solar System, errors in distances are of the order of one part in one hundred million, but beyond its limits only a single star, Barnard's star, has a distance known to better than one part in one hundred, and distances known to one part in twenty from parallaxes are limited to only a few hundred nearby stars. Yet most other distance methods and results must ultimately be calibrated against distances to nearby stars derived from the heliocentric parallax method and its observations and uncertainties.

Fundamental positions and proper motions are derived from absolute catalogs. Taking recent Washington Six-Inch and Seven-Inch Transit observing programs as examples, the observation and compilation of absolute catalogs are discussed. The roles of quasi-absolute and differential catalogs in the formation of fundamental catalogs are also examined. Finally, the current effort to extend the fundamental system to fainter magnitudes than are presently represented in the FK4 is described as well as the expected composition of the forthcoming FK5 catalog.

In this paper are surveyed the combinations of data needed for mass determinations, the impact of radial-velocity and speckle observations, the 1983 catalog of visual-binary orbits, the current material for main-sequence and red-dwarf masses and the controversial methods and results on low mass objects.

The reliability of values of the fundamental properties of stars derived from eclipsing binary analysis is discussed in terms of general concepts. Since the principles involved in the determination of masses and radii are simple, the heart of the matter is the care and judgment with which the relevant spectrographic and photometric observations are obtained and analyzed. Problems in the evaluation of the radiative properties require special attention.

With the advent of speckle interferometry, high angular resolution has begun to play a routine role in the study of binary stars. Speckle and other interferometric techniques not only bring enhanced resolution to this classic and fundamental field but provide an equally important gain in observational accuracy. These methods also offer the potential for performing accurate differential photometry for binary stars of very small angular separation. This paper reviews the achievements of modern interferometric techniques in measuring stellar masses and luminosities and discusses the special calibration problems encountered in binary star interferometry. The future possibilities for very high angular resolution studies of close binaries are also described.

I have been asked to describe the system of spectral classification as it has developed from the original Yerkes Atlas (Morgan, Keenan, Kellman, 1943) until to-day. I use the word “developed” because any system that is to remain useful must be flexible enough to adapt not only to improved techniques of measurement but also to new theoretical insights into the variables that actually determine the energy spectrum of a star in all its fascinating but sometimes frustrating detail.

It is shown that a great breakthrough has occurred in the accuracy of spectroscopic abundance analyses with the introduction of solid state light detectors, such as Reticons and CCDs. Because of uncontrolled systematic errors in photographic photometry, abundances derived from high dispersion photographic spectra can hardly be known with an accuracy better than 0.3 dex. This is well exemplified by the recent finding that the observational scatter is large even in the equivalent widths of the Utrecht Solar Atlas. A fortiori these uncertainties are present in the [Fe/H] stellar abundance Catalogue, chiefly based in its present form on photographic material. For the future the calibration of the [Fe/H] Catalogue with spectra taken with Reticon detectors is recommended. A signal/noise ratio of 300 to 500 is more important than an improvement in spectral resolution with a low signal/noise ratio. Then, the remaining uncertainties in the abundances will mostly reflect inaccuracies in atmospheric parameter determinations of the models and in the assumptions underlying model computations.

The three basic methods for measuring axial rotation of stars had been suggested before the beginning of this century. These are (1) Modulation of starlight due to dark or bright areas on a rotating star; (2) Distortions in the radial velocity curves of eclipsing binary systems; and (3) Line profile analysis. Research in each of these areas is reviewed.

The stellar intensity interferometer at Narrabri Observatory was used to measure the angular diameters of 32 stars in the spectral range 05f to F8 and brighter than about V = +2.6. The results were combined with measurements of absolute flux and, in a few cases parallax, to find the emergent flux, effective temperature, radius and luminosity of the stars. The methods used to calibrate these measurements and their uncertainties are discussed. It is shown that in measuring emergent flux and effective temperature the major uncertainties were most frequently in the measurements of angular size.

The current state of knowledge of angular diameters of stars is reviewed and, based on this review and the requirements for the determination of surface fluxes, effective temperatures, radii and masses, targets of sensitivity, angular resolution and accuracy for future programs of stellar angular diameter measurements are established. Long baseline interferometry is the only technique with the potential to meet all the targets. The necessary improvements in sensitivity, angular resolution and accuracy are promised by the approach adopted in the modern Michelson stellar interferometer under development at the University of Sydney and the prototype instrument, which is currently nearing completion, is briefly described to illustrate how the atmospheric and mechanical problems which have inhibited the development of amplitude interferometry may be overcome using modern technology. This program together with the developments taking place at CERGA lead to the conclusion that the prospects for contributions by high angular resolution measurements to the determination of fundamental stellar quantities during the next decade are excellent.

This paper summarizes the current status of ultraviolet spectrophotometry with emphasis on the instrumental characteristics unique to space observations and on the application of existing data to the calibration of stellar properties. The currently available data bases will be briefly reviewed. When combined with ground based data, ultraviolet observations provide information on effective temperatures and bolometric corrections for early type stars and on the nature of the intervening interstellar medium. The ultraviolet measurements are sensitive to chemical composition differences and provide a powerful tool in discussion of stellar evolution in composite systems. This review concludes with a brief discussion of future directions in instrumentation and analysis.

The absolute fluxes and energy distributions of stars are the foundation of the calibration of fundamental effective temperatures and bolometric corrections. In this paper I will review recent progress in the calibration of absolute fluxes and energy distributions in the visual and IR parts of the spectrum. In the visual, the calibration of the absolute flux and energy distribution of Vega has settled down well, and the remaining difficulties include the lack of a worldwide common list of brighter secondary standard stars, the lack of enough satisfactory fainter secondary standard stars and the possibility of variability in Vega. In the IR, the process of arriving at a dependable and accurate calibration, and of linking it to commonly used photometric systems, is in its infancy. A final, and rather special problem, is the question of the calibration of the Sun. The Sun is a special case both because it is so well studied astrophysically and because its extreme brightness makes it very difficult to calibrate photometrically. Some progress has recently been made on the calibration of the absolute flux and energy distribution of the Sun, and I will discuss this work.

In this review an examination will be made of the experimental conditions which must be satisfied by multicolor photometric systems if the observational parameters are to be correlated unequivocally with physical quantities of the star. An inventory of error sources disturbing such calibrations contains: instability of the natural system, procedures of observation and reduction to outside the atmosphere and correlation with a standard system. The requirements desirable for the preparation of lists of standard stars, for the definition of the pass bands, for the “orthogonality” of the calibrated parameter, for the definition of the domain of validity of the calibration are listed. Some remarks are made on the concepts of “open” or “closed” calibrations.