stellar parameter determination

Stellar parameters such as their mass, luminosity, radius, temperature, metallicity, age, rotation period, and distance can occasionally be determined directly, but more values can be determined with the help of models, such as equations, or charts reflecting many observed stars, or elaborate simulations of stellar processes. Through chains of such determinations, many such parameters can often be determined or approximated.

In some cases even when a parameter cannot be fully determined, a probability distribution of its values can be, through statistically-based studies of entire populations. For example, though the viewing angle of many binary systems' orbits are undetermined, the distribution of viewing angles of all such systems is likely to be evenly distributed, and data from a population of such binary systems still reveals a distribution of any parameters that can be revealed in part by the viewing angle.

The most direct methods of stellar parameter determination:

• parallax - triangulation to measure the distance to nearby stars.
• interferometry and parallax, to measure the radius of nearby giant stars.
• spectroscopy - indications of the star's constituents and temperature (the effective temperature, presuming it is close to black-body radiation, and its spectral temperature, based upon its spectral type).

Less direct methods:

• luminosity, apparent magnitude, and distance are interrelated such that with any two, you have the third.
• luminosity, radius and temperature are similarly interrelated.
• H-R diagram and spectrum shows temperature, and approximate radius, mass, and luminosity.
• photometry gives an approximate temperature, which relates radius to luminosity.

Binary stars observed over time can reveal much:

• visual binary - allows the orbital period to be determined.
• visual binary and astrometry and distance together reveal the orbit size.
• spectroscopic binary - reveals the orbital period and a minimum of the orbital speed, and implies the viewing angle is not face-on.
• double-line spectroscopic binary - also reveals the ratio of orbital speeds.
• eclipsing binary - reveals the orbital period and the viewing angle (i.e., close to edge-on), and the individual spectra.
• orbital speed over time reveals the orbit size.
• ratio of orbital speeds reveals ratio of masses.
• ratio of orbital speeds and viewing angle together reveal actual orbital velocities.
• orbital size and period and the sum of the individual masses are related such that with any two, you have the third.
• orbital size and period and the binary's distance, are related such that with any two, you have the third.

The methods listed here are rather basic and can be thought of as the lower rungs in the ladder leading to the current knowledge of the physics of stars. Advancing knowledge and improving technology have yielded new methods of parameter determination and have also improved the performance of these methods.

See "stellar distance determination", "stellar mass determination", "stellar radius determination", "stellar luminosity determination", "stellar temperature determination", "stellar age determination", "stellar rotation", and "gyrochronology".