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, which may be simple equations, 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 would be determined by also knowing 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 reveal much:

• visual binary - allows the orbital period to be determined.
• visual binary and astrometry give the orbit size if the distance is known.
• spectroscopic binary - shows the orbital period and ratio of masses.
• double-line spectroscopic binary - reveals the ratio of orbital speeds.
• eclipsing binary reveals viewing angle.
• orbital speed over time reveals the orbit size.
• ratio of orbital speeds reveals ratio of masses.
• ratio of orbital speeds and viewing angle reveals actual orbital velocities.
• orbital size and period determine sum of masses, such that with any two, you have the third.
• period and distance determine orbital size, such that with any two, you have the third.
• eclipsing binary reveals individual spectra.

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".