Glossary

Glossary#

Quick reference for key terms used throughout the brutus documentation.

Stellar Evolution#

EEP#
Equivalent Evolutionary Point#: A dimensionless index (typically 202-808) that parameterizes a star’s evolutionary state independent of mass. Key values: 202 (pre-main sequence), 353 (ZAMS), 454 (TAMS), 605 (base of RGB). See Stellar Models and Photometry for details.
ZAMS#
Zero-Age Main Sequence#: The point when a star begins hydrogen fusion in its core (EEP ~ 353). Marks the start of the main sequence phase.
TAMS#
Terminal-Age Main Sequence#: The point when core hydrogen is exhausted (EEP ~ 454). The star then evolves off the main sequence toward the subgiant and red giant phases.
isochrone#: A curve in the HR diagram connecting stars of the same age but different masses. Used for modeling stellar clusters where stars formed together. Compare with evolutionary track.
evolutionary track#: The path a star of fixed initial mass traces through the HR diagram as it evolves. Tracks are parameterized by EEP. Perpendicular to isochrones.
initial mass#: The mass of a star at formation, in solar masses (M☉). Combined with age and metallicity, this determines all other stellar properties.
MIST#: MESA Isochrones and Stellar Tracks - the stellar evolution models used by brutus. Covers 0.1-300 M☉, [Fe/H] from -4.0 to +0.5, all evolutionary phases from pre-MS through AGB.
IMF#
Initial Mass Function#: The probability distribution of stellar masses at birth. brutus uses the Kroupa IMF: P(M) ∝ M⁻¹·³ for 0.08-0.5 M☉, P(M) ∝ M⁻²·³ for 0.5-150 M☉.

Chemical Abundances#

metallicity#
[Fe/H]#: The logarithmic iron abundance relative to solar: [Fe/H] = log₁₀(N_Fe/N_H) - log₁₀(N_Fe/N_H)☉. Solar is [Fe/H] = 0; metal-poor stars have [Fe/H] < 0.
alpha enhancement#
[α/Fe]#: The abundance of alpha-process elements (O, Mg, Si, Ca, Ti) relative to iron, compared to solar. Old halo and thick disk stars typically have [α/Fe] ~ +0.3. Note: Alpha enhancement is not currently exposed as a user-facing parameter in brutus; models assume solar-scaled abundances.

Photometry#

magnitude#: Logarithmic brightness scale: m = -2.5 log₁₀(F/F_ref). Fainter objects have larger (more positive) magnitudes. A difference of 5 magnitudes corresponds to a factor of 100 in brightness.
flux density#: Linear brightness measurement. brutus uses “maggies” internally, where 1 maggie is the flux of a 0th magnitude source. Convert from magnitudes: flux = 10^(-0.4 × mag).
SED#
Spectral Energy Distribution#: The distribution of flux across wavelengths or photometric bands. brutus predicts SEDs from stellar models and compares them to observed photometry.
parallax#: The apparent angular shift of a star due to Earth’s orbital motion, measured in milliarcseconds (mas). Distance in parsecs = 1000 / parallax_mas.
distance modulus#: The difference between apparent and absolute magnitude: μ = m - M = 5 log₁₀(d / 10 pc). A star at 1 kpc has μ = 10 mag.

Extinction & Dust#

extinction#: Wavelength-dependent attenuation of starlight by interstellar dust, in magnitudes. A_V denotes extinction in the V-band (~550 nm). Extinction is stronger at shorter wavelengths.
reddening#: The color change caused by wavelength-dependent extinction. Quantified as color excess E(B-V) = (B-V)_observed - (B-V)_intrinsic.
R_V#: The ratio of total-to-selective extinction: R_V = A_V / E(B-V). Characterizes the dust grain size distribution. Typical values: ~3.1 (diffuse ISM), ~2 (dense molecular clouds), ~5 (diffuse high-latitude).
reddening vector#: The direction a star moves in color-color or color-magnitude space due to dust extinction. Depends on the extinction law and stellar spectrum.

Statistical Inference#

posterior#: The probability distribution of model parameters given the observed data: P(θ|data) ∝ P(data|θ) × P(θ). This is what brutus computes for each star.
prior#: Probability distribution encoding knowledge before seeing the data: P(θ). In brutus: IMF, Galactic structure, dust maps. See Prior Distributions.
likelihood#: The probability of observing the data given model parameters: P(data|θ). Measures how well a model matches observations.
marginalization#: Integrating over nuisance parameters to obtain distributions for parameters of interest. For example, marginalizing over stellar mass to get the distance posterior.
brute force#: The grid-based inference approach that gives brutus its name: systematically evaluate the likelihood at all pre-computed grid points, apply priors, then marginalize. Avoids MCMC convergence issues and guarantees complete parameter space coverage.

brutus Classes#

StarGrid#: A pre-computed grid of stellar models optimized for fast fitting. Stores absolute magnitudes at a 1 kpc reference distance plus reddening coefficients for each model.
BruteForce#: The main fitting class for individual stars. Takes a StarGrid and observed photometry, returns posterior samples for distance, extinction, and stellar parameters.
EEPTracks#: Class for interpolating stellar parameters along evolutionary tracks. Provides predictions for any combination of mass, EEP, and metallicity.
Isochrone#: Class for generating isochrones at specified age and metallicity. Used with StellarPop for cluster modeling.
StellarPop#: Class for generating synthetic photometry for stellar populations. Combines Isochrone predictions with bolometric corrections and extinction.

Note

Model Coverage

MIST models in brutus cover: main sequence stars (0.1-300 M☉), subgiants, red giants, horizontal branch, and asymptotic giant branch stars.

Not covered: white dwarfs, brown dwarfs (< 0.08 M☉), neutron stars, black holes, or very young pre-main-sequence stars (< 1 Myr).