Spectral Classification; Old and Contemporary

TL;DR

This paper reviews the history and evolution of spectral classification, including traditional and automated modern methods, covering spectral types from early classifications to L and T classes, with standards and criteria.

Contribution

It provides a comprehensive overview of spectral classification methods, including historical, traditional, and contemporary automated approaches, extending to new spectral classes.

Findings

Historical spectral classification methods are detailed.

Automated classification approaches are introduced.

Standards for new spectral classes L and T are described.

Abstract

Beginning with a historical account of the spectral classification, its refinement through additional criteria is presented. The line strengths and ratios used in two dimensional classifications of each spectral class are described. A parallel classification scheme for metal-poor stars and the standards used for classification are presented. The extension of spectral classification beyond M to L and T and spectroscopic classification criteria relevant to these classes are described. Contemporary methods of classifications based upon different automated approaches are introduced.