The Classification of Stellar Spectra

The exploration of stellar spectra began in the early 19th century when William Wollaston first noted dark lines in the sunlight spectrum in 1802. He attributed these lines to natural boundaries between colors, but it was Joseph Fraunhofer who, in 1814, made meticulous observations of the solar spectrum and identified around 600 distinct dark lines, measuring 324 of their wavelengths. Fraunhofer's work laid the foundation for modern spectroscopy, revealing that these dark lines were indicative of the elements present in the stars. In 1864, Sir William Huggins matched dark lines in the spectra of stars with those of terrestrial substances, confirming that stars and everyday materials share common elements.

Historical Background and Development of Stellar Classification

Historically, astronomers have endeavored to categorize stars even prior to the discovery of spectral lines. As scientists observed stellar spectra, they identified a limited number of distinct patterns, leading to the realization that classification through spectral features could enhance their understanding of stellar characteristics. The current system of spectral classification emerged in the early 20th century at the Harvard Observatory. Henry Draper initiated this work by capturing the spectrum of Vega in 1872, and following his death, his wife funded continued research that culminated in significant contributions by Annie Jump Cannon between 1918 and 1924.

The Spectral Classification Scheme

The original classification scheme employed capital letters arranged in alphabetical order; however, as knowledge regarding stellar evolution progressed, refinements were made. The resulting classification is documented in the Henry Draper Catalogue (HD) and the Henry Draper Extension (HDE), encompassing the spectra of 225,000 stars down to the ninth magnitude.

The classification primarily depends on spectral lines that correlate with the surface temperatures of stars rather than their chemical compositions or luminosity. Key spectral lines include the hydrogen Balmer lines, neutral and singly ionized helium lines, iron lines, and several important features from titanium oxide and ionized calcium.

Standard Stellar Types and Their Characteristics

The spectral classification consists of seven main types ranked from the highest surface temperatures to the lowest: O, B, A, F, G, K, and M.Here’s a summary of each type:

• O (Blue): > 25,000 K - Exhibits strong lines of singly ionized helium and strong ultraviolet continuum. Example: Lacertra.

• B (Blue): 11,000 - 25,000 K - Shows neutral helium lines in absorption. Example: Rigel.

• A (Blue): 7,500 - 11,000 K - Strong hydrogen lines at maximum strength. Example: Sirius.

• F (Blue to White): 6,000 - 7,500 K - Noticeable metallic lines. Example: Canopus.

• G (White to Yellow): 5,000 - 6,000 K - Contains absorption lines of neutral metallic atoms. Example: The Sun.

• K (Orange to Red): 3,500 - 5,000 K - Dominated by metallic lines. Example: Arcturus.

• M (Red): < 3,500 K - Features noticeable molecular bands of titanium oxide. Example: Betelgeuse.

Each type can have subclasses ranging from 0 to 9 denoting specific characteristics. For instance, the Sun is classified as G2.

Luminosity Classes

While the initial classification focuses on surface temperature, luminosity is also crucial for a complete classification. The Yerkes classification scheme measures star luminosities, categorizing them into six classes:

• Ia: Most luminous supergiants

• Ib: Less luminous supergiants

• II: Luminous giants

• III: Normal giants

• IV: Subgiants

• V: Main sequence stars (dwarfs)

For example, the Sun is classified as a G2V type star.

Additional Spectral Peculiarities

Further categorization includes various spectral peculiarities denoted by lowercase letters. These codes provide insights into specific characteristics such as composite spectra, the presence of emission lines, and irregularities in absorption lines due to rotation. For example, 'e' indicates emission lines, and 'n' denotes broad, nebulous absorption lines.

Special Types of Stars

Within the universe, certain peculiar stars embody unique traits:

• Wolf-Rayet Stars (WR): These stars share characteristics with O type stars but show broad emission lines due to high velocity gas streams ejected from an exposed stellar interior.

• T Tauri Stars (T): These are young stars frequently found in interstellar clouds, demonstrating sporadic brightness changes and exhibiting bright emission lines along with various forbidden lines indicative of low densities.

Understanding these classifications and peculiarities is fundamental in the study of stellar evolution and the physical properties of stars, shedding light on their life cycles and the materials composing them.