The night sky is full of celestial wonders, each star shining with its unique brilliance. But what determines the color, brightness, and behavior of a star? Astrophysicists classify stars into spectral types, a system that reveals crucial details about their temperature, composition, and evolution. Let’s take a closer look at spectral types and what they tell us about the universe.

What Are Spectral Types?

Spectral types categorize stars based on their light spectra, which vary depending on the star’s surface temperature and chemical composition. The main spectral types are labeled O, B, A, F, G, K, and M, with O being the hottest and M the coolest. These types are further divided into subcategories (e.g., G2, M5) for precision.

The Seven Spectral Types

1. O-Type Stars
   • Temperature: >30,000 K
   • Color: Blue
   • Features: Extremely hot, with strong ultraviolet radiation and weak hydrogen lines. Ionized helium lines dominate their spectra.
   • Example: Rare and massive stars often found in young stellar regions.
2. B-Type Stars
   • Temperature: 10,000 – 30,000 K
   • Color: Blue-white
   • Features: Slightly cooler than O-types, with strong helium and moderate hydrogen lines.
   • Example: Rigel in Orion.
3. A-Type Stars
   • Temperature: 7,500 – 10,000 K
   • Color: White
   • Features: Prominent hydrogen lines and some metal lines.
   • Example: Sirius, the brightest star in our night sky.
4. F-Type Stars
   • Temperature: 6,000 – 7,500 K
   • Color: Yellow-white
   • Features: Moderate hydrogen and stronger metallic lines.
   • Example: Procyon in Canis Minor.
5. G-Type Stars
   • Temperature: 5,200 – 6,000 K
   • Color: Yellow
   • Features: Strong metallic lines, moderate hydrogen lines, and often associated with habitable zones.
   • Example: The Sun, a G2 star.
6. K-Type Stars
   • Temperature: 3,700 – 5,200 K
   • Color: Orange
   • Features: Cooler, smaller, and stable for billions of years.
   • Example: Arcturus in Boötes.
7. M-Type Stars
   • Temperature: <3,700 K
   • Color: Red
   • Features: The coolest and most common stars, with strong molecular lines.
   • Example: Proxima Centauri, the closest star to Earth after the Sun.

Why Spectral Types Matter

Spectral types are more than classifications—they help us understand:

• Stellar Evolution: From massive O-types that burn out quickly to long-lived M dwarfs.
• Exoplanet Studies: G, K, and M stars are often studied for their potential to host habitable planets.
• Galactic Mapping: By analyzing the distribution of spectral types, astronomers uncover the structure of galaxies.

A Mnemonic for the Sequence

To remember the spectral sequence, try this classic mnemonic:

“Oh Be A Fine Girl/Guy, Kiss Me.”

Or create your own! For example: “Only Bright Astronomers Find Giant Killer Meteors.”

Final Thoughts

Spectral types are a window into the diversity of stars in the universe. Whether blazing blue O-types or dim red M dwarfs, each star tells a story about its origins and future. The next time you gaze at the stars, think about the secrets hidden in their light—secrets that spectral types help us unlock.