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Stellar Spectral Classification (OBAFGKM) Cheat Sheet
A printable reference covering OBAFGKM spectral classes, star color, temperature, absorption lines, luminosity class, and the H-R diagram for grades 9-12.
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Stellar spectral classification organizes stars by the patterns of light in their spectra. This cheat sheet helps students connect a star’s color, surface temperature, and absorption lines to its spectral class. The OBAFGKM sequence is essential for reading H-R diagrams and comparing stars across the universe. It also supports lessons on stellar evolution, luminosity, and star composition. The main order of spectral types from hottest to coolest is O, B, A, F, G, K, M. Each class is divided into subclasses from 0 to 9, where 0 is hotter and 9 is cooler within the same letter class. A star’s spectrum shows absorption lines caused by elements and molecules in its atmosphere. Luminosity classes, such as V for main sequence and III for giant, describe a star’s size and brightness category.
Key Facts
- The spectral sequence from hottest to coolest is O, B, A, F, G, K, M.
- Surface temperature decreases from O stars above 30,000 K to M stars below about 3,700 K.
- Spectral subclasses run from 0 to 9, so A0 is hotter than A9 and G2 is hotter than G8.
- Star color changes with temperature: O and B stars look blue, A stars look white, G stars look yellow-white, K stars look orange, and M stars look red.
- A star labeled G2 V is a G-type star, subclass 2, on the main sequence.
- Hydrogen Balmer absorption lines are strongest in A-type stars, not in the hottest O-type stars.
- Luminosity classes include I for supergiants, III for giants, and V for main sequence stars.
- On an H-R diagram, temperature usually decreases from left to right while luminosity increases upward.
Vocabulary
- Spectral class
- A category assigned to a star based on its spectrum, mainly related to surface temperature and absorption lines.
- Absorption line
- A dark line in a spectrum caused when atoms or molecules in a star’s atmosphere absorb specific wavelengths of light.
- Surface temperature
- The approximate temperature of a star’s visible outer layer, usually measured in kelvins.
- Luminosity class
- A label that describes a star’s size and brightness category, such as main sequence, giant, or supergiant.
- H-R diagram
- A graph that plots stars by luminosity and temperature or spectral type to show patterns in stellar properties.
- Main sequence
- The long stable stage when a star produces energy by fusing hydrogen into helium in its core.
Common Mistakes to Avoid
- Putting the sequence in alphabetical order is wrong because OBAFGKM is arranged by decreasing surface temperature, not by the alphabet.
- Thinking red stars are hotter than blue stars is wrong because blue stars have higher surface temperatures and red stars are cooler.
- Assuming all stars of the same spectral class have the same brightness is wrong because luminosity also depends on radius and luminosity class.
- Reading subclasses backward is wrong because 0 is hotter than 9 within a spectral letter, so B0 is hotter than B9.
- Using only color to classify a star is unreliable because spectra and absorption lines provide the more precise classification.
Practice Questions
- 1 Put these stars in order from hottest to coolest: K5, A0, O9, G2.
- 2 A star has spectral type M3 V. What is its spectral class, subclass, and luminosity class?
- 3 Which star is hotter, F2 or F8, and how do you know?
- 4 Why can two stars with the same spectral type have very different luminosities?