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?
Understanding Stellar Spectral Classification (OBAFGKM)
A spectrum is more than a colored strip. Dark lines appear when atoms or molecules in a star's outer layers absorb very specific wavelengths. The pattern depends on how much energy the particles have.
At very high temperatures, many atoms lose electrons, so some lines become weak or disappear. At lower temperatures, electrons remain attached and different lines become noticeable. This explains an important result about hydrogen.
Strong hydrogen lines need many atoms with electrons in a particular excited state. Stars at intermediate high temperatures provide those conditions better than the most extreme hot stars. In cool stellar atmospheres, molecules can survive and produce broad absorption bands, especially from titanium oxide.
A star's visible color comes mainly from thermal radiation. Hot surfaces give off a larger share of their light at short wavelengths, while cool surfaces give off more at long wavelengths. Color is useful, but it is not a perfect temperature measurement.
Dust between Earth and a star scatters and absorbs blue light more strongly than red light. This makes a distant hot star look redder than it truly is. Astronomers correct for this effect by comparing observations at several wavelengths.
Students should separate a star's intrinsic color from its observed color. Camera images can be misleading too, since image processing and exposure time can change the displayed colors.
The Roman numeral in a stellar label comes from clues about surface gravity. A compact star has stronger gravity at its surface than a swollen star of similar temperature. Higher gravity produces greater pressure in the atmosphere.
Collisions between particles affect the width and shape of absorption lines. A giant and a main sequence star can therefore have similar temperature-related features but different line widths. The giant has a much larger radius.
Its total light output can be far greater even when its surface is not hotter. Luminosity depends on both emitting surface area and temperature to the fourth power. Astronomers use these line details to estimate a star's physical state without travelling to it.
The H-R diagram becomes more useful when it is treated as a map of changing stars, not just a chart for labels. Most stars spend the longest part of their lives fusing hydrogen in their cores. Their position during this stage is strongly linked to mass.
Massive stars use fuel rapidly, shine intensely, and have short lives. Lower-mass stars burn fuel slowly and remain stable for far longer. When core hydrogen runs low, many stars expand into giants and move to a different region of the diagram.
Some eventually leave behind hot, faint white dwarfs. Real catalogues classify spectra by measuring line ratios and matching them with standard stars. Motion can shift every line slightly through the Doppler effect, so astronomers account for that shift before making a careful classification.