Spectroscopic notation and stellar types help astronomers classify stars using the light they emit and absorb. This cheat sheet covers the OBAFGKM temperature sequence, luminosity classes, spectral lines, and the meaning of labels such as G2 V. Students need these tools to connect a star's spectrum to its temperature, color, size, and evolutionary stage.
These ideas are central to understanding the Hertzsprung-Russell diagram and how stars are studied from Earth.
Key Facts
- The main spectral sequence from hottest to coolest is O, B, A, F, G, K, M.
- Spectral subclasses run from 0 to 9, with 0 hotter than 9 within the same letter class, so A0 is hotter than A9.
- A full stellar classification such as G2 V means spectral class G, subclass 2, and luminosity class V for a main sequence star.
- Wien's law is lambda_max = 2.9 x 10^-3 m K / T, where hotter stars peak at shorter wavelengths.
- The Stefan-Boltzmann law is L = 4 pi R^2 sigma T^4, showing that luminosity depends on radius and surface temperature.
- Luminosity classes are commonly I for supergiants, III for giants, and V for main sequence stars.
- Hydrogen Balmer absorption lines are strongest in A-type stars, not in the hottest O-type stars.
- Cool M-type stars show strong molecular absorption bands, while hot O-type stars show ionized helium lines.
Vocabulary
- Spectrum
- A spectrum is the pattern of light spread by wavelength, often showing bright or dark lines that reveal a star's properties.
- Spectral Class
- A spectral class is a letter category, such as O, B, A, F, G, K, or M, that mainly indicates a star's surface temperature.
- Luminosity Class
- A luminosity class is a Roman numeral category that describes a star's size and brightness compared with stars of similar temperature.
- Absorption Line
- An absorption line is a dark line in a spectrum caused when atoms or molecules absorb specific wavelengths of light.
- Main Sequence
- The main sequence is the long-lived stage when a star fuses hydrogen into helium in its core.
- Hertzsprung-Russell Diagram
- The Hertzsprung-Russell diagram is a graph of stellar luminosity versus temperature or spectral class used to compare star types and evolution.
Common Mistakes to Avoid
- Reading the OBAFGKM sequence backward is wrong because O stars are the hottest and M stars are the coolest.
- Assuming the subclass number means a hotter star is wrong because within one spectral letter, 0 is hotter than 9.
- Ignoring the luminosity class in a label such as K2 III is wrong because K2 gives temperature, while III shows the star is a giant.
- Thinking all strong spectral lines mean high abundance is wrong because line strength also depends on temperature and ionization state.
- Calling a red star low luminosity automatically is wrong because red giants and supergiants can be very luminous due to their large radii.
Practice Questions
- 1 A star is classified as B8 V. What are its spectral class, subclass, and luminosity class?
- 2 Using Wien's law, estimate lambda_max for a star with surface temperature T = 5800 K.
- 3 Which star is hotter, F2 or F8, and why?
- 4 A K-type giant and a K-type main sequence star have similar surface temperatures. Explain why the giant can be much more luminous.
Understanding Spectroscopic Notation & Stellar Types
A stellar spectrum is not simply a rainbow with a few missing pieces. Light first comes from dense, hot layers below a star's visible surface. This produces a nearly continuous spread of wavelengths.
The light then passes through cooler gas above those layers. Atoms, ions, and molecules in that gas absorb only particular wavelengths because their electrons can change energy only in fixed steps. Each absorption line is therefore a clue to the material and physical conditions in the star's atmosphere.
Line strength does not directly measure how much of an element is present. It depends strongly on temperature, density, and the energy state of the atoms.
Hydrogen gives an important example of this temperature effect. Balmer lines require hydrogen electrons to be in a particular excited state before they can absorb visible light. In relatively cool stars, too few atoms reach that state.
In extremely hot stars, many hydrogen atoms lose their electrons entirely, so neutral hydrogen lines weaken again. They are most noticeable at an intermediate temperature range. This is why astronomers must compare many lines rather than identifying a star from one feature alone.
Helium lines, metal lines, and molecular bands provide further checks. The word metal in astronomy means any element heavier than helium.
A star's overall colour provides a useful temperature estimate, but it is not perfect on its own. A hot surface emits more strongly at short wavelengths, while a cool surface emits more strongly at long wavelengths. Wien's law links the peak wavelength to surface temperature.
Dust between Earth and a star can scatter more blue light than red light. This makes a hot star appear redder than it truly is.
Astronomers correct for this effect by studying the spectrum and comparing it with expected colours. They can also use infrared observations, where dust has less influence.
Luminosity classes come from subtle differences in spectral line shape. A giant has a much lower surface gravity than a dwarf star of similar temperature. Its outer gas is less compressed, which changes the widths and strengths of certain lines.
This allows two stars with similar colours to be separated into different size groups. The distinction matters because total light output depends on surface area as well as temperature. Stefan-Boltzmann law shows that luminosity equals surface area times a quantity that rises very rapidly with temperature.
A cool giant can therefore outshine a hotter dwarf because its radius is far larger. When reading a stellar label, pay attention to every part of it.
The temperature subclass and luminosity class together give a much more reliable picture than colour alone. This is especially useful when placing stars on an H-R diagram or comparing nearby stars with distant star clusters.