This cheat sheet covers the stellar spectral classification sequence from hottest to coolest: O, B, A, F, G, K, and M. Students need this reference because spectral class connects a star’s color, temperature, and absorption lines to its physical properties. A clear memory aid helps students quickly recall the order when reading H-R diagrams, comparing stars, or interpreting spectra.
Key Facts
- The standard stellar spectral sequence from hottest to coolest is O, B, A, F, G, K, M.
- A common memory aid for the sequence is: Oh Be A Fine Girl/Guy, Kiss Me.
- O stars are the hottest main spectral class, with surface temperatures above about 30,000 K and blue color.
- M stars are the coolest main spectral class, with surface temperatures below about 3,700 K and red color.
- Star color generally changes from blue to blue-white to white to yellow-white to yellow-orange to orange to red as temperature decreases.
- The Sun is a G-type star, specifically G2, with a surface temperature of about 5,800 K.
- Spectral classes are subdivided using numbers 0 through 9, where 0 is hotter and 9 is cooler within the same letter class.
- Spectral classification is based on absorption lines in a star’s spectrum, which reveal temperature and elements in the star’s atmosphere.
Vocabulary
- Spectral class
- A category assigned to a star based mainly on the pattern of absorption lines in its spectrum and its surface temperature.
- OBAFGKM sequence
- The ordered list of main stellar spectral classes from hottest to coolest: O, B, A, F, G, K, M.
- Surface temperature
- The approximate temperature of a star’s visible outer layer, usually measured in kelvin.
- Absorption line
- A dark line in a star’s spectrum caused when atoms in the star’s atmosphere absorb specific wavelengths of light.
- H-R diagram
- A graph that compares stars by luminosity and temperature or spectral class, showing patterns such as the main sequence.
- Main sequence
- The band on an H-R diagram where stars spend most of their lifetimes fusing hydrogen into helium in their cores.
Common Mistakes to Avoid
- Reversing the sequence as M, K, G, F, A, B, O is wrong when the question asks hottest to coolest because O stars are hottest and M stars are coolest.
- Assuming red stars are hottest is wrong because red light indicates a cooler surface temperature than blue or blue-white light for stars.
- Thinking the spectral letters are alphabetical is wrong because the modern order was reorganized by temperature, not by letter order.
- Forgetting that numbers subdivide each class is wrong because B0 is hotter than B9, and G0 is hotter than G9.
- Using brightness alone to identify spectral class is wrong because apparent brightness depends on distance as well as a star’s actual luminosity and temperature.
Practice Questions
- 1 Write the stellar spectral classification sequence from hottest to coolest.
- 2 A star is classified as K5 and another as K0. Which one is hotter?
- 3 Put these stars in order from hottest to coolest: M2, A0, G2, B8.
- 4 A blue star and a red star appear equally bright from Earth. Explain why color is a better clue than apparent brightness for comparing their surface temperatures.
Understanding Stellar spectral classification sequence from hottest to coolest Memory Aid
A spectrum is made when starlight is spread into its different wavelengths. Dark absorption lines appear because atoms and ions in the outer atmosphere absorb very specific wavelengths. The pattern depends strongly on temperature.
At very high temperatures, many atoms lose electrons, so lines from ionized helium can be important. At moderate temperatures, hydrogen lines become especially strong. In cool stellar atmospheres, atoms can join into molecules, producing broad molecular bands.
This is why astronomers do not classify a star from its color alone. They compare the detailed line pattern with standard reference spectra. The lines give a more reliable temperature estimate than a photograph.
Color comes from the broad glow of a star’s surface, which behaves roughly like a heated object. Hotter surfaces produce more short wavelength light, while cooler surfaces produce more long wavelength light. However, observed color can mislead.
Dust between Earth and a star scatters blue light more effectively than red light. A distant hot star may therefore look redder than it truly is. Astronomers correct for this effect when possible.
Temperature values use kelvin, an absolute temperature scale. A star’s surface temperature is not the temperature at its center. Fusion occurs deep inside, where temperatures are vastly higher.
Spectral type is only one part of a full stellar description. Two stars with similar surface temperatures can have very different sizes and brightnesses. A small main sequence star and a swollen giant can show related spectral features, yet the giant has much more surface area and gives off far more light.
Pressure in a star’s atmosphere changes the width of some absorption lines. This allows astronomers to assign luminosity classes, which separate dwarfs, giants, and supergiants.
On an H R diagram, temperature runs across the graph while luminosity runs up and down. Reading both positions helps students distinguish a cool faint dwarf from a cool bright giant.
The sequence is useful for thinking about a star’s life as well as its appearance. Massive hot stars use their fuel quickly and live relatively short lives. Less massive cool stars burn fuel slowly and can remain stable for extremely long periods.
When reading a spectral label with a number, remember that the number gives a finer temperature position within that letter group. The direction matters more than memorizing every boundary. Check whether a problem gives observed color, corrected color, line strength, luminosity, or temperature.
Each clue tells part of the story. Real stars can be binary systems, variable stars, or chemically unusual objects, so some spectra do not fit a simple textbook pattern perfectly.