4.4: Stellar Classification

Sampling the Stars

Imagine if astronomers had to collect a sample of the Sun to discover its composition. One challenge would be the distance between the Sun and the Earth. Another is the temperature of the Sun. The Sun's atmosphere is around 5700 K, much too hot for a human to survive. It is also too hot for any kind of technology that humans can build to not be quickly destroyed. Fortunately, astronomers are able to determine the composition of the Sun by analyzing its spectrum. Analyzing an object from a distance is called remote sensing.

Analysis of spectra is the key to modern astronomy. This is the only way astronomers can sample the stars, which are significantly farther away than the Sun. Encoded in the light from celestial objects is clear information about the chemical makeup of these objects. Only by understanding what the stars were made of could astronomers begin to form theories about what made them shine and how they evolved.

One spectrum of a star, the Sun, tells us the composition of one star. As astronomers observed the stars, they noted that just as stars have different visible colors, they also have different spectra. Once a scientist collects a large amount of data, such as the spectra of hundreds of stars, the next step is to organize the spectra into groups. Once those groups are created, it is up to the astronomers to determine why stars have different types of spectra. Are they all made of different types of material, or are other physical properties of the stars affecting the spectra?

Organizing Spectral Data

Although many astronomers began collecting spectra of stars, one of the most impressive observing programs took place at the Harvard College Observatory. The observatory's director, Edward Pickering, hired a group of female astronomers, known as the Harvard Computers, to analyze the data. In the 1880s, one of the computers, Williamina Fleming, devised a system to classify roughly 10,000 stars based on the strength of hydrogen absorption lines. Spectra with the strongest lines were classified as A stars, the next strongest B, and so on down the alphabet to O stars, in which the hydrogen lines were very weak.

With Fleming's system, it would be reasonable to conclude that some stars are made of hydrogen and other stars are made of something else. Among the computers there was some disagreement regarding how the stars were organized in classes. When analyzing groups of data, it is good to see if there are other factors that may be affecting the data. The computers not only measured spectra of stars, they also knew the colors, and therefore the temperatures of the stars.

Annie Jump Cannon was another of the computers who managed to classify around 500,000 stars over her lifetime, at a rate of up to three stars per minute. She revised the classification of stars by focusing on just a few letters from the original system: A, B, F, G, K, M, and O. She then rearranged them in order of decreasing temperature into the sequence O, B, A, F, G, K, M. This is how astronomers ended up with a group of letters that doesn't make sense at first glance. This is the system that astronomers still use today.

Spectral Classes

After Annie Jump Cannon created the spectral classes, astronomers created subclasses within each letter and added three more classes L, T, and Y. They were able to do this as telescope and detector technology improved and they gathered more data. Each of these spectral classes is further subdivided into 10 subclasses designated by the numbers 0 through 9. A B0 star is the hottest type of B star while a B9 star is the coolest type of B star and is only slightly hotter than an A0 star. Each star is assigned a letter and number based on its temperature and spectral characteristics. For example, the Sun is G2 while Polaris is an F7. There are further subclasses, but we will not get into those here.

Figure \(\PageIndex{1}\) is a table of images representing each spectral classes. The spectral classes, O to Y, are listed in the first column and the first row is the subclasses 0 to 9. The O0 star is dark blue with a lighter blue center. Moving along the row of O stars, the shade of blue changes from dark to light blue. B0 is slightly lighter blue than O9 and the rest of the B row changes to cyan by B9. This pattern continues through the row of Y stars and is summarized in Table \(\PageIndex{1}\).

Figure \(\PageIndex{1}\): These images of stars are organized by stellar classes. (CC0; BIG DADDY Dunkleosteuss via Wikimedia Commons). Alternative description of Figure \(\PageIndex{1}\).

Now that the stellar classes are clearly defined, new stars can be easily classified by measuring the color of the star. However, the differences in spectra between classes is still important. Figure \(\PageIndex{2}\) includes examples of 13 different stellar spectra ranging from O to M. In spectral type A1 the strongest four lines include one in the red, one in the blue-green, and two in the blue. These are the Balmer lines of hydrogen. These lines weaken at both higher (O and B) and lower (F-M) temperatures. The strong pair of closely spaced lines in the yellow in the cool stars (K and M) is due to neutral sodium. Given an unclassified stellar spectrum, an astronomer can match the pattern of spectral lines to a star whose type has already been determined.

Figure \(\PageIndex{2}\): Spectra of Different Classes. Each class of stars has a different spectrum. The spectral class assigned to each of these stellar spectra is listed on the left. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Alternative description of Figure \(\PageIndex{2}\).

We know that the Sun is made of hydrogen because astronomers identified the spectral lines. However, the spectra of different stellar classes have different absorption lines. The classes B through F have the strongest hydrogen lines of all of the classes. The other classes have weaker hydrogen lines, but have lines from a variety of materials, such as sodium, iron, magnesium, and more listed in Table \(\PageIndex{2}\). Does this mean that different classes of stars are made of different types of materials? As astronomers collected spectral data, other researches collected more data in the lab and also developed more complicated models of electronic transitions in atoms. They eventually learned that the spectral lines that appear depend strongly on the temperature of the material they come from, which is explained in the next section.

Table \(\PageIndex{2}\): Spectral Classes of Stars

Spectral Class	Color	Approximate Temperature (K)	Principal Features	Examples
O	Dark to light blue	> 30,000	Neutral and ionized helium lines, weak hydrogen lines	10 Lacertae, Orion Belt Stars (3 stars)
B	Light blue to cyan	10,000–30,000	Neutral helium lines, strong hydrogen lines	Rigel, Spica
A	Light green to white	7500–10,000	Strongest hydrogen lines, weak ionized calcium lines, weak ionized metal (e.g., iron, magnesium) lines	Sirius, Vega
F	White to yellow	6000–7500	Strong hydrogen lines, strong ionized calcium lines, weak sodium lines, many ionized metal lines	Canopus, Procyon, Polaris
G	Yellow to orange	5200–6000	Weaker hydrogen lines, strong ionized calcium lines, strong sodium lines, many lines of ionized and neutral metals	Sun, Capella
K	Orange to light red	3700–5200	Very weak hydrogen lines, strong ionized calcium lines, strong sodium lines, many lines of neutral metals	Arcturus, Aldebaran
M	Light red to dark red	2400–3700	Strong lines of neutral metals and molecular bands of titanium oxide dominate	Betelgeuse (Orion's Armpit), Antares
L	Red losing brightness	1300–2400	Metal hydride lines, alkali metal lines (e.g., sodium, potassium, rubidium)	Teide 1
T	Magenta, low brightness	700–1300	Methane lines	Gliese 229B
Y	Infrared	< 700	Ammonia lines	WISE 1828+2650

Temperature and Spectra

The spectra of a gas is also dependent on the temperature of a gas. Emission spectra are created when a warm gas releases photons and specific wavelengths. Assuming that, like the Sun, all stars are made of hydrogen, how does temperature affect the spectra of stars. In the atmospheres of the hottest stars, hydrogen atoms are completely ionized. Because the electron and the proton are separated, ionized hydrogen cannot produce absorption lines.

In the atmospheres of the coolest stars, hydrogen atoms have their electrons attached and can switch energy levels to produce lines. However, practically all of the hydrogen atoms are in the lowest energy state in these stars and thus can absorb only those photons able to lift an electron from that first energy level to a higher level. Photons with enough energy to do this lie in the ultraviolet part of the electromagnetic spectrum, and there are very few ultraviolet photons in the radiation from a cool star. What this means is that if you observe the spectrum of a very hot or very cool star with a typical telescope on the surface of Earth, the most common element in that star, hydrogen, will show very weak spectral lines or none at all.

The hydrogen lines in the visible part of the spectrum Balmer lines are strongest in stars with intermediate temperatures. Calculations show that the optimum temperature for producing visible hydrogen lines is about 10,000 K. At this temperature, an large amount of hydrogen atoms are excited to the second energy level. They can then absorb additional photons, rise to higher levels of excitation, and produce a dark absorption line. Similarly, every other chemical element, in each of its possible stages of ionization, has a characteristic temperature at which it is most effective in producing absorption lines in any particular part of the spectrum.

In the hottest O stars, those with temperatures over 28,000 K, only lines of ionized helium and highly ionized atoms of other elements are conspicuous, Figure \(\PageIndex{3}\). Hydrogen lines are strongest in A stars with atmospheric temperatures of about 10,000 K. Ionized metals provide the most conspicuous lines in stars with temperatures from 6000 to 7500 K, spectral type F. In the coolest M stars, below 3500 K, absorption bands of titanium oxide and other molecules are very strong.

Figure \(\PageIndex{3}\): Temperature Effect on Absorption. In this graph, the strengths of absorption lines of different atoms, ions, molecules change as a function of temperature. (CC BY 4.0; Fraknoi, et al. via Openstax Astronomy 2nd ed.). Alternative description of Figure \(\PageIndex{3}\).

Further Exploration: Interactive Activity

• Explore the AstroSims Spectrum Explorer simulation to investigate how absorption and emission lines appear in stellar spectra and how astronomers use spectral patterns to identify elements and determine the properties of stars.
• Use the Sloan Digital Sky Survey Classifying Stars activity to compare stellar spectra, identify absorption lines, and practice assigning spectral classifications based on observed spectral characteristics.

References

Stellar Classification. (2026, Feb 13). In Wikipedia. https://en.wikipedia.org/w/index.php...did=1338195752

Harvard Computers. (2026, March 11). In Wikipedia. https://en.wikipedia.org/w/index.php...did=1343012847

Attributions

This page was adapted from "Sample Spectral Types" in Introduction to Astronomy (Lumen) originally written by Lumen Learning, and published under CC BY 4.0.