Stellar Spectral Classes: O, B, A Revision Notes | A Level AQA Physics

1. Why Stars Come in Different Colours

Look up at the night sky and you will notice that stars are not all the same colour. Rigel glows blue-white; Sirius appears brilliant white. These colour differences are not random — they are a direct window into a star's surface temperature, and they form the basis of stellar spectral classification.

When astronomers pass starlight through a spectrometer, they see a continuous rainbow of colours crossed by dark absorption lines. The precise pattern of those lines — which wavelengths are absorbed and how strongly — is the star's spectral fingerprint. The Harvard classification system sorts stars into lettered classes: O, B, A, F, G, K, M, running from hottest to coolest. This lesson focuses on the three hottest classes: O, B, and A.

The underlying physics is elegant: the temperature of a star's outer atmosphere determines which atoms are ionised and which are in the right energy state to absorb specific wavelengths of light. Master that idea and the whole classification system clicks into place.

2. The Physics Behind Spectral Lines

Before comparing the three classes, you need a firm grip on why temperature controls which lines appear. In a stellar atmosphere, atoms absorb photons only when the photon's energy matches the gap between two electron energy levels. If the temperature is too low, electrons remain in the ground state (n = 1) and can only absorb UV (Lyman-series) photons, not visible Balmer-series photons, so no visible absorption lines appear. If the temperature is too high, the atom is fully ionised — it has no electrons left to make transitions — so it produces no absorption lines at all.

There is therefore a sweet spot for every element: a temperature range at which its atoms are partially excited or partially ionised, placing electrons in exactly the right energy levels to absorb visible-wavelength photons strongly. This is why hydrogen's Balmer series lines peak in A-class stars, not O-class stars, even though hydrogen is the most abundant element in all stars.

With that foundation in place, you can now analyse each class on its own terms — and understand why the spectral differences are physically inevitable rather than arbitrary labels.

3. Class O Stars — The Hottest and Most Luminous

Class O stars sit at the extreme hot end of the sequence. Their surface temperatures exceed 25,000 K, and some reach above 50,000 K. At these temperatures, the radiation peaks in the ultraviolet, but the tail of the Planck curve extends into the visible range, giving O stars a distinctly blue appearance.

The defining spectral feature of O stars is the presence of lines from highly ionised atoms. You will see lines from ionised helium (He II), as well as lines from multiply ionised heavier elements such as oxygen and nitrogen. Hydrogen Balmer lines are present but relatively weak — the extreme temperature has ionised most of the hydrogen, leaving few neutral atoms in the n = 2 level needed to produce strong Balmer absorption.

O stars are rare — they form in small numbers and burn through their fuel at a ferocious rate, giving them lifetimes of only a few million years. Despite their rarity, they are so luminous that they dominate the light output of star-forming regions and are visible across enormous distances.

4. Class B Stars — Hot, Blue-White, and Helium-Rich

Moving down the temperature scale, class B stars have surface temperatures between approximately 10,000 K and 25,000 K. They appear blue-white to the eye. The drop in temperature from O to B has a significant effect on the spectrum: helium is now only singly ionised or neutral rather than doubly ionised, so He II lines disappear and neutral helium (He I) lines become prominent.

Hydrogen Balmer lines are stronger in B stars than in O stars because more hydrogen atoms remain neutral with electrons in the n = 2 level. However, the lines are still not at their maximum strength — that peak comes in class A. Lines from singly ionised metals such as silicon and magnesium also appear in B-class spectra, providing additional diagnostic features.

Rigel, one of the brightest stars in the night sky, is a B8 supergiant — the number after the letter refines the classification within the class, with B0 being the hottest B star and B9 the coolest. This subdivision is worth knowing because exam questions sometimes ask you to compare stars within the same broad class.

5. Class A Stars — Where Hydrogen Dominates

Class A stars have surface temperatures between approximately 7,500 K and 10,000 K and appear white or blue-white. This temperature range is the sweet spot for hydrogen: enough thermal energy exists to excite hydrogen electrons from the ground state up to n = 2, but not so much that the atoms are fully ionised. The result is that hydrogen Balmer absorption lines reach their maximum strength in class A stars.

Helium lines are now weak or absent — the temperature is too low to excite or ionise helium significantly. Lines from singly ionised calcium (Ca II) begin to appear at the cooler end of the A class, hinting at the transition towards class F. Sirius, the brightest star in the night sky as seen from Earth, is an A1 star and shows textbook-strong Balmer lines.

A common exam question asks why hydrogen lines are strongest in A stars rather than in the hottest O stars. The answer always comes back to the same principle: it is not about how much hydrogen is present, but about how many hydrogen atoms have electrons in the n = 2 level at any given moment. In O stars, hydrogen is ionised; in K or M stars, hydrogen electrons sit in the ground state (n = 1) and cannot absorb Balmer-series photons. Class A is the temperature at which the n = 2 population is maximised.

6. Comparing O, B, and A: A Side-by-Side Analysis

Now that each class has been examined individually, it is worth stepping back and comparing them directly — the kind of analysis an examiner will ask you to perform. The three classes form a continuous sequence of decreasing temperature, and every spectral change follows logically from that single variable.

The table below consolidates the key comparison points. When an exam question asks you to compare two spectral classes, you should address temperature, colour, and the specific spectral lines that are present or absent — and crucially, explain why each difference exists in terms of ionisation or excitation.

Worked example: Answering a 'Compare' Question on Spectral Classes

Compare the spectra of a class O star and a class A star. In your answer, refer to the hydrogen Balmer lines and helium lines. [4 marks]

1. Class O: surface temperature > 25,000 K. Class A: surface temperature 7,500–10,000 K. — [B1] Establishing the temperature contrast is the foundation of any spectral comparison answer. Always state the values.
2. Class A shows very strong hydrogen Balmer absorption lines. Class O shows weak Balmer lines. — [B1] State the observation clearly before explaining it.
3. In class A stars, the temperature maximises the number of hydrogen atoms with electrons in the n = 2 level, enabling strong Balmer absorption. In class O stars, hydrogen is largely ionised, so few neutral atoms remain to produce Balmer lines. — [B1] The explanation must link temperature to ionisation/excitation state — this is the mark-winning step.
4. Class O shows ionised helium (He II) lines. Class A shows no significant helium lines. — [B1] He II lines require temperatures above ~25,000 K to ionise helium to the He+ state. At class A temperatures, helium is neither sufficiently ionised nor excited to produce visible absorption lines.

Answer: Class O stars (> 25,000 K) show weak hydrogen Balmer lines because hydrogen is largely ionised, and strong He II lines because helium is doubly ionised. Class A stars (7,500–10,000 K) show maximum-strength hydrogen Balmer lines because the temperature optimally populates the n = 2 level, and no significant helium lines because the temperature is insufficient to ionise or excite helium.

7. Spectral Classes and the Hertzsprung-Russell Diagram

Spectral classification connects directly to the Hertzsprung-Russell (HR) diagram, which plots luminosity against surface temperature (or spectral class). O, B, and A stars all sit on the upper-left portion of the main sequence, where temperatures are highest and luminosities are greatest. Because luminosity scales steeply with temperature (via the Stefan-Boltzmann law, $L = 4\pi r^2 \sigma T^4$), even a modest increase in temperature from class A to class O produces an enormous jump in luminosity.

Red and orange supergiants found in the upper-right region of the HR diagram are typically class K or M stars, which have expanded and cooled dramatically. O and B supergiants, by contrast, remain in the upper-left of the HR diagram — they are hotter and bluer than the main sequence at the same luminosity, sitting above the main sequence due to their greatly enlarged radii. This is a useful reminder that spectral class alone tells you surface temperature, not luminosity class; you need both to fully locate a star on the HR diagram.

8. Exam Technique and Common Mistakes

Questions on spectral classification tend to fall into three categories: recall of properties, explanation of why a spectral feature appears or disappears, and comparison between classes. Each requires a slightly different approach, and the marks are almost always awarded for the physical reasoning rather than the label alone.

When a question asks you to explain a spectral feature, the mark scheme will expect you to link temperature to ionisation or excitation state, and then connect that to the presence or absence of specific absorption lines. Saying 'O stars are hotter' earns no marks on its own — you must say what that temperature does to the atoms in the atmosphere.