Stellar Spectral Classes: F, G Revision Notes | A Level AQA Physics

1. Where F and G Stars Sit in the Stellar Sequence

Picture the full Harvard spectral sequence — O, B, A, F, G, K, M — running from the hottest, most luminous blue giants down to the coolest, dimmest red dwarfs. F and G stars occupy the warm middle ground of this sequence, sitting between the hot white A stars and the cooler orange K stars.

This position matters because it places F and G stars in a temperature regime where hydrogen lines are weakening and metal lines are strengthening — a transition that makes their spectra rich with diagnostic features. The Sun itself is a G-type star, which means everything you learn here connects directly to the star you know best.

Notice how the temperature drops steeply from O to M. F stars span roughly 6000–7500 K, while G stars cover approximately 5200–6000 K. These ranges are not arbitrary — they correspond directly to the energies available to excite or ionise atoms in the stellar atmosphere, which is precisely what controls which absorption lines appear.

2. F-Type Stars: Characteristics and Spectral Features

An F-type star appears yellow-white to the eye. Its surface temperature of roughly 6000–7500 K places it just below the A stars, and that temperature difference has a clear consequence in the spectrum: the strong hydrogen Balmer absorption lines that dominate A-star spectra begin to weaken noticeably in F stars.

Why do the hydrogen lines weaken? At A-star temperatures, the majority of hydrogen atoms are in the first excited state (n = 2), perfectly primed to absorb Balmer photons and produce strong dark lines. As temperature drops into the F range, fewer hydrogen atoms are thermally excited to n = 2, so fewer Balmer absorptions occur and the lines fade.

At the same time, ionised metal lines — particularly ionised calcium (Ca II) — begin to strengthen in F-star spectra. The Ca II K and H lines (at approximately 393 nm and 397 nm respectively) become increasingly prominent as you move from A through F towards G. Ionised iron (Fe II) lines also appear. This shift from hydrogen-dominated to metal-dominated spectra is the defining trend across the F and G classes.

A well-known example of an F-type star is Procyon (F5) and Canopus (F0). Canopus sits near the hot end of the F range, so it still shows moderately strong Balmer lines alongside emerging metal features, while a cooler F5 star like Procyon shows noticeably stronger Ca II lines and weaker hydrogen absorption.

3. G-Type Stars: Characteristics and Spectral Features

G-type stars are yellow in colour, with surface temperatures of approximately 5200–6000 K. The Sun, at around 5778 K, is the canonical G2 star and the reference point for this entire class. At these temperatures, the spectral story shifts decisively: hydrogen Balmer lines are now quite weak, while ionised and neutral metal lines dominate the spectrum.

The Ca II H and K lines reach their greatest strength in G-type stars. This happens because the temperature is ideal for producing singly ionised calcium — hot enough to ionise calcium from its neutral state, but not so hot that it becomes doubly ionised (Ca III), which would not produce the same absorption features. This is a key principle: each spectral class represents a temperature window where a particular ionisation or excitation state is most abundant.

Neutral metal lines — including neutral iron (Fe I), neutral calcium (Ca I), and the sodium D lines — also appear prominently in G-star spectra. Molecular bands are just beginning to form at the cooler end of the G range, foreshadowing the strong molecular features that characterise K and M stars. The overall spectrum of a G star is therefore a rich mixture of ionised and neutral metal lines with very faint hydrogen features.

4. Comparing F and G Stars: A Side-by-Side Analysis

Now that both classes are established, comparing them directly sharpens your exam responses. The differences are systematic and temperature-driven, so you can reason through any comparison question by asking: what does a lower temperature do to atomic excitation and ionisation states?

The trend is clear: moving from F to G, hydrogen lines weaken further, Ca II lines strengthen to their peak, and neutral metal lines become more prominent. Every change traces back to the lower surface temperature reducing the average energy available per atom in the stellar atmosphere.

5. The Physical Reason Behind Spectral Trends

It is worth pausing to consolidate the underlying physics, because exam questions frequently ask you to explain — not just describe — spectral features. The key principle is that the absorption lines visible in a stellar spectrum depend entirely on which energy states are populated in the atoms of the stellar atmosphere.

For an absorption line to appear, atoms must be in the correct lower energy state to absorb the relevant photon. If the temperature is too low, atoms remain in the ground state and cannot produce certain lines. If the temperature is too high, atoms become fully ionised and lose their electrons entirely, again removing those lines. Each spectral class therefore represents a temperature window where a specific atomic state is most populated.

For hydrogen Balmer lines, the optimal temperature is around 10 000 K (A-type). For Ca II lines, the optimal temperature is around 5500–6000 K (G-type). This is why the spectral sequence is not arbitrary — it is a direct readout of stellar surface temperature through the lens of atomic physics.

Worked example: Reasoning Through a Spectral Classification Question

A star is observed to have a yellow-white colour. Its spectrum shows moderately strong hydrogen Balmer lines that are weaker than in an A star, alongside strengthening Ca II H and K lines and the first appearance of ionised iron lines. Identify the spectral class and justify your answer.

1. Yellow-white colour corresponds to a surface temperature of approximately 6000–7500 K. — Colour gives a first estimate of temperature. Yellow-white sits between the white of A stars and the yellow of G stars, pointing to the F range.
2. Balmer lines are present but weaker than in an A star. — Strong Balmer lines peak at ~10 000 K (A-type). Weaker lines indicate fewer hydrogen atoms in the n = 2 state, consistent with a lower temperature — ruling out O, B, and A classes.
3. Ca II H and K lines are strengthening but not yet at maximum strength. — Ca II lines peak in G-type stars. Strengthening but not yet maximal Ca II lines indicate we are approaching but have not yet reached the G range — consistent with F-type.
4. Ionised iron (Fe II) lines are beginning to appear; no molecular bands present. — Fe II lines emerging and no molecular bands further confirm F-type. Molecular bands only appear in K and M stars. G stars show neutral metal lines more prominently than ionised ones.
5. Spectral class: F — All evidence converges: yellow-white colour, moderately weakened Balmer lines, strengthening (not maximal) Ca II lines, and emerging ionised metal lines are all characteristic of F-type stars.

Answer: F-type star. The combination of yellow-white colour (~6000–7500 K), moderately weakened hydrogen Balmer lines, strengthening Ca II H and K lines, and emerging ionised metal lines is characteristic of the F spectral class.

6. The Sun as a G2 Star: Context and Significance

The Sun's classification as G2 is worth understanding in detail, because it appears frequently in exam contexts. The '2' after the G is a subclass digit running from 0 (hottest within the class) to 9 (coolest). G2 therefore sits near the hot end of the G range, close to the F/G boundary, with a surface temperature of approximately 5778 K.

The Sun's spectrum shows the features you would predict from the G-class description: strong Ca II H and K lines, weak hydrogen Balmer lines, and prominent neutral metal absorption lines including Fe I and Ca I. The solar spectrum was historically one of the first to be studied in detail, and many of the absorption lines were identified by Fraunhofer in the early nineteenth century — these are still called Fraunhofer lines.

Alpha Centauri A, the nearest solar-type star, is also classified G2 and has a surface temperature and spectral appearance very similar to the Sun. Comparing these two stars illustrates that spectral class captures intrinsic stellar properties — two stars of the same class, regardless of their distance, will show the same pattern of absorption lines.

7. Exam Technique: Answering Spectral Classification Questions

Spectral classification questions at A-level typically use one of three command words: describe, explain, or compare. Each demands a different response structure, and confusing them is one of the most common ways students lose marks.

A 'describe' question wants observable features in sequence — colour, temperature range, which lines are strong or weak — without requiring you to explain why. An 'explain' question demands the atomic physics mechanism: which energy state is populated, why that state produces the observed line, and why the temperature determines the population. A 'compare' question requires you to address both similarities and differences explicitly, often using connective language such as 'whereas' or 'in contrast to'.

When writing an explanation of why a particular line is strong or weak, always follow this three-part structure: state the temperature, identify the atomic state that temperature produces, and connect that state to the absorption line. For example: 'G-type stars have a surface temperature of approximately 5500 K, which is optimal for producing singly ionised calcium (Ca II); this means a large proportion of calcium atoms are in the Ca II state and can absorb photons at the H and K wavelengths, producing strong absorption lines.'

Mastering F and G spectral classes means understanding the temperature-driven logic that connects surface temperature, atomic physics, and observable spectral features — the core of AQA's classification of stars specification requirement for these two classes.