Stellar Spectral Classes: K, M Revision Notes | A Level AQA Physics

1. Why Star Colour Tells You So Much

Look up at the night sky and you will notice that stars are not all the same colour. Some appear blue-white, others yellow, and some glow a deep orange-red. That colour is not random — it is a direct window into the star's surface temperature, and from temperature we can read an entire story about the star's size, age, and chemistry.

The spectral classification system orders stars by temperature using the sequence O, B, A, F, G, K, M — from hottest to coolest. Our Sun sits at class G. This lesson focuses on the two coolest classes in that sequence: K and M. These are the most numerous types of star in the Milky Way, yet they are often overlooked in favour of the dramatic blue giants.

The key to understanding spectral classes is the absorption spectrum. As radiation travels outward through a star's cooler outer atmosphere, atoms and molecules absorb photons at specific wavelengths. The result is a continuous rainbow spectrum crossed by dark absorption lines — a fingerprint unique to the star's temperature and composition.

2. K-Type Stars: Orange and Iron-Rich

A K-type star has a surface temperature in the range of approximately 3500 K to 5000 K. At these temperatures the star glows orange, and the balance of energy in its photosphere is cool enough for certain atoms to remain un-ionised — meaning they can absorb photons and produce strong spectral lines.

The reason calcium lines dominate in K-type spectra comes down to atomic physics. At around 4000–5000 K, singly ionised calcium (Ca II) is the most abundant absorbing species in the photosphere. The Ca II H and K lines at approximately 393 nm and 397 nm are among the strongest absorption features in the entire visible spectrum for these stars.

Hydrogen lines, by contrast, are weak in K-type spectra. This is not because K-type stars lack hydrogen — hydrogen is the dominant element in virtually all stars. The issue is excitation: at 3500–5000 K, there is insufficient thermal energy to excite hydrogen electrons to the n = 2 level from which Balmer absorption occurs. Most hydrogen atoms sit in their ground state and cannot absorb visible-wavelength photons.

Neutral metal lines — iron (Fe I), magnesium (Mg I), and calcium (Ca I) — also appear prominently in K-type spectra. These neutral species survive at lower temperatures where hotter stars would ionise them completely, stripping away the electrons needed for absorption.

Well-known K-type stars include Arcturus (K1.5 III, a giant) and Epsilon Eridani (K2 V, a main-sequence dwarf). Arcturus is one of the brightest stars visible from Earth, demonstrating that K-type stars can still be highly luminous when they are giants, even though their surface temperature is lower than the Sun's.

3. M-Type Stars: Cool, Red, and Molecular

Drop below approximately 3500 K and you enter the realm of M-type stars. These are the coolest stars on the main sequence, glowing a deep red. They are also by far the most common type of star in the galaxy — estimates suggest that around 70–75% of all stars in the Milky Way are M-type dwarfs, though their low luminosity means none are visible to the naked eye from Earth.

The defining spectral signature of M-type stars is the presence of molecular absorption bands, particularly those of titanium oxide (TiO). At temperatures below about 3500 K, the photosphere is cool enough for molecules to survive without being dissociated by thermal collisions. TiO is a diatomic molecule that produces a series of broad, overlapping absorption bands across the red portion of the visible spectrum, giving M-type stars their characteristic deep red appearance.

This is a crucial distinction from all hotter spectral classes. In O, B, A, F, G, and K stars, the photosphere is too hot for molecules to persist — they are broken apart by energetic collisions. Only in M-type stars (and the even cooler L, T, and Y brown dwarfs, which are beyond the scope of this specification) does molecular chemistry dominate the spectrum.

M-type stars also show strong neutral metal lines (calcium, iron, magnesium) and very weak hydrogen Balmer lines — even weaker than in K-type stars, for the same reason: the temperature is too low to populate the n = 2 hydrogen level. The overall spectral energy distribution peaks in the red and near-infrared, consistent with Wien's displacement law applied to temperatures of 2500–3500 K.

Prominent M-type stars include Proxima Centauri (M5.5 Ve, the nearest star to the Sun), Barnard's Star (M4 V), and the red supergiant Betelgeuse (M1–M2 Ia). Betelgeuse illustrates that M-type classification applies to both tiny, dim red dwarfs and to enormous, luminous supergiants — the spectral class tells you about surface temperature, not about size or luminosity directly.

4. Comparing K and M: A Side-by-Side Analysis

Now that both classes are established, it is worth comparing them directly. Exam questions frequently ask you to compare spectral classes, and the command word 'compare' requires you to address both similarities and differences explicitly.

The most important distinction between K and M spectra is the presence or absence of TiO molecular bands. If you see broad molecular absorption bands in the red region of a spectrum, you are looking at an M-type star. If the spectrum shows strong Ca II lines and neutral metal lines but no molecular bands, you are looking at a K-type star.

Both classes share the characteristic of weak hydrogen lines — this is what separates them from hotter A, F, and G stars where Balmer lines are more prominent. Recognising this shared feature, and then using TiO bands to distinguish M from K, is the core analytical skill the examiner is testing.

5. Why Temperature Controls the Spectrum: The Physical Mechanism

It is worth pausing to understand why temperature determines which lines appear, rather than just memorising the pattern. This deeper understanding is what separates an A-grade answer from a C-grade one when the examiner asks you to 'explain' rather than 'state'.

For an absorption line to appear in a stellar spectrum, two conditions must be met. First, the relevant atom or molecule must be present in the star's outer atmosphere. Second, the atom must be in the correct energy state to absorb a photon of the relevant wavelength. Temperature controls the second condition through the Boltzmann distribution — it determines the population of atoms in each energy level.

At K-type temperatures, calcium is predominantly in the singly ionised state (Ca II), and a significant fraction of Ca II ions are in the ground state from which the H and K line transitions occur. This produces strong absorption. At M-type temperatures, the photosphere is cool enough that even molecules — which require low-energy environments to avoid dissociation — can survive and absorb radiation across broad wavelength bands.

Worked example: Analysing a Stellar Spectrum to Determine Spectral Class

A spectrum shows the following features: dominant broad absorption bands in the red region of the visible spectrum, very weak hydrogen Balmer lines, and strong neutral calcium lines. Determine the spectral class and justify your answer.

1. Dominant broad absorption bands in the red region — Broad bands (as opposed to sharp lines) in the red region are characteristic of molecular absorption. The most common molecule producing such bands in cool stellar spectra is titanium oxide (TiO).
2. Molecules survive only below ~3500 K — Above ~3500 K, thermal collisions dissociate molecules. The presence of TiO bands therefore constrains the surface temperature to below approximately 3500 K.
3. Very weak hydrogen Balmer lines — Weak Balmer lines are consistent with a cool star where insufficient thermal energy populates the n = 2 hydrogen level. This rules out A-type (strong Balmer) and is consistent with K or M.
4. TiO bands present → M-type; TiO bands absent → K-type — K-type stars do not show TiO bands because their temperatures (3500–5000 K) are too high for TiO to survive. The presence of TiO bands here confirms M-type classification.
5. Spectral class: M-type — The combination of dominant TiO molecular bands, very weak hydrogen lines, and strong neutral metal lines is the defining signature of an M-type star with surface temperature below ~3500 K.

Answer: M-type star. The dominant TiO molecular absorption bands in the red region confirm a surface temperature below ~3500 K, which is the defining characteristic of M-type classification.

6. Exam Technique: Getting Full Marks on Spectral Class Questions

AQA questions on spectral classification tend to fall into three patterns: 'state the spectral class from a described spectrum', 'explain why a particular line is strong or weak', and 'compare two spectral classes'. Each requires a slightly different approach.

For 'explain' questions, the mark scheme rewards a chain of reasoning: state the temperature range, link it to the ionisation or excitation state of the relevant atom or molecule, and connect that to the observed spectral feature. A one-line answer such as 'K-type stars are cooler' will not score full marks — you need to explain the mechanism.

For 'compare' questions, you must address both similarities and differences. A common error is to describe only one class in detail and then say the other is 'different'. Name the specific feature, state what it is in each class, and use comparative language: 'stronger in K than in M', 'absent in K but dominant in M'.

A frequent trap in exam answers is confusing the reason for weak hydrogen lines with a low hydrogen abundance. Always clarify that hydrogen is abundant in K and M stars — the lines are weak because of the excitation conditions at low temperature, not because hydrogen is scarce.

Bringing this together: K and M stars are the cool end of the main sequence, united by weak hydrogen lines and strong neutral metal lines, but distinguished by the presence of TiO molecular bands in M-type spectra. Understanding why each feature appears — through the lens of temperature, excitation, and molecular stability — is what the AQA mark scheme rewards at the highest levels.