An A-type star shows very strong hydrogen Balmer absorption lines, while an M-type star of similar mass shows almost none — instead displaying broad TiO molecular bands. What is the most accurate explanation?
AThe A star is far richer in hydrogen than the M star, whose hydrogen has been converted to helium over its lifetime
BBoth stars are about 75% hydrogen, but at different temperatures different quantum states are populated: the A star's ~10,000 K photosphere excites hydrogen to n=2 (enabling Balmer absorption), while the M star's ~3,000 K photosphere leaves hydrogen in the ground state and allows TiO molecules to survive
CThe M star's stronger gravity suppresses hydrogen line formation and allows heavy molecules to dominate
DA stars have active hydrogen fusion in their photospheres, producing emission that appears as strong absorption in cooler surrounding gas
Both stars are approximately 75% hydrogen — composition is not the variable. At ~10,000 K (A stars), conditions are ideal for hydrogen atoms to have electrons in the n=2 state, producing the strongest Balmer absorption of any spectral type. In M stars (~2,500–3,500 K), thermal excitation cannot populate n=2 significantly, so hydrogen lines are weak; at the same time, temperatures are cool enough that TiO molecules survive without thermally dissociating. Spectral differences reflect temperature-driven excitation and ionization physics, not composition.
Question 2 Multiple Choice
Two G5-type stars have identical color and temperature, but one shows broad, smeared spectral lines while the other shows narrow, sharp lines. The narrow-line star is most likely which of the following?
AA white dwarf — extreme gravity compresses the atmosphere and sharpens transitions
BA main-sequence dwarf — its compact, high-pressure atmosphere produces pressure broadening of spectral lines
CA supergiant — its extended, low-density atmosphere has infrequent particle collisions, producing narrow lines
DA neutron star — quantum confinement effects sharpen the spectral features at high density
Luminosity class is physically rooted in surface gravity and atmospheric pressure. Supergiants have extended, low-density atmospheres where atoms collide rarely — producing narrow, sharp lines. Main-sequence dwarfs have compact, high-pressure atmospheres where frequent collisions broaden energy levels through pressure (collisional) broadening, smearing spectral lines. This is why luminosity class can be read directly from line widths: a same-temperature comparison reveals whether a star is a supergiant (Roman numeral I) or a dwarf (V) purely from spectral appearance.
Question 3 True / False
The OBAFGKM spectral sequence is fundamentally a temperature sequence; two stars at opposite ends of the sequence can have nearly identical chemical compositions yet produce completely different spectra.
TTrue
FFalse
Answer: True
Nearly all main-sequence stars are approximately 75% hydrogen and 24% helium by mass, with trace amounts of heavier elements. Yet O stars (>30,000 K) and M stars (<3,500 K) look completely different — one dominated by ionized helium lines, the other by molecular absorption bands. The vastly different temperatures drive different ionization and excitation states, changing which spectral lines are observable even though the underlying composition is nearly identical.
Question 4 True / False
O-type stars show weaker hydrogen absorption lines than A-type stars because O stars have converted more of their hydrogen to helium through nuclear fusion.
TTrue
FFalse
Answer: False
O stars are still approximately 75% hydrogen — the weak Balmer lines reflect temperature physics, not depletion. At temperatures above 30,000 K, nearly all hydrogen is ionized (H+, proton), and ionized hydrogen has no electrons to produce absorption lines. Balmer lines require hydrogen atoms with electrons in the n=2 state, which is maximized around 10,000 K (A stars). Hotter O stars have too much ionization; cooler stars below ~6,000 K have insufficient thermal excitation of n=2. The weak lines in O stars are about ionization state, not hydrogen abundance.
Question 5 Short Answer
A classmate says 'M stars show TiO absorption bands because they have a higher abundance of titanium than other stars.' Explain why this interpretation is wrong and what actually controls TiO band appearance.
Think about your answer, then reveal below.
Model answer: The appearance of TiO bands reflects temperature, not an unusual titanium abundance. TiO is a molecule, and molecules dissociate at high temperatures — in hotter stellar photospheres, the thermal energy is sufficient to break TiO apart into atomic Ti and O. Only in the cool photospheres of M stars (~2,500–3,500 K) is the temperature low enough for TiO molecules to survive intact and produce their characteristic broad absorption bands. The same titanium atoms are present across all spectral types but appear in different forms (atomic Ti+ lines in hotter stars, molecular TiO bands in cool ones) depending purely on photospheric temperature. Spectral line pattern is a temperature diagnostic, not a composition diagnostic.
This is the central insight of the Harvard spectral classification system: nearly all stars have similar compositions, and the dramatic variation in their spectra is explained almost entirely by temperature differences that shift the populations of different atomic and molecular quantum states — exactly as described by the Boltzmann distribution and Saha ionization equation.