The H-R Diagram and Stellar Classification Study Pack

Kibin's free study pack on The H-R Diagram and Stellar Classification includes a 3-section study guide, 8 quiz questions, 10 flashcards, and 1 open-ended Explain review question. Sign up free to track your progress toward mastery, plus upload your own notes and recordings to create personalized study packs organized by course.

Last updated May 21, 2026

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The H-R Diagram and Stellar Classification Study Guide

Decode the H-R diagram by tracing how luminosity, surface temperature, and stellar radius work together to place stars across the main sequence, giant, supergiant, and white dwarf regions. This pack covers the full OBAFGKM spectral classification sequence, the Stefan-Boltzmann relationship, and how stars like our G2 Sun migrate off the main sequence as they age — everything you need to interpret stellar data with confidence.

Key Takeaways

  • •The Hertzsprung-Russell (H-R) diagram plots stars by luminosity (vertical axis) against surface temperature or spectral class (horizontal axis, running from hot on the left to cool on the right), revealing that most stars fall along a diagonal band called the main sequence.
  • •A star's position on the H-R diagram reflects its current physical state: surface temperature determines color and spectral class, while luminosity depends on both temperature and radius through the Stefan-Boltzmann relationship.
  • •The seven spectral classes — O, B, A, F, G, K, M — form a temperature sequence from roughly 50,000 K (O stars) down to below 3,500 K (M stars), each defined by which atomic absorption lines are strongest in the star's spectrum.
  • •Stars spend the majority of their lives on the main sequence, fusing hydrogen in their cores; after hydrogen exhaustion, they migrate off the main sequence to become giants, supergiants, or eventually white dwarfs.
  • •Giants and supergiants occupy the upper-right region of the H-R diagram, while white dwarfs cluster in the lower-left, demonstrating that luminosity class — not just spectral type — is required to fully classify a star.
  • •The Sun is a G2 main-sequence star with a surface temperature near 5,800 K and luminosity defined as 1 solar luminosity (L☉), serving as the standard reference point for comparing other stars.

Architecture of the H-R Diagram

The Hertzsprung-Russell diagram is a two-dimensional map of stellar properties, and understanding what each axis represents is essential before interpreting the patterns it reveals.

Luminosity Axis (Vertical)

  • •Luminosity measures the total energy output of a star per second, expressed in watts or in multiples of the Sun's luminosity (L☉).
  • •The vertical axis spans an enormous range — typically from about 10⁻⁴ L☉ at the bottom to 10⁶ L☉ at the top — so it is plotted on a logarithmic scale.
  • •A star's luminosity depends on both its surface temperature and its physical size, as described by the Stefan-Boltzmann law: L = 4πR²σT⁴, where R is radius and T is surface temperature.

Temperature and Spectral Class Axis (Horizontal)

  • •The horizontal axis represents surface temperature, but it runs in reverse order: the hottest stars (above ~30,000 K) appear on the left and the coolest stars (below ~3,500 K) appear on the right.
  • •Spectral class labels — O, B, A, F, G, K, M — are often marked along this axis because they correspond directly to temperature ranges determined from stellar spectra.
  • •This reversed orientation is a historical artifact: the axis was originally labeled by spectral type before the temperature connection was firmly established.

Radius as an Implicit Third Variable

  • •Because luminosity depends on both temperature and radius, lines of constant stellar radius can be drawn diagonally across the diagram.
  • •Stars of the same temperature but higher luminosity must be physically larger, which is why giants and supergiants appear above main-sequence stars of the same spectral class.
  • •The Sun's radius (R☉) serves as a convenient reference; white dwarfs have radii around 0.01 R☉, while red supergiants can exceed 1,000 R☉.

Spectral Classification: The OBAFGKM System

Stellar spectra contain absorption lines whose identities and strengths are controlled by surface temperature, providing a precise way to sort stars into named categories.

Physical Basis of Spectral Lines

  • •When photons pass through a star's outer atmosphere, atoms absorb specific wavelengths corresponding to electron energy transitions, producing dark absorption lines in the spectrum.
  • •The temperature of the atmosphere determines which atoms are ionized or excited enough to produce detectable lines, so different spectral classes show different dominant features.
  • •For example, hydrogen Balmer lines are strongest in A-type stars (~10,000 K) because that temperature efficiently populates the second energy level of hydrogen; at higher temperatures hydrogen is ionized, and at lower temperatures it is not excited.

Spectral Class Characteristics

  • •O stars (>30,000 K): ionized helium lines dominate; stars appear blue-white and are extremely rare but very luminous.
  • •B stars (10,000–30,000 K): neutral helium lines are prominent; includes many bright stars visible to the naked eye such as Rigel.
  • •A stars (~7,500–10,000 K): hydrogen Balmer lines reach maximum strength; Sirius and Vega are classic examples.
  • •F and G stars (~5,200–7,500 K): ionized calcium H and K lines strengthen; metal lines appear; the Sun (G2, ~5,800 K) falls in this range.
  • •K stars (~3,700–5,200 K): neutral metal lines dominate; molecular bands of titanium oxide begin to appear; Arcturus is a well-known example.
  • •M stars (<3,700 K): strong titanium oxide and other molecular bands define this class; Betelgeuse and Proxima Centauri are both M stars despite enormous differences in size.

Subdivisions Within Each Class

  • •Each spectral class is subdivided on a scale of 0 to 9 (e.g., G0 through G9), with lower numbers indicating hotter temperatures within that class.
  • •The Sun's full spectral designation is G2, placing it toward the hotter end of the G class.

About this Study Pack

Created by Kibin to help students review key concepts, prepare for exams, and study more effectively. This Study Pack was checked for accuracy and curriculum alignment using authoritative educational sources. See sources below.

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