An HR diagram (Hertzsprung-Russell diagram) is a graph that plots stars by their temperature and brightness, revealing patterns that tell astronomers almost everything about how stars live and die. Temperature runs along the horizontal axis (with hotter stars on the left), while brightness runs along the vertical axis (with more luminous stars at the top). When thousands of stars are plotted this way, they don’t scatter randomly. They cluster into distinct groups that correspond to different stages of a star’s life.
What the Axes Represent
The horizontal axis shows a star’s surface temperature, measured in Kelvin. The hottest stars (above 25,000 K) sit on the left side, and the coolest (around 3,000 K) sit on the right. This axis is reversed compared to most graphs, which can be confusing at first. Temperature is closely tied to a star’s color: blue-white stars are the hottest, followed by white, yellow-white, yellow, orange, and red as you move rightward across the diagram.
The vertical axis shows luminosity, or how much total energy a star radiates. This is typically expressed relative to the Sun. A star with 100 solar luminosities puts out 100 times more light than the Sun. Because the range is enormous (from a ten-thousandth of the Sun’s output to a million times greater), the scale is logarithmic, meaning each step represents a multiplication rather than an addition.
Astronomers sometimes swap in closely related measurements. Instead of temperature, the horizontal axis might show spectral class or color index. Instead of luminosity, it might show absolute magnitude (a measure of intrinsic brightness). When color and magnitude are used, the chart is technically called a color-magnitude diagram, but the idea is the same.
Spectral Classes: OBAFGKM
Stars are sorted into spectral classes based on the absorption lines in their light, which directly reflect surface temperature. The classes, from hottest to coolest, follow the sequence O, B, A, F, G, K, M. Generations of astronomy students have memorized them with the mnemonic “Oh Be A Fine Guy/Girl, Kiss Me.”
- O stars: Bluish white, 25,000 to 50,000 K. Extremely rare and short-lived.
- B stars: Bluish white, 10,000 to 25,000 K.
- A stars: White, 7,400 to 10,000 K.
- F stars: Yellow-white, 6,000 to 7,400 K.
- G stars: Yellow, 5,000 to 6,000 K. The Sun is a G star.
- K stars: Yellow to orange, 3,500 to 5,000 K.
- M stars: Red, around 3,000 K. The most common type in the galaxy.
Additional classes extend below M for objects too cool to be true stars. L-type brown dwarfs range from about 1,500 to 2,500 K, T-types from 800 to 1,500 K, and Y-types are cooler still, below 800 K.
The Main Sequence
The most prominent feature on any HR diagram is a broad diagonal band running from the upper left (hot, bright stars) to the lower right (cool, dim stars). This is the main sequence, and it’s where stars spend roughly 90% of their lives. During this phase, a star is in a stable equilibrium, fusing hydrogen into helium in its core. The Sun sits partway along the main sequence with a surface temperature of about 6,000 K and a luminosity of 1 (by definition, since it’s the reference point).
A star’s position along the main sequence is determined almost entirely by its mass. More massive stars sit higher and to the left: they’re hotter and vastly more luminous. Less massive stars sit lower and to the right. The relationship between mass and brightness is steep. Across a broad range of star masses, doubling a star’s mass increases its luminosity by roughly 10 to 16 times or more, depending on the mass range. For stars near the Sun’s mass, the relationship is especially steep: luminosity scales with roughly the 4th to 5th power of mass. This means a star twice the Sun’s mass doesn’t shine twice as bright; it shines closer to 20 times as bright.
This also means massive stars burn through their fuel far faster. A star 10 times the Sun’s mass might exhaust its hydrogen in only 20 million years, while a small red dwarf can keep fusing hydrogen for hundreds of billions of years.
Giants, Supergiants, and White Dwarfs
Three other regions of the HR diagram stand out clearly. Each represents a different stage of stellar life, after a star has left the main sequence.
Red giants occupy the upper right: cool surfaces (hence red) but enormous luminosity because they’ve swelled to huge sizes. When a star like the Sun exhausts the hydrogen in its core, the core contracts and heats up while the outer layers expand dramatically. The result is a star that’s cooler on the surface than the Sun but roughly 100 times more luminous. The Sun will enter this phase in about 5 billion years.
Supergiants sit across the top of the diagram. These are the most luminous stars visible, and they can be hot (upper left) or cool (upper right). They descend from the most massive main-sequence stars, the O and B types, and represent a brief, spectacular phase before those stars end their lives in explosions.
White dwarfs cluster in the lower left: hot surfaces but extremely low luminosity. They’re small, roughly Earth-sized, which is why they’re dim despite being hot. A white dwarf is the exposed core left behind after a star like the Sun sheds its outer layers at the end of the red giant phase. With no fusion occurring, it slowly cools and fades over billions of years.
How Stars Move Across the Diagram
An HR diagram looks like a snapshot, but it also tells a story about time. As a star ages, its temperature and luminosity change, and its position on the diagram shifts. Astronomers call these paths “evolutionary tracks,” though the star isn’t physically moving through space. The diagram is tracking changes in its physical properties.
Consider a star like the Sun. It begins as a collapsing cloud of gas. As a protostar, it’s cool (about 3,500 K) but surprisingly luminous, around 1,000 times the Sun’s current brightness, because it’s so large. On the HR diagram, this places it above and to the right of the Sun’s current position. As it contracts, it heats up and dims, moving down and to the left. During the T Tauri phase (a turbulent pre-stellar stage), it fluctuates in brightness but remains cooler and fainter than its final main-sequence position. Eventually it reaches equilibrium at about 6,000 K and 1 solar luminosity, settling onto the main sequence where it remains for roughly 10 billion years.
After hydrogen is exhausted, the star swells into a red giant (upper right), then sheds its outer layers and collapses into a white dwarf (lower left). A more massive star follows a similar early path but at higher temperatures and luminosities, and its post-main-sequence evolution ends differently, potentially as a neutron star or black hole rather than a white dwarf.
Where the Diagram Came From
The diagram was developed around 1910 through the independent work of Danish astronomer Ejnar Hertzsprung and American astronomer Henry Norris Russell. Hertzsprung noticed that stars of the same spectral type (color) could have very different luminosities, and he used their motions across the sky to estimate which ones were truly brighter. Russell published an influential version of the diagram in 1913 that combined data from nearby stars, the Hyades star cluster, and several groups of stars whose distances could be measured. Early studies of open clusters like the Hyades and Pleiades by Hertzsprung and others actually produced the first color-magnitude diagrams a few years before Russell’s synthesis brought the idea widespread attention.
Why Astronomers Still Use It
The HR diagram isn’t just a teaching tool. It’s a practical instrument for measuring cosmic distances. A technique called spectroscopic parallax uses the diagram to estimate how far away a star is. By analyzing the light from a star, astronomers determine its spectral class (which fixes its horizontal position on the diagram) and the thickness of its spectral lines (which reveals its luminosity class, fixing its vertical position). Together, these give the star’s intrinsic brightness. Comparing that to how bright the star appears from Earth yields its distance. This method works for individual stars far too distant for direct geometric measurements.
The diagram is also essential for understanding star clusters. Because all stars in a cluster formed at roughly the same time and distance, plotting them on an HR diagram reveals the cluster’s age. In a young cluster, even the most massive stars still sit on the main sequence. In an older cluster, the brightest stars have peeled off the main sequence and moved toward the giant and supergiant regions. The point where the main sequence “turns off” tells astronomers precisely how old the cluster is: a cluster whose turn-off point is at B-type stars is roughly 100 million years old, while one whose turn-off is at G-type stars is several billion years old.