Vol. Science · Astronomy · Color lesson

Stars
types & lives

A color-first literacy desk: the Harvard sequence O B A F G K M, why hotter stars look bluer (Wien’s law), what “dwarf / giant / supergiant” means, and two life-path cartoons — Sun-like (planetary nebula → white dwarf) and high-mass (supernova → neutron star or black hole). Pair with the nearest-stars yardstick, the parallax desk that turns Earth’s orbit into that distance, the magnitude desk that turns the distance into apparent brightness, exoplanets, the local sky chart, and a Milky Way starter. The temperature–luminosity picture, with cited stars and Stefan–Boltzmann radius lines, is the HR diagram. How mass sets luminosity and a main-sequence lifetime is the stellar lifetime desk. Where a planet sits in that star’s light is the habitable zone. The full radio-to-gamma ladder — and a Wien peak you can slide — sits on the electromagnetic spectrum desk.

Educational schematic. Temperatures, masses, and lifetimes are rounded NASA / textbook order-of-magnitude ranges — not a research catalog, not an HR-diagram fit, and not NASA imagery. Sizes on the canvases are exaggerated so the lesson stays visible on a phone.

Interactive · Spectral types

Interactive · Life phases

Interactive · HR cartoon

This strip is the cartoon. Cited temperatures, luminosities, and Stefan–Boltzmann radius lines are on the HR diagram.

Literacy · words that keep getting mixed

OBAFGKM

Harvard spectral sequence, hottest to coolest. Mnemonic leftovers exist; the physics is Wien’s law: hotter photospheres peak bluer (often in the ultraviolet), cooler ones redder.

Main sequence

NASA: stars fusing hydrogen to helium in the core — about 90% of the stellar population, from roughly a tenth to ~200 solar masses, living millions to billions of years (the smallest, much longer).

Dwarf / giant / supergiant

Morgan–Keenan luminosity class, not a personality test. V = main-sequence “dwarf” (the Sun is G2V). III = giant. I = supergiant. Same spectral letter can be a compact dwarf or a swollen giant.

Red giant / AGB

After core hydrogen is gone (stars under about eight solar masses), the envelope puffs out. NASA notes red giants often look more orange than red. The asymptotic giant branch is a later, pulsing, mass-losing stage before the planetary nebula.

Planetary nebula

A misleading 18th-century name: not planets. The star’s outer layers drift away as a glowing shell; the leftover core is a white dwarf.

White dwarf

Earth-size remnant, hundreds of thousands of times more massive than Earth. NASA: a teaspoon would outweigh a pickup. No new fusion; it cools for billions of years. The Sun is headed here in about 10 billion years.

Supernova → remnant

Stars about 8–20 solar masses can end in a core-collapse supernova and leave a neutron star (more mass than the Sun packed into a city-scale ball). Still heavier stars can leave a stellar-mass black hole.

Brown dwarfs

Between giant planets and stars. NASA’s teaching range is about 13–80 Jupiter masses. They never settle into sustained core hydrogen fusion the way a main-sequence star does.

Sources · public NASA / ESA / textbook

Figures are teaching toys. A real star’s type, mass, and remaining life need spectra and models. This page will not tell you whether Betelgeuse explodes next year. Earthrise preview: NASA / Bill Anders (public domain).

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