HR Diagram

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Star Noun

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[star]

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Star

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A luminous ball of gas, primarily hydrogen and helium, held together by its own gravity and producing light.

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Example: This image shows our solar system, with the Sun, a star, at the center and the planets orbiting it. It does not illustrate star properties.

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Luminous Adjective

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[loo-mi-nus]

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Luminous

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Describing an object that is bright because it emits or radiates its own light, rather than reflecting it.

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Example: This image compares luminous objects, which produce their own light (like the sun), with non-luminous objects, which only reflect light (like the moon).

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Hertzsprung-Russell (HR) Diagram Noun

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[herts-sprung rus-sel dai-uh-gram]

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Hertzsprung-Russell (HR) Diagram

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A scatter graph of stars plotting their luminosity or absolute magnitude against their surface temperature or spectral class.

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Example: This Hertzsprung-Russell (HR) diagram plots stars by their brightness (luminosity) and temperature, grouping them into types like main sequence, giants, and white dwarfs.

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Luminosity Noun

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[loo-mi-nos-i-tee]

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Luminosity

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The intrinsic brightness of a celestial object, representing the total amount of energy emitted per unit of time.

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Example: This HR diagram plots a star's luminosity (its true brightness) on the vertical axis against its surface temperature on the horizontal axis.

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Absolute Magnitude Noun

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[ab-so-loot mag-ni-tood]

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Absolute Magnitude

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A measure of a celestial object's intrinsic luminosity, defined as its brightness from a standard distance of 10 parsecs.

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Example: This diagram shows that absolute magnitude ('M') is a star's brightness measured from a standard distance (10 parsecs), unlike apparent magnitude ('m') which depends on the actual distance ('d').

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Spectral Class Noun

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[spek-trul klas]

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Spectral Class

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A classification system for stars based on their spectral characteristics, which are primarily determined by their surface temperature.

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Example: This diagram shows an absorption spectrum, a unique "barcode" of dark lines in light. Scientists read these barcodes in starlight to classify stars.

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Main Sequence Noun

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[meyn see-kwens]

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Main Sequence

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A continuous band of stars on the HR diagram where most stars spend the majority of their stable, hydrogen-burning lives.

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Example: This HR diagram shows that Main Sequence stars form a continuous band where hotter, bluer stars are brighter, and cooler, redder stars are dimmer.

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Red Giant Noun

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[red jai-uhnt]

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Red Giant

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A large, luminous star of low or intermediate mass in a late phase of stellar evolution with a cool surface.

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Example: This image shows the enormous size of a star after it becomes a red giant, comparing it to its previous, much smaller size as a main-sequence star.

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Supergiant Noun

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[soo-per-jai-uhnt]

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Supergiant

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A class of extremely massive and luminous stars, located at the top of the Hertzsprung-Russell diagram.

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Example: This image shows the massive size of a supergiant star by comparing it to the orbits of planets in our solar system.

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White Dwarf Noun

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[hwyt dworf]

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White Dwarf

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The small, dense, and hot remnant of a low-to-medium mass star that has exhausted its nuclear fuel.

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Example: This image shows a real white dwarf (the tiny dot indicated by the arrow), which is the hot, dense, and dim core of a star that has run out of fuel.