Stars

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Parallax Noun

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[par-uh-laks]

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Parallax

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The apparent shift in an object's position when viewed from two different lines of sight, used to measure stellar distances.

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Example: As Earth orbits the Sun, a nearby star appears to shift its position against distant background stars. This apparent shift is called stellar parallax.

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Parsec Noun

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[par-sek]

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Parsec

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A unit of astronomical distance, equal to 3.26 light-years, defined by a parallax angle of one arcsecond.

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Example: This diagram shows the parallax angle, the apparent shift of a star when viewed from different points in Earth's orbit. A parsec is a distance defined by this angle.

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Light-year Noun

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[lite-yeer]

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Light-year

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The distance light travels in a vacuum in one Julian year, used for measuring vast distances in space.

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Example: This diagram shows a light-year is a unit of distance by giving a real-world example: the 4.2 light-year distance between our Sun and Proxima Centauri.

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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 total amount of electromagnetic energy emitted per unit of time by a star or other celestial object.

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Example: This diagram shows that as light travels away from a star (S), its energy spreads out over a larger area, making it appear dimmer.

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Apparent Brightness Noun

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[uh-pair-ent brite-ness]

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Apparent Brightness

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The brightness of a celestial object as it is measured from Earth, which depends on its luminosity and distance.

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Example: This diagram shows a scale of apparent magnitude, the measure of apparent brightness. It compares different celestial objects, showing that brighter objects like the Sun have a lower (more negative) magnitude number than dimmer objects like Polaris.

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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 apparent magnitude from a 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 pc), unlike apparent magnitude (m) from any distance (d).

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Apparent Magnitude Noun

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[uh-pair-ent mag-ni-tood]

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Apparent Magnitude

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A measure of a celestial object's brightness as seen by an observer on Earth, on a logarithmic scale.

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Example: This diagram shows the apparent magnitude scale, where brighter objects like the Sun have a lower (negative) value and dimmer objects have a higher (positive) value.

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Blackbody Radiation Noun

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[blak-bod-ee ray-dee-ay-shun]

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Blackbody Radiation

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The thermal electromagnetic radiation emitted by an idealized opaque, non-reflective body, which indicates its surface temperature.

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Example: This graph shows that hotter objects, like blue stars, glow brighter and emit light at shorter wavelengths, while cooler objects, like red stars, are dimmer.

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Photosphere Noun

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[foh-toh-sfeer]

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Photosphere

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The visible surface layer of a star from which the light we see is emitted into space.

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Example: This diagram shows a cross-section of the Sun, identifying the photosphere as the star's visible surface layer located above the convection zone.

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Hertzsprung-Russell (H-R) diagram Noun

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

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Hertzsprung-Russell (H-R) diagram

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A scatter plot of stars showing the relationship between their luminosities and their effective surface temperatures.

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Example: This diagram plots stars on a graph to show the relationship between their temperature (color) and their brightness (luminosity), revealing distinct groups of stars.