Fix the mistake: Stars and Galaxies

The Brightness and Colors of Stars

Among the stars shining in the night sky, those that emit their own light are called stars. Their brightness and colors are important information that indicates their properties.

Basic Knowledge

The brightness of stars is expressed by magnitude.
① Apparent magnitude: The brightness seen from Earth. It is defined so that first-magnitude and sixth-magnitude stars differ by exactly 100 times in brightness. A difference of one magnitude corresponds to approximately 2.5 times (2.512 times).
② Absolute magnitude: The brightness a star would have if it were placed at a distance of 10 parsecs (=32.6 light-years). It represents the star’s intrinsic brightness.
Stars with the same magnitude have the same brightness. The smaller the numerical value of the magnitude, the brighter the star (negative magnitudes also exist: Sirius -1.5 magnitude, the Sun -26.7 magnitude).
The color of a star reflects its surface temperature. Bluish-white stars are hotter, while red stars are cooler (Rigel in Orion = bluish-white・approximately 12000K, Betelgeuse = red・approximately 3500K).

Important Terms
Star（A celestial body that emits its own light. Obtains energy through nuclear fusion）
Apparent magnitude（Brightness seen from Earth. Affected by distance）
Absolute magnitude（Brightness at a distance of 10 parsecs (32.6 light-years)）
Approximately 2.5 times per magnitude（100 times for a difference of 5 magnitudes）
Star color and temperature（Bluish-white>white>yellow>orange>red, in order of decreasing temperature）
Spectral classification（Seven types: O・B・A・F・G・K・M. High temperature→low temperature）

Further Exploration (Background・Significance)

By examining a star’s spectrum (a rainbow-like distribution formed by separating light according to wavelength), we can determine its surface temperature, composition, motion, and other properties.
The Sun is a G2-type star with an absolute magnitude of +4.8. Sirius, the brightest first-magnitude star (in Canis Major), is a very nearby star at approximately 8.6 light-years. It appears bright because it is close (its absolute magnitude is +1.4).
In contrast, Deneb (in Cygnus) is 2600 light-years away but appears at magnitude 1.3; it is a supergiant with an absolute magnitude of -7.2. Even first-magnitude stars can differ greatly in their intrinsic brightness.
The color of stars is based on the principle of blackbody radiation. The higher the temperature of an object, the more strongly it emits short-wavelength (blue) light. This is a physical law shared by molten iron (orange) and stars (blue or red).

Key Points
  • A difference of one magnitude corresponds to approximately 2.5 times the brightness
  • A difference of five magnitudes corresponds to exactly 100 times
  • The smaller the magnitude, the brighter the star (negative values also exist)
  • Apparent magnitude and absolute magnitude are different
  • Absolute magnitude = brightness converted to a distance of 10 parsecs
  • Bluish-white stars→high temperature・red stars→low temperature
  • Spectral types O B A F G K M

Points to Note (Easily Confused)

① Do not confuse apparent magnitude with absolute magnitude. ② The more negative the magnitude, the brighter the star. The Sun is -26.7, the Moon is -12.7, and Sirius is -1.5. ③ A difference of five magnitudes is 100 times, and a difference of one magnitude is approximately 2.5 times. ④ A star’s color indicates its surface temperature: a red star is cool, while a blue star is hot.

Practice

  1. How many times brighter is a first-magnitude star than a sixth-magnitude star?
  2. What is the name of the magnitude expressed by the brightness a star would have at a distance of 10 parsecs?
  3. Which has the higher surface temperature, a bluish-white star or a red star?

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