Astronomy · Rung 7 of 12

The stars

All that reaches us from a star is its light, and almost everything is in that light. Brightness is measured in magnitudes, a scale inherited from Hipparchus where less is brighter and every five numbers is a hundredfold: Sirius sits at −1.5 and the naked eye reaches 6 under a dark sky. But apparent brightness mixes how far away a star is with how powerful it is; to separate them there is absolute magnitude, the brightness it would have at 10 parsecs. Colour gives temperature: blue is hot and red is cold, the opposite of the tap. Hence the sequence O B A F G K M, from 30,000 K to 3,000 K. The Sun is a G2, yellow, ordinary.

Where this sits

Comes after
Distances and scales
Opens up
The life of a star, The Milky Way and the galaxies

Exercises from this rung

Generated by the app, from easiest to hardest

  1. On the magnitude scale, which of these stars is the brightest?

    • Vega, magnitude 0.0
    • One of magnitude 6.0
    • Polaris, magnitude 2.0
    • Sirius, magnitude −1.5

    Why The lower the number, the brighter: the scale comes from Hipparchus, who called the showiest stars “first magnitude” and the barely visible ones “sixth”. It is logarithmic: five magnitudes are exactly a hundredfold, so each one is 2.512 times. The naked eye reaches 6 under a dark sky; from a city, 3 or 4.

    easier

  2. In the spectral sequence O B A F G K M, where does the Sun sit?

    • At G, towards the middle
    • Outside the sequence
    • At O, at the start
    • At M, at the end

    Why The Sun is a G2V: yellow, 5,772 K, utterly ordinary. The sequence runs from O, blue and over 30,000 K, to M, red and under 3,700 K, and the order is not arbitrary: stars were classified by the look of their spectrum before anyone knew temperature was what it sorted. Hence the jumbled letters.

    in between

  3. Deneb looks fainter than Sirius but is vastly more luminous. Which magnitude reveals that?

    • Absolute
    • Spectral
    • Apparent
    • Photometric

    Why Apparent magnitude mixes power and distance. Absolute magnitude is the brightness a star would have at 10 parsecs, and it separates the two. Sirius is 8.6 light years away and Deneb some 2,600: Deneb puts out nearly two hundred thousand times the Sun, and still looks fainter because it is three hundred times further.

    harder