Apparent magnitude

Learning to think in terms of apparent magnitude, the logarithmic scale of star brightness. Each step in magnitude is roughly 2.5 times brighter; 5 magnitude is exactly 100 times brighter.

Some rules of thumb for minimum magnitude visible, some info from here:

  • Naked eye: 6.0 (or 7.0?)
  • 10×50 Binoculars: 10.0 (if your sky is dark)
  • 8″ Orion Dobsonian: 14.2

Some info on how many stars have the given magnitude or above, from WikipediaNSO,

  • 1: -2 to -1 (Sirius)
  • 4: -1 to 0 (Canopus, Arcturus, Alpha Centauri / Rigel Kentaurus)
  • 15: 0 to 1 (Vega, Rigel, Procyon, Betelgeuse, Achernar, …)
  • 48: 1 to 2
  • 171: 2 to 3
  • 513: 3 to 4
  • 1602: 5 to 6
  • 4800: 6 to 7

I’m surprised there’s so few visible stars; under 5000. I thought classical wisdom was the stars were innumerable, infinite. But before telescopes there really were just 5000. I wonder if all of them were mapped and named by ancient astronomers, a complete catalog? Of course you have to correct for what part of the sky is visible, and that changes over time. Still 5000 is a small set of things, not “billions and billions”.

Interesting how the  Wikipedia and Atlas of the Universe lists of brightest stars differ. Some of it is for variable stars, maybe the rest is accounting for binaries or something? I should learn more about SIMBAD and other astro databases.

And brightest magnitudes of the planets:

  • -2.45 to 5.73: Mercury
  • -4.89 to -3.82: Venus
  • -2.91 to 1.84: Mars
  • -2.94 to -1.61: Jupiter
  • -0.49 to 1.47: Saturn
  • 5.32 to 5.95: Uranus
  • 7.78 to 8.02: Neptune
  • 13.65 to ??: Pluto (dammit)

I wonder if it’d be fun to list out the brightest stars and go learn where they all are. I guess when I’m done with that I’d know my constellations. I’d like to start with Capella; it’s an interesting pair of binary stars.

Another fun thing: 3C_273 is the brightest apparent quasar, at 12.9. Would make a fun target for the telescope.

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