October 2017 Discover

OUT THERE

68 DISCOVERMAGAZINE.COM

a lot more about how stars relate

to each other than apparent

magnitude does.

Now when astronomers list the

magnitude of an extended celestial

object — such as a comet, nebula, or

galaxy — they calculate its integrated

magnitude: the brightness it would have

if all its light originated from one point.

Astronomers have assigned to comets

and asteroids a different absolute mag-

nitude system (because it also makes no

sense to imagine them 32.6 light-years

away). The value for those objects is

the brightness each would have to a

theoretical observer standing on the

sun if the object were 1 astronomical

unit away. (An astronomical unit is the

Earth-sun distance.)

Then there are meteors. Believe

it or not, it’s easier to estimate the

magnitude of a faint meteor than CLOCKWISE FROM TOP LEFT: NASA /JPL/SPACE SCIENCE INSTITUTE; NASA/JON MORSE (UNIVERSITY OF COLORADO; ROGER LYNDYS/NOAO/AURA/NSF;

ERNIE MASTROIANNI/DISCOVER

JUPITER AND MARS AT MAXIMUM BRIGHTNESS –2.9

THE PINWHEEL GALAXY (M33) 5.7

COMET IKEYA-SEKI AT MAXIMUM

BRIGHTNESS –10

ETA CARINAE AT MAXIMUM

BRIGHTNESS, 1843 –0.8