October 2017 Discover

BETELGEUSE

Magnitude 0.5

Reddish orange M2 star

EARTH

ACHERNAR

Magnitude 0.5

Blue-white B6 star

140 ly 640 ly

October 2017^ DISCOVER^67

CLOCKWISE FROM TOP LEFT: NASA/ESA/P. CHALLIS AND R. KIRSHNER/(HARVARD-SMITHSONIAN CFA); NASA/GSFC/ASU; ROEN KELLY/DISCOVER; NAS

A/ESA/Z. LEVAY (STS

CI)

Major the Great Dog. This luminary

shines at magnitude –1.47. Then look

40° north-northeast for Wasat. It

glows at magnitude 3.53, which makes

it exactly five magnitudes — or 100

times — fainter than Sirius.

Finding how much brighter one

object is than another certainly is

useful, especially for those of us who

write about observing. But there are

times when you might want to add

magnitudes, like when you know the

brightnesses of both components in a

double star system and you want the

total magnitude, or when two planets

(or a planet and a star) are scheduled

for a close approach.

Fortunately, our readers don’t shy

away from a tiny bit of math. To add

magnitudes, use this formula:

mc = m 2 – 2.5log(10x + 1)

Where mc is the combined

magnitude and m 1 and m 2 are the

magnitudes of the objects, and

x = 0.4(m 2 – m 1 ).

TYPES OF MAGNITUDES

When we look at a star, we see its

apparent magnitude (usually just

called magnitude), or how bright it

appears to us. But that tells us nothing

about its real brightness. Our eyes or

telescopes can’t tell us how big the star

is, how hot it is, or how far away it is.

Astronomers must figure out these

attributes in other ways.

An absolute magnitude exists

whereby we can compare objects to

one another directly. The absolute

magnitude, also called the luminosity,

of a star is the brightness it would

appear from a distance of 10 parsecs

(32.6 light-years). This is a valuable

number because it tells astronomers

SUPERNOVA 1987A 3.0

Stars of different real brightnesses can appear to be the same

brightness in our sky, as with Achernar and Betelgeuse, which both

shine at magnitude 0.5. Achernar, the brightest star in Eridanus, lies

some 140 light-years away. Betelgeuse, the ruddy supergiant in Orion,

is nearly five times more distant and intrinsically much brighter.

Apparent Brightnesses

SUPERNOVA 1006 –7.5 THE FULL MOON –12.7