October 2017 Discover

October 2017^ DISCOVER^69

a fireball. Numerous comparison

stars exist in the 0–6 magnitude

range. The opposite is true for

negative magnitudes. Experienced

meteor watchers use Jupiter, Venus,

and phases of the moon to help in

their estimates.

Within the apparent/absolute

divisions, magnitudes in different

wavelengths are possible. Blue

magnitude, a remnant of blue-

sensitive photographic emulsions,

is one; red is another. Astronomers

even measure magnitudes in invisible

light — ultraviolet and infrared.

Another thing you may see that’s

related to magnitudes is color index.

To determine this, researchers compare

the magnitudes of a star through two

different filters.

The most widely used color index in

astronomy comes when you subtract

a star’s visual magnitude from its

blue magnitude, often written as B–V.

When this value is large, the star is red,

and when it’s small or negative, the star

is blue. The working range of color

indices for stars is about –0.5 to 2.5.

ESTIMATING LIMITING MAGNITUDE

When you’re out observing, you

should always estimate your site’s

limiting magnitude.

This does three things: First, it

reinforces the quality of the sky.

Second, it gives you a way to compare

your observations with others for

months or years, assuming you keep

an observing log. And third, as you

make more estimates, you’ll become

a better observer, more conscious

of little details.

Most limiting magnitude estimates

are done by eye. Some observers

have used a telescope, but only for

special observations or those of

really faint stuff.

Through the years, amateur

astronomers have estimated limiting

magnitudes in three ways. Today,

most values come from observations

of stars near the zenith, directly

overhead. This region lies farthest

from any light pollution, and you’re

also looking through the least amount

of atmosphere.

Some older observers still use

a system called the North Polar

Sequence. As its name suggests, you

simply look at stars near Polaris and

determine the faintest one you can see.

The third technique is rare, but it’s

more accurate than the other two for a

specific task. Let’s say you’re studying

an object far from both the zenith and

Polaris. It may benefit you to estimate

the magnitude limit near the object.

If you do, take our atmosphere out of

the mix by making your estimate at

the same altitude as your target.

A FINAL THOUGHT

Observers take magnitudes into

account in a variety of ways, from

determining their telescope’s cutoff

limit to deciding if a certain object is

worth spending time on. When you’re

with a beginner, however, remember

to point out that magnitudes are

counterintuitive. The smaller the

number, the brighter the object.^ D

Michael E. Bakich is a senior editor at
CLOCKWISE FROM TOP LEFT: NASA/JPL-CALTECH/UCLA/MPS/DLR/IDA; TONY HALLAS; DAN CROWSON Astronomy magazine.

The constellation Ursa Minor and its Little Dipper

asterism offer a great area to compare naked-eye

magnitudes. The dipper’s seven stars range from

Polaris, at magnitude 2.0, to Eta (η) Ursae Minoris, at

magnitude 5.0. (In the photo, magnitudes are given

without a decimal point to avoid confusion with stars.)

Polaris (α)

20

δ

44

ε

42

ζ

43

γ

30

η

50

Kochab (β)

21

1. Average faintest star
   under good sky 6.5


2. Extreme naked-eye limit 7. 5


3. Faintest objects visible
   in 7x50 binoculars 9.5

2

3

MINOR PLANET VESTA AT MAXIMUM

BRIGHTNESS 5.2

OPEN CLUSTER M6 IN THE CONSTELLATION SCORPIUS

1