Zon Mosaic Pro5 User's Guide download pdf (Page 94)

94 Starry Night User’s Guide

•Other Data

The fields that appear in this category

depend greatly on the database that the

object is from. Different databases will

have different information fields.

Star Information Fields: For stars, the

following fields are present:

Variability: This indicates whether a star’s

apparent magnitude (and, therefore, its

brightness) varies with time, and by how

much.

Radius: This measures the star’s radius, in

terms of the Sun’s radius. Red giant stars

can be hundreds of times larger than the

Sun.

Double/Multiple: This field indicates

whether a star is part of a multiple star

system. About half of the stars in the sky

are part of a multiple star system.

Magnitude: This shows a star’s apparent

magnitude

, a value which measures its

brightness in the sky. The idea for

assigning magnitudes originated with

Greek astronomer Hipparchus (190-120

BC). He catalogued all the visible stars he

could see and assigned them magnitudes

from 1 to 6, the brightest stars having a

value of 1.

Contemporary astronomers have refined

Hipparchus’ system, so that the system

now includes larger numbers for dimmer

stars, and negative numbers for very bright

objects. The faintest stars that can be seen

by the Hubble Space Telescope are in the

26 to 28th magnitude range — very dim

stars indeed!

Note: Many newcomers to astronomy get

confused by the fact that the greater an

object’s magnitude, the dimmer it is. An

object with magnitude -5 is much brighter

than an object with magnitude 0, which in

turn is much brighter than an object with

magnitude 5.

The Sun has a magnitude of about -28,

while the brightest star in the night sky is

the dog star Sirius, which has a magnitude

of -1.47. The magnitudes of the planets

change, depending on how close they are

to Earth, but Venus, Mars and Jupiter can

all have lower magnitudes than Sirius. At

the other extreme, Pluto has a magnitude

of about 14, far too dim to be seen with the

naked eye, and even most small

telescopes.

Absolute Magnitude: The magnitude we

normally use when talking about objects is

the apparent magnitude, which compares

how bright they are as seen from Earth.

But this doesn’t tell us much about the

intrinsic brightness of an object, because

the bodies we see in the sky are spread out

over a wide range of distances. The Sun

produces the same amount of light as the

average star in the sky, but it is far, far

brighter than anything else simply because

it is so close to us.

Absolute magnitude

tells us how bright objects would appear if

they were all at the same distance (the

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