   Chapter 11, Problem 68P ### University Physics Volume 3

17th Edition
William Moebs + 1 other
ISBN: 9781938168185

#### Solutions ### University Physics Volume 3

17th Edition
William Moebs + 1 other
ISBN: 9781938168185
Textbook Problem

# The Andromeda Galaxy is the closest large galaxy and is visible to the naked eye. Estimate its brightness relative to the Sun, assuming it has luminosity 1012 times that of the Sun and lies 0.613 Mpc away.

To determine

The brightness of Andromeda Galaxy relative to that of the sun.

Explanation

Given:

The luminosity of Andromeda ( LA ) = 1012 x Luminosity of the Sun ( LS )

The distance of Andromeda from Earth ( dA ) = 0.613 Mpc

Formula used:

The apparent brightness of the sun as observed from the earth is, BS=LS4πds2 where ds the distance of the sun from Earth is.

The apparent brightness of Andromeda as observed from the earth is, BA=LA4πdA2

So the relative brightness of Andromeda with respect to the sun is given by,

Relative Brightness = Apparent Brightness of AndromedaApparent Brightness of Sun

So, RB=BABs or, RB=LALs×( d s d A )2

Calculation:

As per the formula, relative brightness is given by,

RB=LALs×( d s d A )2

We know the distance of the sun from Earth as, ds=1.496×1011 m

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