Universe: Stars And Galaxies
Universe: Stars And Galaxies
6th Edition
ISBN: 9781319115098
Author: Roger Freedman, Robert Geller, William J. Kaufmann
Publisher: W. H. Freeman
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Chapter 8, Problem 47Q
To determine

(a)

The angular distance between the star and its planet as seen from Earth.

Expert Solution
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Answer to Problem 47Q

The angular distance between the star and its planet as seen from Earth is 1.06arcsec.

Explanation of Solution

Given:

The distance of the star from the Earth is d=170ly.

Formula Used:

The small angle formula is given by

α=D(206265)d

Here, D is the angular diameter.

Calculations:

The small angle is calculated as

α=D( 206265)d=( 55au)( 206265)( 170ly× 63240au 1ly )=1.06arcsec

Conclusion:

The angular distance between the star and its planet as seen from Earth is 1.06arcsec.

To determine

(b)

The orbital period of the planet and whether it is possible for the astronomer to observe a complete orbit in one lifetime.

Expert Solution
Check Mark

Answer to Problem 47Q

The orbital period of the planet is 2579.73yr and it is not possible for the astronomer to observe a complete orbit in one lifetime.

Explanation of Solution

Given:

The mass of the star is 0.025 times that of the Sun; that is, M=0.025MSun.

Formula Used:

As per Kepler’s third law, the relation between the orbital period and the orbital distance is given by

P2=(4π2GM)a3

Here, P is the period, G is the gravitational constant and a is the orbital distance.

Calculations:

For the Sun, the relation is given by

PSun2=(4π2GM Sun)aSun3 …… (I)

For the star 2M1207, the relation is given by

P2M12072=(4π2GM 2M1207)a2M12073 …… (II)

Divide equation (II) by equation (I).

P 2M12072P Sun2=( 4 π 2 G M 2M1207 )a 2M12073( 4 π 2 G M Sun )a Sun3P2M12072=( M Sun a 2M1207 3 M 2M1207 a Sun 3 )PSun2P2M1207=( M Sun a 2M1207 3 M 2M1207 a Sun 3 )P Sun2=[ M Sun ( 55au ) 3 ( 0.025 M Sun ) ( 1au ) 3 ] ( 1yr )2

Solve further,

P2M1207=6655000 yr2=2579.73yr

Conclusion:

The orbital period of the planet is 2579.73yr and it is not possible for the astronomer to observe a complete orbit in one lifetime.

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