My Notes Ask Your Teacher Studies of the relationship of the Sun to its galaxy t

Question

My Notes Ask Your Teacher Studies of the relationship of the Sun to its galaxy the Milky Way approximately 250 km/s around the galactic center have revealed that the Sun is located near the outer edge of the galactic disc, about 30,000 light years from the center. The Sun has an orbital speed of (a) What is the period of the Sun's galactic motion? yr (b) What is the mass of the Milky Way galaxy? kg (c) Suppose that the galaxy is made mostly of stars of which the Sun is typical. What is the order of the magnitude of the number of stars in the Milky Way? 1 × 1018 1 × 1011 1 x 1015 1 x1015 O 1 x 109

Explanation / Answer

r = 30,000 light years  

= (30,000 x 3 x 10^8 m/s) (365 x 24 x 3600 s)

= 2.838 x 10^20 m

v = 250 x 10^3 m/s

(A) T = 2 pi r / v = 7.133 x 10^15 s  

T = 2.262 x 10^8 years  

(B) G M m / r^2 = m v^2 / r

G M / r = v^2

(6.67 x 10^-11) (M) / (2.838 x 10^20) = (250 x 10^3)^2

M = 2.66 x 10^41 kg

(c) n = M / mass of sun

n = 1 x 10^11

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