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The Sun, which is \(2.2 \times 10^{20} \mathrm{~m}\) from the center of the Milky Way galaxy, revolves around that center once every \(2.5 \times 10^{8}\) years. Assuming each star in the Galaxy has a mass equal to the Sun's mass of \(2.0 \times 10^{30} \mathrm{~kg},\) the stars are distributed uniformly in a sphere about the galactic center, and the Sun is at the edge of that sphere, estimate the number of stars in the Galaxy.

Short Answer

Expert verified
Approx. 100 billion stars.

Step by step solution

01

Calculate the Angular Velocity

The angular velocity, \( \omega \), can be calculated using the formula: \( \omega = \frac{2\pi}{T} \), where \( T \) is the period of one complete revolution. Here, \( T = 2.5 \times 10^{8} \) years. First, we convert years into seconds, knowing there are \( 3.15 \times 10^{7} \) seconds in a year: \[T = 2.5 \times 10^{8} \times 3.15 \times 10^{7} \ = 7.875 \times 10^{15} \text{ seconds} \]Then, substitute this \( T \) value into the angular velocity equation:\[\omega = \frac{2\pi}{7.875 \times 10^{15}} \approx 7.98 \times 10^{-16} \text{ rad/s} \]
02

Use Centripetal Force Equation

The centripetal force keeping the Sun in orbit is provided by the gravitational force. They can be equated as follows:\[m v^2 / r = \frac{G M m}{r^2} \]Here, \( v \) is the orbital velocity, \( r \) is the distance from the center of the galaxy to the Sun, \( M \) is the total mass within the Sun's orbit, and \( G = 6.674 \times 10^{-11} \ m^3 \ kg^{-1} \ s^{-2} \) is the gravitational constant. Since \( v = r \omega \), substitute \( v \) as:\[r^2\omega^2 = \frac{G M}{r} \]Simplifying gives:\[M = \frac{r^3 \omega^2}{G} \]
03

Substitute Values and Calculate Total Mass

Substitute known values:- \( r = 2.2 \times 10^{20} \text{ m} \)- \( \omega = 7.98 \times 10^{-16} \text{ rad/s} \)- \( G = 6.674 \times 10^{-11} \ m^3/kg/s^2 \)Into the mass equation:\[M = \frac{(2.2 \times 10^{20})^3 \times (7.98 \times 10^{-16})^2}{6.674 \times 10^{-11}} \]Calculate:\[M \approx 1.98 \times 10^{41} \text{ kg}\]
04

Estimate the Number of Stars

To find the number of stars, divide the total mass by the mass of one star (the Sun's mass):\[\text{Number of stars} = \frac{M}{M_{\text{Sun}}} = \frac{1.98 \times 10^{41}}{2.0 \times 10^{30}} \]Calculate the result:\[\text{Number of stars} \approx 9.9 \times 10^{10}\]
05

Conclusion

From our calculations, the estimated number of stars in the Milky Way galaxy is approximately \(9.9 \times 10^{10}\), or around 100 billion stars.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Angular Velocity
Angular velocity is a measure of the rate at which an object rotates around a certain point or axis. In simple terms, it describes how fast an object spins. The angular velocity, often denoted by the Greek letter \( \omega \), is calculated by the formula \( \omega = \frac{2\pi}{T} \), where \( T \) is the period of one complete revolution or cycle. To comprehend this, consider the "period" as the time taken for one full orbit around the center. When dealing with such vast scales as the Sun revolving around the Galactic center, units are crucial. Here, we initially convert from years to seconds, acknowledging the expansive nature of celestial movements. The period in seconds lets us accurately compute \( \omega \), providing the angular velocity in radians per second, which is a unit suitable for such calculations involving rotational dynamics.
Centripetal Force
Centripetal force is the inward force required to make a body follow a curved path. Its role is vital in the context of stellar dynamics, where it keeps celestial bodies, like stars and planets, in their orbits.For the Sun's orbit around the center of the Milky Way, the gravitational force supplies the necessary centripetal force. The equation \( m v^2 / r = \frac{G M m}{r^2} \) helps equate this. On the left side, \( m v^2 / r \) represents the centripetal force, with \( m \) being the mass of the Sun and \( v \) its orbital velocity around the Galactic center.Meanwhile, the right side, \( \frac{G M m}{r^2} \), accounts for gravitational force where \( G \) is the gravitational constant, and \( M \) is the mass inside the Sun's orbit. This equation cleverly links the velocity of the body with the gravitational effect exerted by the total mass inside its orbit.
Gravitational Force
Gravitational force is the attractive force between two masses. It's the reason why planets, stars, and galaxies hold together in stable formations. Here, the core concept deals with the gravitational force acting as the centripetal mechanism that enables the Sun to orbit the Galactic center.Newton’s law of universal gravitation is expressed in the equation \( F = \frac{G M m}{r^2} \), where:- \( F \) represents the force of gravity- \( G \) is the universal gravitational constant- \( M \) and \( m \) are the masses of the two objects- \( r \) is the distance between the centers of the two massesIn the case of the Sun orbiting the Milky Way’s center, the gravitational force exerted by the mass of the stars and other materials within its orbit provides the necessary centripetal force to keep the Sun in motion around the Galactic center.
Orbital Velocity
Orbital velocity is the velocity a body must maintain to orbit around another body without descending into it or flying away. In terms of stellar dynamics, it is crucial for understanding how stars move around the galactic center.The relationship between orbital velocity \( v \) and angular velocity \( \omega \) is defined through the equation \( v = r \omega \). Here, \( r \) is the radius—the distance from the Sun to the center of the Milky Way.Comprehending orbital velocity allows us to realize how gravitational pull and the speed of a celestial body interact. At our distance from the Milky Way center, this delicate balance between centripetal and gravitational forces keeps the Sun, our "stellar anchor," in a stable orbit. Hence, precise calculations of \( v \) are fundamental for estimating the total mass of orbiting bodies or stars through the expression \( M = \frac{r^3 \omega^2}{G} \). This gives insights into the sheer scale and mass distribution in galaxy dynamics.

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Most popular questions from this chapter

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