# A simple astrophysics rough estimate exercise

• shanepitts
In summary, the given estimation method is a reasonable first approximation, but may fall short by at least an order of magnitude due to the assumptions made about the shape of the galaxy and the orbit of the Sun. Other methods of estimation may involve more complicated math and the use of other objects such as globular clusters to obtain a more accurate result.
shanepitts

## Homework Equations

(mv2)/r=(GMm)/r2 --------(1)
r≈8kpc≈2.47×1020m
v≈220kms-1

## The Attempt at a Solution

Using equation (1) and solving for M I got

M=(v2r)/G=(4.84×10102.47×1020)/6.67×10-11

M=1.79×1041 kg

P.S. This is not a homework assignment, it's an exercise from an introductory book I'm reading.

Your result looks fine for the given estimation method.

shanepitts
gneill said:
Your result looks fine for the given estimation method.

Thanks for the quick response

Would the given estimation method be appropriate?
Moreover, would you have any other Newtonian estimation methods to suggest?

shanepitts said:
Thanks for the quick response

Would the given estimation method be appropriate?
Moreover, would you have any other Newtonian estimation methods to suggest?
The method is a reasonable first approximation. It'll fall short by at least an order of magnitude though; Consider the actual shape of the galaxy as opposed to the assumed shape for the method. Also consider that the Sun does not orbit outside the entire galaxy, but rather within it: The galaxy is about 30 kpc across.

Other methods of estimation would involve much more effort and more complicated math. For example, the actual shape is more disk-like than spherical, and does not have a uniform density, making estimating the mass of the outlying portions tricky.

A better estimate might had by using the globular clusters which orbit outside the Milky Way as the "test particles", rather than the Sun.

theodoros.mihos and shanepitts
gneill said:
The method is a reasonable first approximation. It'll fall short by at least an order of magnitude though; Consider the actual shape of the galaxy as opposed to the assumed shape for the method. Also consider that the Sun does not orbit outside the entire galaxy, but rather within it: The galaxy is about 30 kpc across.

Other methods of estimation would involve much more effort and more complicated math. For example, the actual shape is more disk-like than spherical, and does not have a uniform density, making estimating the mass of the outlying portions tricky.

A better estimate might had by using the globular clusters which orbit outside the Milky Way as the "test particles", rather than the Sun.
gneill said:
The method is a reasonable first approximation. It'll fall short by at least an order of magnitude though; Consider the actual shape of the galaxy as opposed to the assumed shape for the method. Also consider that the Sun does not orbit outside the entire galaxy, but rather within it: The galaxy is about 30 kpc across.

Other methods of estimation would involve much more effort and more complicated math. For example, the actual shape is more disk-like than spherical, and does not have a uniform density, making estimating the mass of the outlying portions tricky.

A better estimate might had by using the globular clusters which orbit outside the Milky Way as the "test particles", rather than the Sun.

Thank you

## 1. What is "A simple astrophysics rough estimate exercise"?

A simple astrophysics rough estimate exercise is a scientific exercise that involves making rough calculations and estimations based on basic principles of astrophysics. It is often used to gain a better understanding of complex astronomical phenomena and to make predictions or hypotheses.

## 2. Why is a simple astrophysics rough estimate exercise useful?

A simple astrophysics rough estimate exercise can be useful in several ways. It allows scientists to quickly approximate values and solutions to complex problems, which can then be used as a starting point for more precise and detailed calculations. It also helps to develop critical thinking and problem-solving skills, and can provide a better understanding of the underlying principles of astrophysics.

## 3. What are some examples of simple astrophysics rough estimate exercises?

Some examples of simple astrophysics rough estimate exercises may include calculating the mass of a planet based on its gravitational pull, estimating the distance between two celestial objects using their apparent brightness, or determining the size of a star based on its temperature and luminosity. These exercises often involve using basic equations and principles from astrophysics, such as Newton's law of gravitation and the Stefan-Boltzmann law.

## 4. Can anyone do a simple astrophysics rough estimate exercise?

Yes, anyone with a basic understanding of astrophysics and the necessary equations can do a simple astrophysics rough estimate exercise. However, it is important to keep in mind that these exercises are only rough estimates and may not provide completely accurate results. They are meant to serve as a starting point for further calculations and should not be used as definitive measurements or data.

## 5. How can a simple astrophysics rough estimate exercise be applied in real-life situations?

A simple astrophysics rough estimate exercise can be applied in various real-life situations, such as astronomical research, space exploration, and even everyday life. For example, estimating the distance between two stars can help astronomers determine their relative positions and movements in the sky. Similarly, calculating the size of a planet can provide valuable information for space missions and potential habitats for extraterrestrial life. In everyday life, understanding basic astrophysics principles can help us make sense of astronomical events and phenomena, such as eclipses and meteor showers.

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