Stargazing Celestial objects size comparison

Aug 31, 2021
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I wondered what are the sizes of celestial objects and decided to write this comparison.

It will be written in the form radius / diameter and followed by approximate mass. Every number in this post is estimation I have found on the Internet.

Earth 6.371 km / 12.742 km..... 5.9722 x 10`24 kg
Sun 696.340 km / 1.392.680 km..... 1.989 × 10`30 kg
Milky Way galaxy 52.850 ly / 105.700 ly..... 1.5 trillion solar masses (2 x 1.5 trillion x 10`30kg)
(note: there are various estimates for radius of the Milky Way so this is just provisory)
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Neutron star 10 km / 20km..... 1.4 solar mass
Stellar black hole 3 km / 6 km..... 1 solar mass
Supermassive black hole Sagittarius A 22.000.000 km / 44.000.000 km..... 4 million solar masses.

Conclusion:

Typical neutron star has 1.4 mass of the Sun but it occupies 70.000 times less space making it 70.000 more dense than our Sun.

Stellar black holes are even more dense than neutron stars by the factor of 3.

You want layman comparison? I got a good one: take A4 paper and fold it 70 thousand times to get a sense of a density of a neutron star, or 210.000 times for stellar black holes, compared to the Sun.

Supermassive black hole has 31 times bigger diameter than Sun which sole fact would make us think that it is a huge object. Well, it is, but we must take the mass difference into the account as well: 4 million solar masses in the 31 times bigger diameter or 129.032 higher density than Sun.

Therefore, the density ration between Sun, neutron star, stellar and supermassive black hole is, roughly speaking, about 1 : 70.000 : 210.000 : 129.032 with proportionally stronger gravity.
 
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Aug 31, 2021
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In addition to my "Celestial objects size comparison" post, I calculated how much one tea spoon of neutron star weights. I didn`t want to use available data but made my own figure. As much as you can, always try to find your own example, instead of using already existing ones made by someone else, because in that way you will remember better.

I am starting with previously determined density ratio between Sun and neutron star which is 1:70.000 (one : seventy thousand), for a neutron star with 20km diameter. Mass of the Sun is 2 x 10`30 kg and we divide it with 70.000 to get neutron star mass per km, further division with 1000 will give us neutron star mass per meter, further division with 30 will give us neutron star mass per 3.3cm which is approximately the size of a teaspoon. The result I get is 10`21 kg per 3.3cm. Now I am checking different masses on the Internet to find good comparison. I found it here on space.com: weight of the moon is 7.35 x 10`22 kg. Hence, one teaspoon of a neutron star weights as a moon.

I used 3.3cm teaspoon for calculation. There are different sizes of the teaspoon and results will slightly vary for each.

This numbers are just for illustration purposes. To get exact data we should take volume into the account not just diameter.

Of course, weight is not the same as mass but I sometimes say weight nevertheless. Actually I mean mass.

One teaspoon of a neutron star has a mass of the moon. :)
 
Nov 19, 2021
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Mass of Sun is 2E30 kg.
Mass of Moon is 7E22 kg.
Mass of typical neutron star of 1.4 Solar masses equal 2.8E30 kg.
Diameter of neutron star is 20 km or 2E6 cm, radius is 1E6 cm..
Volume of neutron star is 4/3 * pi * r^3.
Radius is 1E6cm. Volume is thus 4E18 cubic cm.
Mass of 3 cc is thus 3 * 2.8E30 / 4E18 cc or 2E12 kg
This is one part in 35 billion of the Moon's mass.
Your error appears to be when you divide by diameter, you need
to be dividing by the cube of diameter.
 
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Aug 31, 2021
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I wrote: This numbers are just for illustration purposes. To get exact data we should take volume into the account not just diameter.

You reply: Your error appears to be when you divide by diameter, you need
to be dividing by the cube of diameter.


You actually repeated my words.
 
Nov 19, 2021
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Sorry to rub it in but I am not going to state that you were off by a factor of 35 billion without mentioning why.
 
Last edited:
Jan 28, 2022
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I wondered what are the sizes of celestial objects and decided to write this comparison.

It will be written in the form radius / diameter and followed by approximate mass. Every number in this post is estimation I have found on the Internet.

Earth 6.371 km / 12.742 km..... 5.9722 x 10`24 kg
Sun 696.340 km / 1.392.680 km..... 1.989 × 10`30 kg
Milky Way galaxy 52.850 ly / 105.700 ly..... 1.5 trillion solar masses (2 x 1.5 trillion x 10`30kg)
(note: there are various estimates for radius of the Milky Way so this is just provisory)
_____________________________________________

Neutron star 10 km / 20km..... 1.4 solar mass
Stellar black hole 3 km / 6 km..... 1 solar mass
Supermassive black hole Sagittarius A 22.000.000 km / 44.000.000 km..... 4 million solar masses.

Conclusion:

Typical neutron star has 1.4 mass of the Sun but it occupies 70.000 times less space making it 70.000 more dense than our Sun.

Stellar black holes are even more dense than neutron stars by the factor of 3.

You want layman comparison? I got a good one: take A4 paper and fold it 70 thousand times to get a sense of a density of a neutron star, or 210.000 times for stellar black holes, compared to the Sun.

Supermassive black hole has 31 times bigger diameter than Sun which sole fact would make us think that it is a huge object. Well, it is, but we must take the mass difference into the account as well: 4 million solar masses in the 31 times bigger diameter or 129.032 higher density than Sun.

Therefore, the density ration between Sun, neutron star, stellar and supermassive black hole is, roughly speaking, about 1 : 70.000 : 210.000 : 129.032 with proportionally stronger gravity.
Size can mean a lot of things like distance across or how much stuff in it. It also depends on your viewpoint like an atom nucleus must look pretty big to an electron.
 

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