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### the Mathematics of the Milky Way

Posted: **Thu Oct 08, 2015 4:52 pm**

by **AlanConroy**

I am guessing that this is the best place for this question and apologies if not. I am trying to learn the maths behind astronomy. I am told by the OU book 'Introduction to Galaxies and Cosmology' that the estimated mass of the Milky Way is 10 to the power 12 x the mass of the sun. It then goes on to say that the estimated mass of the central bulge is 10 to the power 10 x the mass of the sun. Further the mass of the stellar halo is ONLY 10 to the power 9 x the mass of the sun. Surely the bulge and halo estimates exceed the total estimated mass? Sorry if the answer is obvious but I have spent my life dealing with words and O Level Maths is where I stopped. Could it be the book is just wrong?

### Re: the Mathematics of the Milky Way

Posted: **Fri Oct 09, 2015 6:12 pm**

by **David Frydman**

Hi Alan, welcome.

10 to 12th is more than 10 to 9th or 10th.

Regards, David

### Re: the Mathematics of the Milky Way

Posted: **Fri Oct 09, 2015 6:32 pm**

by **AlanConroy**

Thank you David

Yes I know that but 10 to the 12th is significantly less than 10 to the 9th and 10 tk the 10th combined. The book suggests that 2 components of the Milky Way have greater mass than the estimated total mss

### Re: the Mathematics of the Milky Way

Posted: **Fri Oct 09, 2015 6:49 pm**

by **stella**

No, combining those two gives 1.1x 10^10, which is less than 10^12.

### Re: the Mathematics of the Milky Way

Posted: **Fri Oct 09, 2015 7:48 pm**

by **LeoLion**

This Wiki may give you more of a feel for the 'powers of ten'

https://en.wikipedia.org/wiki/Power_of_10 .Hope this is a help

### Re: the Mathematics of the Milky Way

Posted: **Fri Oct 09, 2015 8:26 pm**

by **AlanConroy**

Thanks everyone. Stella I went back to basics and did it from first principles and now see how it works. I have much work to do