Comments on: Amassing Evidence

By: Alan

Alan — Wed, 10 Jun 2015 23:31:43 +0000

Absolutely – I just wanted to confirm I was reading the abstract correctly – and part of me was concerned we had suddenly lost all those stars! 😉

By: cassiopeiancustardcream

cassiopeiancustardcream — Tue, 09 Jun 2015 22:08:51 +0000

Hi Alan, you’re quite correct, the abstract of the paper by Küpper et al. does state that “.. we determine the Galactic mass within Pal 5’s apogalactic radius of 19 kpc to be (2.1±0.4)×10^{11} M⊙…”
As you point out, this is around 210 billion +/- 40 billion stars.
Given the huge service to science outreach performed by Dr Koberlein, and the monumental volume of science articles he continues to author, day after day, I think we can forgive the error in decimal places on this occasion.
Besides, I think the main point of Brian’s article is to show how the seemingly unrelated topic of evaporation in globular clusters can be reliably used as a guide to the total number of stars in the Milky Way.
Incidently, I was also greatly impressed with the other calculations made by Küpper et al. who found that the distance of the Sun from the Galactic Center is 8.30±0.25 kpc, and the transverse velocity of our solar system is 253±16 km/s.

By: Brian Koberlein

Brian Koberlein — Tue, 09 Jun 2015 22:03:02 +0000

Yeah, I dun goofed. I’ve corrected it now.

By: Alan

Alan — Tue, 09 Jun 2015 21:27:23 +0000

I was thinking 2.1 billion seemed so much smaller than 100-400 billion that I tried a basic web search to find the paper. In an abstract I caught a reference to 2.1 +/- 0.4 10^11 Mo (can’t print the little circular symbol for solar) – this looks like 210 billion +/- 40 billion (i.e. 170-250 billion) solar masses to me. This is much closer – is this correct? If so, I’d say 200 billion stars is a good approximation for the number of stars in the Milky Way – if I’m reading this correctly.

Really enjoying your posts as always. Was funny I was just attempting to explain our solar system’s general location within the Milky Way and how we can only see relatively few stars with the naked eye at night – and lo and behold your post yesterday shows very nicely roughly how far “out” into the Milky Way we can see visually! Thanks! 🙂

By: Jpatrick

Jpatrick — Tue, 09 Jun 2015 17:43:39 +0000

I guess that estimate could be refined by applying a couple other things which are probably known.

2.1 billion solar masses of dust + gas + stars. Not sure how much is stars and how much is dust and gas.

Then, it’s necessary to apply a frequency distribution of stellar masses to the portion that exists as stars. If that distribution is skewed toward lower mass stars, the estimate 2.1 billion might be low.