About one hydrogen atom per cm3 plus a small amount of dust. All in all, it accounts for about 5% of a galaxy’s total mass, which is a surprisingly large fraction.
'Luthon64
So then, given one hydrogen atom per cubic meter, what are the chances that a photon will collide during a 4.3 year journey through space?
Rigil
Yes, and how much “air” is there between us and Alpha Centauri? Presumably it’s negligible, because we can see stars much further away clearly.
And should that be “further away” or “farther away”? I can never remember what the rule is with those two words… 
I think the chance of a photon crashing into a hydrogen atom during a 4.3 light year journey is vanishingly small.
http://forum.skeptic.za.org/fun/interesting-estimates/?action=dlattach;attach=942
1/cm3, not 1/m3 
Ah ok, thanks. That comes to, what, 1,000,000 H-atoms/m3 ? Which improves the chances of a collision to 10-19 to 1 against. Still a pretty safe bet.
Yet another excuse for riding around on a motorcycle is to engage in an alphabet run. The idea is to visit twenty-six towns, each starting with a different letter of the alphabet, and take a picture of your bike at the town’s Welcome To sign. A longer version of the run can be completed sequentially, meaning that you start in a town beginning with A, then you ride (on roads only) to the next town starting with a B, and so on, until ending up at a town starting with Z.
Now, if you were to start off in Adelaide in the Eastern Cape, estimate the minimum distance you have to travel to complete a sequential South African alphabet run.
Rigil
At the rate they are changing place names in this country, you’ll never complete it; by the time you get to your next town on your itinerary, the name will have changed.
Or alternatively, you simply stay in Adelaide, and the name will change through the whole alphabet. And don’t worry, they will put up new Welcome to signs. Judged by what happened here in Pretoria when they changed street names,that is the one thing they really are highly effective at - the new street names were up within a day or two of the decision being made.
LOL; yes that could be a very feasible solution. While hunting for local towns beginning with I - somewhat of a rarity - I was happy to come across Idutywa, only to find it was changed to Dutywa in 2004! Looks like it’ll have to be Irene now, or with a bit of licence, Ibayi.
Rigil
Not to spoil your fun, but Irene isn’t a town. It’s a suburb of Centurion, Pretoria. Try Ixopo instead.
'Luthon64
Thanks … I was in Irene many moons ago to attend a cheese making course. Must have mistaken it for a town in its own right.
Rigil
To date, SAB has reached 7303 taverns and distributed 10 456 600 condoms to them over a period of a year. This has assisted in averting 20,914 new HIV/Aids infections.And not 20 915, mind you. ::):/
Rigil
I have to wonder how many cases of HIV are caused by booze in the first place.