I saw this on Phil Plait’s blog the other day. I’ve watched it over and over. It is just mind blowing!

Watch it here.

How can one save a You Tube movie so that you can look at it off line?

If you Google “Youtube video download”, you will get lots of options. I have just tried this one - Youtube Catcher, and it works brilliantly. Just follow the instructions - paste the video address in the box and click download. You then right-click the link and save the file with a .flv extension. I played it using VLC media player, my preferred player.


Realplayer also does the job nicely. A “download tab” pops up when you hover over the Youtube clip.

Wandapec, many thanks for posting the link, it really is awesome.

But it did give me something to be provisionally sceptical about :wink: , and that is: how can astronomers be so sure of these sizes?

We cannot observe and measure the disk of these stars like planets through a telescope, with Betelgeuse as a possible exception.

We can gather the star’s spectrophotometric data. From that, the scientists can infer the chemical composition, and velocity. Paralax measurements will tell us the distance of closer stars. But size? How on earth can they measure that? Oh, and the mass for that matter.

I know I’m wrong, just not sure in what way. ???


In a nutshell, gravitational redshift. It’s a little more complicated than a direct calculation because it involves several unknowns that are solved for simultaneously as a system from a set of measured observables. Known binary and multiple star systems are often used to confirm calculated parameters.


Thanks 'Luthon for the links. Still complicated stuff and the details will forever escape me, but its nice to at least know the principle.

I’m just glad that we live in a day and age where people are clever and tenacious enough to come up with such data for everyone to marvel at. It makes the faint, unassuming fuzzy in the eyepiece so much more interesting when you can go look up the outrageously huge numbers that describe it!


I found this website that has slides that starts off slow. The measuring etc. starts on about slide# 6.

Hey I know this topic ended eons ago but this is truly awesome and merits reopening the topic and our minds even more. Interactive scale of the universe

If you’re on linux it’s downloaded to /tmp. Just copy it from there to somewhere else (e.g. your desktop) before you close your browser.