Our Local Stars

I'm my comments I got a pretty nice question from a gentleman named Peter who works at the Museum of Science and Technology in Syracuse, New York. He wanted to know how many stars systems there were within 100 light years of the Earth. Why? Well, the first radio transmissions on Earth have had around 100 years to travel out into the Galaxy, so any potential civilizations living in nearby star systems could have picked us up. Could intelligent shades of purple be listening to Marconi's first messages or watching us take our first steps on our neighbour in space? They might be, but first lets work out how many star systems there are. Peter does a pretty good calculation which goes like this:

A number of web sites list the closest 25 stars which fall within 12 light years of Earth. If star density is equal, I estimate that the sphere of radius of 100 light years would include ~ 14,600 individual star systems, which is a very small % of the number in the Milky Way.

NASA's Ask an Astrophysicist came up with the same answer as Peter back in 1998 using data from the Gliese Catalogue 3rd Edition (1991) to work out the average density of stars and then using that to work out how many stars there might be within a volume of radius 100 lightyears.

I love back-of-the-envelope calculations and this got me thinking. There have been a few projects over the last 20 years which have measured accurate distances for many nearby stars. The first of these which springs to mind is Hipparcos which measured positions for over 118,000 stars. However, the main Hipparcos catalogue only contains measurements for all stars over a certain brightness and that means that it misses stars that are faint and closer than 100 lightyears. Tycho would probably be more complete but the uncertainties on the distance measurements (actually the parallax) in that database can be quite large. NASA Ames produced a database named NStars back in 1998 to collect good quality data on all stars closer than 25pc (81.5 lightyears) as this would be useful for the future Terrestrial Planet Finder mission.

The NStars database contains 2633 stellar objects in 2029 systems. However, the authors think that this only represents about 30% of the true number of stars in that volume because there are a lot of faint stars out there that haven't been measured yet. Scaling up the volume they cover to a radius of 100 lightyears from 81.5 lightyears and using this 'best guess' we get an answer of around 12,500 systems (16,200 stars). Of course this is a bit hand-wavy and has pretty big uncertainties on it, but it is a handy rough guide. Putting it in context, the Milky Way galaxy (the one we live in) contains around 100,000,000,000 stars, so Peter is spot on when he says that 16,000 is a tiny fraction.

Getting back to the issue of other civilizations listening in to us, our radio leakage is actually quite weak and once you factor in those huge distances between stars, the chance of another civilization actually being able to detect anything more than 'noise' is pretty small. After all, with the largest of our radio telescopes you can pick up a signal equivalent to a mobile phone (cell phone) on Mars, but the nearest star other than the Sun is more than 500,000 times further away and the phone would appear around 300 billion times fainter from there! You would be hard-pushed to pick that up.

Of course all this was assuming the transmissions spread out into space in all directions. This is generally the case, but in 1974 there was a deliberate transmission directed at the M13 star cluster. This message was sent by the Arecibo telescope and covered quite a small patch of sky so the number of stars it will have passed so far is much, much less than 16,200. There still remains one question: would aliens appreciate The Simpsons?

Posted in astro blog by Stuart on Saturday 08th Jul 2006 (01:06 BST) | Permalink
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