This week Harry’s been hard at work in the University of Hertfordshire, desperately searching for brown dwarf binaries and handling huge clumps of data in a room with no windows. He hasn’t seen sunlight in days, and is turning into some kind of astronomical mole-man. Will he find his pair of stars? Will he get to see sunlight? When did he start writing these little intros? Who is Keyser Soze? Read on to find out…
Monday-Tuesday (Attack of the Spreadsheets)
Who’d have thought that GCSE ICT would come in so handy? The past couple of days have presented me with such functional gems as “=AND(IF(BC4<3,1,0),IF(BF4<3,1,0),OR(AND(IF(BJ4>3,1,0),IF(BJ3>3,1,0)),AND(IF(BK4>3,1,0),IF(BK3>3,1,0))))”,
“=((F7+J7)/2)*(AV7/1000)*4.74” and the catchy little number “=10^((I6/5)+1)”.
Most of my days at the moment are spent on LibreOffice Calc, which is the Linux version of Excel. This means that it is free and it runs very smoothly, but occasionally the functions can go a little bit mad. I’m still having trouble trying to convince it that the cosine of 90° is 0, not 1.457849384E-17. So far I’m not doing very well, but hey-ho!
Wednesday-Thursday (Marvellous Magnitudes and Magical Moduli)
All this spreasheet manipulation hasn’t been in vain though, as we’re finally whittling down the list of candidate stars into something a bit more manageable. If last week was all about proper motion (and it was), then this week is all about colours, magnitudes and distance moduli. These things are essential for working out how far away from us a star actually is. After we’ve done this for a pair of stars, we can then calculate how far away they are from each other! Firstly, we have to consider the different “bands” that the star has been seen in. There are a great many bands of colour, through infra-red into visible and beyond, and these are labelled with letters from the alphabet, but not in alphabetical order (because scientists love to make things difficult). These bands are given as one letter minus another, for example “I-Z” or “J-H”. This is because the light is measured at a certain brightness in the J frequency, then a certain brightness in the H frequency. We can then see what the difference is to give us a band between these two frequencies. From these bands you can then find out the Absolute Magnitude of a star (this is the measurement of how bright a star would be if viewed from 10 parsecs* away, and allows us to have a well defined base value for the brightnesses of all stars) by matching the colour to a star of known magnitude on a chart. This was pretty laborious work, as I had over 200 separate entries, that I had to do twice (for reasons I will explain next week). As you can imagine, this took me a good day of estimating and eye-strain, but it was worth it in the end. Soon we had values for the maximum and minimum absolute magnitudes.
I then needed to get the Distance Modulus. This is a very important number that we use to work out the distance to a star. Luckily it’s really easy to work out! All you do is subtract the absolute magnitude from the apparent magnitude (how bright the star appears to be for us here on earth**), and viola! It’s then a simple matter of dividing it by five, adding one and putting 10 to the power of your new number. This gives you the distance to the star in parsecs, and since the scale is logarithmic (in powers of ten) a small difference in the modulus can make a very big difference in distance! Magnitudes are also logarithmic, so small differences there can make large differences later on. Basically, for such massive objects (stars and the like) you have to be ridiculously precise!
Once the distance has been worked out it takes some simple trigonometry (distance x angle of separation in arcseconds) to find out how far apart the stars are in real space! You can then begin to think about how likely it is that the stars are related, as the bigger the separation, the less likely they’re a pair. Often these objects are many thousands of Astronomical Units (distance from earth to the sun) apart, but some can still be linked together. It’s fantastic to think of two stars, so far apart, yet still whirling through the cosmos as a pair, inescapably linked by nothing more than the forces of gravity. Some of the distances are enough to make your head spin a little, though. In astronomy everything is (surprise-surprise) astronomically big!
It’s enough to make a chap want to take a siesta with a nice glass of Pimms, eh? Chin-chin!
Harry Saban – The Octave Doctor (Phd Pending)
*A parsec is an astronomical unit of distance measured using some clever trigonometry. As you should know, the earth goes round the sun, and this causes stars that are nearer to us to appear to move, relative to the stars that are very far away. This is called the stellar parallax and is used to work out parsecs and the difference is greatest every half a year, because the sun is on the opposite side of the “circle”. If a star appears to move one arcsecond in the sky, it is one parsec away. This means it forms a sort of triangle, with a baseline that is 2 AU wide, and a height of 1 parsec. Wikipedia has some good diagrams, if you’re finding this a bit tricky to visualise (it’s pretty odd).
**Or more precisely, from a satellite orbiting the earth. The brightness would be affected by the atmosphere down here on earth by all those pesky gas particles up there making mischief and generally having a whale of a time.