The Four Ages Of Sand

The ABC of Spectroscopy

As promised (threatened?) here's my attempt to explain the basics of spectroscopy:

 

                                                                          A

As talked about in a previous post, hot things emit radiation. This radiation has long wavelengths for objects less than a few hundred degrees C, and it's not visible to the naked eye.

This radiation is called the Infrared part of the spectrum, and you need a special camera to see it. That's how  you can see criminals glowing in the dark when filmed running across gardens in the dead of night in those Police Stop! videos. When objects get really hot, the wavelengths of the radiation gets shorter, and at a certain point we can see things start to glow. If you've seen glass or metal in a furnace or Bunsen Burner, you'll know what colour it starts glowing; a deep red colour. As it gets hotter still it turns orange, yellow, and eventually blue/white. When it's completely white, most of the radiation is in the very short, (and invisible to us) wavelengths of the ultraviolet part of the spectrum.

 The first important point here is that a hot body emits a continuous spectrum. In other words it produces a whole rainbow of colours without any spaces in it. However it emits the majority of its light in the part of the spectrum proportional to its temperature. 

Summary: the colour of a hot body tells you its temperature. 

For more, see here: http://en.wikipedia.org/wiki/Black_body 

 

                                                                             B

Point A is really great new for astronomers because of course they have no way of visiting a star for the foreseeable future, but simply by looking at a star's colour they can work out its temperature. You can do the same on any clear night; look at the stars and impress your friends by pointing out that the bluer stars are hotter than the yellow ones, and the yellow ones are hotter than the red. 

Spectroscopists just take things a little bit further. By spreading the continuous spectrum of light out, they can identify the exact wavelength where most of the energy is being radiated, and using a simple formula, give you the star's temperature. Actually there are other factors to take into account but that's the basic idea. 

Here's my spectrum of the star Betelgeuse in Orion.  Without any processing you can see that it's brighter at the red end than the blue, which tells you it's a colder star than a blue one. 

And again repeating a previous post,  young stars are hot and old stars are cooler, so you can tell what point a star is in its lifecycle. Compare Sirius to Betelgeuse:

You can see that Sirius is a hot young pup, and Betelgeuse is a tired old dog. 

 

The colour of a star tells you its temperature and its age

 

                                                                     C

If that was all that spectroscopy could do, it wouldn't be the impressive science it is. In fact ,it's been argued that spectroscopy is responsible for 75% of all of our knowledge of the Cosmos. 

How has it managed to achieve this? By a single lucky effect resulting from Quantum Mechanics.  Coming soon...

First graph of Betelgeuse spectrum; thank goodness for RSpec.

I spent some time trying to get to grips with the software that will allow me to read the spectra of the stars I photographed the other day.

I tried IRIS and VSpec, and quite quicky found that the learing curve was too steep for me to climb up. Then I tried RSpec and it all became much easier. Here's my first graph! It's not calibrated yet, so I can't tell you what it all means right now. I was about to make a breakthrough at 6.00 this morning then I had to get ready for work ;-(

I don't want to say too much at the moment, but I do believe I'm on the verge of discovering that stars are made of hydrogen!

Yes, I DO realise that boffins discovered that years ago, but has it occurred to YOU that I'm catching up with them??

For my other RSpec experiments see the Spectroscopy link in Categories (left hand sidebar) 

The Joy of Spectroscopy (finally). First ever spectrum of starlight!

I should have known that when I said on Friday "The skies are clear and nothing can stop me now."(previous post) this was sure to bring the curse of 4AoS down upon me, and indeed the clouds immediately rolled in and stayed rolled in for 4 days. 

Last night the BBC predicted total cloud cover with not the slightest chance of a break, so of course the skies were clear. As is almost always the case, the rare occasions when the clouds do clear also almost always coincides with the rare occasions when you have other things planned, and so it was last night. But I DID get half an hour to myself to drag the telescope out, attach the SA100 Star Analyser, and set up the camera,  and here is my first ever spectrum of starlight:

The star, visible on the right side, is Betelgeuse. 

The spectrum is quite pretty but it's also a long way away from revealing any useful information. The first problem is that it's very overexposed so the subtle detail is lost.

Here's a later attempt of the star Sirius:

This shows another problem which is that the firing of the shutter wobbles the camera, blurring the image (see the star isn't sharp and the line of the spectrum isn't clear) 

After Photoshop:

The really important information revealing the chemical composition of the star is contained in the dark lines (more on that later) but if we compare the spectrum to a better one of Betelgeuse, we can already see something interesting:  

 The spectra look quite different.

What's going on here? 

If you take a metal bar and heat it up the atoms gain energy and they give off radiation with very long wavelengths. This is infrared radiation and with an infrared camera you can see warm objects glowing in the dark. As it gets really hot it will begin to radiate more of its energy in the shorter wavelengths and the bar will glow in the visible spectrum. What colour? A deep red.

As it gets even hotter the body will radiate more energy at shorter wavelengths and it will become more orange-coloured, then yellow, and eventually a blue-white colour. At this point it is giving off most of its energy in the green and blue part of the spectrum.

A new star is very hot and gives off a lot of energy, then as it get old, like most of us, it produces less energy. So by looking again at the 2 spectra together, what can you say about Sirius and Betelgeuse?

Careful now, there may well be a short written test on this stuff in the future.

 

 

I'm sure you noticed that Sirius is much brighter in the higher end of the green and blue area, and much dimmer at the red end. Betelgeuse on the other hand is very bright at the red end and the light falls off at green and blue end.

So which is the old star and which is the young one?

Sirius is very young in star years; 200 - 300 million years old, and it's very hot; around 10,000K at the surface. Betelgeuse on the other hand is right at the end of its life and is around as third as hot as (Sirius  3000 - 3500K)  It is around 10 million years old, which is also very young for a star, so the fact that's it's on its last legs is something of a puzzle. It's probably evolved quickly because of its vast mass. Live fast die young.

Now I need to get to grips with the software which will allow me to callibrate the images and turn them into proper date. Watch this space, though it make take time; I read that the learning curve is steep...

The magic of the Internet, Yahoo User Groups and the SA100

Have I mentioned before how great the Internet is? ;-)  The magic of the times we live in has been impressed upon me yet again recently as I prepare to become a spectroscopist.

I got excited by the idea of spectroscopy 2 weeks ago (see previous post) and I've been busy educating myself ever since; not a difficult task, since it turns out I knew almost nothing about it. 

I received my SA100, which is a very fine grating, designed to split up star light into a spectrum. It goes between a CCD and a telescope, but when I got mine I couldn't find any way of attaching it.

 

I have lots of adaptors and connectors, but still couldn't find a way:

 

 

So I joined a Yahoo User Group. There are thousands of these forums on every subject imaginable, and you are instantly connected with a whole range of knowledgeable, enthusiastic and extremely helpful and friendly people. Whatever you are into, there will be a user group. Try it: 

http://groups.yahoo.com/ 

Actually, there are several spectroscopy groups, even one specifically for the SA100, and one of the guys who have been helping me turns out to be its designer. 

They explained to me that what I was missing was a 1.25" nosepiece (I felt such a fool...)

so it was back to the internet to source one. 

It allowed me fit the SA100 to the camera, but I still could get it onto the scope:

 

 Again, they talked me through it, and finally I got it (I wasn't using my eyepiece adaptor to connect to the field flattener. It's a long story, and probably not of great interest...)

Now I'm good to go. The skies are clear, the batteries are charged, the tripod is aligned, I've downloaded the software ready for processing the images, and nothing apart from lack of skill, knowledge and ability can stop me now.

 The thought that I'm about to analyse starlight to discover what the star is made of is quite incredible I think.  In the long run you can use this set up to measure red shift of galaxies and quasars, which will prove that the Universe is expanding. And all from a garden in Hatfield; who'd have thought?  

Make your own spectrogram for free

I'm too excited to be able to wait for delivery of the equipment (see previous post) which will enable me to get on with becoming an Astronomical Spectroscopist (all those wasted years!) so I looked around on the Net to see if it was possible to make your own simple spectroscope. It is!

Spectroscopy is the science of analysing light. To analyse the light you need to split it up into its constituent parts. You can do that with a prism, a la  Pink Floyd: 

or you can use a diffraction grating. This has a number of technical advantages and is much cheaper to produce, so that's what most spectroscopes use. 

What is diffraction? When you push light through a small hole or slit, it "bends around" the edges. If you have lots of lines close together - a grating - you spread the lightwaves out to create pretty, and incredibly useful, patterns of light. 

Where can you find a diffraction grating with millions of lines?

You probably have one in the room you are in right now. And it's ideal for the job because each line is separated from the next by only 1.6 micrometers. This means there are around 625 lines per millimeter, which is in the range of ordinary laboratory diffraction gratings. 

It's called...

a CD. Hold it up to the light and you'll see flashing rainbows of colour. Congratulations, you are already a hands-on spectroscopist!

Ok, now we need to build our spectroscope. 

We can make it out of card using the plans here:   
I made mine following the instructions on this excellent site:

https://pantherfile.uwm.edu/awschwab/www/specweb.htm

 

 

And here are the results:

This is the spectrum from a table lamp:

 

And this is the sky:

Yes, I've proved that the sky is bluer than a light bulb. More major discoveries to come!

 

The Joy of Spectroscopy

How do we know what the universe is made of? We've been to the Moon, a comet, and a few of the planets, so we've been able to analyse them directly. But what about the Sun, the stars, galaxies, nebulae etc? How do we have any knowledge of the composition of the rest of space? And how do we know that galaxies are rushing away from us at high speed, thus demonstrating that the Universe is expanding? 

It was understood even back in the 19th Century that stars were trillions of miles away, and that since we would never be able to visit them, our knowledge of them would always be restricted and that chemical knowledge of the stars was something that mankind could never possess.  

"On the subject of stars, all investigations which are not ultimately reducible to simple visual observations are ... necessarily denied to us. While we can conceive of the possibility of determining their shapes, their sizes, and their motions, we shall never be able by any means to study their chemical composition or their mineralogical structure ... Our knowledge concerning their gaseous envelopes is necessarily limited to their existence, size ... and refractive power, we shall not at all be able to determine their chemical composition or even their density... I regard any notion concerning the true mean temperature of the various stars as forever denied to us."

 

Auguste Comte, 1842

Comte couldn't have been more wrong:

...only 14 years later, the physicist Kirchhoff discovered that the chemical composition of a gas could be deduced from its electromagnetic spectrum viewed from an arbitrary distance. This method was extended to astronomical bodies by Huggins in 1864, who first used a spectrograph attached to a telescope. Not only have we learned how to determine the chemical composition of the stars and nebulae, but the element helium (the second most abundant in the universe) was first identified in the spectrum of the Sun, rather than in an earthbound laboratory. 
Today we use spectroscopy to measure chemical abundances, temperatures, velocities, rotations, ionization states, magnetic fields, pressure, turbulence, density, and many other properties of distant planets, stars, and galaxies. Some objects studied this way are over 10 billion light years away. Spectroscopy is the richest source of information about the universe. In its laboratory setting, spectroscopy provided the experimental basis for quantum mechanics.

http://www.astro.virginia.edu/class/oconnell/astr121/comte.html

 

So, while astronomers give us the most amazing "pretty pictures" the majority of our knowledge of the universe comes from spectroscopy; the analysis of light.

I've always been fascinated by the idea of spectroscopy but whenever I've tried to read up on the subject, my brain turns to porridge. Also to understand it, you need a good understanding of Chemistry, Maths and Physics, which for me  - and in the correct order - were always the most boring subjects in the world. On top of this, there didn't seem much point getting into it because I'd never be able to DO spectroscopy for myself. 

So what has changed? In the January issue of Astronomy Now, there is an extended article on the chemistry of the stars which is actually quite readable. This sparked my interest and I wondered if Spectroscopy was something accessible to the amateur. It turns out that it's something of a "black art" shunned by most amateur astronomers as being too complicated and technical.

But if you REALLY want to do it, what do you need to become a spectroscopist? Well, it's the usual story; a few years back you could only hope to do astronomical spectroscopy if you were part of the professional community but as in so many areas, computers and digital imaging has opened the doors to amateurs in previously unimaginable ways. 

You need: a powerful telescope, eg. a 10" Cassegrain Schmidt, an f6.3 field reducer to make the magnification smaller, T rings and connectors, a decent DSLR, a computer, spectroscopy software, photo-editing software and a high quality transmission diffraction grating to split up the light

That was the point at which my spectroscopy dreams were shattered. How could I ever acquire thousands of pounds' worth of equipment? ;-( 

But hold on... I already HAVE most of this stuff! Telescope: check. Field reducer: check. DSLR and connectors: check. Computer: check. Software? Free off the internet. 

So all I'm missing is the diffraction grating.  Actually I WAS missing it, but not any more. After lengthy negotiations with Uni M as to where household saving could be made, I was able to place my order for the Paton Hawksley Star Analyser, SA100.

Knowledge of the stars forever denied to me Auguste? Ha, add to cart!