The Four Ages Of Sand

Congratulations to RSpec: Hot Product 2012!

Tom Field  - a regular to 4AoS - created RSpec, a wonderful piece of software which allows you to easily create calibrated profile graphs from raw spectra of stars. As I struggle to get to grips with my new video processing software Cyberlink PowerDirector 10, I'm stuck again by how easy and intuitive Tom has made RSpec.  I was able to produce this kind of thing almost immediately:

The 3 main hydrogen absorption lines in the visual spectrum of Sirius  (previous post)

And even turn images and raw spectra...

Orion Nebula

...into calibrated graphs revealing the chemical structure of M42: ionised  oxygen (OIII forbidden line 5007 A) and Hydrogen (H-Alpha 6560 A) (previous post)

Whereas with PowerDirector, I'm still struggling with the basics, and wondering why it crashes every other time I try to use it.

Well, now Tom has got the recognition he deserves. RSpec has been awarded the prestigious Hot Product award by Sky and Telescope magazine. Congratulations!

This from the RSpec website:

 

 

Chasing rainbows, making pretty pictures, and the joy of RSpec

Spectroscopy isn't supposed to be about "pretty pictures" in fact the first thing you do to a coloured spectrum is convert it to black and white. It's all about the scientific knowlege rather than the aesthetics. However, I do find the specra very beautiful and the idea of "spreading out" the light from a star into its constituent colours and discovering secrets hidden within it is wonderful.

To make a pretty spectrum takes quite a bit of processing. 

There are 6 stages:

1) Produce your raw image.

 

 

2) Rotate and crop the First order spectrum:

 

You can already see 2 of the hydrogen absorption lines in the blue part of the spectrum. These are created when the electrons in the hydrogen absorb specific wavelengths of the spectrum. 

3) Use software to produce a profile.

I'm a total convert to RSpec. It's a wonderfully intuitive program which turns a complicated and potentially frustrating process into an absolute pleasure.

As you can see under the profile, we already have a spectrum, but there are 2 problems with it:  

Because it's much brighter in the centre, you lose the detail at the sides. Sirius has very strong hydrogen lines but these are hardly visible here. Also there are dark lines caused by the non linear nature of the camera's CCD (see previous post)

In particular, notice the big dip in the centre of the graph which produces a wide dark line in the specrum. This is an artifact of the Nikon D60 sensor, and nothing to do with any real features of Sirius.  Also the "noise" in the graph is creating spurious lines. 

So the next stage is to produce a spectrum which has been corrected to take into account the variations and limitations of the CCD.

4) As in the previous post, we do this by subtracting the raw profile from a reference profile and use this to calibrate the image. Now we have a spectrum which is a fairly accurate representation of the actual enegy of the various wavelengths in the light from Sirius. But...

... we have a new problem now. The left side is bright and the right side is dark.

 

5) So we need to normalise the curve using another great feature in RSpec called Spline Smoothing. 

Having taken some time to get my head around spline smoothing, I'd  love to say more about it, but I came to the conclusion that people would naturally fall into one of 2 camps; those who already know what it was, and those who feel quite strongly that if they've got by in life thus far without knowledge of curve nominalisation using spline smoothing, they are happy to continue without going into it in too much detail now...

Anyway, you get the idea from the blue line. The software replots the graph based on how far the points on the profile are above and below the line.

And this produces a normalised graph, which in turn creates a more uniform spectrum:

 

Almost there. We now have a "balanced" spectrum and the hydrogen lines are becoming clearer, but the process has also emphasised the noise. To get rid of this we simply...

6) Perform a new spline smoothing operation to iron out the artifacts.

And now the final result. But first, here's my best effort from my early heady days as a spectroscopist (2 weeks ago to be precise)

I made this by simply stretching the original image in Photoshop, before discovering the joys of RSec.

And this is made using the same raw image.

Ta dah!

The 3 main hydrogen absorption lines in the visual spectrum of Sirius are now lovely and clear, and I'm looking forward to producing a set of spectra covering different classes of star. 

Identifying absorption lines with confidence; instrument response curves revisited

Here's a profile I produced of the hot young star Sirius:

As you know, the "dips" in the graph show where specific wavelengths of light were absorbed by the gas in the upper atmosphere of the star, and these can tell us exactly what the gas is made of. In this case, the dips indicated by the white triangles show 3 of the main lines for Hydrogen. This is what you would expect because a young star is mostly made of hydrogen.

But what about the 2 dips by the green triangles? 

Part of the problem of identifying these is that this graph is uncalibrated; the curves show what the camera sensor (Nikon D60) is sensitive to but this is not linear across the wavelengths. As we saw before (previous post) a camera CCD is not sensitive to lower and higher wavelengths, and also not uniformly sensitive even in the normal visual range.  One way to find out what is real and what is not is to compare the profile with a reference.  Here's the Library Profile of a similar star on RSec:

You can see the problem; they look totally different. The answer is to snip out the high and low wavelengths so that it matches the original profile. And then subtract the library profile from the raw. (Instrument Response Calibration; previous post) 

Here's my profile after adjustment:

This is now a true representation of the various wavelengths. 

Now let's add back in the snipped library profile:

The reference profile (blue) confirms the strong hydrogen lines and also show that the dip by the left hand green triangle is just noise.  The right hand green triangle dip is simply the "remnant" of the huge dip in the raw profile, and is not associated with any real feature.

The upshot is, I won't be making any more spurious claims as to the chemical composition of the stars, because I'll be able carefully check the results against professional spectra. Marvellous!

Elements in Aldebaran and instrument response curves.

First I discovered hydrogen in space, then oxygen. Now I've discovered metal!

Ok, "discovered" is perhaps the wrong word because boffins beat me to it some time ago. But if I'd been born 150 years ago, I'd be hailed as a flippin genius, so meet me half way for goodness sake. 

And actually I DID make a real discovery last week. I found hydrogen lines in the star Betelgeuse. And I published the results on one of the spectroscopy forums. Unfortunately that turned out to be a mistake, because Betelgeuse is an old cold star which has burned its hydrogen up already...

The guys on the forum were very nice about it and pointed out my error without once mentioning the word "idiot."

Anyway, past failures can't get in the way of future success, so ever onwards:

I managed to get a spectrum of Aldebaran last night. It's the bright orange star in the constellation of Taurus,

 

and if you've been reading 4AoS over the last few weeks, you will know what the orange colour means. Yes, it means it a cooler star heading towards the end of its life, having burned up its hydrogen (yeah, yeah, much like Betegeuse. I know) and this means it has expanded to a huge diameter; 44X that of the Sun.

(Wikipedia)

What this also means is that it's had time to produce lots of other elements, including metals so it's a great place to look for the heavier elements.  

I just searched Google to check what metals I should be looking for, compared to hot stars like Sirius. I seached for "star" Sirius" "Aldebran" and "metal" which took me to these guys:

To those now eternally damned, I was given the charge of relaying to you the licentious proliferation of the cult doom metal entity Aldebaran. As I write this, I feel the strands of myself succumbing to it. It came to my attention in 2003 when a trio of metal fiends first felt the emanations from the dog star, Sirius, home of that which shan't be named. 

If you are into the doom metal genre (??) you'll want to go here:

http://www.tvcoast.com/aldebaran/biography.html

Here's the profile from the spectrum photographed last night:  

 The suspected metal is marked with the blue triangle.

 This is the "raw" profile, and the vertical axis represents the strength of light at each wavelength, as recorded by the CCD in the camera. However, the response of the chip to different wavelengths isn't linear. The Nikon D60 loses sensitivity to wavelengths below 5000 and above 6500. Also there's a huge dip between the green and yellow parts of the spectrum (white triangle) which is due to the non-linear response of the camera, and not to any actual feature of the star. 

What to do?

Take the spectrum and divide it by a library spectum of a star of the same type to produce an instrument resonse curve for a particular camera:

Instrument response = Raw Spectrum/ Library Spectrum

This sounds difficult but RSpec makes it simple, and even provides the library. It's a K5III star so I just upload it from the supplied database:

The first thing you notice is that it has a much wider range than the Nikon results, especially at the red end. This isn't surprising because the camera has an IR filter which cuts out most of the light above 6500A. You can get this filter removed for about £200 but then the camera is useless for normal photography. And alsomy blue triangle does seem to be associated with a large dip in the library spectrum, suggesting we're onto something. 

I'll spare you the next bit so here's my spectrum (red line) after being calibrated for instrument response, and with the library profile above it (blue line).  I know there a tendency on 4AoS to overinterpret results but I think we can all agree there is a strong correlation at the marked points, 5184A and  5890A, which confirms the existence in the upper atmosphere of Aldebaran (gulp) TWO elements; Magnesium (Mg) and Sodium (Na)! 

 

 

 

 

 

What is the Orion Nebula M42 made of? And why is it pink? Analysing emission lines with a slitless setup.

As 4AoSers know, I have a fascination for the Great Nebula M42 in Orion.  Until now I've only been able to take pretty pictures of it:

 

But now as a spectroscopist, I can work out what it's made of and explain its colour!

If you have read The ABC of Spectroscopy you will understand that the light from stars will excite the electrons in gas clouds in space, and this causes the electrons to jump up to a higher "orbit" in the atoms. When they fall back down to a lower state they emit photons of light. These photons will be emitted at very specific frequencies, and by measuring these frequencies we can identify the gas.  These lines, which appear as bright coloured lines against a black background, are called emission lines.

(Wikipedia)

Great,  so lets take a spectrum of M42 and make our own emission lines!

What are  we looking at here? On the left of centre is the Trapezium, the 4 stars in the middle of M42, and the main ones exciting the gas in the nebula (only 3 are clearly visible here) Then there are 2 other stars above and one below. 

On the right you can see the various specra associated with each star. You can also see 2 clouds, one blue and one red. This already tells us that M42 is made up of 2 separate gases. We've already discovered something profound about M42, which we couldn't know simply by looking at normal images.

Here's the photo superimposed on the spectra, rotated and resized to the same scale. 

Now we hit something of a brick wall. 

To analyse lines in spectra you need nice sharp emission lines, and to do this the original object needs to be a point source, or a line. With big blobs of stuff, the information is "smeared out" and lost. 

This is why serious spectroscopy is done with a slit; a very fine gap only a few micron wide to isolate a "thin slice" of light.

I'm using a slitless set up because I can't afford the serious kit at the moment.  This is ok for stars because they are point sources anyway, but for extended objects there's not much you can do. 

But then I had a thought. What if I could use Photoshop to artificially isolate a small part of the spectrum?

I selected and boosted a small square in the brightest part of the original image, then selected the same 2 squares in the blue and the red clouds of the "smeared out" spectrum:

I then snipped out this thin slice and put it into the RSpec software. And found.....

....the blue and the red square fit almost perfectly with the emission lines for ionised  oxygen (OIII forbidden line 5007 A) and Hydrogen (H-Alpha 6560 A) 

I've searched around on the Net, but I can't find any other examples of this technique. I've posted the results up on one of the Spectroscopy forums and I'll let you know what they say. In the meantime, I'm very happy to have identified 2 chemical elements in M42.  Exciting times!

Oh, and the colour? M42 is basically a huge cloud of Hydrogen. Hydrogen gives off light at 4 visible frequencies when it gets excited. The main one, as we've seen, is at a frequency of 6560 A, which to our eyes is red:

 

The ABC of Spectroscopy Part D. The Joy of NGC 2392 and forbidden lines.

                                                                      D

 

If you've read Parts A,B and C, on spectroscopy (or you are already a boffin) you will know that all hot bodies radiate a spectrum of wavelengths, and that when this light passes through a gas, the atoms in the gas can absorb some wavelengths leaving dark lines in the spectra. The wavelengths of these dark lines - called absorption lines -  can tell us the chemical composition of the atoms because each atom will absorb only very specific wavelengths, whether in the laboratory or in deep space.

So what is happening here?

(13 second exposure, 1600 ISO)

The quality is rubbish because I only had a very short time to take some shots, and within an hour the clouds had rolled in again.

At the bottom left you can see a star, and on the right is its spectrum.  It looks pretty similar to the colours you would see in a rainbow because, like the Sun, the star is a hot body, radiating light in a continuum of wavelengths. If the quality was better, you would be able to see the dark lines in the spectrum where elecrons had absorbed specific frequencies of light. Above the star and slightly to the right you can see a much fainter star, and in the right corner you can see its spectrum. But there's something very odd about it: at the blue/green intersection of the spectrum is a large blob.

What you are seeing is the Planetary Nebula NGC 2392, also known as the Eskimo Nebula. Here's my previous photo of it, at much higher magnification:

 

It's a star which burned up all its hydrogen and collapsed into a white dwarf, sending out a shell of gas travelling at huge speed into space. It's now about 0.3 of a light year in diameter, and lies at a distance of some 2870 light years. 

The faint star on the left is the star at the centre of the nebula, called HD59088 ,and its spectrum in on the right. And the blob?

The starlight energises the cloud of gas and excites the electrons in it, and they move up to a higher "orbit." When they drop down again to a lower orbit they emit a photon, in other words, light. And it's at one specific frequency 5007 Angstroms (  1×10⁻¹⁰ metres, or 0.1 of a nanometre)  This is called an emission line. 

Here's the funny thing; when this line was first discovered, it did not correspond to any Earthbound elelment, so it was thought to be a new element, and was given the name Nubulium. It wasn't until 1927 that the true identity of this element was discovered. It is..... oxygen! Doubly ionized oxygen actually, which means it has 2 missing electrons, hence ion O²⁺, also known as [O III].

 

Why does oxygen never emit light at this frequency on Earth? Because it's forbidden by quantum theory.  To emit the "forbidden line" of 5007A, the electrons in the oxygen would have to stay in a specific orbit for some time in a metastable state, and this is impossible on Earth.

What's a metastable state and why can't oxygen be in this state on Earth?

I read this on Wikipedia, but it didn't help my understanding much:

It is when something "is in equilibrium (not changing with time) but is susceptible to fall into lower-energy states with only slight interaction." 

As always though, a good image makes it all clear.  The ball at 1 is in a metastable state. It won't move if it's left alone. But the slightest "knock" will tip it over the edge (2) and it will fall down to the lowest state (3)

If the electrons can manage to stay in the high state (1) for long enough, when they eventually fall down to the lowest state, the ground state, they will emit a photon of light at 5007 A. But even is the best vacuums boffins can produce on Earth, there are still lots of the oxygen atoms bouncing around and knocking into each other. This means they never have time to emit the 5007 wavelength because the impact of atom against atom, gives the electron a "knock" and it immediately falls to the ground state. In a planetary nebula though, the gas is very rarified, much more so than the best vacuum we can make, and this means the electrons have time to move from 1 - 3 at a rate which allows the forbidden line to appear. 

Here's my profile graph of the spectrum, produced with the fantastic spectroscopy software RSpec:

The star (HD59088) is on the left and there is a huge peak showing the 0III line at (almost exactly) the 5007 line. (More about the calibration of this another time)

This next profile is produced using some of the features in RSpec which allow you to remove a lot of the background noise and also smooth out the curves using a pixel averaging process called binning. I boosted levels in Photoshop to bring out the spectrum of the star too, and you can see more of the spectral colours as well as the dominating green/blue of the nebula (bottom right)

 It's odd, but capturing this line on a graph and knowing it is produced by light produced by ionised oxygen which has been flying at a speed of 186,000 miles a second for almost 3000 years, just so it can land on my camera's CCD in a garden in Hatfield, and be processed on my laptop, gives me more of a feeling of being connected with something quite magical far out in space,  than actually seeing it through the scope. 

Sirius (serious?) error

I have a habit of overinterpreting my data. This leads me to see things which aren't there. Who can forget my annoucement that I had discovered the first ever Resplendent Quetzal in Venezuela? Or the non-existent Horsehead Nebula in Orion?

It seems I've done it again. Now that I've managed to calibrate the spectra, I can identify elements in stars. After finding Hydrogen in Sirius, I let things go to my head and discovered Magnesium (Mg II) in the spectrum, and posted it up on one of the Spectroscopy forums:

 

However, it's been gently pointed out to me that this is likely to be noise, rather than evidence of Magnesium, so I stacked 2 images together, to average out some of the noise,  and the "Magnesium dip" prompty disappeared.

 

One day I will succeed, but until then I may have to review the  4AoS "publish and be damned" policy.

Chasing rainbows and revealing their secrets

With the sound of Nick's dog still ringing in my ears (see comments, previous post ) I'm now ready to reveal my fully calibrated spectrum of Sirius.

 Just to produce a "rainbow" of light from a star makes me very happy, but there's more; the dark lines are  Fraunhofer lines, and an examination of their frequencies reveals the chemical composition of the star.

Here's the 2D spectrum: 

These are 4 of the Balmer lines, at wavelengths of 410 nm, 434 nm, 486 nm, and 656 nm.  

See previous post for the science but what this really means is that light created in the centre of the star Sirius found its way to the surface and passed through the gaseous regions of its upper atmosphere. Most of the light passed through and made its way out into space, but some very specific frequencies were absorbed by the atoms in the gas. Some of the  light then travelled across space for eight and a half years,  made its way to our garden in Hatfield and revealed the fact that the gas is Hydrogen. And that's a kind of magic.

Addendum: To see much improved version with clear sharp hydrogen lines, see later post (final image) here:

http://www.fouragesofsand.com/2011/02/chasing-rainbows-making-pretty-pictures-and-the-joy-of-rspec.html

 

The ABC of Spectroscopy. Part C

                                                                                C

 

Ok, this is the 3rd part in the Spectroscopy overview. Hopefully you've read Part A and B so you already know that hot bodies emit radiation, and that the wavelength of the radiation relates to its colour.

To understand a spectrogram we need to know a little bit about Quantum Physics. It looks complicated but it's not. 

As David said in his comment in the  previous post: "As well as being a supremely useful way of gathering information from the entire visible universe, spectroscopy was directly involved in the formulation of quantum theory; which radically changed the way we understand everything."

When you were at school you were probably given the impression that atoms are like little solar systems, with electrons flying in orbit like planets around the nucleus. This is totally wrong in so many ways, in particular, electrons don't exist as tiny "balls" of stuff, but rather exist as fuzzy wave-like structures which don't have an exact point-like location in time and space, at least not until they are looked at.  It's been calculated that if they were orbiting the centre of the atom in a classical sense they would lose all their energy and fall into the centre in far less than a billionth of a second. Instead it turns out that they exist (if they can be said to exist at all) in well defined "shells" surrounding the nucleus.

When they have more energy they move up to a higher shell but they always want to return to the lowest shell as soon as possible. Here's a funny thing: when they move from a lower shell into a higher shell, they don't move through the space in between; they instantaneously disappear from one place and appear in another.

The simplest element, and the most abundant one in the universe,  is Hydrogen. Here's a model of the atom:

http://en.wikipedia.org/wiki/Hydrogen_spectral_series

We're interested in the Balmer series in the centre. These are the distances of the various "orbits" of the electrons in Hydrogen atoms.

A nanometer (nm) is one billionth of a metre, by the way. How big is that? Not very big. While you read the sentence  "A nanometer (nm) is one billionth of a metre, by the way" your hair has grown several nanometers. 

Here's the great thing; every element has its own orbital distances. In other words, if you find electrons at the distance from the nucleus of 656nm, 486nm, 434nm and 410nm, you know it's a  Hydrogen atom.  Suddenly the whole visible Universe becomes accessible. You know what everything is made of simply by seeing how far the electrons are from the centre of the atom.  

So how can you measure orbital distances of electrons in atoms of stars countless billions of miles away? With spectroscopy!

All you do is:

Create a spectrum of the starlight:

 Then calibrate it. In other words identify the wavelengths of the visible light.

Then look closely and you should see dark lines in the spectrum:

These lines are the characteristic lines of the 4 visible hydrogen lines. 

http://en.wikipedia.org/wiki/Spectral_line

As soon as you see these lines you know you are looking at hydrogen, whether it's in a laboratory, or from light for a star millions of light years away. What causes the lines? This wasn't understood until the theory of Quantum Physics was developed less than 100 years ago.

The light created by the star moves up to the surface. Then it passes through the gaseous outer layers of the star. Most of the light passes through and comes to us as a continuous spectrum, the electrons in the atoms take the energy from the light at at the specific frequencies corresponding to the distances of the various shells mentioned above,  and they use it to jump to higher levels. So each element in the gas will "absorb"  certain frequencies of light, and by looking at these "missing frequencies" which show up as dark lines  - called Fraunhofer lines, after the guy who first saw them - we can identify the element.  Magic!

(Wikipedia)

Summary: electrons in atoms absorb certain frequencies of light, leaving dark lines in the spectra. These lines can be used to identify the atom 

 

 

The Joy of calibrated Spectrograms

As soon as I read that it was possible for amateurs to do spectroscopy, I couldn't rest until I'd produced my own spectrogram revealing the chemical composition of a star. It's been a long rocky road  but I'm now ready to reveal my first calibrated 2D spectrogram, showing the frequencies hidden within sirius! 

"Wow, but what does it actually mean?" I hear you ask.

We'll worry about that later. For now, let us just rejoice: