Monday, 30 November 2009

The Periodic Table in the stars

The Periodic Table is a wonderful thing. It took many, many years for scientists to put it together properly, and work out why it looks like it does. First they had to sort out which materials were elements, and which not; then, not knowing what made them different, they had to find a way to classify them all. The story goes that Mendeleev found the pattern of atomic weight and properties almost by accident, writing a table for a chemistry textbook - just goes to show that writing down the basics is often very revealing! To be picky, he was not the first to do so; but he was the first to predict the properties of those elements that were missing.

If you're not familiar with the Periodic Table, take a good look at this picture (click here to massively enlarge). From studyhallnotes.

It's in the shape of a large, squashed U. The F-block, at the bottom, should actually be slotted into the D-block and make it expand out at the bottom again, the way the D-block expands out the one above it between Ca (20) and Ga (31). What you have got here is actually something like a set of Russian dolls.

Imagine you're painting a series of dots around a Russian doll, each dot the same size and the same distance apart - like a beaded belt. It follows that you're going to be able to paint more dots around the bigger dolls than the smaller ones. Or if you decided to start painting on silly belts, going up and down and around, you'll be able to get more patterns on the larger dolls . . .

The smallest Russian doll can only fit two beads on its belt. Or to be more precise than that, it has space for two, so it can have either one or two. The next-smallest can have eight: again, to be more precise, it has four two-bead belts, each of which fits a different part of it, but until A level chemistry we say eight, for the sake of simplicity.

Why two, and why eight? What are these numbers? They're due to the electrons surrounding the atom. Electrons all have a negative charge, and when they flow freely between atoms they are electricity. Nobody really knows what negative charge is, other than a property of a particle, which is attracted to positive charge and repelled by like negative charge. Anyway, electrons are stand-offish little beasts who don't like sharing their space with other electrons. They'll put up with one partner if it's got something called the opposite "spin" - and that's why you'll find them in groups of two in an atom.

It's the outside of atoms that shape the Periodic Table. I was bored by the Periodic Table at school until I realised how it was put together - at which I was beside myself with joy! But there's a lot going on on the inside of atoms too. For every electron, there's a proton. Protons, of course, have positive charge and repel each other as well - but adding some neutrons to the mix can settle their arguments. That and the strong nuclear force (scroll down to the next box in the link), which sticks them together. So, for all elements except hydrogen, for every electron and proton there's at least one neutron - often more. Some elements have a lot of neutrons; others stick to a minimum.

All atoms in the Universe were created: either inside stars, or in the Big Bang at the beginning of time. That's not as simple as it sounds. Because the strong nuclear force only operates over very short distances, it's not easy to slam enough protons and neutrons together to build up these complex, stable groups out of such sensitive materials. Neutrons, too, only last about ten or fifteen minutes on their own; usually a proton transforms into a neutron, emitting a positron as it does so. And there are only so many stable arrangements of protons and neutrons . . .

They're wonderfully variable. I'm going to tell you stories about three of them: lithium, carbon, and oxygen - how they are formed, and some very odd places in which they've ended up.

Let's start with lithium. You probably remember it from school: lithium, sodium and potassium all react violently with water, like this:

(From webshots.)

They're also on the far left of the Periodic Table: the "alkali metals". They each have one electron in their outermost "shell", which is a cumbersome arrangement and they're just delighted to get rid of it. But what's strange about lithium is that there is no logical way for stars to synthesise the lithium nucleus. Yup, you heard that correctly: to the best of our knowledge, stars shouldn't be able to make it.

I'll come back to why in a minute, and get on with the story.

Lithium in stars has been bothering scientists for many years. As Wiki says, "Though the amount of lithium in the known universe can be easily calculated, there is a "cosmological lithium discrepancy" in the universe: older stars seem to have less lithium than they should, and younger stars can have far more." Lithium was one of the elements - along with (a lot more) hydrogen and helium - created in the Big Bang. The heat from nuclear fusion reactions in stars destroys lithium - as Invader Xan told us at the zoo, and I'm sure he's right, brown dwarfs retain their lithium because their nuclear fusion never gets going, while a red dwarf and anything hotter "burns" it.

Recently another lithium pattern was discovered: not age, but whether or not a star has planets! Stars with planets have less. Not just a bit less, but if I read these figures correctly, stars without planets have only one thousandth that of stars with planets: the original paper (Irsaelian et al) states that "planet-bearing stars have less than 1% of the primordial Li abundance, while about 50% of the solar analogues without detected planets have on average 10 times more Li".

Why? was my first, stupid reaction. Do the planets eat it all or something?

"The presence of planets may increase the amount of mixing and deepen the convective zone to such an extent that the Li can be burned," says the paper. Planets, tiny as they are in comparison to stars, do still affect their Sun - planets are often detected by their host's "wobble" caused by the gravity of the planet. And stars aren't great convectors. Their currents are so slow that, as Marcus Chown memorably describes it, it takes a human lifetime for a current to cross the face of a wristwatch. Without a disruptive influence, lithium can sit around forever without being affected by the furnace at the core.

"Using our unique, large sample, we can also prove that the reason for this lithium reduction is not related to any other property of the star, such as its age," says Nuno Santos, one of the authors of the paper, in the Science Daily article which covers this story. The BBC article emphasises not the stars, but the protoplanetary disk of material which spins with the star during their early days. The general effect, of course, will be the same.

While lithium was created in the Big Bang, in tiny amounts, due to chance reactions, carbon was not. Carbon has six protons and six neutrons. These are in the form of three alpha particles: the helium nucleus, two protons and two neutrons, the result of hydrogen fusion in the Sun, and so stable that it retains its own identity in a larger nucleus (we can tell this because alpha decay, one of the three forms of radioactivity - the sort that killed poor Litvinenko - is always in the form of alpha particles, rather than single protons or neutrons, or larger clumps of matter). To be picky, carbon has two isotopes - Carbon-13 and Carbon-14 - which have seven and eight neutrons respectively.

Carbon's two states and molecular arrangements, from good old Wiki.

Conditions in the Big Bang, violent as they were, could not have formed such a complex nucleus. The reason for this is that two alpha particles don't easily stick together. If they do, they form beryllium-8 - which is so unstable that it falls apart after only a 1/100 000 000 000th of a second! (Stable beryllium has five neutrons - it's number 4 on the Periodic Table.) Nor do spare protons or neutrons stick to alpha particles.

But the insides of stars are a special place - especially towards the end of their lives. The pressure is drastic: in a full-blown red giant, half the star's mass is squeezed into a billionth of its volume. Such density will cause collisions between helium ash (alpha particles) at a great rate, and with enough energy for abnormal things to happen. In fact, the tiny time beryllium-8 holds together is long enough for a very small amount to survive all the time in the star - and therefore, enough time for another alpha particle to come along and turn it into carbon.

(The whole story is a lot more complicated than that, but when I get around to reviewing Marcus Chown's wonderful "The Magic Furnace", I might tell you more - though my stronger recommendation is read it! Back to this story . . .)

It follows that we'll find carbon in the nebulae and dust drifting through space, the ashes of burnt-out stars. Surprisingly enough, it's also recently been detected as the atmosphere of the faint neutron star known as Cassiopeia A, or Cas A.

The supernova remnant of Cas A, from Chandra. It's expanding at 10 million miles per hour, and is about 50 million degrees Farenheit (anyone care to work that out in Kelvin?).

It's a relatively young supernova, about 330 years old, possibly spotted by Flamsteed. There is definitely a neutron star in there, but it doesn't pulse with the usual radio or X-ray waves. Now, Craig Heinke from Alberta University and Wynn Ho from Southampton University have published a letter in Nature stating that this is because these waves are being absorbed by a 10cm thick atmosphere with a hardness of diamond.

Below that is a layer of iron, and below that neutronium - pure neutrons, squashed down by gravity, so heavy that a teaspoonful of them would weigh millions of tons. (I'd love to see some of that of that stuff.) The atmosphere is stratified, meaning the lightest elements appear on top. Usually for neutron stars, this is hydrogen and helium. For this very young one, it is carbon. Why?

It is thought that the intense heat of the neutron star actually caused the hydrogen and helium to fuse into carbon - which seems pretty paradoxical to me. Same as stars, you should find more carbon in older stars, not younger ones. My best guess is simply that we don't know yet. The abstract of the letter says: "If there is accretion after neutron-star formation, the atmosphere could be composed of light elements (H or He); if no accretion takes place or if thermonuclear reactions occur after accretion, heavy elements (for example, Fe) are expected. Despite detailed searches, observations have been unable to confirm the atmospheric composition of isolated neutron stars." In other words, we need to study more neutron star atmospheres; they say good old Chandra's the only telescope that can do this.

Please let me know your thoughts; and if you're interested in any further reading, you can try Universe Today, Discover Magazine, the Telegraph, and PhysOrg! My mum, meanwhile, has completely stumped me with one question: why is the carbon atmosphere called an atmosphere rather than a surface?

Ready for one more? It's all right, this will be the shortest story. One alpha particle step up from carbon is oxygen. (Two more protons, two more neutrons - except, as usual, in the case of isotopes). The process of making carbon is called the triple-alpha process; after carbon, it is simply called the alpha process, and requires ever more extreme conditions in the cores of stars. Look at the Periodic Table again: it goes carbon, oxygen, neon, magnesium . . . you skip alternate elements, going across from left to right, and the same again in the next row, and so on.

The heavier the elements get, the more extreme the conditions required to make them - in other words, the deeper inside the core of a star. From Wiki again, here is a typical (fairly heavy) star's core at the end of its life:

You can expect a star, then, at the end of its life, to contain some oxygen. Now imagine you find a star with a great deal more oxygen than it ought to have. What might be going on there?

There's a theoretical model. Not all supermassive stars go supernova or become black holes. Stars 7 to 10 times the mass of the Sun might go supernova, or they might form massive white dwarves. At this size, such dwarves should be rich in oxygen and neon - and this seems to be what has happened with two white dwarves in our beloved SDSS.

A paper by G et al reports: "the detection of two white dwarfs with large photospheric oxygen abundances, implying that they are bare oxygen-neon cores and that they may have descended from the most massive progenitors that avoid core-collapse." On the forum (a net through which very little astronomy news slips undetected), there was some discussion about how to find these stars; Els and Dave both contributed possibles. This is the one they agreed on:

SDSS 587742013276094467.

So it seems that stars of a certain mass might puff off enough of their outer layers to avoid gravitational collapse (as occurs with neutron stars and black holes), or a particularly violent death - and lay bare cores of materials that are generally locked deep inside a white dwarf. Their nuclear fusion, though, is over - their fate is to very slowly shine themselves to dimness. (A white dwarf cools exceptionally slowly: a star can only really cool by expanding, which of course white dwarves cannot do. Therefore, they have no easy way to get rid of their heat. To the best of our knowledge, the Universe contains no black dwarves yet - a black dwarf is a white dwarf which has cooled down enough no longer to shine.)

You can read more about the high oxygen white dwarves at PhysOrg and SpaceFellowship.

I love chemistry. I'm afraid I just got carried away.


Michelle Harris said...

Hi Alice,

I would like your permission to use the image of lithium in water for a book I am working on.

The book will be published by Marshall Cavendish Singapore for Brunei high-school students (Science).

Appreciate your urgent reply. Thank you for your kind consideration of my request.

Michelle Harris
Ast. Senior Project Editor
Marshall Cavendish Singapore

Alice said...

Hi Michelle,

It's not my image - I provide a link to the source underneath, which I hope is still live. If it's not, I recommend simply using Google Images to find it, or something similar, and contacting the author.

Best of luck with your book!