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Here at the Young Astronomers we have looked in some detail at stars, the various types that exist as well as their spectra. This short series of posts will deal with what processes occur to power the stars, allows them to shine and how the original materials that made up the first stars got here in the first place.
This post will deal with the composition of stars, metallicity, stellar populations and primordial nucleosynthesis.
First lets look at the material a star is made from, for the sake of ease I will use our own sun – Sol – as an example of a typical star.
All stars are composed primarily of hydrogen and helium with smaller traces of all the other elements. As a star ages the proportion of hydrogen falls (slightly) as it is converted by nuclear reactions into the other elements.
Credit: Peter Clark
Credit: Peter Clark
All stars have a broadly similar composition though the exact balance of components varies from star to star. The ratio of the heavier elements to a stars hydrogen and helium content is a measurement termed metallicity.
Metallicity – Z
The next few lines can be enough to bring a chemist to their knees, so be warned. In astronomy the vast array of elements provided by nature or artificially synthesised in the various particle labs around the world are divided into just two groups, not the many used by chemists: – Metals and non-metals.
Making matters worse there are only two astronomical non-metals with all others (including many of the chemically non-metals) being classed as metals. Why is there such an obtuse system? Well let’s explore the issue.
Hydrogen and helium are the lightest two elements in the periodic table, and are the only two that were formed in any great quantities in the first era of nucleosynthesis (simply put element building) following the formation of the universe in the Big Bang – Primordial Nucleosynthesis (more on this later).
So astronomers make the divide between metals and non-metals not on a chemical basis but on one of initial origin. Non-metals were produced in Primordial Nucleosynthesis and metals were not. Though Helium was and is being produced by stars it is still classed as non-metal as a large quantity was originally produced within this era.
Lithium and beryllium were also produced in small quantities in Primordial Nucleosynthesis thought they aren’t generally considered non-metals though could perhaps be included depending on your definition.
Metallicity is a comparative measure of the metal to non-metal content of any particular star or nebula. It is calculated by comparing the intensity of various spectral lines to derive a ratio. Metallicity values are usually given relative to the Sol. So a star with a metallicity twice that of Sol has twice the relative proportions of heavy metals to hydrogen and helium than Sol.
As well as giving information about a star’s composition its metallicity is also an indirect measure of how old a particular star is. After every generation stars the interstellar medium (ISM) – the nebulous gas and dust from which stars form – is enriched with the dying remnants of stars throwing their atmospheres into space. This debris contains the elements that the star formed over its long life span. This enriches the ISM with metals so the next generation of stars have correspondingly larger metallicities.
Astronomers can use metallicity to divide stars into three groups termed Populations.
Stellar Populations
The three stellar populations are as follows:
- Population I stars are stars of similar or greater metallicity than the sun. In the current epoch they are the most common variety of stars present in the Universe
- Population II stars are the oldest stars currently detected and have very low metallicities. They are all red or orange stars (spectral class K and M) as the other heavier hotter stars born in the same time period have long since depleted their fuel reserves and burnt out.
- Population III stars were the first stars formed after the Big Bang. As such they would have virtually no metals in their structure and for reasons touched on later would have been many times the mass of the Sun. As all such stars would have burnt out within a few million years none have yet been detected as they would only be visible for a very short period of cosmic time and we currently do not have the technology capable of detecting them in the afterglow of the Big Bang observable to us today.
It is worth a note that the Populations are numbered in the reverse order suggested by common sense. Population III stars are essentially the 1st generation of stars with Populations II and I indicating later generations. It is also important to note that a population may contain more than one generation of stars and the line between each is somewhat ambiguous.
Now lets look at how the material to form the original stars was produced in the first place.
Primordial Nucleosynthesis
For a duration of about seventeen minutes, between three and twenty minutes post the Big Bang, the universe had the correct conditions (temperature, pressure and density) to serve as a nuclear fusion reactor; similar to the core of a star. These extreme conditions allowed the soup of sub atomic particles to fuse and in doing so form atomic nuclei (though not atoms as the conditions remained far to energetic for electrons to become associated with these nuclei for about 380,000 years).
Nucleosynthesis was initialized after the majority of sub-atomic particles had been formed following the Big Bang – that is after the slight asymmetry between matter and antimatter became evident, allowing ‘normal’ matter to come to dominate our Universe.
One of the most fascinating thoughts about this process is that it occurred everywhere in the observable universe at the same time. That includes the space where my laptop is sitting as I type this, as well as the space now occupied by your brain.
So what exactly happened during this time? To answer this we must first look at the initial conditions as the process begins.
The two basic building blocks of all atomic nuclei – the proton and the neutron (each composed of three quarks) – had already been produced by in large before the onset of the process. Secondary school chemistry would have you believe that the proton and neutron have the same mass, this I’m sorry to say isn’t entirely true. A neutron weighs in at 1.674927351×10−27 kg whilst a proton is slightly lighter with a mass of 1.672621777×10−27 kg. This tiny difference of 2.305574×10−30 kg can safely be ignored in almost all practical cases (including most if not all secondary school chemistry and physics exams 
) but becomes very important to our story.
As Einstein laid out with is mass-energy equivalence equation E=mc2 (arguably the most well known equation in physics), mass and energy are really two side of the same coin. Mass (under current understanding at least) is the most concentrated form of energy possible, indeed one gram of matter contains the energy released by the detonation of 21.4 kilotons of TNT. If we rearrange the formula we can see why the mass difference between the proton and neutron is so significant.
m=E/c2 – This may not look that much different but it reveals a great deal. For a fixed amount of energy in Joules, the equivalent mass is (tiny though it may be) mathematically calculated by dividing the quantity of energy by the speed of light squared which is about 9×1016ms−1. So for 1000J the equivalent mass is about 1.11×10-14 kg, demonstrating how such a tiny difference in mass allow for such drastic implications as we are going to look at now.
Just after the Big Bang the universe was an exceedingly hot soup. Particles popping into existence at random, before encountering their antimatter partner and annihilating each other in a flash of gamma rays. As stated above, as the universe expanded it cooled rapidly, as it did so the slight difference between probability of a ‘normal’ matter particle being generated and its antimatter opposite (in the favour of the ‘normal’ version) allowed our universe to become dominated by ‘normal’ matter. As the universe cooled these particles (the quarks) joined up to form the familiar protons and neutrons. I’m sure you are now wondering why I waffled on about their differing masses for three paragraphs – I’m getting there!
NGC225 Credit: Ken Crawford
As protons are ever so slightly less massive, they require a lot less energy to generate and so more spontaneously popped into existence from the primordial fireball compared to neutrons. This effect was so significant that the universe had seven times as many protons as it has neutrons at the start and end of this first phase of element building. This explains why the universe has an inordinate amount of hydrogen a whopping 75% by mass1 of all ‘normal’ matter - the simplest element containing just a single proton as its nucleus (for the chemists I am explicitly dealing with the lightest isotope protium here rather than the heavier deuterium and tritium which do indeed contain neutrons).
Adding to this ‘proton bias’, free neutrons (that is to say, neutrons that are not bound into atomic nuclei) are unstable and tend to decay to protons within about 15 minutes give or take a bit. Thankfully for the neutrons the density of the early universe was high enough to allow the majority to be incorporated into stable nuclear configurations within the first few minutes, thus avoiding a neutron deficient scenario where most had already decayed.
Despite helium-4 (the most common isotope of helium) being more stable than either a free proton or neutron, and thus should be relatively easy to form, the process encounters a snag. You can’t simply fuse two free protons and two free neutrons together at once to produce a helium nucleus, the process must first pass through an intermediary step of two deuterium atoms. Deuterium is a heavier isotope of hydrogen containing one proton and one neutron, though unlike helium is somewhat unstable and as such any deuterium that did not immediately collide with another deuterium nucleus was broken back down to its component proton and neutron. This in turn prevented any major nucleosynthesis to occur until after the universe had cooled below about 300 million Kelvin. This restriction for the commencing of the majority of the fusion reactions is termed the Deuterium Bottleneck.
Deuterium formation and breakdown. Red indicates a proton and grey a neutron. Credit: Peter Clark
Once the universe had cooled past this point the reactions kicked into overdrive (as deuterium nuclei are able to remain stable at these temperature) with hydrogen being converted to helium via deuterium at a rate not seen since. Though the fact that we can detect any deuterium at all is very telling. As no known process other than primordial nucleosynthesis could produce anywhere near the detected level of deuterium in the universe today (despite that proportion being quite tiny), meaning that virtually all the deuterium in existence was produced in the first twenty minutes of our universe.
He-4 Production Credit: Peter Clark
Primordial nucleosynthesis is therefore tightly constrained by the level of deuterium present within the universe. If it had continued much past the projected twenty minutes, most perhaps even all deuterium would now be tied up within Helium-4 nuclei. So the detected level of deuterium allows us to determine a great deal about this age of rapid element building.
The brief duration of the process also set up restrictions also set up restrictions on the possible final products. Without any ‘massive’ nuclei specifically a stable nucleus containing 5 or 8 nucleons (being protons or neutrons) rapid build up of any further elements is impossible. Such build up requires extremely rare circumstances that produce even heavier nuclei containing more nucleons, and can only occur in significant numbers over millions of years in the cores of high mass stars.
Two He-4 nuclei can collide and fuse to produce a highly unstable Beryllium-8 nuclei, this under normal circumstances would decay back to the original two He-4 nuclei. This process is exceedingly rapid with the half-life being slightly longer than 6.7×10-13 seconds.
Be-8 Formation and Decay Credit: Peter Clark
However, very occasionally a third He-4 nucleus can collide and fuse with the Be-8 nucleus before it decays. This produces a stable Carbon-12 nucleus which in turn can go on in a whole new series of fusion reactions in turn producing all the heavier elements.
Carbon-12 Production Credit: Peter Clark
This process is incredibly slow taking millions of years for any appreciable masses of carbon to be produced and so only a few very isolated atoms of carbon would have been produced in this epoch of the universe. The process eventually becomes significant allowing for the initiation of the CNO cycle in high mass stars.
Taken together, the current models suggest that beryllium would be the heaviest element produced in any (tiny) quantity outside of exceedingly rare freak events as part of primordial nucleosynthesis, with the remaining heavier elements requiring longer term build up within stars, supernovae and through the action of cosmic rays (cosmic ray spallation) long after this first burst of activity had ground to a halt.
New observations however have detected unusually high levels of boron isotopes in some very ancient red dwarfs. This cannot be explained though standard models as the stars are too old to have formed from sufficiently enriched material to contain such levels of boron (produced almost exclusively in Type Ic supernovae2) and such serve as an indication that our current understanding may be incomplete.
The next post in the series will deal with the internal structure of stars and the processes that allow a star like the Sun shine for several billion years.
Notes:
1 Made even more impressive by the statement that just under 92% of all atoms in the Universe are hydrogen with helium filling up the majority of the remainder at just under 8%
2 http://arxiv.org/abs/1007.0212
The ESA’s Planck Space Telescope completed its mission on Saturday.
The mission was designed to peer into the detail of the cosmic microwave background radiation (CMB) – the residual energy left after the Big Bang.
An artist's impression of the Planck spacecraft with a microwave background Credit: ESA
Planck also used its microwave detectors to gaze at the cold dust within our galaxy and beyond, detecting many new galaxy clusters in the distant universe. Some of these even appear to be interacting and merging to form even larger superclusters.
The first data from Planck was released last year and included the improved catalogue of galaxy clusters, though the first data set on its study of the CMB is yet to be released, though this will be made available to scientists outside the project in the early stages of 2013.
The mission was originally planned to make two surveys of the entirety of the sky over the space of 15 months. Planck performed better than expected and completed five surveys over 30 months, double the original mission expectancy.
The data released so far also reveals that stars in the universe were being formed at one thousand times the current rate, a fairly phenomenal statistic!
The telescope is equipped with two instruments:
- The High Frequency Instrument or HFI
- ow Frequency Instrument or LFI
These two instruments work in tandem to build up a highly accurate map of the CMB. Unfortunately the HFI is now offline as the spacecraft depleted the last of its coolant supply and has now warmed above the critical temperature required for the useful opperation of the detector. The LFI however is still in working order and will continue to provide additional data over the rest of the year.
No doubt the data from Planck will reveal many new interesting features of the universe over the next few years, I for one am very excited!
You can read more here.
This post is by AliceS for the Young Astronomers.
Admin note: Alice would like me to note that this was the first post she produced for the Young Astronomers, and that the content is quite old. She has however made several alterations since the post was originally released.
Hello everyone, and thanks to the Young Astronomers for allowing me aboard despite being a comparatively crumbly non professional astronomer! Now, assuming I can get WordPress to bend to my will, my first post is going to be about words in astronomy that end up not meaning quite what they should, if they don’t want to be misleading. Like all sciences, astronomy is done pretty much in the dark (sorry about that) – and sometimes names stick before we know what we’re actually talking about. Here are a few.
Revolution
When we hear of revolts and revolutions, we think of noisy coup d’etats in which the angry mob displaces the, er, other angry mob – and either things improve for the country in question or they don’t, but in any case, it’s a radical change. But the word “revolution” actually means “going round in a circle”. The Earth completes one revolution round the Sun every – you got it – year. Doesn’t seem a very revolutionary word, does it?
It came from Copernicus. His revolution was, really, the ultimate revolution in Science: the recognition that we are not at the centre of the Universe; that, rather, we revolve around the Sun. The book he wrote (which was only published just before he died, as he knew it wouldn’t be popular!) was called “De revolutionibus orbium coelestium”, or “On the revolutions of the heavenly spheres”. It was a revolution, because it called into question the dogma of the day that the entire Universe was created for us.
Handmade oil painting reproduction of The Copernican System by Andreas Cellarius, devised by Nicolaus Copernicus.
(Picture credit: this online art gallery!)
Which, incidentally, led on to . . .
Planets
The word “planet” comes from Greek, and means “wandering star”. Ancient people had no way to tell a planet from a star, except for its odd motion – moving in comparison to the rest of the stars in the sky, which of course was because all planets orbit the Sun at the same distance – and the fact that they don’t twinkle the way stars do. Both basically looked like points of light, and, with a little hard (if incorrect) thinking, might just as well be fixed to “celestial spheres”. Stars, of course, give off light of their own, not reflect their star’s light as a planet does.
In fact, the Sun and the Moon were also originally called “planets”. Gradually the word “planet” came to mean “world that is not a star” – one discovery that helped this along was Galileo’s sighting of Jupiter’s moons in 1610.
But of course the definition of “planet” has changed more than once since then. People seem to feel for some reason that Pluto has been harmed by no longer being known as a planet. In fact we now know that so many different types of objects orbit the Sun in our Solar System, we need to reclassify them somehow!
So, lots of changes as science progresses. To be fair on the ancient Greeks, they couldn’t planet to happen . . .
Planetary Nebula
This is the name for beautiful nebulae such as the Cat’s Eye Nebula. They are actually nothing to do with planets, but apparently looked like them in the 18th century when telescopes were not poweful enough to tell the difference.
The Cat's Eye nebula, a "planetary nebula" from a star too small to explode as a supernova. Credit: NASA
A planetary nebula is a much more gentle and orderly shell of gas than a supernova remnant. It is created when a small or medium star, like our own Sun, puffs off its outer layers at the end of its life. It’s often very hot, ionised gas, and is therefore an emission nebula – shining with its own light. It also contains elements such as carbon and oxygen, which are essential for forming rocks, planets, and life.
The word “nebulae”, however, does at least mean clouds. Astronomers referred to “spiral nebulae” many years ago, believing these to be beautiful spiral-shaped clouds at the same sort of distances as the stars in our Galaxy. They had no idea that these were galaxies millions of light years away from our own!
Astrology/Astronomy
Once upon a time, these two words meant the same thing. In the days when it was essential to know when to expect floods or plant your crops, and indeed when there were no TVs or streetlights at night, people would have known the sky very well. It would make perfect sense to think, “When such-and-such a constellation rises above that hill, it’s time to plant this out”, or “Oh dear, that one. The weather will be bad soon.” Into the Middle Ages, royals employed professional astrologers. A British tabloid newspaper claimed that Dr Brian May, the Queen guitarist who is also an astronomer, has a PhD in astrology . . .
Any word ending in “-ology” (biology, geology etc) usually means science. However, as the science and the myths separated, they needed two different names. They now have pretty well nothing to do with each other – but a lot of people don’t believe me when I say that!
Nova
The word “nova” implies newness. However, a nova is a star so old that it’s no longer strictly a star. It’s a massive explosion caused by the accretion of gas onto a white dwarf. This explosion makes it look as if a new star has appeared in the sky, hence the name.
This white dwarf is pinching this gas from a nearby star, usually in a binary system; every so often, it acquires enough for fusion to start again. It has to reach about 20 million Kelvin to do this, as a white dwarf is made of extremely compressed material which contains no hydrogen fuel to fuse (otherwise it would still be a star!). In order to make this even simpler, novae are not to be confused with supernovae, although a Type I supernova can result from the same sort of process.
The Big Bang
Time and again I’ve been told almost angrily: “It doesn’t make sense. The Big Bang was an explosion, so how could it create such an ordered Universe?”
The term “Big Bang” was actually coined as a derogatory joke, byFred Hoyle, who preferred the steady state theory (that the Universe remains the same size and had no beginning). He said in the 1960’s on a radio program something along the lines of that he didn’t believe the Universe could have begun in one big bang. The name stuck!
We will never know what sort of noise it made – of course, even if we’d been around to hear it, it would have been so incredibly hot and violent that we’d have been smashed to bits. Certainly everything would have been bumping into each other a lot. There were no atoms and molecules as we know them, let alone solid objects or stars – everything was a seething plasma of atomic nuclei, electrons, and most of all radiation. It’s particles bumping into each other that make noise. But when the Big Bang occurred, any noise that occurred would have been inside it.
That’s because any explosion we think of today is nothing like the Big Bang at all. An explosion happens in one place, and its shock waves – flying shrapnel, for instance – fly out and damage their surroundings. The Big Bang didn’t have any surroundings. It’s easy to think of it as an expanding globe, with a centre and an edge. We think of the edge as rippling through something – perhaps the Earth! – at some point in time.
It sounds like it took place – in, well, a place. Somewhere we could go and visit. From there we’d see the evidence of destruction, perhaps everything rushing away . . .
That is everywhere and nowhere. The Big Bang happened right where you’re sitting. It happened across the room for you, and it happened on the other side of the Universe. It’s quite a mind-blowing thought. But it really wasn’t much like a bomb!
An artist's impression of the size of the Universe at the time of the Big Bang, then inflation, then its expansion. Credit: NASA / WMAP Science Team
It was really quite complex too, with inflation, and a period of darkness (because all the atomic nuclei and electrons were flying around in too disorderly a manner to let light through. This is what happens inside a cloud – there’s too much stuff in the way, so light bounces off everything in random directions and goes any old where. It also means it’s relatively dark).
And guess what else? It wasn’t big at all. It was small. It wasabsolutely tiny – smaller than the head of a needle – perhaps smaller than an atom! How did all this stuff in the Universe today come out of something so small? We don’t know. In fact, theoretically, such an object shouldn’t exist. It’s called a “singularity”, and it means, because it’s too small even to have a size, it must have infinite density. But we know there are black holes which are also singularities – and, really, when we look at the earlier Universe and see how much smaller and hotter it was, and when we do the mathematics, it’s the only conclusion we can come up with.
It’s not only how we began, but it’s an immense – and immensely complicated – puzzle. It’s odd to think that something so huge and important could have such a jokey, normal name. But Universes happen before words do!
Alice
The first stars coalesced from the primordial hydrogen and helium synthesised in the first few minutes after the birth of our Universe just a few hundred million years after the Universe burst into existence.
All stars are born when a cool clump of hydrogen and helium gas condenses under its own gravity. As it does so the central regions of the clump heat up eventually to the millions of Kelvin required for nuclear fusion to occur. Once this critical point is reached the star fuses hydrogen to helium and begins to shine brightly. Stars grow by absorbing more material onto their surfaces from the surrounding nebula and hence they grow in mass. Eventually their increased energy output disperses the surrounding cocoon to the point where the star’s gravity is insufficient to attract anymore material and the star ceases growing.
In the current era of the universe this process is aided by heavier elements such as carbon and oxygen, these aid the cooling of the surrounding gas which allows for it to fall onto the star – if the gas was too hot it would have too much energy and escape off into space. The first generation of stars did not have any heavy elements to aid their formation as they had yet to be produced as such elements are only produced in the final stages of a massive star’s life. Thus they would have had a much shorter time to absorb matter from their surroundings before it escaped their clutches.
This may suggest that the first stars should be much smaller than those currently populating the Universe, though this is not the case. As the density of material in the early Universe would have been much higher they would have more material with their grasp to begin with and so all predictions produce stars that are much more massive than the current average – which is only a fraction of the mass of Sol.
It had previously been though that the first generation of stars would have been the most massive of all, with material equating to several hundred solar masses, similar to the most massive stars currently known – such as those within the Large Magelanic Cloud’s R136 open cluster. This new study casts doubt on this view on the first stellar newborns.
Simulations conducted at NASA’s Jet Propulsion Laboratory in California by a team of researchers lead by Takashi Hosokawa have indicated that such first generation stars would be ‘only’ a few tens of times a massive as Sol.
Whilst they are still significantly more massive than the vast majority of stars in today’s Universe this discovery removes a significant problem for Cosmology.
If the first generation of stars were as massive as previously predicted they would have produced a particular chemical signature of elements during their lives that would be detectably different from those of conventional mass stars – the higher mass equates to a higher core temperature – combine this with the unique blend of elements available to produce the first stars (~75 Hydrogen, ~25% Helium with tiny traces of Lithium, Deuterium and Beryllium) there should be a ‘bar code’ of the particular abundances of various heavy elements (as fusion reaction rates are easily altered by starting conditions) in the oldest of today’s stars that could only be attributed to the existence of such monstrous precursor stars. As of yet this has not been detected.
Whilst this proves to be a problem for the old model of 1st generation stars, the revised masses produced by this study removes it all together – with no such high mass stars the elemental fingerprint would have never have existed easily explaining the lack of detection – there is nothing to detect!
This fingerprint of elements would have been generated by the stars ending their lives as a yet undetected variety of Supernova. The new mass predictions would cause the 1st stars to detonate in a manner akin to their more recent brethren, allowing for the lack of unusual supernova detections to be explained satisfactorily as well.
As we have seen the masses of the 1st stars are now thought to be much lower than first expected but why would they be less massive than previously predicted?
The answer lies in the surroundings of the stars. The new simulations indicate that the region directly around a forming first generation star was heated to temperatures of up to 50,000 Kelvin or 8 ½ times the surface temperature of Sol. This extremely high temperature would have allowed the surrounding gas to disperse much more quickly than previously thought, thus the growing star has less time to absorb material leading to a lower final mass.
The region around one of the first stars as it was forming Image credit: NASA/JPL-Caltech/Kyoto Univ.
The simulation produced a star of just 43 solar masses, a far call from the 1000 solar mass superstars of early predictions (though I should add as predictions became more advanced the mass of the 1st generational stars has been falling continually and this is the latest study in a long line to reduce the value further).
You can read more here
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