The ESO has released a stunning image of the nebula NGC 2736 better known to many as the Pencil Nebula
The nebula is the brightest component of the much larger Vela supernova remnant, unsurprisingly centred on the Vela pulsar. The supernova that created the nebula occurred about 11000 years ago giving the resulting nebula plenty of time to expand and diffuse from the typical appearance of an SNR – a sphere of brightly glowing material.
The nebula is located about 815 light years from Earth in the direction of the southern constellation Vela – the sails.
The initial detonation of the star send the debris that now forms the nebula streaming away at several million kilometres an hour. During the extended time between the initial event and today the material has collided with other components of the interstellar medium – ISM – (the gas and dust found between the stars) slowing it considerably. Despite this the pencil nebula is still hurtling through space at 644,000 kilometres per hour!
The interaction between the nebula and the ISM has produced beautiful twists and folds within the nebula’s structure with the end result being the gorgeous spectacle we see today.
The nebula is about three quarters of a light year in length and was first documented by the British Astronomer John Herschel during his trip to the Cape of Good Hope in South Africa.
You can read more here
NASA’s Chandra X-ray Observatory (along with optical data provided by the ESO and infra-red data supplied by the Spitzer space telescope) has produced a truly amazing image of the star cluster NGC 1929 located within the nebula N44.
The nebula and its star cluster are located in the Milky Way’s largest satellite galaxy – the Large Magellanic Cloud (LMC) – at a distance of 160,000 light years from Earth - 940.6 quadrillion miles.
The star cluster is composed of primarily newborn stars that have only recently been forged from the surrounding material.A great number of these are many times the mass of the sun and produce a precipitous amount of hard radiation and vicious solar winds, before burning out in (on the time scales of the universe) sort order as supernovae generating incredible outpourings of energy.
These shockwaves along with the continual bombardment from radiation and particle stream gouges out massive ‘bubbles’ in the surrounding nebula. The x-ray data provided by Chandra (shown in blue) shows the regions of the nebula that are at the highest temperatures – the areas under the heaviest onslaught of radiation or reeling from one or more shockwaves . The cooler gas and dust as detected by Spitzer is displayed in red with the yellow regions show where the radiation is actually causing the surrounding material to glow in the visible range (this data was collected by the ESO’s Max-Planck telescope).
Astronomers have been having a problem with N44 and other similar ‘superbubbles’ in the LMC for sometime now – they are producing too many x-rays.
Before anyone panics, this is not a medical problem (we aren’t all going to suffer radiation poisoning thanks to a few over-active nebulae in another galaxy), it only refers to the measurements pointing to such nebulae producing more x-rays than could be explained using current knowledge – our knowledge of such objects must be incomplete.
A previous study had suggested that the shockwaves of supernovae impacting the bubble’s walls along with the evaporation of hot material from the sides of the bubble could perhaps explain this anomaly. This set of observations at least doesn’t find any supporting evidence for these ideas though it has been the first time that the observations have been sensitive to distinguish between these and other possibilities so progress is being made.
You can read more here
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.
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.
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.
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!
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.
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.
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.
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.
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.
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%
The different types of nebula consist of different elements in different proportions. Most nebulae that have not been formed by the destruction of dying stars (i.e. SNR (several types -Ia and II – will be described at a later date), planetary nebulae and those generated by Wolf-Rayet stars), contain large amounts of hydrogen gas. These nebulae if given the right conditions to compress and heat up will form the next generation of stars.
All stars, whether they are hypergiants or red dwarfs began their lives as a nebula and rather fittingly as a star dies it returns its material to the cosmos as another nebula. This nebula is either a planetary nebula or a supernova remnant, and it is through this release of matter that the universe is provided with all the elements heavier than hydrogen and helium. This includes all the material that forms the Earth and everything on it, including humans. The oxygen we breathe was formed in the hearts of red giants and the iron in our bloodstream was produced in the final days of a massive star’s existence, right before it ripped itself to pieces as a supernova. It is from this that we get the saying that we are all made of star dust, we quite literally are!
As each generation of stars further enriches the universe by spreading their life’s work as a nebula, the following generation of stars contain more of the heavier elements as there is now more available thanks to the previous generation synthesising (producing) them from hydrogen and helium over the course of their lives. Meaning that each successive stellar generation contains a larger quantity of ‘metals’ – in astrophysics a metal is any element other than hydrogen and helium – this allows different populations of stars to be identified based on their metal content. This variation is due to each successive generation of star forming nebulae contain more and more dust and metals hence creating the different detectable differences in the spectra of the stars they produce.
There are three main populations of stars though I shall keep a description of each for a further post more focused on the topic.
There are several very different types of nebulae but these types will be discussed in depth in further posts.
This is an extension to my post: – Stellar Spectral Classes Explained which can be found here. As I previously explained stars can be placed into groups based on distinguishing features in their spectra. Whilst the main groups have already been discussed there are a few special ones that I think should be given special attention.
Spectral class W stars or Wolf-Rayet stars are spectacular sights to behold. These are high mass stars nearing the end of their lives, and beginning to loose the eternal struggle against gravity. As the star beings to die the nuclear reactions within begin to destabilise, this destabilisation will eventually cause the star to rip its self apart as a supernova explosion blasting all but the core into space at phenomenal speeds and extreme temperatures.
The star can stave of this end by blowing off some its outer layers into space, this is detectable as massive jets of material blasting off into space from a tiny point or shells of material drifting off from its parent star. This mass loss is at best a temporary restpite from the prospect of a supernova and only delays the inevitable the star. In a few short million years this stop gap measure fails to maintain the star’s stability and the unavoidable happens with the star going out with a bang.
As Wolf-Rayet stars are short term evolutions of the rare high mass stars lasting for just a few million years Wolf-Rayet stars are comparatively rare. An example can be found in the Crescent nebula (NGC 6888 – image above).
The nebula formed when the central supergiant began to ‘vent’ its upper atmosphere off to space. The nebula is classed as an emission nebula as it is emitting light of it’s own thanks to the bombardment of ultraviolet light from its parent allowing the nebula to fluoresce as it expands.
As the exact composition or each star is subtly different, along with the countless ways a star can disperse material into space no two Wolf-Rayet nebulae are the same. Indeed with the vast array of factors that influence the overall shape, colour and structure of nebulae radically different results are visible.
Take NGC 2359 for example. Despite being formed in the same way, differing interactions with the interstellar medium have produced a nebula that could not be more different. Its distinctive shape has given rise to its more common name – Thor’s Helmet.
The Wolf-Rayet spectral class is divided into two subgroups: WC and WN. WC stars have their spectra dominated by carbon emission and WN are dominated by Nitrogen.
While not really a spectral class on their own, there are three supergiant stars that I think are stunning enough to get a mention here. One of the most well know supergiant stars is the hypergiant Eta Carinae.
Eta Carinae is located within the small glowing clump half way up the image about three thumb widths in from the left hand side.
Another such hypergiant star is the Pistol star (G0.15-0.05). It is found near the heart of our galaxy – in the central bar rather than one of the spiral arms like Sol or Eta Carinae. It is one of the most luminous stars known to astronomers as it shines with the equivalent output of 4 million
The difference in luminosity is so great the Pistol star releases the same energy Sol does in a year in 20 seconds!!! (This figure is an approximation) It undergoes periodic blasts as it struggles to hold itself together (it is similar to the Eta Carinae system in terms of mass). These blasts have shed stellar material into space which can today be seen as the Pistol nebula (the bright blob at the centre of the image is the star itself).
White dwarfs are the remains of main sequence stars that have lost the majority of there atmosphere to space at the end of the red giant phase. A white dwarf is approximately the size of Earth but as they are the cores of dead stars they are incredibly dense – 1×109 kgm-3 or put differently, if we could extract a one cubic meter of a White dwarf it would ‘weigh’ one million kilograms. This extreme density is a result of confining potentially more than a solar mass of material into a comparatively tiny region of space, think of how large the Sun is compared to the Earth and you will get some idea of the compression required.
Neutron Stars and Pulsars.
Neutron stars are the high density remains of supernovae. They form from the remains of massive stars that have exceeded the Chandrashekar limit. They are composed of exotic degenerate matter and neutrons hence their name. The upper mass limit for a neutron star is approximately 3 solar masses, anything more massive would exceed the Tolman-Openhiemer-Volkof limit and collapse into a black hole (as neutron degeneracy pressure would be unable to support the star against gravity).
A pulsar is a neutron star that has retained enough angular momentum to spin rapidly. They release the majority of their energy in two beams that emanate from their poles. A pulsar can rotate as rapidly as 30 times a second and some rotate even faster than that! When the beams pass in the direction of the Earth the star’s luminosity appears to pulse giving the star there name.
Magnetars are neutron stars with exceptionally powerful magnetic fields. They emit large amounts of X and Gamma rays as a result of this field strength. They are also known as soft gamma repeaters (SGRs) or anomalous X-ray pulsars (AXPs) due to their tendency to emit burst of gamma or X-rays at irregular intervals.
Brown dwarfs now have their own post that goes into some detail.
You can read The Not so Hot Stars by clicking the link.
Sub Brown Dwarfs
Some astronomers feel that a category for ‘failed brown dwarfs’ is needed. This would mean stars that are below the mass limit for brown dwarfs (about 13 times the mass of Jupiter) but significantly above the normal mass of a planet. No such objects have yet been confirmed however spectral Class Y has been suggested, though their is some debate if such objects would be better classified as low mass Brown Dwarfs.
Some of the most spectacular sights in the cosmos come in the form of Planetary Nebulae. The name is a bit of a misnomer – it was first thought that planets formed from such nebulae but now we understand that they are created by the mass release of red giants as they become white dwarfs, however the name has stuck regardless. One of the most famous examples is the Ring nebula or M57.
The Ring Nebula is located in the constellation Lyra at a distance of about 2300 light years from Earth.
Another more delicate but no less beautiful nebula is the Hourglass Nebula – MYCN18.
More information about planetary nebulas and other forms of nebula including a more in depth spectral analysis will be made available through Project Nebula.
- The Worlds with Two Suns | The Young Astronomers on Binary Stars Blitzed – Updated
- Ed.A on Image of the Week – A Peculiar Pencil – 18/09/2012
- Saint on SS 433 – A Magnificent Microquasar
- SS 433 – A Magnificent Microquasar » The Young Astronomers on Binary Stars Blitzed – Updated
- John Fairweather on A Star’s Death Giving Life to a Monster – Recovered
- New Post from @Lightbulb500 - The Worlds With Two Suns - bit.ly/RUQKuk 7 months ago
- We will also be posting about our plans for the next while both here, on the blog and our Facebook page - on.fb.me/RUQCuA 7 months ago
- Sorry for the long delay in posts, we have all been very busy. We will hopefully have a more regular post program shortly. 7 months ago
- Our latest Image of the Week highlights the star cluster NGC 1929 and the surrounding nebula N44 - bit.ly/QbkwY6 - by @Lightbulb500 8 months ago
- New post by @Lightbulb500 - How to Understand Spectra – Part 2 - bit.ly/NveYoX 8 months ago
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