Image of the Week – A Peculiar Pencil – 18/09/2012

The ESO has released a stunning image of the nebula NGC 2736 better known to many as the Pencil Nebula

The Pencil Nebula – NGC 2736 Credit: ESO

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

Image of the Week – A Superb Superbubble – 08/09/2012

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.

N44 and NGC 1929 Credit:X-ray: NASA/CXC/U.Mich./S.Oey, IR: NASA/JPL, Optical: ESO/WFI/2.2-m

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

The Burning Heart of the Stars – Part 1

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

What is a Nebula? – Updated

A nebula is a cloud of dust and gas found within the interstellar medium filling the great voids between the stars within galaxies and star clusters.

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.

One such star forming nebula - The Eagle Nebula Credit NASA, ESA; HST

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!

The Helix Planetary Nebula Credit NASA, ESA; HST Perhaps one day a new star will for from the ashes of the star that produced this lovely sight.

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.

 

Spectral Classes Part 2 – Special Spectral Classes – Updated

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.

Wolf-Rayet

NGC 6888 Credit: NASA

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.

Thor's Helmet Credit: Andrew J Dumbleton & iTelescope.Net

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.

Supergiants

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 and the Homunculus Nebula Credit: Nathan Smith (University of California, Berkeley), and NASAESA

The star is a massive 100  which is close to the theoretical upper mass limit possible for any star. If a star was to be much more massive, it would tear itself apart through radiation pressure – the force produced by the star’s nuclear reactions. Eta Carinae is expected to go supernova within the next few million years. When it explodes the star will shine with many hundreds of time its normal luminosity (potentially being visible in the daytime here on Earth) while the resulting debris may even form a black hole. Whatever matter escapes the formation of the black hole will enrich the surrounding space with the heavier elements required for planet formation and which form the seeds of life.
The star is actually a binary pair with one star orbiting its much more massive partner. The pair are embedded within the Homunculus Nebula – the lobes of material in the above image that are believed to have been released during the supernova impostor event in 1841. During this event the pair brightened to a level just short of that of a real supernova. The stars survived the detonation though even well over a century later their internal structures have not yet fully recovered. This was the prototype event of its class and may be a sign that the star is approaching supernova as a similar incident was recorded in another galaxy two years before the true supernova.
The Homunculus Nebula, and by extension Eta Carinae lie around 7500 light years from Earth in the direction of the southern constellation Carina – The Ship’s Keel – within the larger Carina Nebula.

The Carina Nebula Credit: NASA, ESA, N. Smith (University of California, Berkeley), and The Hubble Heritage Team (STScI/AURA)

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).

The Pistol Star Credit: NASA

Our final supergiant is XX Triangulum (HD 12545). This is a red supergiant star with luminosity of around 100, however its most interesting feature is not its size or colour but its temperature distribution – It has been revealed that one hemisphere is cooler than the other. The cooler hemisphere has a dark region like a sunspot on Sol but at a size that dwarfs Sol. Like the sunspots on our own star, magnetic fields are thought to be responsible for this unusual feature.

HD 12545 Credit to NASA

White Dwarfs.

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 – 1×10kgm-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.

All White dwarfs must have a mass lower than about 1.5 solar masses as this is the Chandrashekar limit – if the star was any more massive the force supporting it against gravity (electron degeneracy pressure) would be overwhelmed and the star would collapse further and then detonate as a type Ia supernova.
White dwarfs are given the spectral classification D. An example of a White Dwarf is Sirius B – the small dim companion to the brightest star in the sky:

The Sirius System - Sirius B is the small dot in the lower left Credit: Credit: NASA, ESA, H. Bond (STScI), and M. Barstow (University of Leicester)

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.

Pulsars slowly slow down and so the period of one pulse cycle (the time taken for the star to rotate once on its axis) increases as the star’s velocity lowers due to drag and eventually (after an exceptionally long time) the star will stop spinning all together. The energy emitted by neutron stars is the release of thermal energy as the star cools – it is not releasing any energy by nuclear processes, this process ended when the star went supernova. Neutron stars and pulsars are usually white and so fall in the F spectral category.

Magnetars

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

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.

Planetary Nebulae

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.

M57 Credit: NASAESA The Hubble Heritage Team (AURA/STScI)

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.

MYCN18 Credit: In image

The nebula is slightly tilted towards us so we are looking down through the top of the formation. The dense green glob and the centre contains the dying star.
Unfortunately planetary nebulae do no last long as the are only tenuous clouds of gas and dust illuminated by their dying parent. After a few ten thousands years the nebulae expand so far they become to diffuse to illuminate and they then disperse into space and fade from view. Thankfully they put on one heck of a show while they can!

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 Enigmatic Eagle

The Eagle Nebula is one of the most well known regions in the universe having been snapped many times over the years by several telescopes including Hubble.

The latest images of the region come from the ESA’s Hershel Infrared Space Observatory and the XXM-Newton X-ray Observatory.

The Eagle Nebula seen by Hershel and XXM-Newton Credits: far-infrared: ESA/Herschel/PACS/SPIRE/Hill, Motte, HOBYS Key Programme Consortium; X-ray: ESA/XMM-Newton/EPIC/XMM-Newton-SOC/Boulanger

This image spans approximately 75 light years across the entirety of the nebula.

This image is a combination of data from both telescopes of the dense central region of the nebula. We can learn more about the information the image displays if we separate the data from each observatory, first lets have a look at the XXM-Newton X-ray data.

XXM data of the Eagle Nebula Credits: ESA/XMM-Newton/EPIC/XMM-Newton-SOC/Boulanger

Each individual dot on the image is an X-ray source with  the various colours indicating the energy of the X-rays being emitted by the source, red being the lowest energy (0.3-1keV) working up through medium energy sources shown in green (1-2keV) to the highest energy sources displayed in blue (2-8keV).

The XXM was observing the area to help determine the source of the Eagle Nebula’s strong emission. One theory suggests that a hidden supernova remnant could be supplying the nebula with large quantities of energy whilst remaining obscured by the nebula’s dense cloud. To help determine if this theory is valid the XXM is scouring the area in an attempt to detect any sign of a faint X-ray emission extending from the central region. The scientists believe that if the XXM doesn’t detect any more emitting material than has already been identified by previous searches using Sptizer and Chandra this will be strong support of the hidden SNR explanation.

Now lets examine the Hershel data:

Hershel's view of the Eagle Nebula Credits: ESA/Herschel/PACS/SPIRE/Hill, Motte, HOBYS Key Programme Consortium

This displays the nebula in infra red wavelengths with 70 microns displayed in blue, 160 microns in green (both of these wavelengths were captured using filters in the PACS – Photodetector Array Camera - instrument) and finally 250 microns in red(images by SPIRE - Spectral and Photometric Imaging Receiver).

All these wavelengths are associated with very cold gas, indeed any gas displayed in blue here is just 40K above absolute zero down to that displayed in red which is a chilly 10K.

The twisted gas tendrils are still collapsing and will continue to form the next generation of stars for quite some time yet before the nebula finally disperses. Perhaps the most  famous region within the nebula are the ‘Pillars of Creation’ which are in the above images which can be viewed just below the central point in the image (the eagle for which the nebula is named is located half way up the image on the left hand side, with its head pointing inwards). Indeed the Pillars are the central feature in one of the most recognisable image in all of astronomy:

The Pillars of Creation as seen by Hubble Credits: NASA/ESA/STScI, Hester & Scowen (Arizona State University)

The Pillars of Creation as seen by Hubble Credits: NASA/ESA/STScI, Hester & Scowen (Arizona State University)

The image was taken by Hubble in visible light using filters that isolate emission from excited hydrogen (Hα), singly ionised sulphur (SII) and doubly ionised oxygen (OIII). For scale, the tallest pillar is approximately four light years in height.

Now if we look at the same region in the infra red part of the spectrum (this time the data is provided by the ESO‘s, VLT’s ANTU telescope using the ISAAC instrument – yes that is quite a lot of acronyms), it looks completely different.

The Pillars of Creation as seen by ANTU Credits: VLT/ISAAC/McCaughrean & Andersen/AIP/ESO

At these wavelengths all but the densest regions of the Pillars are virtually transparent allowing us to gaze in wonder at the clumps of stars forming at the tips.

I leave you with this composite image, containing X-ray, visible and infra red data, enjoy.

Composite image of the Eagle Nebula Credits: far-infrared: ESA/Herschel/PACS/SPIRE/Hill, Motte, HOBYS Key Programme Consortium; ESA/XMM-Newton/EPIC/XMM-Newton-SOC/Boulanger; optical: MPG/ESO; near-infrared: VLT/ISAAC/McCaughrean & Andersen/AIP/ESO

You can read more about this fantastic collection of images here.

A Heavenly Veil – Updated

This truly stunning image of the Eastern Veil SNR was released at the very end of 2010 by the Issac Newton Group of Telescopes.

NGC 6992 Credit: A. Oscoz, D. López, P. Rodríguez-Gil and L. Chinarro

The nebula is located approximately 1470 light years from Earth and was produced by a detonating star that died between 5000 and 8000 years ago.
The nebula is the visible portion of the much larger Cygnus Loop and is divided into several arcs, with the image above showing part of the eastern section. Since it’s formation the remnant has expanded to a size that makes it appear to have a diameter around 6 times that of the full moon, or 36 times it’s area when viewed in the night sky. This translates to roughly 50 light years in physical diameter.

The loop is one of the brightest features in the X-ray skyscape as viewed from Earth. The nebula contains large quantities of hydrogen, sulphur and doubly ionised oxygen (OIII) each of which have been picked up in the filters used by the Newton Telescopes. They are displayed in the image as red, blue and green respectively.

The classification name given to this section is NGC 6992 of the nebula, and the Eastern section is also happens the brightest region of the loop.
The nebula was first observed by William Hershel in September 1784.
As the nebula is part of the Cygnus loop it can be viewed in the constellation Cygnus and is most spectacular when viewed through an OIII filter.

The Western Veil Nebula Credit: Nick Howes

You can read more here.

Image of the Week – A New Look at the Helix – 19/01/2012

I’m sure all of are aware of NGC 7293\Caldwell 63. No? Perhaps if we use its more common name – The Helix Nebula – we can jog your memory a little.

The ESO has used the VISTA telescope shows the nebula in a way that has never been seen before.

VISTA's view of the Helix Nebula Credit: ESO/VISTA/J. Emerson. Acknowledgment: Cambridge Astronomical Survey Unit

This image shows the nebula in infra-red radiation which reveals the details of the cool gas and dust structures within the nebula which ironically aren’t visible in images taken in the visible range of the spectrum. The image reveals the exquisite sub structure of the inner rings as well as the faint trails on the outskirts of the nebula.

The Helix is a planetary nebula produced by a dying star flinging off its outer layers into space. The central star is visible as the tiny blue dot in the centre of the structure, within a few short million years the star will have fully transitioned to a white dwarf and the nebula will dissipate into the interstellar medium leaving nothing but the faint, cooling remnant.

It is loacted in the direction of the contellation Aquarius at 695 (+98/-52) light years from Earth and spans a region of approximately 2.5 light years at its widest point.

A comparison between the Infra-red image and a visible image of the Helix Nebula Credit: ESO/VISTA/J. Emerson. Acknowledgment: Cambridge Astronomical Survey Unit

You can read more here

The Spiral Dance of LL Pegasi – Recovered

Hundreds of light years away in Pegasus there lies two stars locked in a gravitational dance. One star is- as far as astronomers can make out – a blue star and the other, called LL Pegasi, is a much older carbon star nearing the end of its life, and is doing so in a spectacular fashion!

Credit: ESA/NASA & R. Sahai

This is IRAS 23166+1655, it’s a beautiful sight, and one that has sparked a great amount of interest amongst astronomers recently. You can’t see the binary system itself, as it’s shrouded by the spiral of gas and dust, causing the starlight to be completely blocked. But if observed in the near-infrared you can see the two stars, revealing what is going on at the centre.

The carbon star is dying; with every pulsation another layer of the star is thrown off, creating what is known as a pre-planetary nebula; a short period in the stars life just before the final product – a planetary nebula – is created, with the temperature having risen to 30,000 Kelvin, ionizing and lighting up the gas and leaving only the core of the star behind.

With every complete orbit the star makes every 800 years or so in its gravitational dance with its companion another spiral shell is formed, with the material rushing out at 50,000 km per hour!

The ionization has yet to take place to create the final planetary nebula, so how is it that you can see the pre-planetary nebula at all? If you look carefully at the spiral you’ll notice the right side is more lit up than the other, this side of the spiral happens to be the nearest to the galactic plane where the highest concentration of stars is and therefore the highest concentration of starlight. The nebula is lit up by the starlight!

The paper written on LL Pegasi by M. Morris et al has a diagram showing where the galactic plane lies in relation to the nebula, so do have a look! It’s also a very interesting paper to read ;)

You can read more here, and the paper is here (in PDF format)!

The Smothered Supernova – Recovered

As I am sure, many of you know high mass stars end their lives in powerful explosions – supernovae. These explosions are among the most powerful single events in the universe and can be detected across vast distances; one has been detected in a galaxy 3 billion light years away from Earth, but this particular supernova is a little bit out of the ordinary.

The Smothered Supernova - Artist's impression - Credit: - NASA/JPL-Caltech/R. Hurt.

The explosion was detected by astronomers using NASA’s Spitzer Space Telescope whilst they were surveying the distance universe forAGN (Active galactic nuclei). The survey used Spitzer to detect the large amounts of infrared radiation (IR or heat) emitted by the AGN. As they searched through the data, they discovered a very hot area, which was emitting huge amounts of IR radiation from its centre. The astronomers found that the cloud did not fit the standard model of an AGN and data from the galaxies visible light spectrum lacked any sign of an AGN (this was confirmed using data from the ground based Keck Telescope in Hawaii).

It was concluded that the heat source was a very powerful supernova or hypernova. Whilst this is not the first hypernova to be detected, it is unusual in that the vast majority of the energy released in the six-month flare up during the event was in the IR radiation band. More normal supernovae release the majority of their energy in the visible range (along with UV, X and gamma rays).

The astronomers concluded that the explosion must have been muffled somehow, with most of its higher energy light photos being absorbed and converted into IR before being re-radiated. The solution comes from the activity of the star itself. As it is projected to have been around 50 times the mass of the sun, it would have been very unstable as it neared the end of its life. In a final effort to keep itself from blowing apart, it would have shed chunks of its atmosphere into space forming expanding dust shells around itself.

Studies of the area of the galaxy the supernova was detected in show evidence of at least two such shells, an outer one emitted around 300 years before the supernova with the second lying much closer to the star as it was released much closer to the time of the supernova (around 4 years prior to the main event). When the star finally exploded the majority of the energy released as high energy light (visible, UV, X and gamma rays) was absorbed by the dust shells, warmed them up to a temperature of around 1000 kelvin (just above the surface temperature of Venus) and then was re-emitted to the universe as IR radiation.

The star may brighten again in around a decade as the shockwave produced by the supernova smashes the two dust clouds together. Many more such supernovae may be detected in the data provided byNASA’s WISE spacecraft. We may not even have to wait that long for such a supernova to occur considerably closer to home – one of the brightest stars in the Milky Way – Eta Carinae is expected to go supernova in a similar way within the next few millennia.

You can read more here.