Image of the Week – A shocking Image – 3/09/10
The latest Hubble image of SNR 1987A Credit NASA, ESA, K. France (University of Colordo, Boulder), and P. Challis and R. Kirshner (Harvard-Smithsonian Center for Astrophysics)
I recently produced a post detailing the results of the latest ESO observations of the SNR 1987A (you can view my post here and the ESO article here) .
A team working with the Hubble Space Telescope have imaged a debris ring surrounding the more concentrated debris from the supernova event itself.
This debris is thought to have come from a precursor outburst around 20,000 years before the star finally blew itself to bits. The debris has expanded colliding with the interstellar medium and heating it up in the process. Currently this has created a ring of between 30 and 40 ‘hotspots’ – areas of the medium that are particullary hotter than average. These hotspots glow brightly and are clearly visable in the image.
Current ideas about the evolution of supernova remnants suggest that the hotspots will expand as the age and merge together to form a complete ring around the detonation site, though only time will tell – as this type of long term interactions are difficult to predict for example the medium may be slightly denser causing the expansion to slow on one side and giving an oval, again only time will tell.
To learn more about supernovae remnants try here
Read more about this latest image here
We know how black holes form. Don’t we …?
Recently it seems the more we look into the stars and galaxies that populate this universe, objects we used to thing we had figured out quite well, it has become clear that we are not as educated in the workings of the cosmos as we once thought.
The Mystery Magnetar Credit: ESO
The basic results of star death have been thought to be rather simple. A low mass star, that is one with less than about 10 solar masses forms a white dwarf a medium mass star between 10 and 25 solar masses go supernova and leave behind a neutron star\pulsar. The high mass stars (those above 25 solar masses) also go supernova and create a black hole from their cores’. (The most massive stars are actually theorised to explode and leave nothing – the explosion is so forceful it vaporises the entire star – such an explosion is known as a pair instability supernova but that is for another day).
Astronomers using the ESO’s Very Large Telescope (VLT) have made a very interesting discovery while surveying the super star cluster Westerlund 1, that once again shows that perhaps we don’t know as much as we thought we did. Westerlund 1 is the closest super star cluster to Earth yet discovered, it lies between 3.5 and 5 kiloparsecs (or somewhere between around 11,500 and 16,500 light years if you prefer) away in the southern hemisphere constellation Ara – The Altar.
You may be wondering while the distance figure is less than accurate, unfortunately it is down to the distance being measure itself. It is so far for the more accurate parallax measuring system to be used and so other methods must be employed. These methods can give different results based on different conditions hence the rather large estimate range. Despite this the rough estimate puts the cluster at the outer edge of the Milky Way’s galactic Bar which may go someway in explaining how the cluster grew to such proportions – it contains many high mass stars including a large number of highly evolved supergiants.
The cluster can be seen in its full glory in this annotated image from the ESO’s VLT.
Westerlund 1 Credit: ESO
A larger version can be viewed here.
The labelled magnetar is the star of particular interest in this marvellous cluster. A magnetar is a type of supernova remnant; specifically a neutron star with an incredibly strong magnetic field many thousands of times more powerful than the Earth’s own magnetic field, they are very rare as only a handful have been identified in the Milky Way.
The cluster contains stars that formed in a single formation event over a short period of cosmological time between 3.5-5 million years ago. Using this age figure calculated from the rate of stellar evolution in stars of different masses – the more massive a star is the faster it dies, thus the age of the cluster can be determined by measuring the highest mass star in the cluster (this has been achieved by carefully studying binary systems which allows for the mass of the stars to be accurately measured by detecting stight changes in their orbits). This puts an upper limit on the age of the cluster because had it been any older this star to would have gone supernova.
Neutron stars as I stated above are thought to form from ‘progenitor’ stars of between around 10-25 solar masses. Based on age estimates on the cluster (as detailed above) the magnetar’s parent star weighed in at least 40 solar masses! This means it was well above the mass of a star that was thought to collapse into a stellar mass black hole. Whilst the limit is be no means exact, a star that is almost double the rough limit is very unusual and very interesting.
Whilst no one is quite sure how a magnetar forms as apposed to a ‘normal’ neutron star, it does still have the same basic structure and they are still subject to the gravitational forces that formed them and are constantly trying to crush them further into a black hole. As this has clearly not happened in this case, this single star presents a rather large problem. Some may be quick to say that the accepted mass limit for neutron stars in obviously wrong.
However, as this star seems to be the exception to the rule rather than a common occurrence, perhaps it is circumstances that are to blame, rather than a flaw in our understanding. If the parent star had been contained in a binary system its partner may have removed sufficient mass via mass transfer and accretion and in doing so lowered the mass sufficiently to avoid the total gravitational collapse of the star into a black hole.
Though this idea creates a number of questions too, where is the companion star? As far as we can currently tell the magnetar is alone with out a binary partner though it is quite possible that the force of the supernova detonation blew the pair apart, this helps to explain why the magnetar is on the outer edge of the cluster. Though a massive amount of material would have had to be removed, which makes it a difficult idea for some to accept. Perhaps one day we will know for certain, finding the star’s partner would certainly help.
Who knows the next big discovery could be just around the corner.
I leave you with this artist’s impression video of travelling through the cluster to the magnetar.
Video credit to ESO
Read more about the discovery here and more about black holes here
That isn’t an explosion … THIS is an explosion!
Astronomers working with the ESO’s Very Large telescope (VLT) have made detailed observations on a supernova.
Supernovae, the final explosions for dying high mass stars, are relativity common in the universe with at least one occurring somewhere in the universe every second, this figure may be an underestiate on the true number of supernovae as the figure could be closer to 3 suaernovae per second!
Despite their apparent frequency supernovae generally occur rarely in any one particular galaxy with long gaps in between bursts. This is not always the case though particularly in starburst galaxies in which large numbers of high mass stars are created and die in a short period of time.
These new observations are for SN 1987A and allow us to see the debris in a way never before achieved – in 3D.
The video below shows the structure of the debris in wonderful detail. This is an ESO video and more information on it can be found here.
The video shows everything from the outer most layers of the explosion moving in closer and closer towards the centre becoming a more and more hostile place as it does.
In fact it is the innermost area of this supernova that is particularly interesting. As can be seen near the end of the video, the central debris is deformed and unsymmetrical - it bulges out more in one direction than the others.
This kind of detail is only possible to obtain as the VLT has such high resolution and the supernova’s relative proximity to the Earth. At ‘only’ 168,000 light years at the edge of the Tarantula nebula in the Large Magellanic Cloud, it is the closest observed supernova since the invention of the telescope and was first observed as its name suggests in 1987.
The new observations allow astronomers to conclude that the supernova was more powerful in one direction - hence the outward bulge. This agrees with the more recent computer models on the development of supernovae explosions. However the direction of the bulge disagrees with the what the models expect in relation to the other debris so it seems the models aren’t perfect just yet.
Another interesting fact about this supernova is that it was the first to have been preceded by a detectable neutrino radiation pulse. Neutrinos are a type of subatomic particle and are produced in HUGE numbers at the very start of a supernova. Models predict that as much as 99% of a supernova’s energy can be released in a neutrino surge. The first detection of such an event bodes well for this theory.
A final interesting anomaly with this supernova is that its missing its neutron star. As star that created the supernova is believed to have been a B3 supergiant, the result of the supernova should have been a a neutron star but as of yet no such object has been detected. A number of ideas have been put forward, one is that the neutron star is there but is shrouded too deeply in gas and dust for us to detect it. The second idea is that enough matter fell back onto the neutron star after the supernova that it further collapsed into a black hole hence its absence. The third idea is that the star collapsed into a more exotic for of star – a quark star.
For the moment it is impossible to know which one of these ideas is correct or even if any of them are! Hopefully one day we will reveal all this explosion’s secrets.
Project Nebula – Type II Supernovae Remnants
Type II supernovae remnants (SNRs) are formed when a massive star completes its life cycle and detonates in a massive explosion that releases enough energy to allow the events to be detected from across the universe – A Type II supernova.
Below is an image of the Jellyfish SNR as an example of its class.
- The Jellyfish SNR Credit NASA/CXC/NCSU/K.J.Borkowski et al. Chandra
The supernovae detonations are the only sources in the universe of ALL the elements heavier than iron. These all require the massive amounts of energy created by the explosions to be fused from the lighter elements created in stellar nucleosynthesis -which can only produce the elements up to iron – thus SNR contain the densest concentrations of heavy elements in free space. SNRs also contain radioactive elements and as the Earth’s core contains radioisotopes (it is their decay that maintains the earth’s high internal temperature) the Earth was partially formed in the fires of at least one supernova!! Some cosmologists believe it was a nearby supernova that began the gravitational collapse in the nebula that provided the material to form our sun and the other bodies within the solar system.
As the SNR ages the initial energy begins to disperse however the radioactive decay of the shorter half life radioisotopes created from the initial nucleosynthesis. This illuminates the SNR for several weeks and produces even more new elements in the process. Due to the unusually high concentration of the heavy elements make SNRs easily identified from more standard types of nebulosity. The heavy elements produced are scattered through space and can form the seeds of new planets and potentially new life. A famous type II SNR is SR1604 or Kepler’s SN. This was first observed by the astronomer Johannes Kepler as its name suggests, in 1604.
- Kepler’s SNR Credit NASA ESA JHU R.Sankrit W.Blair
Supernovae detonations are surprising common with at least one occurring somewhere in the universe every second.
The vast majority of these occur in the far reaches of the universe and so despite their awesome power pose no risk to the Earth.
However a close range supernova – within around 100 light years – could cause significant problems for us. The vast quantities of charged particles and radiation released by the explosion would overwhelm the Earth’s protective magnetic field causing serious radiation damage to the world’s ecosystem. The planet’s shielding ozone layer would likely be destroyed this would expose any survivors of the initial event to increased levels of solar radiation over the long term elevating the occurrences of cancers and other radiation related health conditions. Thankfully such an event is exceptionally rare with very few stars of high enough mass to go supernova within the danger zone around the solar system. Despite their destructive power they play a vital role in the production and the dispersion of high mass elements (those higher in mass than iron). Two stars that you may want to pay close attention to are Betelgeuse (around 640 light years from Earth) and Eta Carinae (around 7500-8000 light years from Earth), both of which are in the final stage of their lives and are expected to go supernova within the next few thousand years.
However as they are far enough away from the Earth we will be able to enjoy the spectacles without suffering any ill-effects.
To finish here are two images of the spectacular Casseopia SN – Enjoy 
First is the SNR in visible light as seen by the Hubble Space Telescope
Cassiopeia SNR Credit to NASA ESA HST
and perhaps the more spectacular combined, X-ray, visible and Infra-red image.
Cassiopeia SNR Credit: X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA/JPL-Caltech/Steward/O.Krause et al.
The Antennae Galaxies
NASA has just released this beautiful image of the Antennae Galaxies.
The Antennae galaxies - Credit: NASA, ESA, HST, Chandra, Spitzer
This image is actually a combination of several images taken by several space observatories over several years. The image shows data from the Chandra X-ray Observatory (shown in blue), the Hubble Space Telescope (shown in gold and brown), and the Spitzer Infra-red Space Telescope (visible in red).
The Antennae Galaxies are in the process of merging to form one large galaxy. They are located around 62 million light years away from the Earth and take their name from feathery arm like structures visible in wide shots of the area. These were caused by gravitational interactions between the two galaxies throwing material into space as the two merge.
The two began merging around 100 million years ago and the process is still ongoing. The merging of two galaxies is a long process that causes many changes with the galaxies themselves and completely changes their structure. Gravitational influences compress huge volumes of interstellar dust and gas creating a wave of star formation which can spread all over the galaxy. Some of the gold and white areas of this image are areas of intense star formation. Such a wave of star formation bears a heavy price for the galaxies involved. Most if not all of the star forming material is used up in the ‘Starburst’, this in turn means that the galaxies star formation can come to a quick end, never to be rekindled.
As in all galaxies high mass stars are formed and die very quickly in merging galaxies, they explodes as supernovae and distribute a huge amount of mineral wealth enriching the surrounding area and providing the dust that can form planets. In the Antennae galaxies a large portion of this debris is still hot enough to produce large amounts of X-ray radiation and so can be picked up by Chandra and shown in blue on the image.
The areas of red are the warm star forming regions of the galaxies. Unsurprisingly the most active areas are in the overlap between the two galaxies, showing the beginnings of a potential starburst that is yet to spread to the main galaxies, yet millions or stars have already been formed.
The bright points of light are actually produced by black holes and neutron stars accreting and absorbing matter, the matter becomes extremely hot and emits large amount of radiation before finally becoming absorbed. Some of the black holes may have as much as 100 times the mass of the sun.
Our galaxy may seem dull in comparison but it may one day be just as chaotic. In around 4.5 billion years the Andromeda galaxy may collide with the Milky Way (our galaxy) with the results being just as spectacular. However the two galaxies may miss each other, or brush close past it is currently impossible to tell. The effects on the Solar System are not something to be concerned about as any chance of direct effect of the system itself is very remote, though it may be flung out of the galaxy completely. It is probable by this time that the Earth will have been uninhabitable for billions of years due to the increasing warmth of the sun. In any case it is not something to be concerned over 
.
Read more about the Antennae Galaxies here
Image of the Week 06/08/10 – WR 22
This week’s image comes from the ESO, and is of the star WR 22 and its surroundings.
WR 22 Credit ESO
The image was captured by the ESO’s La Silla Observatory in Chile.
The main focus of the image is the star WR 22 which is in the heart of the image. For those of you with some knowledge of how stars of named may have deduced that this is a Wolf-Rayet star. If you have you are correct. For those readers who are unfamiliar Wolf-Rayet stars, they are bright blue giant stars that are sheding vast quantities of their atmosphere into space in a vain attempt to remain stable. Soon enough Wolf-Rayet stars detonate as supernovae. They are spectral class W stars and can be designated by WR followed by a number.
This particular star is located in the Carina Nebula (NGC 3372), the same nebula that houses the monster star Eta Carinae. It is around 5000 light years from Earth and can sometimes be seen with the naked eye if conditions are good. As the nebula’s name suggests it can be found in the Southern Constellation Carina.
WR 22 is around 70 times the size of our sun and is located within a binary system.
As well as the star itself this image contains some of the surrounding nebula, containing mostly hydrogen that has been ionised by the harsh ultraviolet light of nearby high mass stars including WR 22 (this is shown as pink in the image). The nebula also has darker, denser, dustier regions that may be forming new stars within, some of which can be seen in this image.
Finally – Thanks to Alice who stepped in to do last week’s IOTW whilst I was on holiday and did a marvellous job .
Revolutionary Astronomical Words
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”. Apart from the odd motions planets made in comparison to the rest of the stars in the sky – which they did because they, like the Earth, were orbiting the Sun – there was no way of knowing in those days that they were any different from stars. Both looked like points of light. But stars are millions of times more massive than planets, and they give off light for entirely different reasons. To be fair on the ancient world, 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 were named as such 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.
(Picture 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 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, by Fred 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.
(Picture credit: good old Wiki.)
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 was absolutely 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
Project Nebula: Types Of Nebulae
As mentioned in the informational introduction to the project, there are many different types of nebulae and each type is very different from the last. This project aims to provide information on all the types of nebulae. What follows is a list of the types of nebulae covered in this project along with a brief description of each. Each type of nebula will be covered in more detailed in the relevant section of the project.
Absorption Nebula
Elephant Trunk. Credit to Spritzer NASA/JPL-Caltech/W. Reach (SSC/Caltech)
These are dense clouds of dust and gas that are dense enough to block out the light from stars and other brighter nebula situated behind the absorption nebula itself. Very large absorption nebulae and clusters (groups of nebulae) are associated with Giant molecular clouds – large volumes of interstellar dust and gas often associated with star forming. Dense but isolated absorption nebulae are known as Bok Globules (see below). The examples of ‘standard’ absorption nebulae used in this project are – The Cone Nebula, Horsehead Nebula, and the Elephant Trunk Nebula.
Bok Globules
Are isolated dense nebulae as stated above. They contain no more than a few hundred solar masses, given the right conditions undergo gravitational collapse, and form new stars. The Bok globule looked at in this project is IC 2944.
Emission Nebulae
Emission nebulae are so called as their components emit light making the nebula itself luminous. This can be caused by ultraviolet florescence – a hot star (or a group of stars – usually of spectral class O or B) found within the nebula emits large quantities of ultraviolet radiation. This collides with the particles and ‘excites’ the (gives them extra energy). This in turn causes them to emit light as a way of shedding their extra energy and the nebula glows as a result. The emission nebulae covered in this project are: – The Carina nebula, The Flaming Star Nebula (which is also a reflection nebula), The Lagoon Nebula, the Rosette Nebula and the Tarantula Nebula.
Reflection Nebula
Reflection nebulae do just as you would expect – they reflect the light of nearby stars. This light does not carry enough energy to ionise and excite the material of the nebula and so the nebula does not emit its own light but scatters the light falling on to it. Reflection nebulae can be found intertwined with intertwined with emission nebula, such features are known as diffuse nebulae. The reflection nebulae looked at in this project are: – Corona Australis Complex and the Rho Ophiuchi Complex.
Planetary Nebulae
Cat's Eye Nebula Credit; NASA, ESA, HST, Chaldra
These nebulae form from the outer layers of dying stars. They are often seen to take the form of intricate shapes and take various colours. It is important to note that some of the images of planetary nebula have been false coloured to highlight certain chemical elements within them and thus these pictures do not accurately represent the true colours of the nebula. Interestingly these nebulae contain high proportions of relatively rare elements such as oxygen and sulphur (sulfur). The planetary nebulae covered in the project are: – The Ant nebula, The Cat’s eye nebula, the Eskimo nebula and the Dumbbell nebula.
Proto-planetary Nebulae
These are young planetary nebulae still receiving matter from the dying star that created them. Due to their youth many have very unusual shapes and structures. The two covered in the project are: – The Red Rectangle and Gomez’s Hamburger.
Nova Debris
The project touches briefly on the debris created by a Nova explosion. Nova Cygni 1992 is the only example of such an object covered in the project. These are formed when a white dwarf accretes enough matter off is binary partner to ignite a burst of nuclear fusion in its outer layers which blows off into space.
Supernovae Remnants (SNRs)
The project also covers two types of debris left by supernovae (SN) of different classes. The project covers those created by Types Ia and II supernovae. SNRs are very interesting as they contain the highest concentrations of heavy elements (those more atomically massive than Iron) in the universe as such elements are only formed within the fires of supernovae.
The Type Ia supernovae examined are Tycho’s SN and SN 1994D
The project covers three Type II SN these are the Cassiopeia SN, IC 443 and the Jellyfish SNR.
Proto-planetary Disks
These disks of dust and gas surround young stars and can accrete to form planets. Whilst technically not nebulae they have been included to demonstrate how planets can form in a similar way to stars using different mediums.
The two examined are the disks surrounding Beta Pictoris and HD 107146.
Hannah has previously written a post on the new planet discovered orbiting Beta Pictoris which can be viewed here.
Herbig-Haro Objects
Herbig-Haro objects or HHs are the result of the bipolar out flowing of material from protostars and very young main sequence stars. In simple terms they are formed when their parent star ejects dust and gas (this is possible as young stars have yet to fully stabilise), this material is forced rapidly away from its parent and collides with the interstellar medium. This then heats up and emits light – the HH itself.
The HHs covered in the project are HH1, HH2, HH32 and HH47.
Wolf-Rayet Generated Nebulae
These are the resulting clouds of gas and dust formed as Wolf-Rayet stars shed mass to space. The one studied in the project is the Crescent Nebula.
Special Examples
Helix Nebula Credit; Spritzer, NASA
Four Nebulae have been selected for a more in-depth study in the project for their beauty and splendour. These four will have special posts written covering each nebula separately and in detail. The four nebulae selected for this section are: -
- The Crab Nebula
- The Orion Nebula
- The Eagle Nebula
- The Helix Nebula
We hope this post has provided a brief outline of the types of nebulae and of the aims of this project. The project is now entering its main informational phase in examining each of the nebular types individually and in detail.
Why not to marry a white dwarf *star*.
This is a follow up to the post -Binary Stars Blitzed (http://ya.astroleague.org/?p=491).
Binary star systems can operate normally for billions of years without any problems, the difficulties arise when the more massive star begins to die. As it expands into a red giant the less massive star may be completely enveloped by its partner’s outer layers, whilst this does not significantly affect the star it is a sign of things to come. After the more massive has collapsed into a white dwarf (assuming it didn’t go supernova (Type II) – which would have blown the other star out of the system) the partnership develops into something that can be devastating. Depending on the physical type and the mass of the still main sequence star various interactions may occur.
As the main sequence star expands into its red giant stage it fills its Roche lobe (the area of space in which its gravity is the strongest gravitational force present). If it exceeds the lobe matter transfer will occur and the white dwarf can begin to take on mass from its partner (this process is also detailed in Binary Stars: – Blitzed http://ya.astroleague.org/?p=491). Eventually enough mass falls onto the surface of the white dwarf for a burst of nuclear fusion to occur. This causes a drastic increase in the luminosity of the white dwarf which can go from a luminosity of around 0.02 suns (2% the luminosity of our sun) to over 100 suns in just a few hours (10000% the luminosity of our sun). The fusion does not last as it is only occurring in the uppermost region of the white dwarf which will slowly return to its normal brightness. This ‘flash fusion’ is known as a Nova. The white dwarf and its partner are largely unaffected by this explosion and the process of mass transfer can continue and the cycle of novas can continue many times. There is a large amount of debris created by a nova at this expands outwards after the event occurs. The next two images show the aftermath of a nova created by the binary star system Nova Cygni 1992. The first image was taken in 1993 fifteen months after the initial explosion and the second taken seven months after the first. It is clear that the debris expanded into a wave which travels through the surrounding space heating it and emitting large amounts of EM radiation in the process
Nova Cygni 1992 - Credit NASA, ESA,; HST Special Credit: F. Paresce, R. Jedrzejewski STScI
The term nova comes from the Greek word for new. This name came about as ancient Greek astronomers thought that a nova was the birth of a new star more recent scientific studies allow us to understand the really nature of these events but the name has stuck.
Some white dwarfs go too far however. The mass transfer between a giant star and a white dwarf can be maintained for several million years as the amount of matter transferred is comparatively small. If the mass transfer is very much larger than normal enough matter can be sucked onto the white dwarf to push it past the Chandrasekhar limit. The resulting runaway nuclear fusion reaction rips the star apart entirely leaving only dust and gas behind in a remnant that expands and cools. As a star has exploded this is classed as a supernova but the force of the explosion is much greater than a ‘normal’ Type II supernova so these events fill the Type IA class of supernovae. The white dwarf’s partner star may be destroyed in the blast wave however it seems more common that the star is ejected from its position in space and travels outward from the explosion site as a rogue star.
SN 1994D Credit NASA, ESA; HST
These explosions are so powerful and emit so much energy that they can temporarily outshine their host galaxy (the galaxy in which the supernova takes place). One such example is the Type IA supernova that took place in the outer reaches of the lenticular galaxy NGC 4526. The supernova (SN1994D) is clearly visible in the image to the right. The explosion itself shone with the equivalent brightness of 5 billion Sols!
Tycho's SNR Credits in image
Perhaps the most famous example of a Type IA supernova is the Tycho’s supernova remnant or SN 1572. This supernova was documented by the astronomer Tycho Brahe four hundred years ago and thanks to History detailed and accurate records of the event today’s astronomers have been able to calculate the position of the remnant. The remnant can be seen below. The image has been taken in the X-ray band of the Em spectrum – this helps us understand just how powerful this explosion was and still is today. X-rays are only produced by objects that are at temperatures of several million Kelvin – the fact that the nebula is still emitting large quantities of X-rays today and that it has expanded to a size of 24 light years across in just 400 years (24 light years is a BIG distance) is tantamount to the massive energy released by the initial blast.
An artist’s impression of a Type Ia supernova
Artist’s Impression of a Type IA Supernova Credit for the video is to the ESO Source http://www.eso.org/public/videos/eso0943b/
I prepared a diagram to show the final possible steps in a star’s life cycle. The diagram follows below but please note all copyrights are reserved.
Description above. Copyrights reserved
Project Nebula – What is a nebula?
This is the informational introduction to Project Nebula and covers the generals on nebulae.
As with all sections of Project Nebula this post has been co-written by PeterC and HannahH.
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 both types covered in this project (Ia and II), 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 is was formed in the hearts of red giants and the iron in our blood stream was formed in the final days of a massive star’s existence 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 via spreading their life’s work or heavier elements as a nebula, the following generation of stars contain more of the heavier elements as the previous generation have synthesised (created) them from hydrogen and helium. As they form from the ashes of their predecessors, and this means 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 populations.
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.
There are three main stellar populations.
Population III – These were the stars formed at the very beginning of the universe and if current theories are correct they where many hundreds of times more massive than the sun and are thought to have contained no metals (except for small traces of lithium as this to was formed at the start of the universe in very small quantities). Due to their high mass these stars would have quickly expended their supply of hydrogen and then went supernovae. As they where the first generation of stars they formed from the initial products from the creation of the universe, and despite their name which may be counterintuitive to some as it could be misunderstood as describing the most metal rich stars. This however is incorrect as the populations are named in order of their discovery, not their increasing metallicity. It is understood that all population III stars will have gone supernova many billions of years ago and now may only exist in the light coming from very distant galaxies which itself is billions of years old. Currently no population III stars have been detected, however many astrophysicists are currently looking for possible candidates to confirm or disprove their existence and thus too find conclusive evidence that the current theories are correct or as a way of disproving them. If any such stars are detected close to the Milky Way it would mean that they existed much more recently that thought, and thus the accepted age of the universe as around 13.5 billion years would be grossly inaccurate.
Population II stars formed from the nebulae created from the death of the population III stars. As such they contain some metals, but are still considered metal poor. Only the longer lived, low mass stars of this population are expected to still exist as the high mass stars have long since burnt out. Population II stars are most commonly found in globular clusters, and in the halos of galaxies as here the level of enrichment is lower as there is a smaller number of high mass stars and thus a smaller number of supernovae and so fewer heavy elements being distributed amongst the interstellar medium.
The globular cluster NGC 6752 Credit ESO Source - See below
After the population II stars began to die the current generation of stars began to form from their ashes. Population I stars are the most common variety in today’s universe and are considered relatively metal rich. Our sun, Sol, is a population I star but despite being considered metal rich currently Sol is only around 1.6% metals the rest being mostly hydrogen with some helium. Population I stars are found mostly in the spiral arms (which also contain some population II stars) of spiral galaxies (like Sol in the Orion arm of the Milky Way) or more generally closer to the centres of galaxies than population II stars. This graduation is because, in the more matter concentrated areas of galactic centres there are more likely to more massive stars being born and thus a higher rate of supernovae and thus a faster increase in the levels of metals in the star forming nebulae near the galactic core.
In the same way, a smaller galaxy is more likely to contain a higher proportion of population II stars than a larger galaxy as there is a smaller number of star deaths in a given tie giving a slower enrichment rate and a lower level of metallicity.
There are several very different types of nebulae but these types are discussed in depth in the next section of the project.
A nebula can hold two of the most important jobs in the universe – to create new stars from molecular hydrogen or it can spread the nuclear products of a star’s life into the wider universe enriching the surroundings and allowing for planets and life to form. It is for this reason that Hannah and myself decided to give nebulae the chance to be viewed by more people as the wonders that they truly are.
ESO Image sourced from – http://www.eso.org/public/archives/images/medium/eso0107a.jpg
