The Young Astronomers
Currently viewing the tag: "Star"
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
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.
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.
, 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 – 1×109 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.
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.
When we look up at the sky we see many thousands of stars twinkling away serenely. In towns and cities many of the stars look almost the same, with the only real difference being how bright each individual mote of light appears. Travel further from the plague of light pollution and look up again, more differences are now apparent. Not only does each star shine at its own brightness each has its own colour. I have previously explained how the colour of a star allows us to calculate its temperature, with this post I hope to go into more detail about spectra how can be used to tell us many more things about the diverse range of objects that fill the universe.
So first of all …
What Exactly is a Spectrum?
In the simplest terms possible, a spectrum (plural – spectra) is a way of examining the light coming from an object. Depending on what wavelengths of light are present, that is another way of saying what colours of light their are, along with their relative proportions you can tell a great deal about the object from which the light is coming from or passing through, in a lot of cases in astronomy – both!
There are three broad types of spectra:
- Black body
- Emission and
- Absorption
Black Body Spectra
A black body spectrum is the pattern of electromagnetic radiation given out by a perfectly black object – that is one that absorbs all incoming electromagnetic radiation perfectly – that is dependent on its temperature.
A black body spectrum is a smooth, continuous spectrum with no irregularities. Moving from the longer wavelengths of lower energy to the shorter more energetic, the black body spectrum increases slowly, then increases more sharply to a peak before dropping off quickly. Increasing the temperature of the black body raises the peak and shifts the spectrum to the left – that is to the more energetic wavelengths. You can see more about how temperature affects a black body spectrum by opening the applet below. Note: It will open a new tab in your browser and may require a plugin to be installed to work properly.
| Click to Open |
For those interested, the peak wavelength of a black body spectrum, that is the wavelength that is emitted most strongly at a particular temperature can be determined by using Wien’s displacement law -
Where:
Lambda max is the peak wavelength in meters
T is the temperature of the black body in Kelvin and
b is Wien’s displacement constant = 0.0028978 meter Kelvins
Absorption Spectra
A perfect black body does not exist in nature, there will always be little bumps or dips in the spectrum. In fact stars show this exceptionally well, each have a spectrum very close to that of a black body but they also show features that deviate from the perfectly formed curve of a true black body.
Lets have a look at an example, the Sun has an effective temperature of near enough 5780 degrees Kelvin (5777K to be particular), so if the sun was a black body we would expect its spectrum to look like this:
The Black Body Spectrum of an Object at 5777 Degrees Kelvin Created using material sourced from here
In reality however the Sun’s spectrum looks like:
Solar Spectrum at Sea Level Source
Clearly these two spectra are very different what could be the cause?
Well the main cause is our own atmosphere, the gases within the atmosphere absorb a good portion of the incoming radiation that arrives from the sun before it reaches detectors on the ground. This accounts for the large gaps in the spectrum at the longer infra-red wavelengths as well as the overall shortening of the spectrum – i.e. less light reaches the detector across all wavelengths. Some of you may be wondering what does a spectrum like this mean in simpler terms. These troughs in the spectrum correspond to wavelengths of light that are ‘missing’ from the true spectrum, in other words some colours are missing
If we where to obtain a spectrum of the sun from outside the atmosphere of the Earth it would be much more similar to the black body spectrum we have already seen. However there would still not be a perfect match, there would still be troughs in the spectrum where less light is reaching us than could be expected and by using a prisim or diffraction grating we can see these peaks in a physical way:
Credit: Robert L. Kurucz
These dips in the spectrum correspond to the presence of specific elements (or compounds) within the atmosphere of the star. Such lines always occur at the same wavelength and each element has a specific pattern of absorption lines giving each a specific bar code if you will.
What causes these absorptions ?
As I have said each element gives its own pattern of absorption lines, but I have yet to delve into how each line is caused and why the lines for each element is different.
The answer lies within the structure of the atom.
A simplistic version of the atom is a dense nucleus, ‘orbited’ by electrons at varying distances. While this is inaccurate in a large number of ways, it will be sufficient for our purposes if we alter it slightly.
First we must imagine the nucleus at the bottom of a deep and very wide well. We now imagine that the sides of the well are not vertical but sloped outwards, with the well being wider at its surface than at the nucleus. We also must imagine that the sides of the well are not smoothly sloped but stepped, with flat regions and steep slopes separating the steps. The steps also become closer together the further we move from the nucleus.
While this may seem somewhat contrived, it allows us to have a picture about what is going on when we see absorption lines.
First let’s look at hydrogen – we have only one proton in the nucleus and one electron on the first step from the nucleus. Under normal circumstances the electron will move around the atom on the first step without interference. However, if we supply the electron with a specific quantum of energy – electrons are very ‘picky’ supply them with too little energy they won’t do what you want them to do, and if you give them too much they won’t do what you want either – the electron will absorb that energy and jump to the next step from the nucleus. In terms of a spectrum that particular energy is provided by a certain wavelength of light as energy is related to wavelength by:
Where:
h is Planck’s constant - 6.63 × 10-34 m2 kg / s
c is the speed of light in a vacuum – 3×10-34 ms-1
and lambda is wavelength in meters
.
So the electrons of hydrogen will absorb a certain wavelength of light, and we see this as an absorption feature within the spectrum. Now suppose the electron wants to jump from the first step to the third, as the jump is bigger it will require a larger amount of energy and so a shorter wavelength of light is removed from the spectrum. Any series or combination of jumps is possible as long as the electron jumps from a fixed step to the other – I should now point out that each step corresponds to a fixed quantum energy level which isn’t physical in any sense of the word – so a series of different absorption features are produced in the spectrum.
When more complicated elements are involved for example Oxygen with its seven electrons more complex spectral absorptions are produced (and each form of an ion has a different set than its parent atom as the removal of an electron alters the energy levels significantly – changes the steps so to speak – so understandably a stellar spectrum is a mess of different spectral absorptions.
SDSS ID 1237659132207497317 Credit: SDSS
Spectrum for SDSS ID 1237659132207497317
Emission Spectra
When we look at the spectrum of an energetic galaxy for example Mrk 1018 we see that it is significantly different from the blackbody.
Mrk 1018 Credit: SDSS
The Spectrum of Mrk 1018 Credit: SDSS
We can see the hall marks of absorption lines of Magnesium (Mg) and singling ionised Calcium (Ca II) but we also see that there are lines where the spectral emission is stronger than could be expected – doubly ionised Oxygen for example (OIII).
Thankfully we can explain this phenomenon using our step analogy. We have seen that absorption features are explained when an electron is given energy and it jumps up a step (energy level), in the case of emission a photon of specific energy is released when an electron drops from a higher energy level to a lower one. We can think of this as a ball on the step above falling under gravity to a lower one. In this case the ball will convert gravitational potential energy to kinetic energy. In the case of an electron it is electrostatic potential energy being converted into light energy. As the size of the drop between the steps determines the balls final kinetic energy so too does the wavelength of light emitted by the atom when an electron decays to a lower energy level depends on the energy difference between the two energy levels.
As the energy levels are fixed the emitted wavelengths of light is always the same, and corresponds to the same set of wavelengths as that particular atom or ion absorbs at.
In my next post in this guide we will be looking at what individual absorption and emission features can tell us specifically about the object under study.
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.
AAPOD
Portal to the Universe
