The Young Astronomers
From the monthly archives: December 2011
Unlike the more traditional Hubble discoveries which tend to be rather large; detailed images of entire galaxies for example, these targets have been quite a bit smaller, and much closer to home.
It has long been known that the area of space beyond Neptune is filled with small icy bodies (with Eris being the most massive). It has been estimated that there are thousands of chunks of rock and ice, but as such objects are small and very far from the sun they are difficult to detect. Thanks to new detection techniques and the mighty observing power of Hubble, astronomers have added another 14 trans-Neptunian (TNO’s) objects to the growing list.
These 14 lie in a tiny patch of the sky and range in size from 40-100km across, as this is the first stage of the analysis it is likely to produce many hundreds more objects.
Perhaps the most interesting object found in this first batch is a binary object (two TNO’s orbiting each other), very similar to a small scale model of Pluto and Charon.
You can read more about these discoveries here
NASA’s WISE mission has produced a stunning image of the area of space surrounding the star Pi Scorpii (or Π Scorpii if you prefer), specifically the reflection nebula DC 129.
DG 129 and Pi Scorpii Credit: NASA/JPL-Caltech/WISE Team
The nebula sits in the middle of the constellation Scorpios’ ‘claw’ (the linking of stars into constellations with apparently ‘recognisable’ shapes is something that I fail to grasp).
It is located approximately 500 light years from the Earth and is classed as a reflection nebula as it is reflecting the light of nearby stars rather than producing its own light.
As I’m particularly interested in stellar astrophysics (the study of stars) I find Π Scorpii to be particularly interesting. In this image, Π Scorpii is the star on the right partially obscured by a greenish ‘fog’, but we see as one star is actually three – a ternary system.
The nebula DG 129 was first catalogued by the German astronomers, Johann Dorschner and Josef Gürtler, in 1963.
This image was obtained after spacecraft had depleted most of its coolant and had started to warm up rendering its longest wavelength detector (sensitive to 22 micron infra- red radiation) useless. Due to this failure, this image is composed of data from three filters rather than four. They are 3.4 microns (blue in the image), 4.6 microns (green in the image), and finally 12-micron (shown in red).
You can find more about this image here.
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.
Its that time of year again when we have a look back at all the wondrous astronomical images that have been released over the past twelve months and pick our favourites.
Obviously there are going to be some differences in opinions with some of our readers feeling like their favourite image of the year has been robbed of the top spot. Whilst we would love to feature every image, clearly that is neither practical and somewhat defeats the purpose of having a competition to pick the best. Though if you have a differing opinion we would love to hear about it, either in the comments section of the post, on our Facebook page or via our forum, but without further ado lets begin!
Best Image From Within the Solar System
This year’s winner is: - Ulyxis Rupes as observed by the ESA’s Mars Express
Springtime at Ulyxis Rupes Credits: ESA/DLR/FU Berlin (G. Neukum)
Ulyxis Rupes is a region near the Martian south pole (though the south pole itself is a little over 1000km further south). The poles of Mars are dynamic areas of the Red Planet constantly changing along with the Martian seasons. The image shows an ice field along with delicate sand dunes and numerous other interesting features.
This image was taken during the Southern Hemisphere’s Spring with the region slowly warming and the ice thinning. This warming, along with its distance from the south pole itself means the ice is rather thin, at just 500m deep compared to some other polar regions where it can reach 3.5km.
The image was taken by the ESA Mars Express’ High Resolution Stereo Camera. The Mars Express has been in orbit of Mars for 8 and a half years and continues its work of studying the Planet and mapping it in extraordinary detail.
You can read more about this image here
Best Image From Within Our Galaxy – Runner Up
Our runner up in this category is IC 2944 – The Running Chicken Nebula
IC 2944 Credit: ESO
IC 2944 is an emission nebula glowing from the harsh bombardment of the ultraviolet light produced by the hot young stars that have been birthed by the nebula’s dusty clouds.
It is located around 6500 light years from Earth in the direction of the constellation Centaurus - The Centaur.
The red glow indicates the familiar presence of excited hydrogen, a feature common in and around such star forming emission nebulae. Star formation is evidenced further by the presence of Bok Globules – the dark black objects in the image particularly concentrated in the top right corner around the cluster of bright blue stars. These are small dense regions of gas and dust that are collapsing to form the next generation of stars.
Unfortunately, such beautiful emission nebulae are short lived in astronomical terms, lasting just a few million years before their gas has either been used to forge stars or blown out from the area by fierce stellar winds. The most massive of stars will burn out in flashes as they rapidly chew through their supply of hydrogen briefly lighting up the area again as a supernova and glowing remnant.
The image was produced using data from the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory.
You can read more here.
Best Image From Within Our Galaxy
Our winner in this category is the glorious star forming region S106
S106 Credit: NASA ESA, Hubble Heritage Team (STScI/AURA)
The nebula is located within the constellation Cygnus – The Swan – at a distance of about 2000 light years from Earth.
The fantastic bubbles of material, with the intricate ripples of gas and dust within the surrounding nebula are caused by the young star S106 IR.
This stellar youngster is undergoing the final stages of its formation process – sucking up material from the surrounding area. Despite still undergoing its formation, S106 IR is already 15 times the mass of our of Sun.
Rather like someone whose eyes are too big for their belly, this young star is firing some of this material back off into space accompanied by large amounts of radiation that is shocking and exciting the nebula making it glow brightly.
The blue regions of emission in this image are the result of superheated hydrogen glowing at about 10,000 degrees. The cloud is only two light years across at its widest point making it a small stellar nursery (the much more famous Orion nebula is 24 light years across).
S106 is located in the direction of the constellation Cygnus and is around 1900 light years away from where you are sitting.
The image was produced from data collected by Hubble’s Wide Field Camera 3.
You can see a wider view of the entire nebula below -
S106 - Hubble and Subaru Composite Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA) and NAOJ
You can read more about the images here
Best Extragalactic Image – Runner Up
Our runner up in this category is this ESO image of the Leo Triplet
The Leo Triplet Credit: ESO/INAF-VST/OmegaCAM. Acknowledgement: OmegaCen/Astro-WISE/Kapteyn Institute
At around 35 million light years from Earth, in the direction of, you guessed it, Leo – The Lion. Such a distance my seem large though it is a stone’s throw on terms of the universe.
All three members of the Triplet (sometimes called the M66 group) are in fact spiral galaxies not dissimilar to our own Milky Way. They each appear so different as they are visible to us from different angles. NGC 3628 is seen edge on at the left of the image whereas M 65 (in the top right hand corner) and M66 (in the bottom right) are closer to being face on and so allow us to peer at their spiral structures unhindered.
The image also contains many other galaxies that lie much further away from us. Along with many stars that lie within our own Milky Way as well as a few asteroid streaks produced by small objects in our Solar System.
The image was produced by the ESO using the Very Large Telescope’s Telescope for Surveys (try say that ten times quickly), mercifully abbreviated to VST.
It was snapped as part of a survey designed to find illusive small objects, such as Brown Dwarfs and Black Holes within the Milky Way’s halo, objects normally to small and dim to be picked out but can be identified through gravitational microlensing. It will also peer deep into the universe to help expand our knowledge of the illusive dark matter.
You can read more here
We will be seeing the winner of the extragalactic section later, but now we move on to our amateur section,
Best Amateur Astrophotograph – Runner Up
Our runner up this year is by Milly Howes.
The ISS by Milly Howes Credit: Milly and Nick Howes
Milly took this image of an ISS pass during the STS-131 shuttle mission with her dad’s Canon EOS10D on the 26th of February 2011.
A lovely shot once again illustrating that you don’t need thousands of pounds worth of equipment to take beautiful astrophotographs.
The ISS is the largest inhabited space station ever produced by humanity. It has been occupied continuously for over 11 years.
It zips around the planet every 91 minutes at an altitude of about 380 km.
The International Space Station Credit: NASA
Best Amateur Astrophotograph
The best amateur astrophotograph of this year is this fantastic image of the Andromeda Galaxy by Nick Howes.
The Andromeda Galaxy Credit: Nick Howes
The Andromeda galaxy or M31, is the largest galaxy in our Local Group. Andromeda is a spiral galaxy similar to the Milky Way, it is located about 2.5 million light years from Earth in the direction of the constellation that shares its name.
Andromeda and the Milky Way are on a collision course and will collide in a few billion years producing a larger elliptical after many million years of gravitational distortions.
The Andromeda galaxy is the furtherest object that can be reliably observed with the naked eye. In dark skies, away from city lights it appears as a milky patch against the black of the sky.
Best Extragalactic Image and Overall 2011 Winner
This year’s best extragalactic image, and the overall winner of this year’s competition is this magnificent Hubble image of Arp 273 -
Arp 273 Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)
Arp 273 is a pair of interacting spiral galaxies, the larger upper one on its own is UGC 1810 with the smaller lower member of the pair called UGC 1813. The pair lie 340 million light years away from us in the direction of the constellation Andromeda – The Princess.
The smaller UGC 1813 is believed to have passed through the larger galaxy, off to one side twisting the larger galaxy into a shape resembling the head of a flowering rose – as evidenced by the off centre ring structure in UGC 1810.
The smaller galaxy has only 20% the mass of the larger, though the interaction has caused the larger’s spiral arms to unfurl and for a stellar bridge of material to be thrown out thousands of light years into the void between the two.
Interactions like these generally cause starbursts to occur in both galaxies with the smaller experiencing a burst first and after a short delay a starburst is also set up in the larger galaxy. This is thought to be due to the different quantities of interstellar gas within high and low mass galaxies. In general, low mass galaxies have more gas and dust not bound into stars than high mass galaxies so it is easier for a low mass galaxy to form stars than a high mass galaxy.
A smaller third spiral can also be seen within the arms of UGC 1810. Astronomers have noted that the spiral arm changes from being ordered and reddish - indicating lots of middle age and old stars – on one side of the small spiral to being blue and clumpy on the other – indicating large numbers or recently formed high mass stars.
The image was taken using Hubble’s Wide Field Camera 3 on December 17th 2010 and had a total exposure time of 5.9 hours.
Hubble is a joint project between NASA and the ESA.
You can read more here.
That’s it for this year’s competition folks I hope you enjoyed the images for this year, and only one thing remains:
Merry Christmas
from all of us here at Sigma Orionis and the Young Astronomers
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)!
AAPOD
Portal to the Universe
