The Young Astronomers
Currently viewing the tag: "Spitzer"
NASA’s Chandra X-ray Observatory (along with optical data provided by the ESO and infra-red data supplied by the Spitzer space telescope) has produced a truly amazing image of the star cluster NGC 1929 located within the nebula N44.
N44 and NGC 1929 Credit:X-ray: NASA/CXC/U.Mich./S.Oey, IR: NASA/JPL, Optical: ESO/WFI/2.2-m
The nebula and its star cluster are located in the Milky Way’s largest satellite galaxy – the Large Magellanic Cloud (LMC) – at a distance of 160,000 light years from Earth - 940.6 quadrillion miles.
The star cluster is composed of primarily newborn stars that have only recently been forged from the surrounding material.A great number of these are many times the mass of the sun and produce a precipitous amount of hard radiation and vicious solar winds, before burning out in (on the time scales of the universe) sort order as supernovae generating incredible outpourings of energy.
These shockwaves along with the continual bombardment from radiation and particle stream gouges out massive ‘bubbles’ in the surrounding nebula. The x-ray data provided by Chandra (shown in blue) shows the regions of the nebula that are at the highest temperatures – the areas under the heaviest onslaught of radiation or reeling from one or more shockwaves . The cooler gas and dust as detected by Spitzer is displayed in red with the yellow regions show where the radiation is actually causing the surrounding material to glow in the visible range (this data was collected by the ESO’s Max-Planck telescope).
Astronomers have been having a problem with N44 and other similar ‘superbubbles’ in the LMC for sometime now – they are producing too many x-rays.
Before anyone panics, this is not a medical problem (we aren’t all going to suffer radiation poisoning thanks to a few over-active nebulae in another galaxy), it only refers to the measurements pointing to such nebulae producing more x-rays than could be explained using current knowledge – our knowledge of such objects must be incomplete.
A previous study had suggested that the shockwaves of supernovae impacting the bubble’s walls along with the evaporation of hot material from the sides of the bubble could perhaps explain this anomaly. This set of observations at least doesn’t find any supporting evidence for these ideas though it has been the first time that the observations have been sensitive to distinguish between these and other possibilities so progress is being made.
You can read more here
For this post we will be looking at the star 55 Cancri
Credit: SDSS
and a slightly less gaudy version after some calibration by your’s truly
Original data credit: SDSS Calibration: Peter Clark
Background
55 Cancri is located reasonably to close to the Earth at just 40.3 light years (+/- 0.3ly) in the direction of the Zodiacal constellation Cancer – The Crab. The system is actually a gravitationally bound, detached binary with the bright primary star – 55 Cancri A – separated from its much dimmer partner – 55 Cancri B – by a distance of 1065 AU (nearly 99 billion miles!).
The primary star is about 95% the mass of Sol and is thus slightly cooler and dimmer (though it can be viewed from Earth by the naked eye with clear very dark skies) whilst the secondary is a cool red dwarf only 13% the mass of Sol and only 0.76% as bright (and is thus only visible through a telescope).
The primary star has nearly double the amount of iron content (186% to be exact) compared to Sol classing it as a rare “super metal-rich” or SMR (with an astronomical metal being any element other than hydrogen and helium).
This abnormally high metal content makes it difficult for astronomers to accurately age the star as the models that would normally be used are more uncertain when dealing with stars of this chemical constitution. As such the estimates range from between 5.5 and 8.7 billion years old depending on what study you choose to look at.
Whilst the star itself is certainly interesting, the bodies in its orbit are worthy on mention in there own right.
There are currently five known exoplanets within the system (all of which orbit the primary star). The first – 55 Cancri Ab – was announced in 1997 with 55 Cancri Ac and 55 Cancri Ad following in 2002. 55 Cancri Ae was announced after the detection of its transits across the disk of its parent star and was revealed to the public in 2004 making the 55 Cancri the first known to have 4 planets (other than our own of course!). The completion of this planetary set came in 2007 with the discovery of 55 Cancri Af adding another milestone as the system was the first detected to contain 5 planets.
The two most interesting of this collection are e & f (I think we have reached the stage where repeating 55 Cancri Ax ad nauseum has become unnecessary) and we shall have a look at each in turn.
Rather than going with alphabetical order I will first deal with f (as the most recent and interesting news centres around new observations of e and I will save the best to last!)
55 Cancri Af
f was the first planet to be detected that spends the entirety of its orbit within its system’s ‘habitable’ zone – read as the region of space surrounding a star where ambient temperatures could allow for liquid water to exist on the surface of a planet – unfortunately the planet itself is unlikely to harbour life as we know it. With a minimum mass of about 0.144 times the mass of Jupiter – half the mass of Saturn – (with the actual figure likely to be 25% larger at 0.18 Jupiter masses), it is most likely a gas giant and as such would have no solid surface on which liquid water could collect or for life to evolve. If the planet has one or more large, atmosphere swathed moons (current technology cannot say one way or the other about their existence) life may have a suitable location to develop.
f orbits the primary star once every 261 days and you can see its orbit displayed on this NASA graphic relative to our solar system
Credit: NASA
55 Cancri Ae
e is the closest planet to the primary star and has an orbital period of just 18 hours. It is just under 7 times the mass of the Earth and is likely to be a terrestrial ‘Super-Earth’ – a rocky world more massive than Earth but less massive than Neptune – though it is a very different beast than Earth.
The new observations from NASA’s Spitzer Space Telescope have been collected by analysing the infra-red light given off by the planet itself – rather than observing how the light from the parent star was altered as the planet track across its disk as seen from Earth.
The observations expose a very interesting world indeed. As could be expected from a planet that orbits just 0.015 AU from its star e is rather warm. Indeed its sunward side is likely to reach 2000 degrees Kelvin – hot enough to melt iron and titanium. The observations also show that the planet is most likely dark in colour whilst also supporting previous ideas about the planet’s structure.
All current observations suggest the planet has a large rocky core surrounded by a layer of supercritical water – the best way of describing this is that the water is under so much pressure and is so warm it can’t make up its mind if it wants to behave as a liquid or a gas – covered over by a blanket of conventional steam.
One of the lead astronomers working on the observations – Michaël Gillon of Université de Liège in Belgium – had this to say about the planet,
It could be very similar to Neptune, if you pulled Neptune in toward our sun and watched its atmosphere boil away
I will conclude with this artist’s impression of the planet beside its parent star
Credit: NASA/JPL-Caltech
You can read more about the latest observations here
This post was originally produced as an Object of the Day for the Galaxy Zoo Forum
The Orion nebula is the closest region of large scale star formation to Earth sitting just 1340 light years from where you are reading this post.
The nebula is in the process of birthing the next generation of stars, with many still cocooned within the clouds from which they are forming, from peering eyes. Well that’s in the visible spectrum at least. Using infra red observations we can looks through the obsuring dust as if it isn’t there at all.
This is exactly what astronomers using the Sptizer and Hershel Space telescopes have done to produce this gorgeous image:
The Orion Nebula in I-R Credit: NASA/ESA/JPL-Caltech/IRAM
The rainbow effect is due to the combination of different sets of observations through different filters. by combining the individual images the compound image can reveal the nebula in stunning detail with each colour displaying a different wavelength of I-R radiation. Using two telescopes also has advantages, as Sptizer is designed to observe at shorter wavelengths than Hershel and so by combining the two sets of data astronomers can get a more complete view of what is going on.
In this case the data revealed something very unusual indeed. Several of the young protostars have been flickering wildly, with their brightness fluctuating by as much as 20% in just a few weeks. Based on the cool temperatures of the material involved, the fluctuations had to occur far from the hot regions near the growing star, but such material should be far enough away from the star to spend years or even centuries in a slow decaying orbit before accreating onto the star’s surface.
Currently the explanation for how such a process could be so drastically accelerated is still up for debate though there are several suggestions. The other material may not be evenly distributed around the star, with some regions being more densely occupied than others. That may allow some of the denser clumps or filaments to collide with an inner, warmer shell of material causing the flare ups. It could also be caused by material piling up at the edge of the inner disk and so casting a shadow on the outer disk.
The universe is truly huge. It stretches for at least 13.5 billion light years (1.277×1023 km) in every direction (considerably further in fact), and its massive size poses an equally large problem how can you accurately measure how far away objects are?
Over such large distances conventitional methods of measuring distances (like using the time taken for light to hit and object and reflect back) become impractical and very, very slow.
Thankfully astronomers can use other methods developed over the years to deal with these monumental universal distances. The three main methods they can achieve this are:
- Parallax
- Cepheid variables
- and Redshift
Parallax uses changes in the relative positions of celestial objects throughout the year to estimate how far away they are using trigonometry. A star that is closer to the Earth will appear to move more than a star that is further away. The same principle applies when you look out of the window in a moving car, objects close to you wizz past while those in the distance move lazily across your field of vision for some time (provided you don’t move your head of course).
Parallax is only useful for distances that are relatively small as after a certain point (16,000 light years distant – which may seem like a huge distance but it is actually less than 1% of our Galaxy’s diameter, tiny in comparison with the rest of the universe) the relative position of a distant object changes so little it is impossible to measure with any accuracy.
The standard unit of parallax is the Parsec or Parallax Arcsecond. Without going into too much detail of how it is calculated, a star that is one parsec away will have apparently changed in position by a fixed amount in the sky. A star that is two parsecs away will have appeared to have moved 1/2 as much and so on and so forth.
One parsec is approximately equal to 3.26 light years, which puts Earth’s nearest star (after the sun) Alpha Proxima Centauri at 1.29 parsecs away (4.2 light years).
Parallax forms the ‘bottom rung’ on the universal distance ladder (covering the shortest distances), skipping over the middle rung for now and moving to top – redshift.
Not only is the universe really, really big it is expanding – quickly. This is a direct result of the initial explosion that sparked our universe’s formation – the Big Bang. This was not an explosion in any conventional sense, it did not expand from a single point, instead it happened everywhere in the universe at once. This means that even today every single object in the universe is moving away from everything else (yes it is mind-blowing). This expansion is constant no matter where you are in the universe (as far as we can tell though this is the subject of some debate) and has been measured at 70.4 km\s/Mpc (within a small range of uncertainty). Or in plain English, if two objects are a million parsecs away from each other they will be moving away from each other at 70.4 kilometres per second.
The Expanding Universe Credit: NASA / WMAP Science Team
Now at first glance this may not seem all that useful in determining distances, but over large distances it is a very powerful method indeed.
As this rate of expansion is fixed and is occurring throughout the universe, and most importantly everything is moving away from everything else (thanks to the space around it literally stretching), an object that is 2 million parsecs away will be moving at 140.8 km\s, an object that is 3 million parsecs away will be whizzing away from us at 211.2 km\s and so on.
That means we can infer the distance of an object by measuring its relative velocity. The only question is how do we measure the velocity of something that is literally half way across the universe? The answer comes from a somewhat unusual source.
Light as most people know travels at a fixed speed (3×108 m\s) in a vacuum. Regardless of what you try to do it, its speed is constant. As the universe expands you would expect light to appear to slow down as it has to travel further, in actual fact it stretches with the universe.
This manifests as in increase in the light’s wavelength (the distance between two adjacent, identical points on a wave). An increase in wavelength means the light has ‘lost’ energy and appearers redder hence the term redshift. Redshift is calculated by measuring the difference between observed spectral features in cellestial objects and comparing them to measurements taken here on Earth i.e. where redshift = 0. Redshift can be shortened to z and is the only practical method of determining distances to objects far into the universe.
So far we have dealt with Parallax which is used for very short distances (in universal standards), and Redshift which deals with very large distances. Whilst these are each very useful to have there is a reasonable gap between the two meaning that some objects within our own galaxy could not be distanced with any accuracy, and we would have no way of checking the reliability of either as there is practically no overlap in the measurement range of the two. This means that the results obtained for both methods could not be compared and checked to see if they agreed with each other.
Thankfully there is a middle rung on the cosmological distance lader – the Cepheid variables.
Cepheid variables (more accurately Population I Cepheids or Classical Cepheids) take their name from the second to be discovered - Delta Cephei.
Classical Cepheids are all yellow supergiant stars that fall in the spectral class bracket F6 – K2 (For information spectral classes of stars see here and here).
They were once main sequence B class stars like many of those found within the Pleiades open star cluster (right).
The PleiadesM45 Credit: Tad Denton/Adam Block/NOAO/AURA/NSF
These stars are all somewhere between 4 and 20 times the mass of the sun and spent just a few million years as a main sequence star fusing hydrogen into helium, before departing the main sequence and evolving into the supergiants we see today.
Such yellow supergiants are inherently unstable, having regular pulsations as the interior of the star changes in cycles which alter how much radiation the star retains causing it to swell and then contract as radiation is released into space at a faster rate causing it to return to its original size.
This cycle repeats again and again, in a relationship that is directly linked to the luminosity of the star. The more luminous the star the longer its pulsation period (the time taken for one expansion phase followed by one contraction phase). As this is fixed for all Cepheid variables regardless of their distance from Earth, two stars that have the same pulsation period with differing luminosities are at different distances.
That means that relative distances can be calculated by comparing Cepheids. Whilst that would be useful in itself there is more to the Cepheids, some of them are close enough to have their distance calculated by Parallax and other methods. So with a starting point for comparison the actual distances of all Cepheids regardless of their distance can be calculated accurately.
Cepheids have also been used to refine our results and estimates of important cosmological constants including values for the Hubble constant – a measure of how the universe is expanding.
So far around 700 classical Cepheids have been identified within the Milky Way, with an overall total of several thousand known as far out as NGC 4603 – 100 million light years away. Cepheids have been detected, and have had their properties measured, at distances at which redshift has become detectable bridging the gap between the two measurements and providing a way of comparing and confirming calculated distances for the very close and very distant universe.
For our mathematically minded readers (anyone else is welcome to have a look but if the site of an equation makes you cringe you are more than welcome to skip on ahead) I will now detail how to first calculate the luminosity of a classical Cepheid and then how to use the luminosity to calculate how far away the star is from Earth.
For the first stage all that is required is the pulsation period of the star. This is obtained by monitoring the star’s brightness over a long period of time and identifying the time for the star to dim from its peak brightness to its dimmest and to brighten back to its peak again. This is recorded for several cycles and averaged to reduce uncertainties in the measurements and to minimise the effect of any mild random fluctuations in the star itself.
The value for the period – P – is then substituted into the equation:
It looks rather complicated but the equation really isn’t, if you are careful it can even be done in one step using a calculator!
In this case M is the mean absolute magnitude of the star – a measure of distance adjusted brightness.
The second stage in the calculation is essentially a comparison between the mean absoultude magnitude and the mean apparent (average observed magnitude over the full pulsation cycle). Is this case the larger the difference the greater the distance involved – as of course, the further away an object is the dimmer it appears.
The equation we use in this case is:
Where:
- M is the calculated mean absolute magnitude
- m is the observed mean apparent magnitude
- and D is the distance in parsecs
This formula can in turn be rearranged to give an equation where D is in the subject:
And all you need do is plug in the numbers.
For example lets imagine we measure Cepheid to have a period of 51 days and a mean apparent magnitude of 18.5.
Its mean absolute magnitude would be:
-2.78(log51)-1.35 = M = -6.10 (Try it yourself if you don’t believe me!)
So now if we substitute M and m into the distance equation we get:
D = 104.92Parsecs
Which can also be written as: 83200 Parsecs or roughly 1.6 times the distance to the Large Magellanic Cloud.
There you go, not so scary after all!
An example of a Cepheid Variable (left) with its spectrum (below).
SDSS ID: 1237646647833788777 Credit: SDSS, DR8
Recently the distances calculated using Cepheids have been called into some doubt with concerns over the stars themselves.
It has long been though that their may be some change in the period of the star’s pulsations. This was predicted as stars slowly use mass over time as they blow out a fraction of their mass as their solar wind and in the emission of thermal and electromagnetic radiation (heat and light).
Spectrum of SDSS ID: 1237646647833788777 Credit: SDSS, DR8
This mass would then form a shell of material surrounding the star. This dust would absorb visible light from its parent star and re-emit it in the infra-red frequency range. Thus a star would appear dimmer than it truly is in visible light and brighter in infra-red light. Despite the differences being quite small it would introduce large uncertainties in the calculated distance of the star which in turn would cause serious inaccuracies in the values calculated from such measurements, thus proof of this mass loss and a way to compensate for it is vital in maintaining the integrity of the cosmic distance ladder and cosmology as a whole.
Recently, astrophysicists using NASA’s Sptizer Space Telescope have confirmed this mass loss by taking detailed images of the class’ namesake Delta Cephei. They show an intense bow shock surrounding the star, caused by the high speed interactions between the star’s stellar wind and the surrounding interstellar gas and dust.
The Bow Shock Surrounding Dela Cephei Credit: In image
Other observations also show similar bow shocks surrounding at least 25% of currently known Cepheids.
With these new measurements of mass loss a correction program can now be used to compensate for the reduction in luminosity of the variables and so allow for better measures of distances and more accurate calculation of the variables that define our universe.
You can read more on Cepheid Variables here
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
