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This post was originally produced as an Object of the Day for the Galaxy Zoo forum.
In this post I will be looking at three star systems that share a common theme, they are PSR B1620-26, Kepler-16 and Kepler-47 with the common tie being that all three systems are centred on a binary pair of stars – two stars orbiting one another.[1]
Lets begin by taking a look at PSR B1620-26. The system is located 12400 light years away in the direction of the constellation Scorpius – The Scorpion – within the globular cluster M4. M4 is a reasonably loose association of stars that is around 75 light years across. M4 holds the honour of being the first globular cluster to have any of its component stars resolved into isolated objects. M4 is one of the brightest globular clusters in the sky located just west of the α Scorpii – Antares – taking up roughly as much space in the sky as the full moon.
Credit: Kitt Peak National Observatory 0.9-meter telescope, National Optical Astronomy Observatories; courtesy M. Bolte (Universityof California, Santa Cruz)
The PSR B1620-26 is a highly evolved system containing both a white dwarf and a pulsar (spinning neutron star), indeed the system is estimated to be around 12.2 billion years old (compared to our own solar system which is estimated to be about 4.5 billion years old) potentially making it one of the oldest planet containing star systems in the Milky Way (or perhaps even the universe as a whole).
The planet (PSR B1620-26 b) orbits both stars making it a circumbinary planet. It was first announced in 1993 by a team that was studying the Doppler shifts of the system. At first they thought they were looking at a binary pulsar system (with the white dwarf being identified later) though their results showed that there was a third body within the system. When they calculated this unknown object’s mass they found that it was too small to be a star and thus identified it as a planet – one of the first outside our own solar system to be announced though official confirmation had to wait until 2000 (the first planets detected outside our own solar system orbit PSR B1257+12 – another pulsar).
PSR B1620-26 b is about two and a half times the mass of Jupiter and takes about a hundred years to orbit its parent stars.
The star system as a whole is thought to have had a rather unusual history that you can see documented in this NASA graphic.
Credit: NASA and A. Feild (STScI)
The system’s pulsar is 1.35 solar masses and is rotating at about 100 times a second! The white dwarf is considerably less massive (0.35 solar masses) and the pair of stars orbit each other at an average distance of one AU.
The system faces an uncertain future, it is continuing its approach to the core of M4 and as it does so the density of stars surrounding the system will increase. Why is this so you may be thinking? A common way of thinking about globular clusters is that they are essentially self contained spheres of stars. Whilst this is broadly accurate, the stars are not spread evenly through the sphere. Stars are most densely clustered in the centre and become more widely spaced moving out.
As the surrounding area becomes more and more crowded the chances of a close encounter between two star systems also increases. Within the next billion or so years the system is very likely to have another such encounter with the most likely scenario being that the planet (as it is the least massive body in the system) being ejected into deep space fated to wander the stars alone.
Next lets continue with Kepler-16 (I should here note that if we are to follow the full naming convention, the system should be refereed to as Kepler-16 (AB) to show that we are taking about both stars though that is going to rapidly become tedious for everyone involved I shall keep to the shortened version and you can assume that I am referring to both stars when I don’t identify otherwise). Kepler-16 located 196 light years from Earth in the direction of the constellation Cygnus – The Swan.
The system is centred on two small, dim stars – The primary (16A) is an orange dwarf of spectral class KV. it is just 69% the mass of Sol and only a fraction of the brightness. It counterpart is even smaller at only a fifth the mass of Sol making it a MV class red dwarf.
The pair orbit one another in just 41 days and are separated by just 0.22AU – 22% of the average distance between the Earth and the Sun – which is smaller than Mercury’s orbit which sits between 0.31-0.47AU (the range is due to Mercury’s rather eccentric orbit).
Now to the planet itself Kepler-16 (AB) – b catchy isn’t it ::) so for brevity – 16b
16b is a gas giant a third of the mass of Jupiter and 3/4 its radius. This was the first circumbinary planet detected via the transit method – the reduction in the amount of light coming from the parent star as the planet passes in front of it as observed from Earth.
16b transits both of its systems stars, and they themselves transit each other, I admit that is more than slightly challenging to visualise so here is a visual representation with the two stars in the centre and 16b shown as a small blue\purple dot.
Credit: Silver Spoon (Wikipedia user)
Our final system of the day – Kepler 47
This system has only recently had its planets confirmed by the team working on the Kepler mission and marks their first discovery of a multiple star system with more than one transiting planet.
The system can be found at a distance of 4900 light years from Earth in the direction of Cygnus. Both planets are circumbinary orbiting their parent stars. Both of which are smaller than the Sun with the secondary star just 1% as bright as Sol
The innermost planet (47b) orbits once every 50 days and would thus be much too hot for life as we know it to survive on. The outer planet (47c) orbits once every 303 days and this places it at the outer edge of the systems habitable zone. Life like ours is not expected to have developed on 47c as it is predicted to be a gas giant similar in size to Neptune, though perhaps one of its moons (if it has any!) could be suitable.
47b on the right has three times the radius of the Earth
47c on the left is quite similar to Neptune
Credit: NASA/JPL-Caltech/T. Pyle
The most important aspect of the discovery is that it proves that multiple planet systems can indeed form around binary stars. Under current planetary formation models such systems are very difficult to form and suffer from stability issues throughout their existence. Furthermore, as at least one such planet is within its systems habitable zone it is evidence that such orbital configurations are potentially stable and thus the number of locations for life similar to our own to develop has just been increased!
The Kepler 47 system
Credit: NASA/JPL-Caltech/T. Pyle
[1]For those interested you can read more about binary stars and the various types that exist from my post for the Young Astronomers – Binary Stars Blitzed.
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
This post is part of the Young Astronomers Databank Project.
In the first part of this guide I looked at the basics of spectra with particular focus on black-body spectra. In this section I will take a look at a few specific emission features and what they can tell us about an object.
So lets begin our journey …
Spectral Notation
As each element and ion produces its own set of spectral lines scientists have created a standard notation of showing which line is produced by each atom or ion.
Any produced by an atom is labelled as – (x)I where (x) is the chemical symbol of the atom involved e.g. a line produced by the atom sulphur would be labelled as SI on a spectral chart.
As mentioned above ions also produce spectral lines. In astronomy we are nearly always dealing with positive ions (those which have had one or more electrons knocked off) even with those elements, that under normal circumstances on Earth, would take on additional electrons to become negative. This is because in astronomy where emission lines are produced the energies involved are much higher than on Earth and so it is much more likely that an electron (or perhaps two or even three) is knocked off rather than attracted towards the atom. As such ions with a positive are much more common in astronomy than those with a negative charge.
An ion with a +1 charge (one electron has been stripped off) produces lines labelled as (x)II with (x) again standing for the chemical symbol of the atom involved.
An ion with a +2 charge follows the same pattern – (x)III
and so the pattern continues
Let’s now look at two specific elements:
Hydrogen
Hydrogen produces a series of emission lines that fall within the visible section of the electromagnetic spectrum (it also produces several others that lie outside the visible range). This set of lines is called the Balmer series after the scientist who first described them Johann Jakob Balmer.
They are all produced by electron transitions to the second energy level. When displayed using a hydrogen discharge tube (a cylinder of pure hydrogen through which an electric current is passed to excite the atoms) and a spectroscope the lines can be seen like so:
The Visible Spectrum of Hydrogen Credit: National Institute of Standards and Technology
As is typical in science the rule we just learned about spectral notation doesn’t apply in this case.
For historical reasons the lines of atomic hydrogen in the visible region are named as H followed by a Greek letter. Hα is the lowest energy transition – red- moving through Hβ – which is a blue-green – and then Hγ, Hδ and Hε all being shades of purple (Hδ and Hε are sometimes classed as being ultraviolet rather than visible spectral features but still follow the same naming pattern).
Hydrogen Alpha – Hα
This emission feature occurs at 6562.8 Angstroms (656.28 nm). It is produced when an electron in a hydrogen atom decays from the third energy level to the second producing a photon with 1.9 eV (where one eV is 1.66×10-23 J) of energy.
Hα is a hallmark of star forming regions. It appears pink to the human eye (its the bright red line in the image above), and is displayed as such in most professionally produced images such as this one captured by the ESO’s MPG telescope of the Large Magellanic Cloud (LMC)
N44 in the Large Magellanic Cloud Credit: ESO
Hα is also a marker for AGN activity, with most such galaxies (including quasars) display strong Hα emission.
Oxygen
Oxygen is the third most common element in the Milky Way making up about 10,400 parts per million in terms of mass.
As oxygen has eight electrons rather than hydrogen’s solitary one, oxygen’s spectrum is much more complex with a great deal more lines than that of hydrogen ( hydrogen has 5 spectral lines between 4000-7000 Å (roughly the visible range) compared to oxygen’s 73! Using simplistic terms because oxygen has more electrons there are more available energy levels for those electrons to jump into, that in turn means more energy level transitions are possible and so giving rise to more spectral lines, as each line represents a possible transition.
The visible spectrum of oxygen looks like:
The Visible Spectrum of Oxygen Credit: National Institute of Standards and Technology
Doubly Ionised Oxygen – OIII
Another hallmark of star forming and active regions. A blue-green line that can be exceptionally intense in certain circumstances and can completely dominate the colour of some galaxies.
In most images containing OIII data it is displayed as either green or blue such as this Hubble image of the nearby active galaxy NGC 6822 (OIII is shown as green in this particular example)
In the next post in this series I will be taking a look at a few specific absorption features and what the colour of an object in general can reveal.
Not too long ago, astronomers figured out that most large galaxies – including our own – have a supermassive black hole (an enormous object which can be billions of times the Sun’s mass) right in its core. Along with this astonishing finding, an unsettling question was raised: Which came first, galaxies or the supermassive black holes in their cores?
Artist’s concept of a supermassive black hole and its accretion disc at the centre of a galaxy. (Credit: NASA/JPL-Caltech)
The cosmic chicken-and-the-egg problem
Scientists know that there is an intrinsic relationship between the galaxy and its black hole. It’s believed that black holes and galaxies grow together, in a kind of partnership. As the black hole swallows the material in the surrounding area, it becomes more and more massive, and the more massive a black hole is, the more radiation it blasts out. This powerful radiation heats up the clouds of gas in the galaxy, but it’s still unclear whether this radiation boosts or prevents the formation of new stars.
By studying the nearby galaxies, astronomers also figured out that there is an elegant relation between the mass of the supermassive black holes and the giant cloud of stars and gas in the core of their galaxies (nicknamed “bulge”): no matter how big is the galaxy, the black hole is always about 700 times more massive than the bulge. Although this suggests that galaxies and their black holes evolve together, scientists weren’t sure whether this rule was obeyed in the early universe.
Looking back in time
This image, taken by the Very Large Array radio telescope, shows the gas clouds of a galaxy as it appeared just 870 million years after the Big Bang. It’s used to analyse the motion of the gases orbiting the ancient black hole hidden in core. (Credit: NRAO/AUI/NSF)
In order to see the first black holes, astronomers need to look as deep into space as possible. The problem is that it’s not easy to observe an ancient galaxies in details, mainly because of their high redshift. The most easily seen galaxies in higher redshifts are quasars, due to their extreme brightness. Nevertheless, separating the radiation emitted by the supermassive black hole from the glare of the quasar is a tough challenge.
To calculate the size of the Milky Way’s supermassive black hole – as we cannot actually “see” it, – astronomers needed to measure the speed of the stars orbiting in the heart of the galaxy: the faster the stars move around the black hole, the more massive it is. Measuring the size of a supermassive black hole hosted in another galaxy is an even more complex task: the scientists need to analyse the infrared light coming from the centre of the galaxy. Moreover, the data needs to be carefully filtered, in order to separate the radiation emitted by the black hole from the light coming from other objects in the galaxy.
Using powerful radio telescopes, such as the Very Large Array (VLA) in U.S. and the Atacama Large Millimeter/sub-millimeter Array (ALMA) in Chile, along with image-filtering processes, astronomers were finally able to get accurate measurements of the mass of these early black holes and had a big surprise.
In the galaxies observed during the studies, the black holes were only 30 times less massive than the bulge of their host galaxy, breaking the rule we see in the modern galaxies. It appears to solve the chicken-and-the-egg mystery, indicating that black holes actually came first.
Despite this major discovery, the origins of the supermassive black holes remains one of the most hotly debated topics in astronomy. Many undergoing projects are trying to pile up evidences to either underpin or contest this theory, and better explain when and how this mysterious relation came about.
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