Sunday, March 31, 2013

Made from the stars


Stages in Star and planet Formation

From star-stuff, to stars to us. As the saying goes,
‘We are all made from star-stuff,’
 but how did we get that way?

The universe is a vast and sparsely populated place. Even within our galaxy things are spread out. The closest star to us is over four light years away with little else occupying the space in between. Yet that gap is not entirely empty the space between the stars, the interstellar medium, is filled with gas and dust at densities lower than the vacuums we can create on Earth. This gas and dust is mostly hydrogen and helium, some of which was left over from the big bang, but it also contains heavier elements like oxygen, carbon, and nitrogen, formed in ancient stars then spread out across space in the explosive event of their death.

This gas and dust can come together under its own gravity to for slightly denser regions called giant molecular clouds that can be seen throughout our galaxy. You can even see some of them in the night sky with the naked eye.
Orion's sword
Just below the Belt of Orion on the left is Orion’s sword, where the central region is the Orion nebula. This massive molecular cloud complex is the birthplace of stars and future planetary systems and yet it is a trillion times less dense than our own atmosphere and thinner than the wispiest clouds observed on the Earth.

These clouds are so cold, just 10 degrees above absolute zero, allowing the atoms to clump together into molecules. Winds and turbulence in the clouds can cause bigger clumps to form in knots. Due to the low temperatures the clumps can grow and grow, and as they acquire more mass they attract more and more clumps. This process can take around 10 million years eventually forming a clump so large that the pressures at the center heat it up forming a proto-star at the heart of the collapsing cloud. These regions are dense enough to block any optical light; millimeter and sub-millimeter observations have helped to reveal how this process occurs searching for the emission from the in-falling material that makes up the proto-star.

The proto-star begins gathering more dust and gas, spinning and collapsing further.  After a few million years of accretion the central mass becomes dense enough to ignite the core, fusing hydrogen into helium. Strong stellar winds erupt from the poles; blasting away the gas and dust of the surrounding cloud, halting the in fall of mass to the star.

The rings of Saturn form distinct bands of material
That stars fate is now sealed in the mass it managed to accumulate and is at the start of its journey along the main sequence. Surrounding the star is a disk of accreted matter that forms the proto-planetary disk. It is here that a stellar system can form and our journey from star-stuff to life can begin.

The gravity of the star pulls on the particles in the disk. As they make their journey in towards the star they hit each other and in some cases join together forming grains. Over several thousand years they grow in size and mass turning into pebbles and rocks orbiting in a plane around the central star. These clump into irregular objects forming bands around the star. Much like the structures seen in the rings orbiting Saturn.

It not until a series of violent mergers and collisions between these rocks, over millions of years, that large terrestrial worlds emerge capable of holding onto any gasses to form an atmosphere. The final fate of these strange new worlds will depend upon the temperature and type and quantity of material in the forming region.

In the last few decades our theory of planet formation has been taken down and shook up. With the discovery of extra-solar giant planets in places contrary to those in our own solar system we have been forced back to the drawing board. Determining how the surrounding environment during formation impacts the resultant system is a major topic in astrophysics with many groups of scientists scratching their heads and imagining anew.

So keep watching this space, reality always has a new way of surprising us.







WHAT’S NEXT?

There are some great links that I found very useful so I have included them here for you to explore.


National Radio Astronomy Observatory –

University of Exeter - Astrophysics group - star formation -

NASA astrophysics – Stars –

Tuesday, March 26, 2013

What to do when your PhD supervisor is gone?

Apart from submit a paper, start a new project, register for a conference, and sleep in until nine. The only thing left to do is play around on photoshop.

Thought I would just post up some I made up last night whilst the code on my new project is running.
Hug the Sun

Pale blue dot, on a moat of dust, suspended in a sunbeam

Fighter Pilot Snoopy taking on space

Mr Fredrickson goes Up on Tatooine

The Clanger living it large

Second star on the right and straight on to morning

Hey you found Nemo

Ninjas!

I hope you all have a good week.

Just keep asking. What's Next?

Saturday, March 16, 2013

Exoplanets 101


Our Sun in all of its glory is just one of over 200 billion stars in our galaxy, the Milky Way. As a very ordinary and perhaps boring star it harbors an extraordinary system of 8 planets, over 150 moons, a host of sun grazing comets and a plethora of dwarf worlds and asteroids all orbiting at its gravitational mercy. Our solar system has provided us with the most detailed information about planets and their environments all conveniently located in our back yard.

But ours is not the only planetary system out there.

Alien worlds have long been embedded in our collective consciousness be it through film, TV, or literature. Yet it was not until 1995 that science fiction truly became science fact when Mayor and Queloz discovered the first alien planet orbiting a star similar to our Sun just 50 light years away. But this new discovery was not quite what everyone was expecting.

As a stars gravity pulls a planet around in its orbit, the planet also pulls on the star. The bigger the planet and the closer it is to the star the greater the pull it has. This causes the star to wobble backwards and forwards as the planet makes its way from one side to the other side of the star each time it orbits. By observing the stars spectrum we can see the shift in the absorption limes as the star is forced backwards and forwards by the planet.

The Radial Velocity method
From this type of observation Mayor and Queloz revealed a world that was astonishingly similar in mass to that of Jupiter, the largest planet in our solar system, in an orbit that took it eight times closer to its star than Mercury is to the Sun at just 7.8 million kilometers from its star compared to our comfortable 149 million kilometers.

This exoplanet, 51 Pegasi b, sparked interest around the world and provided a number of scientists, who had strange ideas about observing planets orbiting other stars, some hard data to stand on.

One of those scientists was Dr William Borucki, who since 1984 had been designing and submitting a mission proposal to NASA for a space telescope dedicated to hunting for exoplanets. I would, however, not be until 1995 with the first concrete observations and a long development period that Borucki’s vision would be accepted and launched by NASA.
That mission was the Kepler space telescope which, since its launch in 2009 has been the most prolific source of headlines getting exoplanets in the media and out to the general public.

Kepler relies on the chance phenomenon of the exoplanetary system being aligned 90 degrees to our own, meaning that part of the planets orbit takes it between its star and us, similar to the transits of Mercury and Venus in our own solar system. As the exoplanet transits it blocks out a small fraction of the starlight. The amount of light that is blocked can tell us a lot about the planet and its environment such as its radius from the amount of light blocked and the length of its orbit from observations of multiple transits.
This method, however, still favors the close-in giant planets that appear to populate our galaxy, as the larger the planet relative to the size of the star the more light they will block out and the easier it is to observer, and the closer they are to their star the shorter their year is so the greater number of transits that can be observed in a short amount of instrument time.
This means that in order to observer an exoplanet roughly the same size as the Earth orbiting a star similar to the Sun with a one year orbit, we need continuous observations of a large number of stars over a number of years. That is exactly what Kepler is doing now and with every additional year of the mission we explore a different sub-set of potential exoplanets.

The transit of exoplanets offers us a unique opportunity to observe the planets atmosphere. As the planet passes between its star and us, some of the starlight shines through the planets upper atmosphere before reaching us. Like on the Earth, different parts of the atmosphere absorb different amounts of the starlight at certain wavelengths. So imprinted on the observed starlight, as weak absorption lines, are the tale-tale signatures of various gasses allowing us to build up a spectrum of the exoplanets atmosphere. Ultimately helping us determine the nature of the environment in which it exists and the potential to work out how it formed.
This is unfortunately not as easy as it sounds and although a number of chemical species have been identified in a handful of worlds the application of this technique to exoplanets is still in its infancy.


The study of exoplanets is at a fantastic stage of discovery, where curiosity and ingenuity combine, taking us from the Wright brothers flying a few feet across a field to Amy Johnson flying solo half way around the world.
Be assured there are going to be more unbelievable discoveries coming our way from the exoplanet community in the not so distant future so keep an ear out. 





WHAT'S NEXT?



PICTURE CREDIT: 
   NASA CalTech
   Radial Velocity Gif from http://optroastro.free.fr/techexo.php 
   NASA/Kepler image can be found here: http://kepler.nasa.gov/images/201205planet_size_comparison-full.jpg
   D. K. Sing University of Exeter

Saturday, March 2, 2013

Answering the call


As an observational astrophysicist at some point in your career you will have to write a proposal of your observations to get time on your desired telescope/instrument.

When a call for proposals is sent out the first thing that seems to happen, after a perhaps brief acknowledgement and perhaps a scribble in your diary, is that your forget all about it. After all you have over a month before the deadline so it won’t be a problem. Oh how wrong you were.

Proposals are not as easy to write as you think they might be. Some people have the knack for it but others have a fight on their hands each and every time. You really have to beg, borrow, and steal to get time on a telescope these days. Well not so much of the stealing, or the burrowing, mostly it is just begging lots and lots of carefully worded specific yet vague begging.

The first thing to understand is that your proposal will be sent out to a number of different people. These will be mostly other scientists but most will not work in your field and instead of studying exoplanets they might be specialized in dark matter or something. So you need to bring them up to speed without being in any way condescending, hopefully this is not a hard thing to do. 
You have to grab the reader’s attention in the first paragraph or even the first sentence. This is your elevator pitch, where you have the time it takes to get from the bottom to the top floor to sell your idea to a stranger in a lift.

Once you have got their attention you need to quickly get to the meat of the problem that your observations intend to investigate and justify how they will potentially solve this problem. The operative word there is, justify. Why should they give you precious time on their instrument, how is your research relevant not only to your field of research but also to a wider picture.
In our case recently with the Hubble Space Telescope proposals that were submitted on Friday we had to justify our selection of planets that we wanted to investigate further, what was it that made these planets unique in order for them to supply a wider understanding of exoplanetary populations and the environments in which they live.

The most important part of the proposal after you have grabbed their attention is in the figures. Do the images that you have carefully chosen to include contain all of the information that they need to convey? Are the messages clear and the images crisp? Do they tell the reviewer that the investigation you plan to conduct is even possible?  

Previous observations conducted using HST/WFC3 (not for scientific use!!)
And last but not least. Get someone else to read it! If you cannot sell the idea to one of you colleagues with your proposal you will not sell it to the reviewers. The language will need a few refinements before it is ready to go out. If you have written the proposal with a number of other co-authors make sure that the tone is consistent and it flows from one persons section to the other without substantial gaps or repeats.  In most proposals you will have strict restrictions on the number of pages used for each section or in total so that the reviewers do not have to comb through a 20-page proposal that is only asking for say two days worth of data; so be concise and clear.

The deadline …
Less than ten years ago proposals had to be submitted in paper hard-copy and the deadline was an arrival date at the desk of the man in charge, so depending on your relative to him your deadline becomes very important. As we were clicking the big shiny button to submit our HST proposal my supervisor told me a story of meeting not only the submission deadline but the FedEx deadline so that it would arrive on time. Explaining that if that deadline was missed, as it was one year while he was a PhD student in Tucson, Arizona, the senior PhD student would then have to hop on the Red Eye up to the Goddard Space Flight Center in Maryland to deliver the paperwork.  Luckily proposals nowadays are all submitted electronically, so the deadline is just a frantic communication between the writers and their computer and perhaps a little bit of running around the department to get everything in place.

My advice for those who are planning to go into observational astronomy or who are just starting out; start writing proposals as soon as you can. It takes a while to get the tone and the degree of begging at just the right level while also having ideas that spark interest in others inside and out side your field of study.
Good Luck!



POST IMAGE from the MAST online data archive showing the HST usage in 2007