Tuesday, December 10, 2013

Observing on Mauna Kea



Over the past week I have delved into the life of a ground based astronomer in an effort to gain some practical knowledge of the instrumentation involved and an understanding of the observing process.

Having previously work only with space based data – and as they refuse to send me to see one of those in action – this was my first time seeing a professional telescope.
During the week I was able to observe with both Subaru and the James Clerk Maxwell Telescope (JCMT), two vastly different telescopes all in one glorious place. That place, Mauna Kea.

Mauna Kea is one of the worlds leading observatories with 13 telescopes covering the near UV through sub-millimeter out to radio with contributions from 10 different countries it is a true collaborative effort.

After acclimatizing to the advanced altitude at Hale Pohaku astronomer center, which sits at 2800m on the side of Mauna Kea, you head up to the Summit, a further 1400m above sea level. This altitude gives the mountain ideal conditions for observing sitting above the lower cloud deck and surrounded by cool dry air you get a nearly unobstructed view of the universe.

Well at least in theory!!

On my first observing night I joined some of our collaborators on the Japanese Subaru telescope. Subaru is one of the largest telescopes on the mountain and contains the largest single mirror reflector measuring 8.2m across.
In true Japanese style it is a technological and engineering marvel. Our technician for the night, Daniel Birchall, described it as and over engineered playground for the technicians. As a result it is quite a hands off telescope for visiting observers, apart from the full on tour you can take before the sunsets and you get to see all of the robotic arms and instruments that control this ginormous ‘scope.
The observations that we were taking with Subaru were not only time dependent, as we were observing the transit of an exoplanet, but also required very high precision measurements. So a clear sky is critical.
Unfortunately that night the clouds decided to roll in and with exposures of 15 minutes per image any breaks in the cloud would end up being combined with those of a cloud free sky.
Over the course of the 12 hour observing shift we eagerly checked each of the exposures for the maximum count level hoping that we would collect enough photons to do some science with the data. I think in the end we just about got there, but only just.

While I was up observing with Subaru another observer with the University of Exeter was heading up to do her own observations on the JCMT, and she kindly offered to let me tag along for a few nights and take a look. While the science they do is not in my specific field of study it is a great idea to get a look in at how they take their data and operate their telescope to expand your understanding of the instruments that can be used for astronomy.

Now in contrast to the over engineered telescope of Subaru the JCMT is a different kind of engineering marvel. There is something beautifully British about the whole thing and it is one of a kind in the sun-millimeter astronomy world. JCMT is a 15m dish sitting inside an enormous dome in what is called sub-millimeter valley on the summit of Mauna Kea. From the giant dish to the control room that rotates along with the telescope it appears as if it is from a different time and has a fantastic cobbled together space junk feel to it, like something out of Firefly.
Now unlike Subaru’s measurements clouds would not be too much of a factor for JCMT, however, what we got over the next two nights was. Humidity.
The top of Mauna Kea was shrouded in fog! And with the humidity over 95% we could not even open up the dome.
On the second night our support astronomer, Will, lit up the inside of the dome and walked me through how they take measurements but setting up a mock observation. This let me see how the entire place rotated along with the telescope and how the instruments are all aligned with the dish by moving the secondary reflectors. The best thing about it was that I now had a better understanding of the work my old office mate does and after 2 years of learning about it from her the final pieces clicked into place.

I think the Brit in me liked the industrial feel of the JCMT, but I certainly look forward to getting to use the Subaru telescope for transit observations in the future. Hopefully with better weather conditions.



Wednesday, October 9, 2013

The Birth of Our Solar System

Artists impression of the late heavy bombardment
of our solar system

4.5 billion years ago, long before Homo habilis first picked up a pointed stone and used it as a tool, something truly spectacular occurred that would change the space around it forever. The Sun was born!

Imbedded in a fluffy cloud of gas and dust compressed under its own gravitational pull the core of the Sun burst into existence igniting a fusion reaction that would and will continue to fuel it for over 10 billion years.

At the start of its life a star rotates very quickly, while there is no way to really know how fast the Sun's early rotation rate was the impact that it would have had on the surrounding disk is also hard to determine. We do, however, know that over time a star will loose its angular momentum through outflows and winds reducing its angular rotation over time, or 'spinning-down'. Using a technique called Gyrochronology we can estimate that at the age of 100 million years the Sun would have been rotating over 10 times its current ~25 day rotation period, so we can only assume that in the lifetime of solar system formation or disk dissipation the Sun had a much faster rotation rate.

This large rapidly rotating mass at the center of the cloud spins the material surrounding it causing it to flatten into a disk – like spinning out a pizza base from a ball of dough. By observing other pre-main-sequence stars and measuring the dust emission in the infrared and mm wavelengths we can estimate what the Sun might have looked like shrouded by its protoplanetary disk.
From observations it is estimated that it takes around 7-10 million years for the protoplanetary disk to dissipate potential forming a planetary system invisible to our current instruments.

The evolution of solids in the protoplanetary disk is a multi-stage process:
First the gas and dust of the disk condense to form micron-sized particles, 100 times smaller than the thickness of a sheet of paper, to cm sized oxide and silicate grains. Over the next few million years evaporation and recondensation will be the dominant process in the disk.
From studies of meteorites and asteroids it is estimated that this high-temperature nebula process lasted between 3-5 million years before larger asteroid like bodies formed.
These asteroid-like bodies would have then later formed bodies capable of retaining their own heat or substantial radioactive material. Over the next 2-3 million years through collisions and gravitational interactions planetesimals emerged. Followed by a chaotic period of ‘shock processing’ or ‘heavy bombardment’ where the material fought its way into stable orbits or was chucked out of the solar system entirely – potentially forming some of the comets that come back to visit their original home every few hundred years.  

Theoretical models of protoplanetary disks to early solar systems help us understand how material is likely to behave within the disk and the likelihood of forming planetary systems that are stable like our own. With the discovery of such systems over the last few decades an increased effort has been applied to such simulations to determine if we really are the exception to the rule, which thus far appears very different to our own.

The nature of the very early solar environment is still largely a mystery but scientists are working with renewed vigor from analysis of meteorite to observations and simulations of young protoplanetary disks. In an effort to answer the question; where did it start and how did we get here? 

A time line of the protoplanetary disk and its different stages
What’s next?

Ian Czekala from the astrobites team has a good review article on Protoplanetary disks and their evolution - http://astrobites.org/2011/03/11/review-article-protoplanetary-disks-and-their-evolution/

If you want to know a bit more about gyrochronology and the methods used here is the paper written by Sydney Barnes explaining in detail the technique used
Gyrochronology: S. Barnes 2007 -  http://arxiv.org/abs/0704.3068


 Title image: ART BY DANA BERRY; SOURCES: HAROLD LEVISON AND DAN DURDA, SWRI.

Tuesday, September 17, 2013

The Edge – you don’t really know where it is until you fall off.


The news about Voyager 1 last week got me thinking about what we define as the edge. While something like the edge of a cliff is well understood, something you perhaps don’t want to be finding out by walking off of it, the edge of other things are slightly more hand-wavy.
Is the need for definition just human natures quest for solid answers, a more refined interpretation of the world around us, or is it always a physical boundary?

Astrophysics is one of those subjects plagued by hand-wavy definitions of ‘the edge’. Stars, atmospheres, nebulae, solar systems, even the Universe itself have only vague definitions of the edge.

The dictionary defines the ‘edge’ as - The outside limit of an object, area, or surface; a place or part farthest away from the center of something.
So with this in mind there is the possibility of multiple ‘edges’ for any one thing.

The Sun and stars for example have a visible surface called the photosphere a layer in the solar atmosphere where light in the visible wavelengths emerges. So this is the inner edge from which light emerges from a star, but where does it stop, does it stop? As you go farther and farther away from a star the amount of light you see will reduce with distance. But there will be no point at which the light will get weaker and stop. This gives the stars an inner edge but not an outer one.
“Ahhh, but…!” I hear you cry. Electromagnetic radiation is not the only defining physical parameter associated with stars. What about their gravity, how do we define the edge of a stars gravitational influence?

Again this has no defined edge. As you increase your distance from an object the effect of its gravity diminishes but does not completely go away. In fact it follows a simple inverse square law meaning if you double your distance the gravitational pull of the object reduces to ¼. For an object like you or I this means very little but for something as massive as the Sun it can have far reaching consequences. A more common definition of the edge of the Sun’s gravitational effect is when that of another star takes over. For our Sun this stretch’s out way beyond an expanse of icy bodies called the Oort Cloud over 100,000 AU from the central star or over 1.5 light years. That is 1.5 times the distance light can travel in one year, and light travels really fast.

So if light has no outer edge and neither does gravity what is it that separates us from interstellar space?

This leads us to another edge. Formed by the separation of one magnetic region from another, producing a physical boundary to an otherwise handwavy realm.
The sun’s magnetic field.
While planetary magnetic fields like the Earth’s are shaped by the solar wind, causing a tadpole like tail in the magnetic field to drag behind the dark side of the planet, the Sun’s is relatively spherical forming something called the heliopause. This is the edge that defines the boundary between our Sun and interstellar space.
It is this giant magnetic bubble, this defined edge, which Voyager 1 has been attempting to breach, trying to reach out a human hand into interstellar space.

This is the edge. Voyager 1 marks the edge. The edge of humanity as it endeavors to explore the
unknown. Voyager 1 is the farthest from the Sun any man-made object has ever been and it will continue on its journey into the reaches of the solar system long after we are able to communicate with it. Maybe one day we will go pick it up but until then it will continue to test the boundaries of our Suns influence on its profound voyage from the Earth.

Next step the edge of the universe. Whatever that may be.



What’s Next?
There have been some great articles about Voyager 1’s journey but I think these two really speak to the facts the most.

NASA JPL – How do we know when Voyager reaches interstellar space?

and

IEEE’s – Voyager hasen’t really left the solar system, but that’s OK

I hope you had fun let me know what you think by leaving a comment.
Thanks.

middle and final image - NASA/JPL

Monday, August 26, 2013

Ocean worlds?

All of Earth's water, liquid fresh water, and water in lakes and rivers  

"How inappropriate to call this planet Earth when it is clearly ocean" - Arthur C. Clarke

But is it?

While the surface of our pale blue dot is almost 71% ocean, accounting for over 96% of the total water of our planet, it only goes so deep. According to the USGS (United States Geological Survey) the total volume of the oceans if contained in a sphere would be around 1,385km in diameter, or 1/10 the size of the Earth. Not very big.
The remaining 4% of the Earths water is locked up in the ground, glaciers, and in fresh water lakes and rivers. The main image is from the USGS website showing the Earths water in a series of spheres relative to the waterless sphere of the Earth. The tiny sphere hovering over Georgia, US that you can just make out is all of the water that we use everyday in surface water lakes and rivers.
But considering the distinctive voids where ocean once was perhaps we can forgive Arthur C. Clarke for his outside perspective of it all. 

It is only once we consider the ~6400km of land beneath our feet that we see how meager the contribution of water is to our planet. It is in fact just 0.023% of the total mass.

So ocean it may have, but ocean world it is not.

Pacific ocean from space -
you could be forgiven for thinking it was a ocean world
What about the rest of our solar system? Water is an incredibly abundant substance throughout the galaxy and probably the universe so if it is not on the Earth where is it.
The gas giants in our solar system are as much as 40% water most of this is in the form of ices. In fact a huge portion of the solar systems water is locked up as ice be that in the shadowy depths of craters on Mercury, the polar caps of mars, or the hurricane force winds of Neptune.
The search for liquid water in our solar system takes us 628 million km away to Europa, a moon of Jupiter. Although no direct evidence has been found of liquid oceans beneath the moons frozen exterior it is likely that internal heating and radioactive decay could maintain a body of water beneath the shell of ice.



But is water is so abundant what about other solar systems, could exoplanets be covered in liquid oceans?

There have been a number of reports over the past few years, of potential ocean covered exoplanets Super Earths with no land mass, but water as far as the eye can see.
The first of these was GJ 1214b a planet 2.6 times the radius of the Earth with a mass 6.55 times that of our own planet, giving it a bulk density of 1.87g/cm3, 33% of the Earths.
GJ 1214b orbits a small dwarf star 20% the size of our Sun this means that although small the planet makes a large difference in the amount of light we receive as it transits its star. Thus we are able to look at its atmosphere as it transits through transmission spectroscopy – looking for the atmospheric imprint on the planets radius as it transits its star. A number of studies have been done on GJ 1214b with little luck as to defining characteristic absorbers in its atmosphere. Rogers et al (2010) makes an important distinction for this potential ocean world – “an important conclusion from this investigation is that, under most of the conditions we considered, GJ 1214b would not have liquid water.” With an investigation by Berta et al. 2011 showing a flat transmission spectrum and concluding “The high temperatures and high pressures would form exotic materials like ‘hot ice’ or ‘superfluid water’ – substances that are completely alien to our everyday experience.” So don’t pack your dive gear just yet.

More recently the NASA Kepler team announced a number of planets Kepler-62e and Kepler -62f as well as Kepler-69b and Kepler-69c as potential ocean worlds. Although there were a number of press releases regarding these discoveries there is little to no evidence to support any strong conclusions as we do not even know the mass of these two planets just the radius relative to the star and the orbital period and distance. Relatively little information when we consider the hidden nature of our own solar system planets when they are right at out fingertips.

Follow up observations of exoplanets and our own solar system, with missions like Juno which will reach Jupiter in 2015, will continue to reveal the nature of waters role in our universe as well as opening up many new discoveries for the future.


This is a fantastic info graphic by Stephanie Fox showing the places in our solar system where water is present


What's next?

USGS - How much water is there on Earth - http://ga.water.usgs.gov/edu/earthhowmuch.html

Cfa press release for Kepler water worlds - http://www.cfa.harvard.edu/news/2013/pr201311.html



IMAGES: Main image credit: Howard Perlman, USGS; globe illustration by Jack Cook, Woods Hole Oceanographic Institution (©); Adam Nieman.

NASA

Our watery solar system info graphic by Stephanie Fox