Thursday, 30 October 2014

The Lagoon Nebula

This week I've been studying a young star cluster known as NGC 6530, which is embedded within a famous region known as the Lagoon Nebula. This is one of the most famous nebulae in the sky, and a very attractive target for astrophotographers. Below you can see an image of the Lagoon Nebula that I made, and I think you'll agree that the nebula really does look very lagoon-like!

The Lagoon Nebula as imaged by VPHAS+ (Credit: Nick Wright)

The data for this image comes from the VPHAS+ survey (the southern-hemisphere counterpart to the IPHAS survey), which is being run from the European Southern Observatory's VLT Survey Telescope (the VST) in Chile. We compiled this data for ESO last year to help them with a press release, which they used to produce their own image that you can see here. Their image is nice, but I greatly prefer my own because I think we've retained the lagoon-like swirling clouds of gas much better than they have. What do you think?

The NGC 6530 cluster in the centre of the Lagoon Nebula
The Lagoon Nebula is interesting for astronomers like myself because it's a region where stars are actively forming, and you can even see a cluster of stars that have already formed in the centre of the nebula. This cluster, known as NGC 6530 is about 2 million years old (which is quite young for stars!) and contains many thousands of stars, though only the brightest few dozen can be seen in this image.

The nebula can be found in the constellation of Sagittarius, and despite being about 5000 light years from Earth can actually be seen with the naked eye from a very dark sight (though you'd be better off with a pair of binoculars). Unfortunately, even with binoculars the nebula doesn't look as red and purple as these images suggest, but more of a greyish colour. The reason for this is that under low light conditions human eyes are not good at distinguishing colours, and so most faint things appear grey.

The reddish hue of the Lagoon Nebula seen in most astronomical images comes from the fact that most of the light we see comes from a bright emission line known as ('H alpha'), which is a very prominent emission line from the element Hydrogen that can be found in the red part of the electromagnetic spectrum (hence why we astronomers colour these images red to reproduce their real colour).

The 'Hourglass Nebula" in the centre of the Lagoon Nebula
The nebula is illuminated by a number of very massive, young stars that ionise the hydrogen in the nebula and cause it to glow. It is amazing to think that this entire nebula, approximately 100 light years across, can be illuminated by just a handful of bright stars - but these stars are really bright!

There's a lot of small scale structures within the nebula that this image reveals, including a number of prominent dust globules that are silhouetted against the bright nebula. Perhaps the most famous structure within the Lagoon Nebula is the Hourglass Nebula at its heart. This is a very dense and compact ionised nebula where stars are still forming and which is being ionised by a very young and still embedded, massive star. The Hourglass nebula was actually discovered by the astronomer John Herschel, son of William Herschel, so it's quite exciting to be studying a region previously studied by such a famous astronomer!

This is all particularly timely because of a recent paper that presents high-resolution Hubble Space Telescope images of the hourglass nebula (see below). These observations are much higher resolution than our VPHAS+ images and they're much more detailed, but they only cover a small part of the entire Lagoon Nebula. That is one of the great advantages of large-scale surveys such as VPHAS+, they cover everything!

The Hourglass Nebula as seen by the Hubble Space Telescope
(Credit: Maiz Apellaniz et al. 2014)
The detail in this image is really impressive, and the authors of the paper have done some interesting science with it, detecting evidence of the massive star Herschel 36 being a binary system amongst other results. All of this makes the Lagoon Nebula a really interesting scientific target, which makes me feel very lucky to be studying it at the moment!

Friday, 24 October 2014

Inflatable Solar System

It's been an exciting week because this week I received a package in the post containing a new inflatable Solar System!

I've actually never owned an inflatable Solar System before, but this will be replacing a rather flimsy cardboard Solar System I used to own (yes, you can have a 'cardboard' Solar System!). I have to say that this inflatable one is much better than my previous home-made Solar System, and will probably be much more durable too.

I bought this because I've recently been visiting some schools in my local area to give talks and I needed this model Solar System for one of the activities I do with the school children.

I use this activity to break up the talk and provide some interaction between me and the children. We talk about the different planets and their properties, and then I get some volunteers to come up and pretend to be the Solar System.

Visiting local schools and giving talks to school children is something I love doing because it gives me an opportunity to share my love of astronomy and astrophysics with people. Most people love to hear about science, in particular astronomy, so it can be a really enjoyable experience.

Plus, communicating astronomy with school children encourages them to take up scientific subjects in school and pursue them in college, and a scientifically literate workforce is good for everyone.

Sunday, 19 October 2014

The largest astronomical catalogs ever made! (Part II of II)

In a previous post I introduced the 'Top 10' largest astronomical catalogs that are currently and publicly available for astronomers (and anyone) to download and use. For a quick reminder, here's the list:
  1. The US Naval Observatory (USNO-B1) all-sky catalog, 1046 million entries (2003)
  2. The Guide Star Catalog (GSC v2.3) all-sky catalog, 946 million entries (2006)
  3. The Sloan Digital Sky Survey (SDSS DR9), 933 million entries (2012)
  4. The Wide-field Infrared Survey Explorer (WISE) all-sky catalog, 748 million entries (2013)
  5. The United Kingdom Infrared Deep Sky Survey (UKIDSS), 727 million entries (2012)
  6. The 2 Micron All Sky Survey (2MASS), 471 million entries (2003)
  7. The Deep Near Infrared Survey (DENIS) of the southern sky, 355 million entries (2005)
  8. The INT Photometric H-Alpha Survey (IPHAS), 219 million entries (2014)
  9. The USNO CCD Astrograph Catalog v4 (UCAC4), 114 million entries (2012)
  10. The Galactic Legacy Infrared Mid-Plane Survey (GLIMPSE), 104 million entries (2008)
In that post I discussed five of these surveys (including the IPHAS survey that I work on - read more about that here) that all include astronomical objects observed in the visible part of the electromagnetic spectrum (that is the part that we can see with our own eyes). The other five of these surveys all include measurements made outside of this part of the spectrum, and its those I'd like to talk about today.

The electromagnetic spectrum, with wavelength increasing to the right, and frequency (or energy) increasing to the left. (Credit: NASA)

These five surveys are WISE, UKIDSS, 2MASS, DENIS, and GLIMPSE, and they're all infrared photometric surveys, which means they measure how bright astronomical sources are in the infrared. The infrared part of the electromagnetic spectrum includes radiation with a longer wavelength than that of the light we can see, though not as long as microwave radiation or radio waves. You may be aware that infrared radiation is what we commonly know of as heat or warmth. Any object with a temperature around body temperature or up to a few hundred Celsius will radiate energy in the infrared.

Astronomers often divide up the infrared part of the electromagnetic spectrum into the near-, mid- and far-infrared regions, ordered by how far they are from the visible part of the spectrum. The definition of these three regions isn't important, but originates in the different technologies required to detect radiation in each region (such as different CCD detectors).

The near-infrared is the most easily accessible part of the infrared spectrum, and three of these surveys cover this region. Perhaps the most commonly-used of these is 2MASS, an all-sky near-infrared survey, which operated from 1997 to 2001 at two observatories in Arizona and Chile. The 2MASS all-sky view of the Milky Way is a stunning panorama of our galaxy, aided by the ability of near-IR radiation to penetrate the dust that would otherwise absorb optically visible radiation.

The 2MASS all-sky view of the Milky Way (Credit: IPAC)

The DENIS survey was also a deep near-IR survey that finished in 2001, but which only covered the southern sky. The UKIDSS survey(s) are a series of very deep near-IR surveys (deeper than both 2MASS and DENIS) on the United Kingdom Infra-Red Telescope (UKIRT) in Hawaii that targeted certain areas of the sky for different science goals. Some of these areas include regions of our galaxy where stars are forming and other areas look away from our galaxy out into the darkest regions of space where other galaxies are easily studied.

The remaining two surveys, WISE and GLIMPSE, are mid-infrared surveys, operating at slightly longer wavelengths than the near-IR surveys.

The centre of our Milky Way galaxy seen in the mid-infrared from GLIMPSE (Credit: Spitzer GLIMPSE team)

GLIMPSE was a survey of the Milky Way galaxy with NASA's Spitzer Space Telescope, and WISE is also a NASA space telescope that performed an all-sky infrared survey. Because the Earth's atmosphere absorbs most mid-infrared radiation, mid-infrared telescopes have to operate from above the atmosphere.

An artist's impression of NASA's
Spitzer Space Telescope in orbit
(Credit: IPAC)
A further complication for mid-infrared telescopes is that the telescopes themselves have to be cooled to very low temperatures to prevent them from radiating infrared photons themselves! This is because, even in space objects will radiate infrared radiation depending on their temperature. These photons would contaminate the measurements made, and so the entire telescope has to be cooled to incredibly low temperatures, much lower than anything in our solar system, to prevent them from radiating significantly large amounts of infrared radiation. Both space telescopes used cryostats filled with liquid hydrogen to achieve this, allowing them to be cooled to less than -263 C (under 10 K).

That brings our list of the largest astronomical catalogs to a close, for the time being at least. Astronomy is moving ever so rapidly towards bigger and bigger catalogs and it is almost certain that these catalogs will be eclipsed in the near future. The most likely candidates to take the throne of largest astronomical catalog will probably come from either ESA's Gaia satellite or the Large Synoptic Survey Telescope (LSST). More on these in a future post!

Sunday, 12 October 2014

The largest astronomical catalogs ever made! (Part I)

In a previous post I discussed a huge data release from one of the surveys I've worked on over the last 10 years, which included a catalog of over 200 million sources. This is a BIG catalog of data, but how does it compare to other large astronomical surveys? I wanted to find out!

To do this I searched the two main astronomical data archives, the Vizier and IRSA online databases. My criteria for including a catalog was that it had to be a single catalog with the same information and measurements made in the same way for all the entries in the catalog (usually referred to as 'sources', because they are the source of the light we are measuring). It could be a catalog of any type of object (stars, planets, galaxies) and it could be from data obtained at any wavelength (optical, infrared, ultraviolet) by any telescope in the world.

So here's the 'Top 10' astronomical catalogs currently (and publicly) available:
  1. The US Naval Observatory (USNO-B1) all-sky catalog, 1046 million entries (2003)
  2. The Guide Star Catalog (GSC v2.3) all-sky catalog, 946 million entries (2006)
  3. The Sloan Digital Sky Survey (SDSS DR9), 933 million entries (2012)
  4. The Wide-field Infrared Survey Explorer (WISE) all-sky catalog, 748 million entries (2013)
  5. The United Kingdom Infrared Deep Sky Survey (UKIDSS), 727 million entries (2012)
  6. The 2 Micron All Sky Survey (2MASS), 471 million entries (2003)
  7. The Deep Near Infrared Survey (DENIS) of the southern sky, 355 million entries (2005)
  8. The INT Photometric H-Alpha Survey (IPHAS), 219 million entries (2014)
  9. The USNO CCD Astrograph Catalog v4 (UCAC4), 114 million entries (2012)
  10. The Galactic Legacy Infrared Mid-Plane Survey (GLIMPSE), 104 million entries (2008)
It's great to see that our survey, IPHAS, has made it into the Top 10! Most of these names probably won't mean much to non-astronomers, but to astronomers these are the big catalogs that many of us use regularly. They represent hundreds of nights of telescope time and thousands of hours of valuable work.

They are primarily (all but one) photometric surveys, which as I mentioned in a previous post is pretty much the easiest measurement an astronomer can make, it's simply a measure of how bright an object is. You don't have to measure the shape of the object, or its exact position, just how bright it is - simply take a picture and 'count' up the amount of light!

Photographic (negative) plate showing the galaxy M33, taken
by Edwin Hubble in 1926 (Credit: University of Arizona)
Interestingly the first two of these catalogs, USNO-B1 and GSC, are compiled not from CCD observations (as are the other photometric catalogs on this list), but from photometry extracted from photographic plates. During the 20th century the night sky was routinely photographed from various observatories around the world and many of the photographic plates have been carefully stored since then in telescope and university archives.

Putting this data together into a single uniform catalog (or two catalogs with heavy overlaps) represents an important and valuable achievement, and this data has been useful for many studies.

These two photographic surveys used photographic plates sensitive to either red or blue light, so both are known as 'optical' surveys (meaning they are sensitive to light in the optical part of the electromagnetic spectrum). The same is also true for three other surveys on this list: SDSS (3rd), IPHAS (8th), and UCAC4 (9th), though these surveys all use CCD observations.

CCDs are superior to photographic plates for many reasons, so why are these modern CCD catalogs smaller than the catalogs compiled from photographic observations? Well this is mostly because the first two of these CCD surveys only covered a small area on the sky: SDSS is focussed on the Galactic halo (where it is easier to observe other galaxies, which was the goal of their survey) and IPHAS covers only the Northern Galactic Plane (the part of our own galaxy visible from the northern hemisphere). Compared to the entire all-sky area of 41,253 square degrees, these two surveys only cover 14,555 and 1800 square degrees, respectively.

The Galactic Plane of our Milky Way galaxy (Credit: Imgur)

The final optical survey, UCAC4, is principally a proper-motion survey and not a photometric survey. The objective here is not to measure how bright the sources are, but how fast the object is moving across the sky (known as its 'proper motion').

The simplest way to do this is to take two images of the same area of the sky separated by a few years. You then measure the positions of all the objects in each image and calculate how far they've moved between the images. In truth it is much more complex than this, because you're measuring objects moving across a curved surface (the sky is a curved surface) and most of our cameras and detectors are not as perfect as we'd like them to be (i.e. a straight line across the sky might not be perfectly straight on the image you record because the optics of the camera might distort it).

Because of these difficulties it isn't always possible to measure a proper motion for every source, hence the reason this catalog is smaller than the other optical all-sky catalogs. Despite these difficulties though this is a very large proper motion catalog and it will be many years before it is eclipsed in terms of size!

Next time we'll look at the other five catalogs on this list, which have all been produced from observations outside of the visible part of the electromagnetic spectrum.

Tuesday, 7 October 2014

And the Nobel Prize goes to...

The winners of this year's Nobel Prize in Physics were announced today, and while the award hasn't gone to my first choice, Vera Rubin (or any astrophysicist for that matter), it has gone to a trio of very worthy winners. The winners are Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura for their work in the development of blue LEDs (light emitting diodes).

Why is this so important? Well, it is thought that about 20-30% of the electricity in the world is consumed in electrical lighting, and since LEDs are about 10 times more energy efficient and last about 100 times longer than traditional incandescent light bulbs, the worldwide adoption of LEDs will significantly reduce the world's energy consumption (and therefore its carbon dioxide emission).

So while this wasn't my first choice for the Nobel Prize, it is surely a deserved award!

Sunday, 5 October 2014

My nomination for this year's Nobel Prize in Physics

This coming week will bring with it the announcements of the winners of this year's Nobel Prizes, with the physics prize due to be announced on Tuesday.

There has been considerable talk about the Nobel Prize in Physics this year, thanks in part to the announcements coming from the BICEP2 team who claimed to have discovered the tell-tale signature of gravitational waves emitted during an important phase of the early Universe known as inflation. That result has since been questioned by measurements from the PLANCK telescope, so there is unlikely to be a Nobel Prize in the offing for the BICEP2 team this year.

So that leaves the question, who will win the Nobel Prize in Physics this year? There are many strong candidates, and I can't list them all here, but I'd like to mention one strong candidate who I think is very deserving, Vera Rubin.

Vera Rubin (Credit: UC Berkeley)
Vera Rubin was responsible for a number of pioneering observations in the 1960s and 70s that laid down the case for spiral galaxies containing a large mass of undetected matter. The presence of this dark matter was inferred from the rates at which these galaxies were seen to be rotating, which Vera Rubin calculated by measuring the speeds that the stars in the galaxies were moving (using the method I described in a previous post).

Rubin found that the stars at the edges of the galaxies were moving a lot faster than expected if these stars were gravitationally bound to the galaxy, based upon estimates of the mass of the galaxy from counting up all the visible matter (stars, nebulae, and the like). Rubin didn't just find this in one galaxy, but multiple galaxies, leading up to her seminal 1980 paper which presented results for 21 galaxies, all displaying the same pattern of missing mass.

Galaxy rotation curves: predicted (A) versus observed (B).
For the velocities  in (B) to be so much higher than predicted (A) at large distances,
requires a considerable amount of 'missing mass'. (Credit: Penn State University)

Since these pivotal observations there have been two prominent theories to explain these observations (and hundreds more observations since). The first is that the laws of physics (particularly the law of gravity) may somehow be different in these galaxies, which is often referred to as Modified Newtonian Dynamics (MOND). The more popular explanation among astronomers though, is that these galaxies (and our own) contain a large amount of mass that doesn't emit any light, known as dark matter.

Vera Rubin's pioneering work effectively laid down the observational basis for the existence of dark matter. There had been hints of the existence of such matter previously. Both Jan Oort in 1932, Fritz Zwicky in 1933, and Horace Babcock in 1939 had uncovered similar effects, but there were other explanations at the time that meant the results were never conclusive. Rubin's work was both thorough and conclusive, and has certainly stood the test of time.

In fact the evidence for dark matter has improved since that day to the point that the vast majority of astronomers consider it almost a fundamental truth that the majority of the mass in the Universe is in some dark form that we cannot directly observe. There is evidence from studies of hot gas in galaxy clusters, gravitational lensing of background galaxies by foreground dark matter condensations, and even from the cosmic microwave background radiation.

The Bullet Cluster of galaxies. Blue show the distribution of dark matter,
red shows the distribution of 'normal' matter (Credit: NASA).

Perhaps the most direct evidence to date for dark matter comes from observations of the Bullet Cluster of galaxies that shows dark matter and normal baryonic matter spatially separated because of a collision between galaxies. Because the dark matter in the Bullet Cluster is not co-spatial with the distribution of normal matter, these observations cannot be explained by modified theories of gravity.

And now to add to all of this there were hints this year that astronomers might have actually made a direct detection of dark matter!

So with all of this evidence I think Vera Rubin would be highly deserving of a Nobel Prize. Dark matter is the most important astronomical discovery of the last half century, and although we don't fully know what it is made of, we know it exists!

Awarding Vera Rubin a Nobel Prize would also provide an excellent role model for women in all areas of physics. In over a century there have only been two female recipients of the Nobel Prize in Physics, Marie Curie and Maria Goeppert-Mayer, meaning that they are outnumbered 100-1 by male recipients. Adding Vera Rubin to that list would help to reduce that discrepancy, as well as recognising her important work.

Friday, 3 October 2014

Searching for stars that run away

So it's observing proposal season and Wednesday was the ESO deadline, which means panicked proposal writing and last-minute scribbling all round.

ESO is the European Southern Observatory, Europe's premier observatory in the southern hemisphere, which operates some of the largest and most advanced telescopes in the world. This includes the four behemoths that make up the VLT, the Very Large Telescope (except there's four of them), each housing a 8.2m mirror and some of the best astronomical instruments in the world.

ESO's Very Large Telescope(s) in Chile (Credit: Wikimedia Commons)

One of the proposals we submitted is part of work by my colleague Mike Mohr-Smith to improve the census of massive stars in our galaxy and identify where they formed. Massive stars are very rare and live very short lives (on astronomical timescales at least), but affect the evolution of other stars and the galaxy as a whole in very important ways. How massive stars form is a major unanswered question in astronomy, and since they live such short lives finding these stars and tracing them back to their birth-sites is an important avenue of research.

So we've recently identified a number of hitherto-undiscovered massive stars in the vicinity of one of the young massive star clusters in our galaxy, Westerlund 2. The most massive stars we know of are nearly always found deep within star clusters, and some people have suggested that they can only form in such environments, so finding massive stars near a massive star cluster, but not within it, is very interesting.

The massive star cluster Westerlund 2 (Credit: Robert Gendler)

So the question arises, did these stars form outside of the cluster (which would make them very special) or did they form in the cluster but have since been ejected? The first step in answering this question is to measure the speed these stars are moving relative to the cluster. If the stars have been ejected (known as runaway stars) they should be moving very fast away from the cluster, but if they formed in isolation their velocities will be much lower.

By taking high resolution spectroscopy of these stars we can measure their speeds by observing the shifts in the positions of known spectral lines due to the Doppler effect. This is the same effect that causes the pitch of a siren to change as a vehicle moves towards you and then away from you, but instead of affecting sound waves it is shifting light waves.

Representation of the Doppler-shift effect on spectral lines
(Credit: University of Virginia)
This image shows a spectrum of light, which is light split into its constituent parts using a prism. What was originally white light has now been split into all the colours of the rainbow (it's actually the same effect that causes a rainbow!).

On top of the rainbow you can see dark lines, which are known as spectral lines. These are caused by atoms of different elements absorbing light at certain wavelengths (in certain parts of the spectrum). When an object is moving away from us, it's light is redshifted, meaning spectral lines shift towards the red part of the spectrum, while when an object is moving towards us it's light is blueshifted, meaning spectral lines shift towards the blue part of the spectrum. The faster an object is moving relative to us, the greater the shift in the position of the spectral lines. So by observing the spectrum of light from a star, measuring the positions of it's spectral lines and comparing them to the positions we know they should be at we can determine how fast the star is moving towards or away from us.

So our objective is to use one of the instruments on the VLT to acquire high-resolution spectroscopy, measure the positions of the spectral lines and therefore the speed the stars are moving relative to the stars in the cluster. With this information we can answer the question of whether these very massive stars formed inside the cluster or outside of the cluster.

That's our plan at least, and that's what we've written to the friendly people at ESO asking them if we can use their telescope to do this project. I'll let you know what they say!