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 Hα ('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!

A new catalogue of over 200 million stars in the Milky Way galaxy

Recently one of the surveys that I'm a member of made a huge data release including over 200 million stars from our own galaxy, the Milky Way. We've made the data available to the entire world, not just the astronomical community, but everyone in the world! Astronomers love to share data!

The Isaac Newton Telescope on La Palma (Credit: ING)

The survey in question is the INT Photometric H-Alpha Survey (IPHAS), which has, over the last 11 years been using the Isaac Newton Telescope on the island of La Palma to survey our own galaxy to an unprecedented level of detail.

IPHAS is a photometric survey, meaning the goal of the survey is to measure photometry for all the sources it observes (astronomers refer to most objects as 'sources' until they're sure exactly what they are: stars, planets or galaxies). Photometry is the easiest measurement astronomers can make, since all you have to do is measure how bright an object is using a certain filter. IPHAS uses two 'broad' filters and one 'narrow' Hα filter (more on the awesomeness of this filter and what you can do with it in a future post).

The Milky Way (Credit: Wikimedia Commons)

Our own galaxy is visible to us as a glowing band of light stretching across the sky, known as the Milky Way. It appears this way to us because we (the Earth, the Sun and the entire solar system) are within the galaxy, and so we see the galaxy all around us, but concentrated in a band of light that makes up the disk of our galaxy. Astronomers refer to this as the Galactic Plane.

Surveying our galaxy is not easy because the stars can often be very crowded together (hence why they often appear blurred as a bright band of light instead of individual stars), so to do the job properly requires a high-resolution telescope and instrument, but also one that could invest a lot of time and effort into a single project such as this.

In 2003 the Isaac Newton Telescope was just such a telescope and so the IPHAS collaboration was born, led by Janet Drew (currently at the University of Hertfordshire where I am also based). I joined the collaboration in 2004 when I started my PhD at University College London and have been involved in the survey ever since, participating in the telescope observations, analysing data, and writing papers. Its been great to be involved in such a large and exciting project such as this.

After 11 years of taking data, sorting and organising the data, and doing some exciting science as well, the catalog is finally finished. It contains information on 219 million detected objects, the vast majority of which are stars in our own galaxy. It's an example of what astronomers (and data scientists) refer to as 'big data': huge catalogs of data, observations and measurements that can be sorted, filtered and analysed en masse.

One example of this is the image shown below, which was put together by IPHAS astronomer Hywel Farnhill. This map shows part of the Galactic Plane that IPHAS has been surveying, mostly covering the constellation of Cygnus, but stretching to Sagitta on the right.

IPHAS stellar density map (Credit: Hywel Farnhill)

This map is a stellar density map, showing the density of stars across the Galactic Plane, i.e. the number of stars detected by the IPHAS survey in each area of the plane, and its really detailed! Click here to see a high-resolution version of the image, zoom into it, and have a look at the exquisite level of detail that can be seen. The brightest parts are where we detect the most stars and the darkest parts are where we detect the least stars.

Most of this structure is actually due to the obscuring effects of interstellar dust, and not actually the distribution of stars in our galaxy (though this does contribute). Interstellar dust, which is mostly very small silicate rocks (similar to those found on Earth), absorbs starlight, particularly at visible wavelengths, and leads to the fine structure that you can see in this image. Tracing the distribution of this dust is really important to astronomers because of the huge influence it can have when it absorbs the light from the stars and galaxies we are trying to study.

You can read more about the survey on the IPHAS website, learn about the data release itself in this published paper by IPHAS astronomer and catalog maestro Geert Barentsen, or read the press release from the Royal Astronomical Society. If you'd like to access the data (and please do) you can get the entire set of data, for free, from the Vizier web archive.