A bright bubble around a dying star

The bright bubble around Luminous Blue Variable star G24.73+0.69

A few days ago Milky Way Project user suelaine posted an image of this pretty bubble on the Talk forum, asking whether it was a supernova. As supernovae – or rather, the debris that’s left behind after they explode – often have this kind of shape, I initially thought she was right. But when I looked up the coordinates on SIMBAD – the astronomer’s guide to the Galactic sky – I discovered it was a beautiful example of a more peculiar type of object: a Luminous Blue Variable star (LBV).

[As an aside, the SIMBAD page for the object is a little confusing, as it’s identified on there as a Be star – a rapidly rotating B star. But when you look down the page at the references, it’s clear that the star has since been identified as an LBV star, probably even more massive than originally thought.]

LBV stars are massive stars, often with a few tens of times the mass of the Sun, that are approaching the end of their lifetimes. The fuel in their cores, needed to maintain nuclear fusion, is running out. This makes them unstable, causing them to flare up at intervals. As they’re not able to hold on to their outer layers, powerful winds eject matter into the surrounding interstellar medium during these eruptions, and the star can be sen to brighten significantly over several months. LBVs are on their way to exploding as supernovae.

The evolution of such massive stars once they run out of fuel proceeds very quickly, so these objects are extremely rare: only around a dozen are firmly known in the Milky Way Galaxy. The best-known examples are Eta Carinae and the Pistol Star,  perhaps the most luminous star in the entire Galaxy. Because there are so few LBVs to study, there’s a lot about them we don’t know.

This particular LBV, prosaically known as G24.73+0.69 (its galactic coordinates), was discovered in 2003, and lies at a distance of around 5 kpc, or 16000 lightyears. As well as the compact orange bubble this larger view of its surroundings shows that there is a second, larger shell, more bipolar in shape than a true ellipse. The star and its environment were studied in detail in a very recent paper by Argentinian astronomers Petriella, Paron and Giacani. They discovered a dense molecular shell tracing the outer bipolar nebula. They suggest that the inner compact bubble is the result of an LBV eruption, and the outer bipolar shell perhaps caused by more gradual mass loss during the star’s “regular” lifetime.

Interestingly, they also find evidence that perhaps new stars are forming near the lobes of the larger shell. They suggest this may be triggered star formation in the swept-up gas, but their observations can’t confirm that.

I didn’t know much about LBVs myself, so I was pretty excited with this find. Keep posting your interesting objects to the forum – perhaps we can find more LBVs or other cool types of objects.

If you’re interested in learning more about this interesting star, here’s the full paper reference:

Petriella, Paron & Giacani. The molecular gas around the Luminous Blue Variable Star G24.73+0.69. Astronomy & Astrophysics vol. 538, A14 (2012) [pdf available from Arxiv]

Exploring the MWP: Coordinates and SIMBAD

One of the most common questions posted on Milky Way Talk is “What is [that thing] in this image?”, and science team members try to respond to some of those where we can. The galactic plane is so incredibly rich at these infrared wavelengths and the Galaxy is so vast that even with the combined experience of the whole science team we usually don’t know the answer.

To help everyone out, we’ve created a new tool that lets you search one of the world’s best astronomical databases from within the Milky Way Project. SIMBAD is a huge astronomical database, maintained by the Centre de Données astronomiques de Strasbourg (CDS) and contains 7 million astronomical objects documented in the literature. When astronomers want to see what is known about any part of the sky, many of them start with a SIMBAD search. Our new Coordinates Tool lets you search the images from the MWP for SIMBAD data, to help show you what different objects are.

The Coordinates Tool

You access the new Coordinates Tool directly at http://www.milkywayproject.org/tools/coordinates/

This will take you to a default page, exploring the area around the coordinates 0, 0. The MWP images use the galactic coordinate system, which expresses positions in galactic latitude and longitude – concepts that should be familiar if you know about geo-coordinates here on Earth. The “equator” of the galactic coordinate system (the latitude = 0 position) is roughly coincident with the disk of the galactic plane.

Looking down on the galactic plane (Image: NASA/JPL-Caltech/R. Hurt)

In this picture, the galactic latitude tells you how much an object lies above or below the plane of the Galaxy, and the longitude specifies the angle away from the Galactic Centre. The above image shows a schematic diagram of what we think the Milky Way Galaxy looks like, with an indication of our own location and a galactic longitude grid. Galactic latitude runs from -90 to 90 degrees, and longitude from 0 to 360 degrees, although sometimes you may also see it noted as -180 to 180 degrees.

To search the area around any set of coordinates, you simply include them in the URL for example, to search one of my favourite regions, at longitude 18.4 degrees and latitude 0.2 degrees, you would visit

http://www.milkywayproject.org/tools/coordinates/19.0/0.0/

This will display one of the MWP images that containing those coordinates (see below). It will also list the other MWP images containing these coordinates. This lets you explore the region at different scales and in different contexts. The specified coordinates are shown on the image with a box. A link to the image’s Talk page is also included.

You’ll see that as you move the cursor around the image, coordinates are displayed to help you navigate. You can double click on any point to jump to that centre and see the images available. By default a small, square box is drawn onto the target area. If you want to draw a specific box you can give the width and height (in arc minutes) as URL parameters:

http://www.milkywayproject.org/tools/coordinates/19.0/0.0/?h=15&w=30&zoo_id=AMW102de6d

You can also reach the Coordinate Tool from the main Explore page. just double click on the map to just to more detail on that region. A link has also been placed on the images in the My Galaxy section of the site, for logged in users.

SIMBAD

Also present on the Coordinate Tool is a button with the words ‘SIMBAD Search’. Clicking this performs a SIMBAD search on the current viewing area and displays the results directly on the image. Here’s an example from the URL I gave above:

Any objects SIMBAD finds in the astronomical literature are displayed as circles. If,you hover your mouse over them you will see their object name and type. Clicking on these objects takes you to the objects page on the SIMBAD site, where you can find out more.

Many of the objects found in the MWP will be stars – the galaxy is full of them! – and many will be IRAS and 2MASS objects – these names derive from previous infrared surveys that mapped the regions covered by the MWP data. In the above image you can see one 2MASS object near the centre of the bubble on the right:

The SIMBAD page for 2MASS J18252813-1224187 explains that may be an Asymptotic Giant Branch (AGB) star. These objects are interesting, and there are plant of them to be found in the MWP images.

Some interesting regions that are worth a SIMBAD lookup with the tool include the rim of a broken bubble, a dying star and the pulsating heart of a gorgeous bubble.

This tool is still a bit rough around the edges, but we are keen to invite comments and ideas from anyone that would like to try it out. You can either leave comments on this blog post, or email us on team@milkywayproject.org. We have more updates on the way!

There Is No Right and Wrong

Continuing this series of posts, answering questions from Milky Way Talk, MWP user UncleClover writes:

I’m a very fast learner, but only if there’s some sort of feedback going on letting me know what I’m doing right and what I’m doing wrong. Once an image that we notated has been processed by a professional, is there any way for us to look it up and find out how accurate or inaccurate our notations were?

Feedback wouldn’t even need to necessarily be an extra step, just having access to your own prior notations and being able to compare them to the “final conclusions” of the professional analysis would be useful.

And in a similar vein, user katieofoz writes:

I was just wondering about a few thing that someone here might be able to answer.

• Im assuming the data collected for each image is compiled and if a similar marking shows up a certain number of times someone (with actual qualifications) looks at the image and classifies it? Is this right?
• From the data collected is there anything that looks at how often a person is correct in their markings? Like some sort of way of ranking the user based on their observations?
• What is done with the compiled data afterwards? Is it based off the average of observations or are areas looked at by qualified individuals to map it more accurately?

An important thing I learnt during my PhD is that there’s an awful lot of work involved in research that you really don’t need a PhD for. Or even a degree. Just a pair of eyes will do, and a few fingers. The tasks you’re performing for Milky Way Project are a great example of that. I may well have a PhD, but I’m no better than any of our users at finding or drawing bubbles. So really, we’re all experts in this project, and as long as you’re genuinely drawing what you believe is a bubble, or a knotty thing, or a dark cloud, there is no “right” and “wrong”.

To be clear, all we’re doing in the science team is providing you with the dataset, and then gathering up all the drawings of all the users and merging all that information into a consistent catalog of objects. We implement a few quality control measures, for example we consider a new user’s first few drawings to be practice drawings and don’t enter those into our dataset – as UncleClover says, there is a learning process.

We make no judgment on whether a drawing is “right” or “wrong”, and we don’t process individual images or users’ drawings. Once you’ve had a little practice, every click counts. All the clicks go into a (giant!) database, and a computer program written by Robert Simpson analyses all the data.

This program essentially scans the whole are of the sky covered by our images and locates clusters of ellipses that users have drawn (you can see an example image with all the various drawings made my volunteers, above). Where we find more than a given number in a small region, we mark this as a bubble. The bubble’s properties that go into the catalog are calculated from averages of the individual users’ classifications – as katieofoz rightly suggests. So whether a bubble you drew ends up in the catalog really just depends on whether lots of other people agreed with you. Given that there’s no real “right” or “wrong” in MWP, we don’t rank the users. It’s not a competition! But our processing algorithm does allow the classifications by experienced users to count more heavily than those of newbies.

Rob wrote more about the procedure earlier on this blog, and of course this will all be described in a lot of detail in the forthcoming paper.

If you haven’t done so already and you’re interested in knowing more about the infrared bubbles in the interstellar medium, I’d invite you to read the original bubbles papers of 2006 and 2007, written by our science team members Ed Churchwell (U Wisconsin-Madison), Matt Povich (Penn State), Bob Benjamin (U Wisconsin-Whitewater), Barbara Whitney (U Wisconsin/Space Science Institute) and their collaborators. This paper is packed with information on the Spitzer surveys we’ve taken our data from,the difficulties in picking out bubbles and what the bubbles might physically represent. The references are below, and the paper should be available via the ADS link.

Churchwell, E., Povich, M., Allen, D., Taylor, M., Meade, M., Babler, B., Indebetouw, R., Watson, C., Whitney, B., Wolfire, M., Bania, T., Benjamin, R., Clemens, D., Cohen, M., Cyganowski, C., Jackson, J., Kobulnicky, H., Mathis, J., Mercer, E., Stolovy, S., Uzpen, B., Watson, D., & Wolff, M. (2006). The Bubbling Galactic Disk The Astrophysical Journal, 649 (2), 759-778 DOI: 10.1086/507015 [ADS]

Churchwell, E., Watson, D., Povich, M., Taylor, M., Babler, B., Meade, M., Benjamin, R., Indebetouw, R., & Whitney, B. (2007). The Bubbling Galactic Disk. II. The Inner 20o The Astrophysical Journal, 670 (1), 428-441 DOI: 10.1086/521646 [ADS]

Zooming and Drawing

Continuing this series of posts, answering questions from Milky Way Talk, here’s one from MWP user broomrider1970, who asked about a zoom option for the images. In fact there’s a whole thread about this on Talk.

When we first started planning the Milky Way Project (MWP) and began testing the interface, we actually had quite a lengthy discussion about a zoom option. In fact, digging through my email, it was my very first question to the developers. As it turns out, adding the zoom option is first of all quite challenging technically. Secondly, and more importantly, giving users the ability to zoom in on images gives us an extra level of uncertainty when we’re trying to process the classifications. By keeping the image static we know for sure that all the users saw the same image in the same way, and we know exactly what the minimum and maximum bubbles sizes are for each image. So it was really an issue of ensuring consistency in the classifications.

Next up: user chairstar asked:

Since I’ve found so many areas of interest – not just IDRC’s but other things like green knots, which do not have a round or square shape, I’d like to see if there is a way of allowing us to ACTUALLY drawing “lines” around irregularly shaped areas, as opposed to being constrained to the classic square and round shape that we are to use now.

Perhaps incorporating a drawing software, allowing free-hand lines could be put in.

Again, this is something we thought about. Particularly for the infrared dark clouds (IRDCs), which tend to have complex filamentary shapes, we wanted to have some kind of polygon- or freehand-drawing tool. But as with the zoom option, having freehand drawings as classifications makes it very challenging for us to merge all these drawings into a consistent catalog of objects. I’m also not sure how we would codify the information captured in freehand drawings in an easily accessible format.

In addition, for the specific case of IRDCs, it wasn’t clear that this would give us better results than an automatic detection algorithm. Several existing IRDC catalogs detected dark clouds with algorithms, and these actually do quite a good job. Not wasting people’s time on tasks that are done just as well by a computer is a core principle of the Zooniverse’s citizen science philosophy.

Bok Globules and Herbig Haro Objects

The Milky Way Project science team are currently busy laying what we hope is the final hand on our first publication. In this paper, we’ll describe the project and why we decided to take the citizen science approach for the task of identifying bubble structures in the Galaxy. We will also present our first results from the hundreds of thousands of classifications we’ve logged on the site, and how our new bubble catalog might be useful for further studies of star formation and the interstellar medium. As we’re big fans of open data sharing, the paper will of course be made publicly available via Arxiv.

I spotted a bunch of interesting questions on the Milky Way Project Talk forums recently and wanted to take some time to jot down a few answers. Here the first one (or in fact, two).

User Ken Koester asks:

1. Is the resolution of these images such that we ought to be able to detect Herbig Haro objects?
2. Bok globules are pretty cold; do they still show as black in these images?

Herbig-Haro objects were first discovered in the early 1950s by, not surprisingly, astronomers George Herbig and Guillermo Haro, who spotted optical nebulosity in active star forming regions like Orion. Decades of further observations in the optical, infrared and radio have since established that these “nebulae” are bright knotty streams or jets, sometimes with a very marked bipolar and narrow shape, streaming out from newly forming stars. This type of powerful outflow of material is a typical feature of star formation at all different masses.

The optical radiation seen from these Herbig-Haro flows arises when material in the outflow powers into the quiescent medium surrounding the new star, causing shocks in the gas. Such shocks are also commonly seen in the infrared. In the Spitzer bands of observation, shocks are particularly prominent in the 4.5 μm channel – the blue channel in our Milky Way Project images. Outflows come in variety of size, and of course their apparent size depends on how far away they are. But the resolution of Spitzer is certainly sufficiently high to spot them.

If you want to know more about these objects, there are two excellent review articles from Annual Reviews of Astronomy & Astrophysics on the topic:

Schwartz, R. (1983). Herbig-Haro Objects Annual Review of Astronomy and Astrophysics, 21 (1), 209-237 DOI: 10.1146/annurev.aa.21.090183.001233 [ADS]

Reipurth, B., & Bally, J. (2001). Herbig-Haro Flows: Probes of Early Stellar Evolution, Annual Review of Astronomy and Astrophysics, 39 (1), 403-455 DOI: 10.1146/annurev.astro.39.1.403 [ADS]

Bok globules are very dense and compact cloudlets that are forming new stars in their interiors. They appear black in optical and infrared images, as they’re too cold to emit any radiation shortward of around 100 μm. To study what’s happening on the inside, we have to observe at those wavelengths and beyond, which are covered by e.g. the Herschel Space Observatory and millimeter telescopes such as IRAM or ALMA. They should be visible in our images, as they appear dark in all our colour channels. But not many are as distinct-looking as the ones we see in beautiful Hubble images like this one, so they can be very hard to spot.

The Spitzer image below of a giant Herbig-Haro flow inside a Bok globule towards the constellation of Vela, at a distance of 350 pc (1140 lightyrs), combines light at 3.6 µm (blue), 4.5 and 5.8 µm (green) and 8.0 µm (red). The colours used are a little different than in our Milky Way Project images, which use 4.5/8/24 µm respectively. The image below looks at an area on the sky of 10.2 x 6.5 arcminutes, which is just slightly closer in than the highest zoom level of the MWP images (18 x 9 arcmin). Compared to other H-H objects, HH46/47 is really enormous, so other outflows are likely to appear much smaller.

Spitzer’s view of a giant Herbig Haro flow, HH46/47, inside a Bok globule (Image: NASA/JPL-Caltech/A. Noriega-Crespo (SSC/Caltech), Digital Sky Survey).