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Nebula commentaries completed

Submitted by Kevin Jardine on 19 September, 2013 - 09:03

Eight years ago I started work on a commentary on the Sharpless nebula catalog and eventually expanded to the Gum and RCW catalogs as well. Together these catalogs cover almost all of the prominent emission nebulae of the Milky Way visible in hydrogen-alpha. (These catalogs overlook a few fainter large objects and miss many smaller objects. There are other catalogs describing smaller emission nebulae such as the BFS and Bran catalogs. At some point I will look at these.)

It took me longer than expected but today the Sharpless, Gum and RCW commentaries are complete. Over the years the database and Python code I was using to present the commentaries became bitrotted so now I am using a new Haskell-based system to generate static pages from an off-line database. The resulting pages display faster and more reliably.

I have used the new Haskell-based system to improve the format of the commentaries. There are now proper Wikipedia-style footnotes, links to each nebula in the Milky Way Explorer, and a selection of distance estimates from the scientific literature instead of a single estimate. I have updated the commentaries to use the latest research and to improve the images. Of course, updating the commentaries will be an ongoing task.

There are a total of 313 Sharpless objects, 209 RCW objects and 97 Gum objects. There are more objects in the RCW and Gum catalogs than catalog numbers because both of these catalogs describe subnebulae (for RCW in the notes and for Gum in the main catalog). Sometimes these subnebulae identify the brightest parts of a larger object, but often they identify separate objects.

There is considerable overlap between the three catalogs as this Venn diagram shows:

Because of the overlaps, there are 483 distinct objects in the three catalogs. There are actually fewer nebulae than this, because some catalog entries simply designate nebulous regions that contain separate objects described in other catalogs, and in the case of the RCW catalog, there appear to be a number of objects that are unidentifiable or simply do not exist. More details can be found in the commentaries.

Stewart Lane Sharpless (1926 - 2013)

Submitted by Kevin Jardine on 17 September, 2013 - 09:31

Stewart Sharpless, the creator of the Sharpless nebula catalog, died on January 19, 2013 at the age of 86. A bit sadly, I have seen no obituaries in scientific publications yet for Sharpless. I found out about his death recently by accident while searching for information on Sharpless and encountered this guestbook page with the details of his death.

His page at the University of Rochester describing him as a "professor emeritus" has been removed, but I see that he is still listed as of today at the International Astronomical Union and is still described as alive on his English Wikipedia page (but the German version correctly reports his death).

I find it sad when scientists die or retire unacknowledged. I notice that Veta Avedisova has been removed from the staff listing at the Institute of Astronomy, Russian Academy of Sciences. I hope in her case that she has a new job or is perhaps beginning a long and happy retirement!

Conclusions on mapping the Milky Way

Submitted by Kevin Jardine on 7 June, 2013 - 15:26

A few days ago I announced the Velocity Explorer, an interactive tool for exploring the velocity of gas in the galactic plane. As part of that announcement I mentioned that I had used Velocity Explorer to create a model of the Milky Way:

and even a partial map:

Maps derived from velocity data can't be treated very seriously for many reasons that essentially boil down to the fact that there is no straightforward relationship between velocity and distance. A more reliable map will be possible only when many more maser parallax observations become available, especially from the southern hemisphere.

Nevertheless, I think I can derive several interesting conclusions from the exercise. Of course since these conclusions are based on my (unreliable) map, they may be incorrect. However, in most cases they don't depend upon a distance-velocity relationship and so may very well be real.

The spur system is elaborate

The map shows many short bridges or "feathers" between the spiral arms. This is not surprising as such structures are common in spiral galaxies. What is a bit more unusual is the elaborate system of large spurs on the near side of the galaxy. These include the Orion, Vela and Cygnet spurs as well as the Perseus bridge and at least four other structures. This means that as maser parallax data becomes available the process of mapping objects located between the spiral arms will not be simple as we cannot assume that the masers are associated with one or two large spurs. They will in fact be part of an elaborate hydrogen web.

The velocity data shows a double ring

Instead of a simple ring (sometimes called the "near and far 3kpc arms"), surrounding the bar, the LAB velocity data clearly shows a more complex double ring structure. It is not clear at this point how this maps into a physical structure but it seems unlikely that the bar is surrounded by a simple elliptical ring.

There is anomalous velocity in the anticentre too

The Velocity Explorer shows at least two distinct bands of clouds with strongly negative velocities towards the outer galaxy.

There are major kinks in the Perseus and Sagittarius arms

The velocity data as described in the section on the spiral arms suggests that the Perseus and Sagittarius arms are not simple logarithmic spirals but have more elaborate shapes. In particular, a major bend in the Perseus arm between about 220° - 235° means that we are looking down rather than across the arm in this direction and will likely see an overdensity of spiral tracers.

There appears to be a large complex of clouds in the outer first quadrant

There is a continuous velocity spread between the Norma and Centaurus arms from 37° to 47° as can be seen in this Velocity Explorer image. I have labelled this region the Centaurus confluence on the map. It appears to be a region where the Milky Way's spiral structure breaks down and the Norma and Centaurus arms merge into a large flocculent cloud complex.

What is a spiral arm?

Submitted by Kevin Jardine on 7 June, 2013 - 08:08

Many galactic astronomers use the term "arm" in an inconsistent and confusing way. As a good example, this image of the Milky Way on the Chandra website shows eight arms. In a more recent example, this press release promotes the (in my view) ill-advised term "Local Arm" for the structure this website calls the "Orion Spur".

This confusion is becoming a greater problem as we get closer to the great dream of mapping the Milky Way. For example, it is increasingly the case that the question "How many spiral arms does the Milky Way have?" has no clear answer not because we lack data, but because astronomers disagree over what an arm is, or use the term imprecisely.

So today I'm proposing a definition:

A spiral arm is a large scale structure of atomic hydrogen that:

  • lies outside the galactic centre region,
  • is warmer and denser than its surroundings, and
  • wraps around the galactic centre for more than 180°.

In an astronomical context we can use the word "arm" as a short form of "spiral arm" when no confusion will result. We should not use the word "arm" to refer to any other galactic structure than a "spiral arm".

The emphasis on atomic hydrogen makes sense to me because atomic hydrogen is the basic building block of all galaxies and including the reference to atomic hydrogen avoids arguments that a spiral arm is not an arm because it lacks other tracers like molecular gas, red giants or HII regions.

The definition precludes smaller structures that may exist in the galactic bar and the near and far 3kpc "arms" ("ring" or "ring structures" may be better terms for what has been called the 3kpc "arms").

By this definition, the evidence suggests that the Milky Way has four spiral arms.

Also by this definition the Orion Spur does not appear to be an arm. Although it is a large structure, it is found (so far as we currently know) entirely or almost entirely on the near side of the galaxy. The concept of spiral in a spiral arm implies that it wraps around the galactic centre and the Orion Spur does not appear to do this. I also think that it is confusing in an astronomical context to use the term "arm" when we do not mean "spiral arm".

(Whether "Orion Spur" is an appropriate name for this structure is a different question which I'll address in a future blog post.)

Announcing the Velocity Explorer

Submitted by Kevin Jardine on 3 June, 2013 - 17:58

About a year ago, the Harvard astronomer Thomas Dame suggested that I might consider experimenting with the 3D visualisation of velocity data. As mentioned in the Velocity section of this site, in the 1950s there was tremendous excitement about the idea of using a rotational model of the Milky Way combined with velocity data to produce a map of the galaxy.

This effort ran into various problems and by the 1970s had been largely abandoned for a number of reasons, some scientific and some cultural (even astronomy has its fashions).

However, recently mapping the galaxy has come back into fashion, partly because of improved and new sources of data, and partly because increased computing power has made much more sophisticated data analysis possible.

Today I'm announcing the Velocity Explorer, an interactive tool for exploring gas velocity in the galactic plane. The Velocity Explorer images were created using a marching cubes algorithm that is more typically used by medical researchers analysing MRI data to visualise tissue structures in the brain and other parts of the body.

It turns out that isosurfaces of constant gas temperature in the Milky Way are a bit like human tissue structures and can be analysed by similar tools.

There is a detailed introduction to the Velocity Explorer here:

I'm pretty sure that the Velocity Explorer is not really what Dame had in mind (I think he was more interested in rotating and otherwise manipulating individual cloud complexes). However, it does fit closely with the overall goal of this site, which is to present research on mapping the Milky Way.

I've used the Velocity Explorer to derive a model for the Milky Way, described here:

and even a partial map:

The model and map were fun to produce and the process I used to create them is described in detail, but they should not be treated too seriously. The problems astronomers faced in the 1950s when using velocity data to map the galaxy are still around. The only reliable way to map the galaxy is radio parallax.

Still, I think that the Velocity Explorer may be a useful tool for professional astronomers and may even suggest good parallax targets for radio astronomers. And the new model of the galaxy described on this site might spark some interesting debate, which can only be good.


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