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.

Patrick Moore and the future of UK astronomy

Submitted by Kevin Jardine on 10 December, 2012 - 16:02

Patrick Moore, the long serving host of the BBC's astronomy program The Sky at Night has died. I don't remember hearing much about Moore when I lived in Canada, but I certainly did when I moved to Europe and started relying more on the BBC as a source of news. Moore was known as a tireless promoter of amateur astronomy and the public awareness of astronomy, as well as being a harmless eccentric in his political and economic views.

Moore certainly was a tireless promoter of astronomy, but I'm not sure that his political and economic views were either harmless or eccentric. I'll explain below.

As noted by Sarah Kendrew and Phil Plait, Moore was admired and valued by professional astronomers both because his detailed knowledge of the latest science informed his public outreach, and because he inspired a number of his listeners to become professional astronomers during his long career.

I first heard about Moore during the lead up to the solar eclipse of August 11, 1999. Moore spent the eclipse period in Cornwall, seeing nothing, because (as had been projected by weather forecasters) the sky was clouded over. I was puzzled by a BBC interview he did next to a group of disappointed British amateur astronomers in which he expressed sadness at missing the eclipse. I wondered why Moore had not read the weather forecast and gone to southern Germany (as I had - to Augsburg) or further east, where the skies were clear.

I think part of the reason was that Moore was showing solidarity with British amateur astronomers, many of whom lacked the time or money to travel elsewhere. But I think that Moore's unwillingness to travel outside the UK was also a reflection of something larger - a profound nostalgia for a time long ago (before 1950, possibly before 1900) when the English rarely left their island and carried out world class astronomy even in the outskirts of London.

I think that it is this nostalgia for a time when the sun never set on the British empire, when men were men and women stayed at home, that was the root cause of his hostility to the European Union, his support for the UK Independence Party, and his strongly stated views against immigration and better careers for women.

I don't think that this nostalgia was either harmless or eccentric. It was not eccentric, because it is in fact surprisingly common in the UK. While most supporters of the UK Independence Party may not share Moore's views on gender roles, they do share his Little Englander xenophobia and fear of international cooperation, especially through the institutions of the European Union. This nostalgia is not harmless because its prevalence is doing great harm to the UK's international standing, even within the field of astronomy.

The sun now does set on the British empire, and world class astronomy now largely takes place far from England, in the cold dry deserts of Chile, the peaks of volcanoes in Hawaii or in orbiting space telescopes. Even networks of radio telescopes in Europe like EVN require close international cooperation to run.

Most, if not all good science requires expensive instruments and the cooperation of many talented professionals around the world. Without institutions of international cooperation like the European Union, the UK and other European countries would soon be reduced to second class status, falling far behind the United States and (increasingly) China.

Moore's all-too-common political nostalgia was not only bad for the UK's future in an increasingly interdependent world. It was also bad for the practice of astronomy of which Moore was so passionate an advocate. Astronomy is a subject that has no limits and no borders. I hope that its advocates in future will try to have none either.

Mapping the universe with a split SKA

Submitted by Kevin Jardine on 25 May, 2012 - 17:47

The biggest astronomy news today is not the Dragon docking (though that is a great bit of technology), but the decision to split the giant Square Kilometre Array radio telescope between South Africa and Australia / New Zealand.

The current news stories about this (to its credit, BBC made SKA its lead story) are a bit vague about the implications beyond political buy-in and increasing the cost by about 10%.

The decision was unexpected but is great news for science for at least one major reason: astrometry.

A paper published in 2004 by Ed Fomalont and Mark Reid, Microarcsecond astrometry using the SKA, recommended that the giant radio telescope array be distributed over about 5000 km because such a large radio baseline would allow much more accurate measurements of the positions of the objects it studies.

Splitting SKA between Australia / New Zealand and South Africa produces a baseline even longer than that - more than 10 thousand kilometres.

As explained in this article on parallax, by measuring the tiny shifts in the position of an object in the sky as the Earth orbits the Sun, astronomers can determine its distance.

Distance measurements are essential in mapping the Milky Way and the universe as a whole.

The 2004 article goes on to point out the enormous baselines possible with orbiting radio telescopes (perhaps a 100 thousand kilometres). I suspect that with such huge baselines, astronomers would be able to create 3D maps of many galaxies beyond the Milky Way.

The Rise of WISE

Submitted by Kevin Jardine on 14 March, 2012 - 12:27

Today the first new infrared map of the entire sky in 30 years was released by the WISE team. The previous all-sky infrared map, created by the IRAS satellite in the 1980s, revolutionised astronomy by stripping the veil of obscuring dust away and allowing us to view thousands of objects for the first time.

Since the release of the IRAS data, more detailed infrared observations of a narrow strip surrounding the galactic plane and other selected objects have been carried out by other infrared satellites, including MSX, Spitzer and Herschel. However, much of the sky beyond the galactic plane has been little explored. WISE's sharp view now dramatically improves the previous IRAS results and opens up a huge new opportunity for galactic exploration. This is especially true for local objects within the Gould belt, which can be found anywhere in the sky, but also even in our Milky Way's spiral arms, which are warped and can wander well away from the "official" galactic plane.

Currently the maps are only available in small images appropriate for astronomers studying specific objects. I'll certainly be taking advantage of these to improve the images on this site. In January the WISE team released a breathtaking "supermosaic" of the sky in the direction of Cepheus and Cassiopeia. I hope that the WISE team will be producing more such supermosaics in the future, or perhaps even an interface that allows anyone to explore the entire sky in infrared!

Correction: NASA has released one new supermosaic, an enormous 19080x9598 image of the entire WISE data set. You can find it as the highest resolution version (300MB, below the main image) available here:


Even this enormous image is a tiny fraction of the full WISE data set, but it serves as a useful overview. I'll look into reprojecting it for the Milky Way Explorer on this site.


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