4.1 The Milky Way and the Galactic plane
What does our own Galaxy look like? Figure 14 shows one perspective. If you are lucky enough to find yourself on a clear night in a very dark location, you will be able to see a smooth band of light stretching across the sky, with dark clumpy regions caused by clouds of gas and dust that block starlight from reaching us.
The following activity allows you to view our Galaxy from some other perspectives.
Activity 3 Exploring the multiwavelength Milky Way
Astronomers at Cardiff University have produced a web tool called Chromoscope, used for exploring maps of our Galaxy taken in many different parts of the spectrum. You will use this tool to complete this activity.
First, open the Chromoscope website [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] in a separate browser tab or window, so that you can return to the instructions here. The default view is a visible light image of our Milky Way, which you can pan round and zoom in on. The full image spans an area of 186 × 20 degrees: a section of sky hundreds to thousands of times larger than the images of individual stars, galaxies and diffuse objects seen in this course. It was made by astrophotographer Nick Risinger, who carefully sewed together over 37 000 individual exposures.
A menu allows you to view this same field in images captured using light from different parts of the electromagnetic spectrum. Alternatively, there are keyboard shortcuts, which you can find more about by pressing ‘h’ for ‘Help’ (or from the ‘Help’ link in the bottom-left corner of the website).
Task 1
Explore the visible light image of the Milky Way and comment on any differences between this map compared to the image in Figure 14. Why do you think it has a different shape?
Discussion
The Milky Way appears as a straight line across the centre of the image, rather than as a curved line across the sky. The images here are projected into Galactic coordinates, which means that all positions along the Milky Way’s disc have a latitude of zero and so fall on a line. Whereas the curve in Figure 14 comes from a different mapping to a 2D image of the curved surface of the ‘celestial sphere’.
Task 2
Now switch between the visible and X-ray images. Note what happens to the dark regions near to the Galactic equator when you switch between the two views. Suggest an explanation for any similarities or differences between the two images.
Discussion
The dark regions remain dark in the X-ray image. This tells us that gas and dust clouds that absorb and block starlight in the visible image are blocking the X-ray light as well as the optical. Absorption of X-ray light can be a useful tool for studying gas and dust.
You may have noticed some large dark stripes in the X-ray images: these are areas of sky where there is no usable data to make the image.
Task 3
Now look at the near-infrared, far-infrared and images. Note how the infrared images differ from the visible (and X-ray) images – why do you think this is? What features can you identify in the image that are not present in the others?
Discussion
The Galactic equator glows very strongly in infrared light. The dark features seen in visible and X-ray light have disappeared. This is because the gas and dust clouds that produce them are warm and glow in infrared light, rather than absorbing it.
There are a number of bright swirly regions in the image not visible at other wavelengths (they are slightly visible in the optical image – this is because the filter is contained within the R wavelength band and the R band is one of several visible light filters). These are star-forming regions, full of young stars that are ionising the surrounding gas.