Summer is upon us in the northern hemisphere – and clear summer nights can be a great time to view the superb sight of our home galaxy, the Milky Way, arching right across the sky, if you can get away from light-polluted areas. With the naked eye you can see a well-defined cloudy arc with some brighter and some dark patches as seen in this beautiful image. And if you have the chance to look at it through even a small telescope, you can see that it is made up of thousands upon thousands of stars from very bright to very dim.
How do we know it's a spiral?
Astronomers tell us that the Milky Way is a disc-like galaxy with a central bulge and several spiral arms, similar to spiral galaxies such as this one. Yet what we can see in the sky is simply a broad band of stars, nothing like a spiral. How can this be?
Of course, the first issue is that our star and solar system are located within the galaxy, so we are looking at it from within – naturally we are unable to view its overall shape. The fact that we see it as a band is evidence that it is a disc and we are within the disc – looking at the band we are viewing through the plane of the galaxy so we see many stars, but if we look in other directions we are looking outside our galaxy so we see far fewer stars. The brightest part of the band, located in the direction of the constellation Sagitarrius, contains the highest density of stars, so this is the location of the galaxy’s central bulge.
Another issue is that much of the visible light from the galaxy’s stars is blocked by thick lanes of dust – the black regions that you can see in the band of light above. Astronomers have got around this problem by using radio telescopes to detect the radio emission from hydrogen, as unlike visible light, radio emission is not blocked by dust. Mapping of the density distribution of hydrogen using this technique revealed a spiral structure in the disc, which has been confirmed using infra-red observations (another wavelength range not blocked by dust) by the Spitzer Space Telescope.
Nevertheless, picking out the detail of the spiral arms via an edge-on viewpoint through the disc of the Galaxy has proved difficult, and there has been a lot of discussion about the exact arrangement of the arms – for example, are there two or four? And is our solar system located in one of the major arms or a minor spur? This artists concept shows the layout based on research as of 2017, with two major arms and several minor arms and spurs.
Another problem in finding out more about the layout of our galaxy is the difficulty in observing the other side, beyond the bright central bulge. However, radio observations are having some success – a network of radio antennae across the USA known as the VLBA (Very Long Baseline Array) has managed to observe some star forming regions calculated to be 66,000 light years away, putting them firmly on the far side of the Galactic Centre.
Despite the progress made to date on figuring out the actual form of our home galaxy, there are still many questions and uncertainties, but all will hopefully become a lot clearer in the next few years as data from the impressive Gaia mission gets analysed. Gaia is a space observatory that since 2014 has been mapping millions of stars in the Milky Way with amazing precision. It is expected to map around 1% of all the stars in the galaxy over its 5-year mission, both their location and their movement – enabling a huge leap forward in our understanding of the layout of the Milky Way once all the data is analysed. However, there will still be plenty of unanswered questions as Gaia operates at visible wavelengths only, meaning that it still can’t see stars hidden behind that pesky dust.
Find out more:
https://phys.org/news/2015-05-evidence-milky-spiral-arms.html
https://www.sciencenews.org/blog/science-ticker/solar-system-sits-within-major-spiral-arm-milky-way
https://www.sciencemag.org/news/2017/10/astronomers-map-milky-way-s-most-distant-outpost
https://public.nrao.edu/telescopes/vlba/
http://sci.esa.int/gaia/
https://www.youtube.com/watch?v=yq0zyA6Yr7o Video explaining how we know the Milky Way is a spiral
Image Credits
1. https://www.eso.org/public/images/airglow_ats_beletsky-CC/ Y. Beletsky/ESO. The Milky Way stretched out behind one of the Auxiliary Telescopes of ESO's Very Large Telescope (VLT) at the Paranal Observatory in Chile. 4th December 2018 2. https://solarsystem.nasa.gov/resources/285/the-milky-way-galaxy/ Source: NASA/JPL-Caltech/R. Hurt (SSC/Caltech) Published: November 8, 2017
3. https://public.nrao.edu/gallery/vlba-global-locations/ NRAO/AUI/NSF