Author Archives: Allen Versfeld

About Allen Versfeld

Allen is an amateur astronomer, an IT professional, a podcaster, a father of five growing kids, a barely competent chess player, and the owner and publisher of the Urban Astronomer podcast and website. He has formerly served as Director of the Astrophotography and Citizen Science sections of the Astronomical Society of South Africa. You can find his personal social media feed on Mastodon!

Green Bean Galaxies

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Astronomers working with European Southern Observatory (ESO) telescopes have discovered an incredibly rare class of galaxy, known as Green Bean galaxies, for their distinctive and unusual colour. While Astronomer Mischa Schirmer of the Gemini Observatory was searching for distant galaxy clusters, he came across an object in an image from the Canada-France-Hawaii Telescope that made no sense: it looked like a galaxy, but it was bright green.  After reporting his find, he was granted special observing time at short notice … Continue reading →

Galaxies could get the Pluto treatment

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The great galaxy in Andromeda It seems strange that a field as dynamic and old as Astronomy should be burdened with so many sloppy definitions.  It’s a consequence of the rapid advancement of the field, where each new instrument yanks aside yet another curtain to reveal previously unimagined objects that challenge established models.  Common terms, like ‘Galaxy’ and ‘Planet’ get coined as casual labels to describe vaguely understood lights in the sky, and then as our understanding deepens we find … Continue reading →

How big is the Milky Way Galaxy?

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Our galaxy contains over 200 billion stars, most of which are arranged in a disc containing a core and spiral arms.  This disc is about 100 thousand light years across, but is only about 1000 light years thick.  A small percentage of the stars are distributed around the Halo, which is a spherical cloud marking the boundaries of the galaxy.  The halo completely envelops the disc, and is also home to many globular clusters.     Comments?  Questions?  Why not mail … Continue reading →

What is a galaxy?

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As it stands, there is no formal definition for what makes a galaxy because, up till recently, astronomers have simply known them when they see them.  This will probably be changing soon, as some astronomers have proposed a formal definition which will then be voted upon at a congress of the International Astronomical Union.  Until then, we’ll stick with the traditional “Island Universe” definition: A galaxy is a vast structure composed of hundreds of billions of stars and enormous clouds … Continue reading →

Milky Way surrounded by hot gas

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Recent observations by NASA’s Chandra have found much of the Milky Way’s missing mass in the form of a vast halo surrounding the galaxy. It has been known for many decades that the amount of material which we can see in a galaxy is not enough to account for the speeds at which various stars and clouds orbit the centre. Objects towards the edge should move slowly and objects near the core should move rapidly, which is what we observe … Continue reading →