In the previous discussion about lumens (https://physbang.com/2025/12/30/what-are-lumens/) some numbers were quoted without any explanation. This post adds a bit more detail to those numbers. Firstly, the maximum theoretical output of a light source was stated to be 683 lumens-per-watt. Why that number? The answer is to be found in the definition of the candela, which … Continue reading What are lumens? (part 2)
What are lumens?
Modern light sources are rated in lumens or, more importantly, lumens per watt. The first figure expresses the brightness of the source whereas the second indicates its efficiency. But what, exactly, is a lumen? To answer that question it is useful to recall that the first man-made light sources relied on heat and were very … Continue reading What are lumens?
Mammoth effort to image our galaxy in infra-red
Astronomers working on the European Southern Observatory's (ESO's) Visible and Infra-Red Telescope for Astronomy (VISTA) have today published the biggest ever dataset of infra-red images of the Milky Way. A few of the images captured by the ESO VISTA project. Full details about this composite can be found at https://www.eso.org/public/images/eso2413a/ Importantly, capturing images using infra-red … Continue reading Mammoth effort to image our galaxy in infra-red
The coldest exoplanet ever seen
Most exoplanets (planets that are outside the Solar System) are detected indirectly. Either a temporary and regular dimming is measured as they pass in front of the host star or their orbit causes a wobble that produces a periodic red-shift in the star's spectrum. Only a small number of planets have then gone on to … Continue reading The coldest exoplanet ever seen
Chandra: the Universe as seen in X-rays
Chandra is the unsung hero of space telescopes. Far better known is the Hubble Space Telescope, which is rightly famous for the spectacular images it has produced. But Hubble was only ever intended to be one arm of a four-pronged approach to astronomical observations, another of which is provided by Chandra. Whereas Hubble produces images … Continue reading Chandra: the Universe as seen in X-rays
Shapes of Shadows
First a correction: this post is really about the shapes of the bright patches between shadows but "shapes of shadows" sounds much better so please excuse my decision to go for artistic alliteration! If you have ever walked under some leafy trees on a bright summer's day, with the sun high in the sky, you … Continue reading Shapes of Shadows
Mr Tarrant's physbang 'blog 