The Physics of Visible Star Light

Globular cluster NGC 6717. The colour of light from the stars is a general indicator of surface temperature.

The colour of a star tells us how hot it burns. From the dull red of Arcturus to the brilliant blue of Rigel, you can actually see these subtle colour differences with your own eyes when looking up at the night sky.

Just like an iron cast into the blacksmith’s forge, which slowly changes from red to white hot, stars emit light at different frequencies depending on their overall luminosity and energy output.

The same relationship between colour and temperature is noted when metal objects are heated.

The Planck-Einstein equation E = hf is a basic way of understanding this. E is energy, f is frequency and h is the famous Planck’s constant. Higher frequency light (blue) is more energetic than lower frequency light (red) and therefore hotter and more luminous stars tend to appear more blue. Meanwhile cooler stars whose external atmospheric envelopes has expanded (red giants like Betelgeuse) appear redder.

A simple way to highlight the colour of star light is to take your smartphone camera or DSLR and manually defocus it on a target star. This will emphasise the colour and you can even produce beautiful star trails like the one below by taking a movie or long exposure star trail.

Star trail image credit – Amanda Cross

Betelgeuse’s Great Dimming

The mystery surrounding the dramatic dimming of red supergiant Betelgeuse, observed over a period of several weeks back in winter 2019, has finally been resolved. In astronomy circles this event is now known as Betelgeuse’s Great Dimming.

A paper published in Nature by a team based at the VLT (Very Large Telescope) has concluded that one part of Betelgeuse underwent a temporary convective cooling process on part of its photosphere, allowing a previously ejected cloud of stellar plasma to condense into a kind of opaque nebulosity, obscuring some of the light from the star over a period of time.

Enormous stars like Betelgeuse have highly turbulent and dynamic surfaces and this event has helped us better understand the periodic mass loss that red supergiants experience in their last stages of evolution, as they literally ‘puff away’ vast swathes of their extended atmosphere into space.

It’s fair to say we’re still at the very earliest stages of truly understanding the complex processes controlling these dying stars.

I feel very privileged and humbled to have witnessed this exciting event (with others) from the surface of a tiny world over 550 light years away, and by extension 550 years after it actually happened.

If you want a deeper dive I’ve included the nature paper link here.

How Astronomers use the EM Spectrum

This is Bode’s galaxy (M81), an easily accessible island universe in Ursa Major that’s visible all year round from mid to high northern latitudes. It contains approximately 250 billion stars.

It lies over 10 million light years away and has a relatively close galactic companion – the M82 Cigar galaxy. Both of these galaxies can be framed in a low power telescopic eyepiece, and you can even see them very faintly in binoculars if your skies are suitably dark.

I thought I’d use this galaxy to highlight how astronomers use the full electromagnetic spectrum of light to study galaxies and their evolution. Pictured below, therefore, are images of M81 viewed in different wavelengths of light – from X-rays to radio waves (spanning short to long wavelengths). I’ve provided a very brief description of some of the galactic features revealed by each band of light.

X-rays: a central bright patch is revealed, suggestive of a supermassive black hole within the galactic nucleus. The other bright patches correspond to X-ray binary systems.

Ultraviolet: highlights young hot stars and therefore areas of active star formation within the spiral arms of the galaxy.

Optical and Infrared: Shows the bulk of the stellar population and areas of obscuring dust and nebulosity that will seed ongoing star formation.

Radio: Reveals supernova remnants and large H2 regions of ionised gas in the vicinity of very active stellar populations.

As humans our eyes have evolved to see a very narrow band of the full EM spectrum. This evolution is tied to the fact our particular star (the Sun) releases its peak energy in these wavelengths. I always like to imagine how other species, perhaps evolving next to giant sources of x-rays, might have sensory apparatus totally blind to visible light.

New Information on Betelgeuse

Betelgeuse, shown here as the top left star in the shoulder of Orion

Some interesting new information has emerged on Betelgeuse, the red supergiant that marks the left ‘armpit’ of Orion the Hunter.

In summary:

1. It’s still burning Helium in its core so unlikely to go supernova until around 100,000 years.

2. It’s not as massive as previously thought. Earlier studies had shown its radius would extend to the orbit ofJupiter if placed in our solar system. This new data suggests its real radius is 60% of this.

3. It’s closer to Earth than previously measured, at 530 light years. This is 25% closer than we previously thought.

New data published in the Astrophysical Journal. Further reading here.

Watch a Supernova Explosion in a Distant Galaxy


See a supernova explosion in a distant galaxy over 50 million light years away.

Berto Monard witnessed Supernova 2015F in spiral galaxy NGC 2442 in March 2015, although the actual event happened 50 million years ago, long before human beings inhabited planet Earth.

Supernovae like this produce so much light energy they can briefly out shine the accumulated light from the entire galaxy. For this reason they can be witnessed even with moderately sized back garden telescopes, if you’re lucky enough to be pointing in the right direction at the right time!

Video Credit & Copyright: Changsu Choi & Myungshin Im (Seoul National University)
Source: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)

Milky Way Images


The Milky Way over the Callanish Stones on the Isle of Lewis.  Jupiter and Saturn can be seen in this shot low above the horizon. By Emma Rennie of Callanish Digital Design.


Another stunning Milky Way shot by Christopher Cogan taken from Muie in Sutherland in the far north of Scotland.

Two stunning Milky Way images taken last night from the Scottish Highlands (and Islands). Both show the bright region of the Milky Way in the vicinity of the Summer Triangle, looking south.

If you imagine our Milky Way as a vast disk of stars, these views are peering further ‘into’ the disk, where the density of stars and stellar matter is greater, and hence brighter. Contrast this with the fainter regions we see in Winter near Orion, when we peer ‘out’ of the galactic disk.

The dark lanes you can see are part of the Cygnus Rift – a region containing vast clouds of dust that obscure some of the light from the billions of stars in the background.

With the Moon well out of the way and proper darkness returning late at night, now is a great time to go out and see the Milky Way for yourself.