Every star in the night sky has a different brightness. This is simple to state, but there are a variety of overlapping factors that contribute to the appearance of the stars, and which one seems to be the brightest to us changes over long timescales. All stars have an intrinsic brightness, which astronomers call luminosity. The variability in luminosity can be absolutely staggering. Some stars are millions of times more luminous than the Sun; others are tens of thousands of times less luminous than the Sun. All stars are not created equally bright, yet even the most luminous stars can be muted into obscurity by the immense distances that separate them from us.

As starlight spreads out into the galaxy, its intensity is spread ever thinner. Imagine a lightbulb in an open field. Form a distance of one metre, its light is spread over the area of a sphere with a one metre radius (12.6 m²) and it appears bright. The area of a sphere is proportional to the square of its radius, and so this intensity falls off at an exponential rate. From 100 metres away, the luminosity of the bulb is spread over an area of 125,664 m². At this distance, the bulb appears 100² or 10,000 times fainter than it would from one metre away. So it is with stars too, and the distances are very substantial in the open field of interstellar space. Many of the most luminous stars we can see in the night sky are very far from us, and appear almost unnoticeable by eye. 

There is one final factor which affects the apparent brightness of stars in the night sky: variability. All stars change brightness periodically as the reaction rates that power their nuclear cores go up and down. In some cases, stars exhibit striking variability. By accident, the brightest star in our night sky is not highly variable, but from many vantage points in the Milky Way, a nearby variable star will regularly give up the top spot and reclaim it again.

So, we should expect our brightest night time star to be both close to us, relatively speaking, and fairly luminous. This is exactly what we find with Sirius (α Canis Majoris) which holds the title. Sirius is one of the Sun's neighbours at a distance of 8.6 light-years from us. It's also a powerful star shining 25.4 times brighter than the Sun. In combination, these factors grant Sirius a visual apparent magnitude of -1.46. It is by no means bright enough to challenge the brilliance of Venus, Mars, Mercury or Jupiter at their best, but it is twice as bright as the runner-up, Canopus (α Carinae) and both of these outshine Saturn.

The intense brightness of Sirius, coupled with its location in the constellation Canis Major leads to a curious, widely observed phenomenon. In the Northern Hemisphere, Sirius is best seen throughout the winter months, but from populous mid-northern latitudes, it remains quite low in the southern sky. On nights with turbulent air, the star appears to dance wildly and flicker with an iridescent quality. It has earned the nickname "Rainbow Star" due to the impressive range of colours it displays. This collage made from a sequence of deliberately defocused images of Sirius illustrates the diversity of colour seen over a period of just a few seconds.

We know from observations that Sirius itself doesn't change colour. The star's surface temperature of 9,940 K (9,666 °C or 17,432 °F) indicates that its prevailing colour is blueish-white, so the rainbow effect is entirely due to the Earth's atmosphere. Since the star is seen close to the horizon, its light passes through a relatively thick "slice" of the atmosphere to reach the ground. The air acts like a prism, dispersing different colours of light by way of diffraction, and as it moves and flows above us, this fluid prism continuously alters the star's appearance. The twinkling and splaying of light indicates that the air is turbulent and less suitable for astronomers looking to make precise observations, but there is no denying that the effect is visually beautiful.

Sirius is a magnificent that has played numerous cultural roles over the long history of stargazing, and it will remain generally the brightest in our night sky for at least another 200,000 years. As Sirius drifts away from us, Vega (α Lyrae) is poised to become the brightest star for our descendants in the distant future. The occasional supernova will momentarily take the top spot, but for now the steady Sirius enjoys pole position, and the world will look very different before that changes.

Image credits: Tom Kerss