Astronomers sometimes blithely talk about ‘bright’ stars.  If an exoplanet transits a bright star, we will be able to measure the the parameters of the planet, eg mass and radius to greater precision, and perhaps to sniff its atmosphere.   But in everyday terms, how bright are these stars?

Astronomers use the magnitude scale to describe star brightness, with the larger the number the dimmer. A magnitude difference of 1 between stars means there is a factor of about 2.51 times brightness between them.  Polaris is mag 2, and Alpha Centauri is -0.3  (yes, magnitudes can be negative).  Go out to a really dark sky site and the dimmest star visible to the naked eye is mag 6.  I’ve had difficulty making out mag 10 stars with an 8″ Newtonian telescope from the suburbs.

Of the transiting planets discovered to date very few are brighter than mag 10.  This is 4 magnitudes dimmer than the dimmest star visible to the naked eye, or 2.51⁴ = 40 times dimmer.  Another way to look at to look at this is to consider a candle flame.   Place this 70 km away (Perth to Mandurah, or Boston to Providence, ignoring the atmosphere and dust) and you’d see it as being as bright as a ‘bright’ mag 10 star.

The distribution of discovered planets pictured above is somewhat distorted because many of them were made from space – the Kepler telescope.  Observations from space benefit from greater stability and therefore ability to detect planets around fainter stars.  Nevertheless ground-based surveys have gone down to mag 15.  HATS-6b is an example.

This is 2.51⁹ = 4000 times dimmer than the dimmest naked eye star.  Placing our candle 600 km away (Boston to Washington DC) would do the trick.

So the astronomer’s ‘bright’ star is actually not very bright at all.  It’s amazing how much information we get from such minute scraps of light.

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