The Sky Guide challenge
Can you observe and estimate the variability of Delta Cephei?
It’s easy to spot Delta Cephei with the naked eye
Delta (d) Cephei is a famous variable star in the constellation of Cepheus, the King. At this time of year it’s well placed, providing a good opportunity to see it ‘doing its thing’ which is, of course, varying in brightness. This month’s challenge is to record this variability to reveal the true nature of this incredibly important star.
At this point, you may be wondering why we’re elevating the status of Delta Cephei so much, after all, there are plenty of other stars in the night sky that exhibit variability. While that’s perfectly true, Delta Cephei holds a special place because it’s the prototype of an important class of variable stars called Cepheid variables.
Interestingly, it wasn’t the first of its kind identified. That honour goes to Eta (e) Aquilae, which was discovered on 10 September 1784. Delta Cephei was first recorded by John Goodricke on 19 October 1784, with regular follow up observations made through to the end of that year and into the first part of 1785. His write up of the variability of Delta Cephei was formally published on 1 January 1786.
The star’s variability is intrinsic, meaning it comes from within. It pulsates radially so that both its diameter and luminosity vary with time. The period of variability is stable in both frequency and amplitude. In 1912, Henrietta Swan Leavitt identified that the pulsation period of a Cepheid variable was directly linked to the star’s luminosity.
This discovery was profound and the reason why classic Cepheids have such an elevated status. Basically, once you have identified a Cepheid, by determining its variability period you can work out what luminosity it should have. This is determined by Swan Leavitt’s discovery. Knowing how bright a star should appear can then be used to determine how distant that star must be in order to look as bright as it does to us. For this reason, Cepheid variables are sometimes referred to as ‘standard candles’.
Your challenge is simply to estimate the brightness of Delta Cephei using our comparison chart (left) and plot this variability on a graph with magnitude along the vertical axis and time along the horizontal axis. Ideally, you’ll need to do this over a period of a month or more to get a decent light curve. This will show you the variability repeating itself. Weather will play a part here of course, but as Delta Cephei is easy to spot with the naked eye, it’s possible to make an observation fairly quickly if any clear gaps appear. If you manage to determine the period, you’ve just repeated a fundamental observation which changed the way we figure out how far external galaxies are from us.