Cepheid Variables and Cosmology
Cepheid Variable are young and medium sized, as far as stars go. Despite this fact, they’ve blasted through much of their Hydrogen, and have moved on to burning Helium. The heat from these nuclear reactions causes the star’s gas to pulsate radially, causing the associated fluctuations in brightness.
The star’s core is so hot that its’ surrounding helium gas become doubly ionized - both electrons are stripped from the helium atoms. This plasma of doubly ionized helium is very, very opaque to light. It’s very hard for the photons to escape the envelope of the star into space.
Eventually, this causes the buildup of pressure, and the star balloons in size. This expansion cools things down a bit, allowing the electrons to recombine with the helium nuclei. That, in turn, makes the star’s gas more transparent, which lets all those built up photons escape.
As the star’s gas cools it shrinks again. The helium atoms get ionized by the heat of the star’s innards and the process starts all over again. In other words, it’s a gigantic heat engine.
But from our perspective, the star’s brightness merely waxes and wanes.
(plot created by ThomasK Vbg cc3-a-sa)
The star’s pulsing frequency and brightnesss - the period and amplitude of its oscillations - are strongly linked. This fact has been worked out in theoretical models as well as direct observations with known stars. Essentially, this means that astronomers are able to tell how far away a Cepheid Variable star is by seeing how fast it pulsates. By looking for these variable stars in other objects: star clusters, dwarf galaxies, etc, a measure of the distance to those objects can be observed.
Knowing the distance to objects at all sorts of distances is key to understanding our sense of place in the universe. You see, the universe itself is expanding! Expanding like a big explosion - set off during the Big Bang. To understand that expansion, we need to know not only WHAT else is out there, but HOW FAST it’s moving away from us.
We have many ways to measure the rate of expansion, Cepheid variables are just one tool. There are many others. The biggest success in modern cosmology - the study of this expansion - is that all these tools basically agree with each other. To a point.
The biggest problem in modern cosmology is that there is a slight discrepancy between the expansion at the earliest moments of the universe and today.
Plugging this discrepancy into the equations that govern the dynamics of the universe, we find it consistent with - but not necessarily evidence for - a difference between what dark energy looked like in the Early universe, and what it looks like today. Theoretical physicists have a lot of fun models that could explain such variations in dark energy, so any definitive observation would certainly be big news. As things stand, more data in needed to be sure the difference is real.
