Image of Omicron Ceti (Mira)
Source: Digitized Sky Survey 2
Looking up at the night sky it may seem that while the stars do move around, they remain the same brightness all year round. This was a common belief pushed by philosophers like Aristotle, believing that stars are eternal and invariable. However in 1638 Johannes Howards observed the star Omicron Ceti, now known as Mira, pulsating regularly over the course of 11 months. This discovery alongside Tycho’s Supernova in Cassiopeia marked the first step towards a revolution in astronomy, breaking the notion of the immutability of the heavens.
While the history of these stars is quite vast, but today’s blog post will be focusing on how exactly stars pulsate. There are many different mechanisms that can cause a star to dim and brighten. Some are extrinsic, such as a secondary object eclipsing the primary one (Similar to planetary transits), or through rotating, putting a side with more sunspots into view. The more interesting however are the stars the pulsate due to their intrinsic properties. There’s quite of few of them however, see: RR Lyrae, T Tauri, etc, so I’ll focus on Mira types.
The attributes that define a Mira Variable are their more distinctive red color alongside very long periods of pulsation (~100 day+). The change they undergo is quite drastic going from 5 to 30,000 times their current brightness over the given period. These objects are very old red giants, which by nature undergo both hydrogen and helium fusion. For these stars are constantly expanding and contraction due to these fusion reactions not being in complete balance with gravitational interactions, which causes increases and decreases in the stars overall temperature and magnitude. One tidbit as well is that our sun is currently on track to becoming this type of star once it goes into the red giant phase. Some of these variables also undergo shifts in their period due to the re-ignition of inert hydrogen within the star causing faster oscillations. Below is a graph showing the fluctuating magnitude of the star χ Cygni:
Source: Wikipedia
There’s something I would like to preface however, and it’s that variable stars do not necessarily have to change periodically, they are simply stars that change in brightness. A non-periodic example is a cataclysmic variable star, which are the byproduct of a white dwarf and main sequence star binary system. The white dwarf’s greater density allows it to distort the other star until it begins to consume the others matter. Eventually the dwarf gets massive enough to star burning hydrogen, rapidly getting brighter, and becoming visible in the night sky. Eventually the white dwarf consumes enough mass that it turns into a type 1a Supernova, which is a particularly helpful tool when measuring distances in astronomy. In fact, Cepheids another type variable star are useful in measuring distances.
Depiction of a Cataclysmic Variable
Source: Nasa
Ultimately, Variable stars are a particularly interesting subset of stars that pulsate for a variety of reasons. For us they are helpful tools in understanding both stellar composition and measuring out to the farthest outreaches of the universe.
• Noah Herrero
