Dr. Jason Lisle
[Originally published as the second section of Stellar Astronomy: Part 5 – Variable Stars]
Long-Period Variable Stars
Mira variable stars are red giants that pulsate with a period longer than eighty days and up to nearly three years in some cases. They are named after their prototype, Mira, in the constellation Cetus. Mira sometimes appears as a fairly bright magnitude 2 star, and other times it drops well below naked eye visibility to magnitude 10, depending on what stage it is in its 332-day cycle. The variable nature of Mira has been known since 1596, and possibly much earlier.
Some stars vary their brightness in a way that is not fully predictable. These are called semiregular variable stars. They tend to have a long period, more than twenty days, and often hundreds if not thousands of days. But the way in which they vary their brightness is not consistent from one cycle to the next and therefore not completely predictable.
Betelgeuse, the bright red star in the constellation Orion, is a semiregular variable star. Betelgeuse dimmed rather dramatically in 2019–2020, changing the appearance of its constellation. The reason for such variability is multifaceted and not fully known. In addition to pulsations, large starspots (cooler, dimmer regions on the star’s surface) rotating in and out of view may contribute to variability. Also, material orbiting the disk of the star may obscure some light for a time.
Cataclysmic Variables
Perhaps the most noteworthy variables are cataclysmic variable stars which experience a rapid, dramatic increase in brightness before dropping back to an otherwise consistently dim state. Several varieties of cataclysmic variables exist. Novae (the plural of nova) have been known since ancient times. Such stars rapidly brighten and then slowly fade over a period of time.
Many of the stars that undergo a nova are initially below naked eye visibility but become visible only at the time of their nova. They therefore appear as “new” stars in our sky, albeit temporary ones. This is where the name originates: “nova” means “new.”
Most novae are unpredictable.
A very few recur quasi-regularly. One example of the latter is the star T Coronae Borealis (T CrB). This star undergoes a nova once every eighty years (give or take a year or two). And it is expected to do so this year, sometime between now and September! If this happens, it will temporarily change the look of the constellation Corona Borealis for several days. Only about ten recurrent novae are known in our galaxy.
A “new” star may appear in the red circle sometime this year. The star is already there of course, but is normally too faint to be seen by the unaided eye. The star is expected to become a nova sometime between now and September, attaining naked eye visibility for a week or so.
The cause of many novae is thought to be fusion initiated by mass transfer from a star to a white dwarf. A white dwarf is an object with the mass of the sun but compressed into a spheroid about the size of the earth. Some white dwarfs orbit close enough to a “normal” star that they can pull gas away from the outer layers of the star.
Such gas piles onto the surface of the white dwarf; this produces a great deal of heat. When the temperature reaches a sufficiently high level, the hydrogen gas undergoes nuclear fusion, resulting in a powerful explosion. This is thought to be why T CrB radically brightens every eighty years; that’s how long it takes the white dwarf to accumulate sufficient gas and temperature to undergo surface fusion.
Supernovae
The brightest type of cataclysmic variable is a supernova. These events involve an explosion resulting in the complete destruction of a star. Supernovae are so energetic that they are briefly as bright as an entire galaxy. For this reason, even a small telescope can reveal supernovae in nearby galaxies. The explosion takes a few weeks to reach maximum brightness and then fades over the course of several months.
A supernova in our own galaxy occurs roughly once in a century on average. The last two occurred in 1572 and 1604. So, we are “overdue” for another one, but they do not occur with any regularity.1 Supernovae in our own galaxy are often bright enough to be seen in broad daylight for several weeks. Although the probability of seeing a supernova in any given galaxy in a given year is rather low, there are many relatively nearby galaxies. Thus, anyone with a backyard telescope is likely to be able to see a supernova within a timespan of a few years. I have seen several.
There are two types of supernovae designated by a Roman numeral: type I and type II. These are distinguished observationally by spectroscopic analysis. Type I supernovae lack the spectral feature of hydrogen, whereas type II supernovae possess the hydrogen signature. Type I supernovae are further divided into three subclasses: types Ia, Ib, and Ic, based on the spectral signature of silicon and the way in which they fade over time.
Astronomers believe that types II, Ib, and Ic supernovae are each caused by core collapse in a massive star. The idea is that runaway fusion takes place in the stellar core, resulting in the production of heavy elements up to iron. These reactions produce so much energy that the outer portions of the star are blown into space. The inner portions continue to fuse elements heavier than iron, but such reactions absorb energy rather than releasing it. And so, the core collapses in on itself.
These types of supernovae occur primarily in the disk of spiral galaxies, though rarely in elliptical galaxies. Our current understanding of physics suggests that only stars much more massive than the sun can experience this type of event.
Type Ia supernovae are different. They are thought to involve a white dwarf that is closely orbiting another star and is gravitationally accumulating some of its gas. But a white dwarf can only be as massive as roughly 1.44 solar masses, called the “Chandrasekhar” limit.2 Beyond this limit, gravity is so strong that mutual electron repulsion is insufficient to prevent the white dwarf from collapsing in on itself. This collapse initiates fusion of carbon which releases such enormous amounts of energy that it results in the white dwarf blowing itself apart.
These type Ia supernovae are scientifically useful because they are standard candles – they all have about the same luminosity. This is due to the fact that all come from a white dwarf that has just exceeded its mass limit of 1.44 solar masses. Thus, whenever we detect a type Ia supernova, we can compare its apparent brightness with its known luminosity and compute the distance. And since supernovae are briefly as bright as the galaxy in which they reside, they can be detected at distances as far as the farthest known galaxies.
Conclusions
Unlike speculative ideas about stellar evolution over long time periods, the kinds of stellar changes described in this article can be directly observed. These are part of observational science and are therefore testable and repeatable in the present. Variable stars serve as important tests of our ideas about physics. And many of them serve as standard candles – allowing us to compute the distance to an object of known brightness.
Variable stars also remind us of the uniqueness of our solar system.
Many stars change their luminosity by an enormous factor over a few years, or months, or even hours. But the sun doesn’t. It is remarkably stable. And this is a design feature. If the sun experienced radical pulsations that strongly affected its luminosity, this would be fatal for life on Earth. But the Lord made the sun unusually stable so that it could provide heat and light for the planet that God formed to be inhabited (Genesis 1:14-19; Isaiah 45:18). More to come.
References
- Supernovae are unpredictable. The once-per-century statistic is based on an average. Thus, there is an approximately 1% chance that a supernova will happen in our galaxy each year. The fact that we have not had one in 400 years does not make it any more likely to happen this century than any other century.
- There is some slight variation on this number due to other factors. For example, if the white dwarf is rapidly rotating, it can slightly exceed this limit before collapsing.
