How do stars change over time? In parts 3 and 4, we discussed some of the scenarios secular astronomers have proposed for stellar evolution. These included the now-discredited idea that stars evolve along the main sequence from blue to red, along with modern ideas of giants and supergiants being aged stars. Unfortunately, such long-term changes cannot be observed and therefore are beyond the scope of operational science. Nonetheless, some stars change in ways that have been observed in history, and some types of changes are even observable in the present. In particular, stars can change in luminosity, appearing either brighter or fainter over time. These are called variable stars.
Eclipsing Binaries
The change in apparent magnitude[1] of a star can be either intrinsic to the star itself or merely due to our perspective on Earth. In the latter category are eclipsing binary stars. These are two stars that orbit their common center of mass and in which one star passes in front of the other (as seen from our solar system). Since the star in front is blocking some of the light from the background star, we see the combined light drop for a short while. The main dimming occurs when the fainter star passes in front of the bright star. This is called the primary eclipse. Half an orbit later we perceive a less noticeable secondary eclipse when the fainter star passes behind the brighter one.
In order for us to perceive an eclipsing binary, the orbital plane of the two stars must be nearly edge-on relative to our solar system. For this reason, the two stars in an eclipsing binary are usually very close to each other. This allows a greater range of angles of tilt where one star can still eclipse the other as seen from Earth. The greater the distance between the two stars, the closer to exactly edge-on the system must be relative to us in order to see an eclipse.
Since most eclipsing binaries have a very tight orbit, their orbital period tends to be small, usually a matter of days. This also means that most eclipsing binaries cannot be visually distinguished in even our most powerful telescopes; they appear as a single point of light. Nonetheless, we can use spectroscopy to confirm that two stars are present and indeed orbit with a period matching their mutual eclipses.
One of the most well-known eclipsing binaries and the first to be discovered is Algol. Astronomers have known that it is variable since 1667 (and perhaps much earlier), but they did not initially know why. Algol drops in brightness by a noticeable 1.3 magnitudes every 2.86 days for a period of about ten hours.[2] Its name is Arabic, meaning “head of the demon,” and it is nicknamed the “demon star.” Perhaps ancient people called it this because of its habit of “winking” every 2.86 days. Algol is easy to see with the unaided eye in the northern constellation Perseus.
The two eclipsing stars of Algol are class B8V (blue main sequence) and K0IV (red subgiant), respectively. This is perplexing from a secular perspective because the main sequence star is more massive than the red subgiant. But the maximum lifespan of a star on the main sequence is inversely related to its mass. In other words, the blue star should have left the main sequence before its less massive companion. This problem is called the Algol paradox. The standard explanation is that the blue star was once less massive. However, when the other star entered the subgiant phase, it transferred some of its mass to the less massive star via accretion, converting it into a blue main sequence star. This explanation is plausible but is difficult to prove.
Pulsating Variables
Many other stars change their true brightness by pulsating – by changing their physical size. Recall from the previous article that size and temperature determine the luminosity of a star. If a star swells in size, its surface area increases. Furthermore, its surface temperature changes as well (due to the ideal gas law). This results in a change in luminosity. Depending on the size of the radial pulsation, the change in brightness can be dramatic.
There are many varieties of pulsating variable stars. Depending on the mechanism, such pulsations can be regular or irregular. They can be rapid (on the order of a day) or long-term (years). Cepheids are one of the most well-known (and scientifically useful) regular variable stars. They are named after one of the first members to be discovered: Delta Cephei, a fourth-magnitude star in the constellation Cephus. Each Cepheid has its own very regular pulsation period. Depending on the star, the period can be between 1.5 and 50 days. The North Star (Polaris) is a low amplitude Cepheid with a period of about 4 days.
What makes Cepheids particularly scientifically useful is that there is a relationship between their pulsation period and their average luminosity. The brighter the Cepheid, the longer its pulsation period.[3] The mathematical relationship was discovered in 1912 by Henrietta Leavitt. It is easy to measure the pulsation period of a Cepheid by observing the star’s brightness over time. Once we know its period, we can compute its true luminosity. And by measuring its apparent brightness, we can compute its distance. This is what makes Cepheids so useful in astronomy – they are “standard candles” (objects whose intrinsic brightness is known) that can serve as distance markers. And this method of measuring distance works far beyond those distances obtainable by the parallax method. Indeed, Cepheids are very luminous stars and can therefore be seen to great distances. A sufficiently large telescope can detect them even in other galaxies. This was how the distance to the Andromeda galaxy was first measured.
Likewise, RR Lyrae stars are pulsating variable stars, similar to Cepheids, but not as luminous and with a more rapid period which is between 7 and 14 hours.[4] They are named after the first star of their type to be discovered, RR Lyrae, which is also the brightest (as seen in our sky). Like Cepheids, RR Lyrae stars are useful as standard candles because all RR Lyrae stars are approximately the same (average) luminosity. Refinements in the estimated luminosity can be obtained by analyzing their light curves (the measure of brightness over time) in multiple wavelengths, although this is more complex than with Cepheids. These RR Lyrae stars are often found in globular star clusters and are therefore helpful in determining the distance to such clusters.
Why Pulsate?
Today, we have a pretty good understanding of why Cepheids and RR Lyrae stars (and other similar rapidly pulsating stars) pulsate as they do. It is due to the ionization of helium. When helium is non-ionized, it has two electrons which can orbit only at certain distances / energy levels from the atomic nucleus. Thus, neutral helium can only absorb those specific frequencies of light that correspond to an energy difference between two electron levels (see the second article in this series, on spectroscopy). Otherwise, neutral helium is essentially transparent, allowing the light to flow freely from the lower layers of the star to the surface and into space. However, RR Lyrae and Cepheid stars are at the right size and temperature that helium is ionized below the surface. Since the electrons have been stripped away from the nucleus, they are free to absorb light of any energy, converting it into thermal energy which heats the ionized gas. In other words, ionized helium is opaque, which absorbs light, heating the gas.
The heated gas expands (due to the ideal gas law), causing the outer layers of the star to swell up. This increased volume causes the gas to cool, which results in the electrons rebinding to their nuclei, and the helium becomes neutral. This causes it to become transparent, and the light flows freely through the gas. The helium acts like a release valve. Without the extra pressure due to the energy of absorbing light, the outer layers of the star contract under the force of gravity. This contraction reheats the gas (again due to the ideal gas law), causing it to re-ionize. The gas again becomes opaque, which allows it to absorb light, heat, and thereby re-expand. And the process repeats. This method of pulsation is called the kappa mechanism because kappa is the Greek letter used by physicists to indicate opacity.
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 energic 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.[5] 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 are 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.[6] 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.
[1] Recall that apparent magnitude refers to the brightness of a star as it appears in our sky on Earth. The system is backward in the sense that brighter stars have a lower magnitude. The faintest stars visible to the unaided eye have an apparent magnitude of around 6, whereas the brightest stars are around 0 or even slightly negative.
[2] This is the primary eclipse. The secondary eclipse is not noticeable to the unaided eye but can be detected by instrumentation.
[3] We now know that there are two families of Cepheids: classical Cepheids and type II Cepheids. Each family obeys a period-luminosity relation, but the relation is different for the two families, with type II Cepheids being somewhat fainter. Several subcategories also exist.
[4] I have done photometric observations on RR Lyrae, measuring its brightness as a function of time. In a field like astronomy where most things look exactly the same night after night, and century after century, it is amazing to see a star change its brightness on such a rapid timescale.
[5] 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.
[6] 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.