Variable Stars and the Stories They Tell

The AAVSO podcast is featured on 365 Days of Astronomy again this month.

Today we look at the many reasons variable stars are so interesting to astronomers and the impact variable star research has had on astronomy.

Stellar Evolution
The study of variable stars permeates all branches of astronomy, but first and foremost, the study of variable stars is all about the secret life of stars. How they are born, the way they live and interact with the space around them, how they evolve throughout their lifetime, and ultimately, how they die.

So, what are stars and how are they formed?
A star is a massive object, typically comprised of mostly Hydrogen gas. In the beginning, a star condenses from a cloud of interstellar gases. This cloud gets perturbed enough by some external force that slightly denser pockets of material begin to form within it. Gravity causes these pockets to contract and as their rotation speeds up, causing in-falling material to form a disk we call an accretion disk. Eventually, there is a massive protostar at the center of this rotating cloud surrounded by a lumpy disk that will either be blown away by the star’s wind or become planets.

This protostar continues to grow, and after perhaps 100,000 years or so it is massive enough for nuclear fusion to ignite in its core. From this point on, the life story of our star is the perpetual balancing act between the force of gravity’s contraction and nuclear burning’s expansion trying to blow the star apart.

T Tauri stars are pre-main sequence stars that have recently emerged from this opaque envelope of stellar formation. Our own Sun presumably passed through this T Tauri stage some 4 1/2 billion years ago. Therefore, these stars offer us a peak into the early history of our own Sun and solar system, as well as other planetary systems.

Fortunately for us, most stars live out the majority of their lives peacefully converting hydrogen to helium for billions of years without any major fluctuations or changes. But as stars age and have converted more and more of their original mass from hydrogen into helium, changes begin to take place internally that change the size, temperature and behavior of the star.

As they approach middle age, many stars begin to swell as a reaction to their changing internal structure. A star like our Sun will increase its diameter by a hundred times or more, becoming what we call a red giant. Many of these red giant stars are variable stars, changing in brightness more or less regularly as their atmospheres expand and then contract on timescales of months or years. We’ve learned a lot about star’s internal workings, their atmospheres and stellar winds by studying pulsating red giants.

The stellar aging process also creates some of the most intriguing and beautiful celestial objects. Planetary nebulae are formed when a red giant star ejects its outer layers as clouds of luminescent gas. The Ring Nebula in Lyra and the Cat’s Eye Nebula in Draco are examples of the death throes of swollen, giant stars.

Even this phase is just a fleeting moment in the star’s history. In the end, the nebula will dissipate, and all that is left of our once proud star is a tiny, dense, hot white dwarf, the remnant of the core of our old, evolved star. 95% of all stars that we see in our own galaxy will ultimately become a white dwarf. That includes our Sun.

Not all stars expire so gracefully. Some super-massive stars die in violent explosions we call supernovae. These catastrophic explosions destroy the star and the energy released in the blast can outshine the entire galaxy the star resides in for weeks on end. The famous variable star Eta Carinae may be destroyed one day in exactly this manner.

One of the bi-products of all these dying stars is the heavier elements in the universe that make up our planet, the oceans, the atmosphere we breathe and the very stuff we are made of. All the heavy elements in the universe were created inside the bowels of stars that have long since blown up or blown off their atmospheres. We are indeed “stardust”, and we owe our very existence to variable stars.

Properties of Stars- Distance
Variable stars also reveal a lot about stars in other ways. Take distance, for example.

Astronomy is all about the distance to things in the universe. Variable stars are inextricably woven into the story of our determining the distances to stars and other galaxies.

For many years, the best tool in the astronomical tool bag was the Cepheid period-luminosity relationship. Cepheids are very bright, massive variables with periods of 1 -70 days. The longer the period of the Cepheid, the more luminous it is. Once we know the period of the variable, we know how bright it really is, its absolute magnitude. When we measure a star’s apparent magnitude, how bright it looks to us from a great distance, and compare it to its absolute magnitude, we can calculate the actual distance to the star mathematically. This allows Cepheids to be used as ‘standard candles’ for distance determination. Edwin Hubble used Cepheids in the Andromeda galaxy to make the first estimate of its distance, which led to the realization that it was another galaxy in its own right, hundreds of millions of light years away, and not just a nebula in our own galaxy.

Supernovae are used in much the same way today, to measure the distances to galaxies billions of light years from us.

Mass is the most important quantity of a star. A star’s initial mass largely determines its life cycle. Large, hot stars use up their fuel quickly and may only live for millions of years. Small miserly red dwarfs burn up their resources slowly, and may last for tens of billions of years.

But how do you weigh a star?
Binary stars are of great importance to astronomers because they provide the only means of directly determining the masses of stars other than our Sun. To find the mass of a binary system we need to apply Kepler's Laws. One of the important pieces of information we need to obtain to do this is the period of the stars orbiting each other. Fortunately, eclipsing binaries, another type of variable star, reveal the orbital period of systems by dimming periodically as one star passes in front of the other. Using this information can yield the total mass of the system and from there we can determine the masses of the individual components of the binary.

Nowadays we are using this same method to study planets around other stars. By measuring the minute dip in light output from a star as a planet passes in front of it from our point of view, and then figuring out the orbital period of the planet by observing multiple eclipses, we can determine the size, density, mass and other characteristics of these extra-terrestrial planets.

Binary star evolution can take some pretty wild turns off the normal evolutionary path of single stars. This is another important branch of astronomical research, and nearly all the players in this game are exotic variable stars of one type or another. The evolution of one member of a pair may have dramatic consequences for its partner over the lifetimes of these systems, as they exchange mass and evolve from one type of pair into another over time.

Large Scale Phenomena
Variable stars can also give us a better understanding of larger scale phenomena in the universe. For example, accretion disks in cataclysmic variables teach us things that can aid in our understanding of star formation, planet formation, galaxy formation, active galactic nuclei and the environments close to super-massive black holes in the hearts of distant galaxies. Because these variable star disks are so much closer and brighter than the disk around a distant black hole, they are easier to study and model.

Variable stars are everywhere. They are spread throughout our galaxy, they reside in other galaxies and they are interwoven into the history of our understanding of the Restless Universe we live in.

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