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
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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Ironically, there is still an astronomical link. Dome C is considered to be one of the best potential sites for a new observatory on the face of our planet. For one thing, the Sun never gets higher than 38 degrees above the horizon, so there is a lot of night time for observing from the south polar region. Even better, there is almost no infrared sky glow, the air is extremely dry, there is almost no aerosol or dust, and no light pollution. The Antarctic Plateau is the largest desert on Earth, so there is very little precipitation and a very high percentage of cloud-free time. Surprisingly, the wind is also quite mild at Dome C, averaging a mere 6 mph in winter. That is a good thing, considering the average annual temperature is -55C, with lows of -80C and balmy highs in the -25C range. Who needs wind chill when it's that cold?
The opportunities to do exciting, results-oriented science exploration and discovery from Antarctica is is almost as mind-numbing as the night time temperatures resident astronomers and technicians will have to bear to perform the work.
Image credit: NASA Education and Public Outreach at Sonoma State University - Aurore Simonnet
Subsequently, measurements taken with the Very Large Array, imaging in the radio regime at 8.5 GHz, revealed two sources, separated by a mere 0.97 arcseconds. The individual sources were too small to resolve. In a paper by J. M. Wrobel and A. Laor (http://lanl.arxiv.org/abs/
Today the call came in. A multi-national team of astronomers report on a deep K-band image taken at the European Southern Observatory using the VLT and an instrument called HAWK-1. The image shows that the object is composed of two sources at a separation of 1". Both sources consist of a nucleus plus an extended emission. These results strongly suggest that SDSS J1536+0441 is a pair of quasars, separated by 5.3 kiloparsecs, which is consistent with the recent independent finding of two compact radio sources by Wrobel and Laor.
In 2004, Oksanen received the AAVSO Directors Award for his work in variable star research. In October 2007, Oksanen was the first to find optical afterglow of GRB 071010B, which had been detected by the Swift satellite only 17 minutes earlier.
You know the weather has been poor if I'm showing pictures of the moon on my blog instead of light curves or pictures of exploding cataclysmic variables.
Another famous major crater on the moon is Copernicus. Unlike Plato, Copernicus has not been filled in by lava. It is 3km deep and 93km across. It has three central peaks, towering up to 1.2 km over the rough floor of the crater.
Dr. Bobbie Vaile has been described as "a computer-packing scientific evangelist who was convinced that even physics can be fun." In 1995 she was awarded the Unsung Hero of Australian Science Award in recognition of her enthusiastic and often unconventional efforts to make "hard" science easy.
The AAVSO podcast is featured on 365 Days of Astronomy again this month.
Spica, also known as Alpha Virginis, is the brightest star in the constellation Virgo. Because it lies on the ecliptic, the path of the Sun, Moon and planets across the sky, it is sometimes occulted (eclipsed) by the Moon.
Spica is an ellipsoidal variable. Ellipsoidal variables are binary systems where the two components are close enough to distort their shapes into elongated, egg-shaped stars. As they rotate around each other they show us varying amounts of combined surface area. When we see them both from the side we see the maximum amount of surface area. Since brightness is directly related to the amount of surface area throwing light our way we see the pair at its brightest when we see them both from the side. When they rotate around to the point we are looking at them from the end of one or the other star, we see the least amount of surface area shining at us, so the star appears slightly dimmer.
