It is said that astronomy is one of two physical sciences in which the solitary amateur can make serious contributions, the other science being paleontology. For example, it should be readily apparent that an amateur nuclear physicist or an amateur genetic biologist will have great difficulties in contributing to their respective pastime regardless of their talent. This is not so for the amateur astronomer. Every night under the sky provides an opportunity to make important and serious contributions to the science of astronomy. More importantly, since this is a hobby, it can be immensely enjoyable.
The observation and study of variable stars are as old as humankind and as fresh as the latest supernova.
Some reflection upon past variable star observations will serve as the beginning of our journey. For a few moments let us consider a nearby supernova that would have produced a brilliant star in ancient man's sky and how that event must have attracted his attention. Such an event could not have been ignored since nearby supernovae are very bright. We know that ancient humans observed these events because records of these bright stars have been found, for example, in Chinese and Native American records. In response to the sudden brightening of these cataclysmic events, history records the beginning of wars and the end of wars. Emperors of great kingdoms were crowned and some unlucky few were executed in response to the changes seen in stars. A complete understanding of how humankind's history has been shaped and altered as a result of the varying brightness of stars may never be known but it certainly can provide for some intellectual consideration. For example, a more recent stellar event occurred in 1572 when Tycho Brahe noticed a bright supernova in the constellation Cassiopeia. At such a late date in the history of humanity, this was the first time Western civilization became aware of variable stars. As a result, history followed a new path.
It took a few centuries longer, until the 1800s, before a German astronomer named Friedrich Wilhelm August Argelander (Figure 1.2) began the first, serious study of variable stars. Because of his efforts, F.W.A. Argelander is considered by some to be the father of variable-star observation.
In 1843 Argelander published a catalog of the stars as part of his study of variable stars that were visible to the naked eye and also created a unique method for estimating the brightness of the stars in relation to one another. His method for the estimation of the brightness of stars is called the Argelander stepwise estimation method.' Using his new catalog and his new method of comparison, within eleven years Argelander measured the position and brightness of 324,198 stars between +90 and -2f declination with his assistants Eduard Schonfeld and Aldalbert Kriiger.
Subsequently, in 1863 Argelander published a catalog known as the Bonner Durchmusterung, abbreviated as "BD." In that same year Argelander became the founder of the Astronomical Society and together with
'This method of estimating the brightness of variable stars, among others, will be described later in this book.
F.W.A. Argelander (1799-1875).
F.W.A. Argelander (1799-1875).
Wilhelm Foerster, and others, began completing a survey of the celestial sky. In 1887 the society independently published a catalog of stars between 803 and -23° declination containing approximately 200,000 stars. This catalog is the Astronomische Gesellschaft Katalog (AGK). Argelander died on February 17, 1875, but his assistant Eduard Schönfeld extended the catalog by 133,659 stars within the zone ranging from -2° to -22° declination.
The southern sky was also mapped and, beginning in 1892, under the direction of J.M. Thome at the observatory of Cordoba in Argentina, 578,802 stars from declination -22° to -90 were collected as the Cordoba Durchmusterung (CD). This catalog was published in 1914. Together with the Bonner Durchmusterung, these catalogs built a compendium of more than one million stars down to 10th magnitude, a measure of star brightness explained later in the book.
Over the centuries, beginning with the effort of Argelander, the study of variable stars has secured the interest of many astronomers. Today, thousands of amateur astronomers from around the world observe and study variable stars. Some do it privately for personal satisfaction and intellectual curiosity while others belong to organized clubs or groups and conduct organized campaigns targeting hundreds of stars and collecting thousands of observations. As a result of the Internet, it is possible for amateur astronomers to share their observations with each other and in some cases with professional astronomers from around the world.
Most astronomers would agree that the study of variable stars is crucial to the overall effort of trying to understand the Universe. The assorted categories of variable stars represent stars in various stages of evolution. For example, the eruptive young T Tauri stars allow us to observe the birth of stars as they evolve from their protostar phase and enter adolescence. Supernova explosions bear witness to the violent death of gigantic stars as they produce as much energy in a few short seconds as our Sun will produce over its entire 10 billion year life. Binary stars, gravitationally bound in the orbits predicted by Johannes Kepler, provide a method of measuring a star's mass and allow us to judge the size of stars too distant to visit. Long-period pulsating stars named for the red giant that was first observed by the German astronomer David Fabricius in 1594, Mira -"the Wonderful," allow astrophysicists an opportunity to contemplate the interior workings of ancient stars. The energetic outbursts from dwarf novae furnish some of the best opportunities to study accretion disks and the underlying processes that may have contributed to the formation of our solar system and even the galaxies.
If you find the notion of personally participating in such a journey intriguing, you can hardly embark upon a more rewarding endeavor than the observation and study of variable stars. Come and join the company of astronomers such as Tycho Brahe, David Fabricius, F.W.A. Argelander and thousands of amateur astronomers from around the world. This is a journey of exploration in search for clues that help explain the workings of the cosmos. Come and explore the Universe with your own eyes. This is an invitation for you to become a participant in this great exploration and to play an important role in the history of variable-star observing.
Since variable stars are stars, a basic understanding of how stars work will help you understand why variable stars are variable because not all variable stars are variable for the same reasons. By understanding why and how stars vary in brightness you may more easily select the type of variable star that you want to observe or study. The choice will be yours to make and an informed decision may allow you to avoid some serious frustration.
Consider the Mira-type variable star R Leonis that has a period of slightly more than 300 days. On the other hand, the delta Scuti-type variable star AZ Canis Majoris has a period of only 2 hours and 17 minutes. And the cataclysmic dwarf nova X Leonis goes into outburst about every 17 days. Interesting or confusing? Before you spend a year watching a star slowly vary in brightness or miss the opportunity to catch a much faster star in action, a little time spent understanding why variable stars act the way that they do may save you some valuable time later.
Using thermonuclear reactions, stars produce energy by converting light elements into heavy elements. Most stars convert hydrogen into helium to produce their energy. Hydrogen is the most abundant element in the Universe, and understandably, stars are composed mostly of hydrogen. It is believed by nearly all cosmologists that at the beginning of the Universe the only thing that existed besides energy was a lot of hydrogen, a little helium and a smidgen of deuterium and lithium. It's no wonder that stars are composed of hydrogen and helium if the early Universe was composed mostly of these two elements. Stars have been converting hydrogen into helium to produce energy since the beginning of time. But stars do not just convert hydrogen into helium. They convert helium into carbon and oxygen and nitrogen and eventually, even heavier elements. Except for the hydrogen and helium present at the beginning of the Universe, everything else is composed of the heavier elements that were formed over billions of years within the cores of countless stars. This process is called nucleosynthesis.
Over time, gravity brings hydrogen together into huge clouds that are light-years in diameter. You can go outside on any clear night, look into the sky and find these hydrogen clouds. The Orion nebula, the Trifid nebula and the region around Rho Ophiuchus, the Eagle nebula, the Tarantula nebula, the Cone nebula and the Lagoon nebula are a few of the places in the sky where you can easily observe these hydrogen clouds tonight with binoculars or a telescope. Our Solar System - the planets within, our Sun, you and me -is made from the stuff found within these huge hydrogen clouds. Carl Sagan called it "star stuff."
Eventually, still under the influence of gravity, the hydrogen cloud collapses even farther. After millions of years, the collapsing cloud crushes the hydrogen until its temperature and density are sufficiently high to cause the individual atoms to combine. Hydrogen atoms composed of one proton and one electron fuse to form helium atoms. This process is not as simple as it sounds. A wild dance that strips the electron from its partner and then forces the proton to combine with a free electron to become a neutron takes place. Eventually, proton-neutron pairs bond with a fast-moving free proton to form helium-3. In time, two helium-3 nuclei come together to form helium-4 and release two protons that return to the dance floor to look for new partners. This is a hot dance and it takes temperatures above a million degrees for all of this to happen. Relying upon this process, our Sun converts about 4 million tons of hydrogen to helium every second. It's been doing so for about 4.5 billion years and it will continue to do so for three or four billion years.
It was not until Albert Einstein explained that mass and energy are interchangeable2 that astronomers understood this amazing process that powers the stars. If you come across a science book old enough, you will find that people once considered that coal may have been the source of the Sun's energy. We know better now, in part, thanks to Dr. Einstein.
After this wild dance that combines hydrogen atoms to form helium atoms, a little mass is lost from the original hydrogen atom. This lost mass is converted into energy and the force that initially brought the hydrogen atoms together, gravity, would collapse the hydrogen cloud into an incredibly tiny object except for this energy. Ultimately, the energy produced from the nuclear fusion of hydrogen exerts enough pressure to stop the complete gravitational collapse of the hydro
2Hinstein's famous equation, E = mc2, that says energy is equivalent to mass.
gen cloud. When the inward collapsing hydrogen cloud is balanced by the outward energy pressure from nuclear reactions, a star is born. More importantly, if the outward pressure produced by the nuclear fusion exactly matches the inward gravitational collapse of the star, a stable star is born. However, not all stars are stable. Unstable stars pulsate, contracting and expanding in an attempt to find a balanced existence. During the convulsions that they experience in this attempt to find a harmonious balance within their lives, they vary in brightness; sometimes dramatically!
Most stars are using hydrogen as their source of energy and all of the stars that are using hydrogen for their energy source are related in a sense. It will be obvious once you start observing stars that not all stars are the same size, or the same temperature, or the same color. Some stars, like Betelgeuse in the constellation of Orion, and Mu Cephei, found in the constellation of Cepheus, are so big that they would fill a large portion of our solar system, gobbling up all of the inner planets and the asteroid belt and extending out even to the orbit of Saturn. Other stars like the white dwarf companion to Sirius are as small as the Earth, and neutron stars are only the size of a city. There are stars a hundred times hotter than our Sun and stars cool enough to contain water molecules. Some stars are deep blue, some are yellow and others are blood red. In contrast to this diversity, all stars that use hydrogen for their energy source are grouped together because of that one common characteristic. These are hydrogen-burning3 stars and hydrogen-burners are called wain sequence stars. Our Sun is a main sequence star because it is using hydrogen as its source of fuel.
Main sequence stars are usually stable stars although many vary in brightness. There is always an exception to everything but when talking about stellar evolution the main sequence is usually a good place to start since many of those stars are stable and most stars spend a large percentage of their lives as main sequence stars. Throughout this book the main sequence will be used as a point of departure when we investigate the different types of variable stars. It will be easier to understand the nature of the many different types of variable stars if we can start with a common reference point.
3"Burning" is a term used to mean thermonuclear reactions.
Now, back to the main sequence. As a star consumes its supply of hydrogen, a core composed of heavier elements forms. The production of these heavier elements starts with the fusion of hydrogen. Some of the older stars have converted a significant percentage of their original hydrogen into heavier elements and they are now using these heavier elements for energy. Eventually, if our understanding of the Universe is approximately correct, there will be no more hydrogen. It will all be converted into heavier elements and eventually those elements will be converted into even heavier elements.
As a star switches from using hydrogen as its primary source of energy to the heavier elements like helium, several changes occur within the star. First, it shrinks and gets hotter.
Remember, it takes temperatures above a million degrees to force hydrogen to fuse into helium but this is relatively cool for stars. At these high temperatures, with much of the hydrogen converted to helium, the star is beginning to grow hungry for a new energy source. The energy provided by hydrogen is dwindling but the star is not hot enough to cause helium to fuse. Helium fusion takes a temperature greater than 20 million degrees. The star is not hot enough so the helium is not providing any energy. It's just sitting there in the core of this hungry star. As the star begins its fast, it also begins to shrink; however, it doesn't lose weight. Gravity, the eternal force, has been patiently waiting, perhaps billions of years, for all of this to happen. Time is of no consequence to gravity and since the birth of this star, gravity has been waiting to compel the star's inward contraction. As the star cools, its thermonuclear pressure decreases and the star begins to collapse and shrink.
The star begins its collapse toward oblivion, but it also begins to get hotter, just as in the early stages of this star's birth when gravity first brought the hydrogen together and crushed it. Now, gravity is crushing the helium core, raising its temperature and density. Eventually, while the remaining hydrogen is still fusing within a thin shell surrounding the star's central region, the core reaches 20 million degrees. At just the right time, at just the right temperature and density, the helium dramatically flashes into a much hotter energy source for the star. The core of the star is now producing energy again but at much higher temperatures than when hydrogen was the source of fuel. This is the core of a helium-burning star and a helium-burning star is much hotter than a hydrogen-burning star.
The thermonuclear pressure from the hot helium core not only stops the inward contraction of the star, it actually pushes the outer atmosphere of the star outward. The star grows! It is larger in diameter now than when it was only fusing hydrogen. Its mass has not changed. Nothing new has been added. It's just larger because the much hotter core has pushed the outer edge of the star farther away, and as a result this outer edge is cooler than before. Because the outer atmosphere of the star is pushed farther away from the core, it has cooled somewhat. The core is hotter but the outer edge of the star is located farther away and as a result, it is cooler than before. Now the star has changed color. Because the outer atmosphere of the star is cooler it has become more red. Now, the star is evolving off the main sequence since it no longer uses hydrogen as its primary source of energy. As stars move off the main sequence, they are said to evolve. During this complex process, evolving stars move through evolutionary stages that have been given fascinating names such as the instability strip, the forbidden region, and the asymptotic giant branch. Perilous times await evolving stars.
The process described, to some approximation, continues as the star produces heavier elements and then in turn begins to use these heavier elements as an energy source. In many cases, evolving stars are unstable and a myriad of variable stars develop from these aging stars. The evolution of stars is an important part of the overall story of variable stars.
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