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The Life Cycle of Stars

How stars are born, evolve and die
by

Jessica Barton

on 9 February 2012

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Transcript of The Life Cycle of Stars

The Life Cycle of Stars What different types of stars are there? The history of what we know Such plots were thereafter named Hetzsprung-Russell or H-R diagrams. How do stars form?...How do they die? Main sequence stars are very stable and convert hydrogen to helium for many millions or billions of years. For stars at least 8 times more massive than the Sun, the core of the star can fuse carbon into neon, then neon into oxygen, then oxygen into silicon, then silicon into iron. In the middle of all the debris from the supernova explosion is an incredibly dense neutron star. Variable stars are stars that change in brightness. Some variable stars are intrinsic variables. This means they get brighter or dimmer because the star itself is changing. Some variable stars are pulsating variables Some variable stars are eruptive variables Where do stars live? Spiral galaxies Elliptical galaxies Irregular galaxies There are many different types of pulsating variable stars. Some of them vary in brightness by as much as 100 times, and some have cycles that repeat as often as every few days, while others vary over months or years. In most cases these stars pulsate because they are at the end of their lives and have become unstable. Eruptive variables are likely to have very irregular cycles. They include protostars, which in the process of becoming main sequence stars, often have variations in their brightness. Some variable stars are extrinsic variables. This means they appear to get brighter or dimmer because of something outside the star. The smaller stars are the most common. M stars make up 76% of all main sequence stars. The giant, hot O stars make up only 0.00003% of all main sequence stars. Much of the work on the project was done by Annie Jump Cannon, Williamina Fleming and Antonia Maury and Edward Pickering. In the early 1900s, a team of astronomers at Harvard College Observatory started a project to examine the spectra of hundreds of thousands of stars. They adapted an existing spectral class system which had assigned stars a letter from A to O. A spectrum from a star gives astronomers information about the chemical composition and temperature of a star. Here we can see the different spectral lines from the 7 different types of stars. What are stars? Stars are giant balls of mostly hydrogen and helium gas. They are so big, that gravity puts all of the gas under very high pressure. This pressure is enough that hydrogen atoms can fuse into helium atoms. The outward pressure from fusion balances the inward pressure from gravity in the star. The energy from the fusion happening in the center of the star travels outward in the form of radiation, heat and light. Open clusters are groups of hundreds of stars that probably formed from the same cloud of dust and gas. Astronomers study open clusters to learn about the life cycles of stars, because the stars in an open cluster are all about the same age. Smaller stars live longer than larger stars. O stars live only a few million years, while M stars live as long as 75 billion years! Our Sun is a type G star and will spend about 10 billion years on the main sequence before it becomes a red giant. What else can stars do? Extrinsic variables often have a companion star, and as the stars orbit each other, they appear to get brighter and dimmer as they eclipse each other. White dwarfs that are part of a binary system may also experience eruptions as they take matter from their companion star. Giants and supergiants lose their matter relatively easily and may also experience eruptions. Credit: Casey Reed Most stars are found in galaxies... Within galaxies, some stars are in open clusters. Others are in ancient, very dense clusters of stars called globular clusters. Stars form from clouds of dust and gas, sometimes called "stellar nurseries" Gravity and the momentum of the cloud begin to pull dust and gas to a central point which becomes a protostar Once the interior of the protostar has reached a high enough temperature for hydrogen to fuse into helium, the star becomes stable and is called a main sequence star. Tiny protostars with less than 0.08 the mass of our Sun never reach temperatures high enough for fusion to occur and are called brown dwarfs. Giant protostars with masses over 200 times the mass of our Sun have tremendous internal pressure which overwhelms gravity and these protostars usually expel their outer layers into space before they ever get a chance to become main sequence stars! Sometimes this never happens... Once a main sequence star has used up all of its hydrogen, its outer layers expand and becomes a red giant. The red giant fuses helium to carbon in its core. Eventually the red giant can no longer hold together and the outer layers are ejected into space. These ejected layers form a glowing cloud of debris around the core of the star. The cloud is called a planetary nebula, and the core is called a white dwarf. A white dwarf is so dense that one teaspoon of white dwarf material would weigh 5 tons on earth! The white dwarf slowly cools until it can no longer be seen and is then called a black dwarf. Iron cannot release energy by fusion because it requires a larger input of energy than it releases, so the core collapses releasing a huge amount of energy and an explosion called a supernova. Sometimes neutron stars are massive enough that they collapse further and form black holes! There are 7 main different types of stars O, B, A, F, G, K, and M A star's mass and composition determine what type of star it is. In 1911, Ejnar Hertzsprung (from Denmark), plotted a graph of stars' magnitudes against their color. These plots confirmed that there is a relationship between a star’s luminosity and its temperature, and that stars fall into distinct groups. Independently in 1913, Henry Russell (USA), constructed a plot of stars’ magnitudes against their spectral class. Stars spend the majority of their lives on the main sequence. At the end of their lives, stars move off the main sequence as they change in size and brightness. But remember...A star's position in space does not change - only its position on the H-R diagram! The system they came up with is the system we use today to classify stars! The new system reordered the classes into the order OBAFGKM where O stars are the hottest and each successive class is cooler with M being the coolest stars. Credit: Aurore Simonnet/Sonoma State University/NASA The carbon in our bodies and the oxygen we breathe came into being as a result of star deaths. All of the heavier metals on Earth, like gold and nickel were formed in ancient supernova explosions!
A brief history of stars Stars in the very early universe, right after the big bang, had no metal in them at all, only hydrogen and helium. These stars were extremely massive and did not last long. They all ended up in supernovas which created the first metal in the universe. These stars are still theoretical and are called Population III stars. The stars that formed next formed from the remnants of the population III stars. These all had some metal in them, though little compared to most of the stars we see today. Some of this second generation of stars are still observable in the universe and are called population II stars. Our Sun and most of the stars in our galaxy have more metal than stars of the previous population and are called population I stars. For more information please visit lcogt.net We keep you in the dark! In small stars like our Sun or smaller, the fusion happens by the proton-proton chain. In stars larger than our sun, the fusion happens by the CNO cycle. Let's compare stars This is an H-R Diagram
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