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----------------------- 2238 - Age of Stars
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- How old are the stars? Even for our Sun we confidently say is 5 million years old. The Universe is 13,850 million years old. So there must be a lot of stars older than our Sun But, how do we know the ages of these stars?
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- For years, astronomers have used a method of determining the age of stars that has a 10% to 20% margin of error. Later in his Review we will explain these established methods. Now, a team of scientists has developed a new technique to determine the age of stars with a margin of error of only 3% to 5%.
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- Current star-dating techniques rely on observing stars on the “main sequence” This new technique looks at stars that have begun to ‘die’, which means they are exhausting their supply of hydrogen.
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- Scientists usually can only tell the age of a star by figuring out the age of the population they are a part of, the galaxy or cluster of galaxies. They know the age of some individual stars, but mostly we know the age of star clusters rather than the individual stars themselves.
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- The reasons for that are fairly complex, but our star-dating techniques have led to some strange, rather obviously impossible conclusions, like finding star clusters in the Milky Way that are older than the Milky Way itself. The math is obviously a bit inaccurate.
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- The new methods make measurements of white dwarfs, rather than on main sequence stars. White dwarfs are remnants of stars that have left the main sequence after running out of fuel. Our own Sun will end its life as a white dwarf in another 5 million years. The life of our Sun is expected to be 10 million years.
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- The new technique measure the mass, surface temperature, and whether its atmosphere has hydrogen or helium. Helium radiates heat away from the star more readily than hydrogen. Therefore the ratio of these two gases is a measure of time duration.
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- The star’s mass matters because objects with greater mass have more energy and take longer to cool. This is why a cup of coffee stays hot longer than a teaspoon of coffee.
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- Surface temperature, like spent coals in a campfire that’s gone out, offer clues to how long ago the fire died. Finally, knowing whether there is hydrogen or helium at the surface is important because helium radiates heat away from the star more readily than hydrogen.
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- A star’s mass is still key to determining its age, and it’s still difficult, especially for large populations of white dwarfs. By measuring the positional and radial velocity of about 1 billion stars in the Milky Way and in the Local Group. Measurements of star distances can be made with extreme accuracy.
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- Measurements of star distances can be made with great accuracy by determining the radius of stars based on their brightness. The information on the star’s mass-to-radius ratio is used to determine the mass.
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- Next, figure out the star’s metallicity. Metallicity refers to the abundance of different chemical elements in the star.
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- In archaeology, we use carbon-dating to determine the age of all kinds of objects: tools, structures, fossils, Stone-Age sites. The ages of things give us an understanding of the timeline of events on Earth. The same is true for the Universe.
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- Astronomers have found starsof all different ages from just born to just dieing at 13 billion years old. Our Sun is 4.6 billion years old and will not die until it is 10 billion years old.
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- How do we know how old our Sun is?
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- Meteorites, that were asteroids that land on the surface of Earth, which were part of the accretion disk of dust and gas that formed the Sun and circled the Sun to create the planets, were analyzed using radioactive dating.
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- Radioactive elements decay at a specific rate from a heavier element to a lighter element. By measuring the ratio of the two in a primitive meteorite we can calculate how long it has been decaying. The oldest meteorites are 4.6 billion years old. Therefore the Sun must be 4.6 billion years old. ( See Review # 1554 to learn about these asteroids formations in the early Solar System.)
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- The Sun is half way through its lifetime as it is expected to live for 10 billion years.
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- How do we know how long the Sun will live?
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- The Sun is powered by nuclear fusion. The center of the Sun, the core, is so dense, and so hot, the core reaches 27,000,000 F which is hot enough to cause a thermonuclear reaction of hydrogen nuclei. Hydrogen nuclei fuse into helium nuclei.
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- Not all the matter of hydrogen ends up as matter of helium. Some of the matter gets converted to radiation energy according to E = mc^2. The radiation energy pushes outward from the core against the gravity that is pushing inwards toward the core. The two forces are in balance as long as the Sun does not run out of hydrogen fuel. However, the gravity is relentless.
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- The Sun is fusing 620,000,000 tons of hydrogen every second. That sounds like a lot but the Sun is very, very big. At this rate the Sun will have enough fuel to shine for 10 billion years.
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---------------------- 4,567,000,000 years old is our Sun today
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---------------------- 5,3 billion years old, just 700 million years from now, Earth will absorb all the carbon dioxide in the atmosphere and all the plants will die because they can no longer live on photosynthesis. Animals will go extinct too. That includes us.
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-------------------- 6.2 billion years old all water will boil away including all life , even bacteria.
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-------------------- 10 billion years old the Sun will have burned all its fuel and will begin transforming into a Red Giant Star.
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------------------- 12.2 billion years old the Red Giant Sun will expand to where it will swallow up Mercury, Venus, and Earth.
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-------------------- 12.4 billion years old and the Red Giant Sun will transform into a planetary nebula ejecting outer layers of gas into outer space and leaving behind a White Dwarf Star. The White Dwarf will cool down as a hot cinder.
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- Our Sun will not be big enough to every start fusion again. It is a dead star. Bigger stars can end up as Neutron Stars or Blackholes but not our Sun.
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- Our Sun is a medium size star, one Solar Mass. Every star’s lifetime is different depending on how big it is.
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- Stars that are 10% to size of our Sun shine 1/1,000th as bright and will live 100 times longer, 1,000 billion years.
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- Stars that are 500,000 times bigger than our Sun shine so brightly that their lifetimes are 10,000 times shorter. Only a 1,000,000 years of lifetime before going supernova.
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- Our Milky Way Galaxy has 200,000,000,000 stars of all sizes and all ages. Planets have already been discovered orbiting over 1,000 of these stars. It is estimated that there must be 200,000,000,000 planets in the Milky Way Galaxy alone. These planets are the same age as their stars. If we know the age of the star we may be able to predict if there is life on its planets.
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- Planets that are 1 billion years old could have microbe life.
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- Planets that are 3 billion years old could have primitive life in its oceans and on its land.
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- Planets that are 4.6 billion years old could have intelligent life, like humans, or like orangutans. Or, smarter than humans. It all depends on the speed of evolution and the myriad of factors that affect it.
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- The stars in the range of 1 to 5 billion years old all look alike, like our Sun. We can not tell the age from the color, color is the surface temperature of the star. Young stars are red, they get blue-white as they get older then red again when they die. Medium life spans look like our Sun are all the same color. We need another way to tell age besides color, or surface temperature and size.
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- One discovery that helps us are Star Clusters. Clusters of thousands of stars that were all born at the same time. They all are different sizes. They all have the same composition. They all are the same distance away. They all have the same ages.
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- All stars have a different spin rate. Our Sun spins one rotation in 25 days. Younger stars spin faster and older stars spin slower. Maybe spin rates can correlate to the ages of these medium size stars.
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- The Kepler Space telescope that has been discovering these 1,000 planets by measuring precisely the brightness of stars and detecting the dimming of the star when the planet passes in front of the star.
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- The Kepler Telescope stares at one spot in the sky measuring the brightness of 160,000 stars every 30 minutes. If a particular star dims a planet may be passing in front of it or a sunspot may be rotating around the surface. The orbits and periods are measured carefully to determine what exactly caused the star to dim. It could be a binary star with a Red Dwarf passing in front.
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- When further analysis determines the dimness is a sunspot the rotation period can be measured to get the spin rate of the star. If we focus on a Star Cluster where all the ages are the same we can determine what size stars have what rates of spin. Measuring spin rates can than be used to know the age of the orbiting planets.
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- Those planets with the ages 1 to 5 billion years deserve greater study. What is the temperature of these planets? Could there be liquid water. What is the composition of the atmosphere? Are we alone? Should we send a radio signal to say “Hello”? New methods for determining the ages of stars is advancing. T more we learn bout stars the more we learn about us.
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- (Note 1:) To learn the surface temperature of a star from its color.: The emitted power from each square meter of the surface of a star is directly proportional to the forth power of the Temperature:
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------------------------- Power = Constant * T Kelvin^4
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------------------------- Constant = 5.7*10^-8 watts / (m^2*Kelvin^4)
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------------------------ To find the total power multiply by the surface area of the star.
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- (Note 2): The color of the star is the peak wavelength of the energy-density function. The wavelength is indirectly proportional to Temperature.
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---------------------- Wavelength at maximum = Constant / Temperature
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----------------------- Wavelength = 2,900,000 nanometers * Kelvin / Kelvin
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----------------------- w = 2,900,000 / T
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---------------------- Max. wavelength for our Sun = 500 nanometers
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---------------------- Max. frequency for our Sun = 600,000,000,000,000 cycles per second.
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---------------------- w * f = c = 300,000,000 meters per second
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---------------------- Max color is blue green, but the blue scatters out in our atmosphere and we see a yellow Sun.
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---------------------- The full range of radiation wavelengths of our Sun ranges from 70 nanometers to 4,800 nanometers, from far ultraviolet to far infrared. The peak occurs at 500 nanometers of the blackbody energy-density curve. That is why the sky is blue and the Sun is yellow, and middle - aged.
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- We have several million years yet to live before the Sun gets hotter and evaporates all the oceans. Make the most of it.
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- (More reviews on this subject are available if requested. Review 1822 lists 12 more reviews on the subject.)
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- January 20, 2019 1555
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-------------------------- Sunday, January 20, 2019 --------------------------
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