How Old Is It - 04 - How Old are Stars

according to the lambda cold dark matter Big Bang Theory the universe is around 13.8 billion years old first generation stars would have contained hydrogen and a little helium and virtually nothing else because that is all it existed at the time they formed very few of these Methuselah first generation stars are still around [Music] what we see around us today are second-generation stars including our own Sun in this segment of the how old is it video book will examine what we know about how old these stars are to understand how old a star is we need to take a closer look at how stars form we cannot observe the whole formation process for a single star it takes millions of years but we can learn a great deal by observing star formation at different stages starting with the molecular clouds that hold the matter that is transformed into the Stars thousands of giant molecular clouds exist in the disk of our galaxy each has hundreds of thousands to a few million solar masses of material we've seen some of these clouds in the star birth nebula segments of how far away is it video book giant molecular clouds can be as large as six hundred light years wide this illustration is two hundred light years in diameter they contain mostly hydrogen and some helium but they are also seated with some heavier elements such as oxygen carbon iron and others from the dusty remains of earlier generation stars that ended as planetary nebula or supernova remnants the clouds are dense relative to the rest of the gas between the stars the interstellar medium but are still much less dense than the atmosphere of a planet typical cloud densities are around a billion molecules per cubic meter that might sound like a lot but each cubic meter of air at the surface of the earth has 10,000 trillion times more than that to determine the temperature of a molecular cloud we measure the incoming radiation power from a sizable area and use the stefan-boltzmann law to calculate the temperature results show that clouds like these are around 10 Kelvin that's extremely cold at these low temperatures atoms combine to form molecules such as molecules of hydrogen that's h2 or water h2o over other molecules have been observed in these clouds these clouds are in hydrostatic equilibrium and ruled by the ideal gas laws at every point inside the cloud the weak outward gas pressure is equal to the weak inward gravitational force we don't know what triggers a collapse one thought is that it happens when clouds collide with other clouds another theory has it that supernova remnant wave fronts can do it here we see the remnant exert a force that compresses a cloud to the point where it creates an imbalance in the gas pressure versus gravitational force in favor of the gravitational force another theory suggests that collapse is triggered when a cloud passes into a galaxy spiral arm in any case once started the collapse becomes extremely chaotic and the chaos will continue until a new hydrostatic equilibrium is established in our example the cloud has collapsed to a hundred and eighty Lightyear diameter observations and computer simulations indicate that such a compression would lead to the cloud breaking up into fragments of various sizes and shapes within two million years each of these fragments continue to collapse over a twenty million year period they form planet sized objects brown dwarfs and stars of all masses in this way collapsing giant molecular clouds create star clusters the new hot stars radiation and shock waves push away lighter surrounding gas and dust and illuminate denser surroundings creating a site like this one NGC 602 is at the center of the star birth emission nebula in 90 in the small Magellanic Cloud orbiting the Milky Way Hubble detected over 5600 stars a key point to remember for our how old is it purposes is that these molecular cloud collapses always create star clusters with star counts that range from a few to hundreds of thousands of stars depending on the amount of matter in the original collapsing molecular cloud we never see single stars being formed all the stars in the cluster will be approximately the same age it will be a globular cluster like ma T if the gravitational force due to the total mass of all the stars is enough to bind them together or it will be an open star cluster like the Pleiades these stars still have some of their cloud fragment material in their vicinity the Pleiades stars create this reflection nebula these stars are all the same age the fact that they are still close together indicates that they are very young over the next 250 million years these stars will drift so far apart that observers viewing these stars at that time won't be able to link them together stars not associated with any cluster are called field stars just stars in a field of stars our Sun is a field star it has drifted far from the cluster it was formed in now let's consider fragment large enough to create a star the size of our Sun the primary force on the rotating fragment is gravity pulling it towards the center but because the subset of dust and gas that's rotating around the center of gravity follows the conservation of angular momentum laws matter there will increase its velocity as its distance from the center decreases this effectively slows the region's collapse into the center in fact if the velocity of a small piece reached orbital speeds it would not fall into the forming star at all all effect is for matter above the plane of rotation to move down and for the matter below the plane of rotation to move up the entire fragment morphs into a disk structure around the core called a circumstellar disk matter accretes into the central object via this disk in the early stages of the collapse into the core the temperatures remain low because generated radiation energy is able to escape the compression continues until the core is dense enough to hold on to the energy this takes around 30,000 years at that point the core has enough mass density to capture generated photons and the temperature begins to rise another 100 thousand years and the core temperature reaches ten thousand degrees Kelvin at this temperature the object begins to shine by a normal non-nuclear means it's now a protostar so far the core has only about 1% of its final mass stars develop out of these fragments and three observable phases protostars remain sequence stars and main sequence stars protostars remain shrouded in the dust and gas clouds that created them the actual protostars can only be seen by infrared telescopes the Eagle Nebula is a good example of this it contains large numbers of forming stars some of these proto stars can be seen with Hubble's near-infrared image here we see the trapezium cluster of four stars at the heart of the Orion Nebula if we zoom in we can see several protostars around one of the larger stars they are the white object streaming material away from the central star here's one more example this infrared Hubble Space Telescope image shows a protostar just 950 light-years away protostar is the bright object with fan-like beams of light flowing from it is letting off flashes of light every twenty five point three days this time-lapse movie shows a pulse of light emitting from the protostar most if not all of this light is being scattered off the circumstellar disk the observational evidence along with computer simulations indicate that the protostar phase can last up to a few tens of millions of years continuing to accumulate mass as of collapse causes the protostars diameter to shrink significantly and its core temperature rises to five million Kelvin some astronomers point out that this is hot enough for some hydrogen fusion in its core this makes it a true star but because it is still growing by accreting large amounts of material from its surroundings is not yet stable astronomer call these stars pre main sequence stars or young stellar objects during this phase a strong solar wind forms pushing back on the gas and dust surrounding it a typical transition for a star with three times the mass of our Sun or less is through a teatari phase named after the star teatari the orange star at the center of this photograph is teatari it's the prototype for the teatari class of variables here we see it surrounded by a dusty yellow cloud named the Hinn's variable nebula a typical characteristic of t-tauri stars are Jets of high speed gas and dust streaming from both poles a strong magnetic fields guide matter from the star circumstellar disk into the core we can see these Jets and star birth nebula here's a striking example from the Carina Nebula you can see the Jets at the top of this mystic mountain here's a view of a multiple star system called XC tari its neighbor HL tari and V 1213 tari just 450 light years away these young stellar objects are illuminating the entire region exit ari is actually a binary star system it is expelling hot bubbles of gas into the surrounding space gasps from an unseen disc around one or both of the stars is channeling through magnetic fields surrounding the binary system and forced out into space at nearly five hundred and forty thousand kilometers per hour or 300 thousand miles per hour this outflow which is only about 30 years old extends nearly 96 billion kilometres or 60 billion miles from the star stars can remain in the teatari phase for as long as a hundred million years and reached 10 million Kelvin at their cores as their solar winds pick up they disperse the remaining gas and dust around them back into the interstellar medium this ends mass accumulation and the star settles into hydrostatic equilibrium at the mass of our Sun the core temperature reaches 15 million degrees Kelvin and it has ninety-nine point eight percent of all the collapsing molecular cloud fragments matter leaving only 0.2% left over for planets moons asteroids and comets it is now a main-sequence star here's the hertzsprung-russell or HR diagram we covered in the distant star segment of the how far away is a video book the long diagonal line represents the main sequence for stars and hydrostatic equilibrium burning hydrogen the lower right red stars are cooled low mass stars that are a fraction of the mass of the Sun the middle yellow and orange stars are closer to the mass of the Sun and the upper left blue and white stars of a hot high mass stars many times more massive than the Sun proto stars that are many times the mass of the Sun evolves so rapidly that they show up at the high blue end of the main sequence in a short amount of time for them there is no teatari phase T Tauri stars would start here on the diagram as they stabilize they shrink in size increase in temperature start fusing hydrogen in large quantities and migrate to the main sequence the more mass of the young stellar object the higher up on the main sequence the eventual star will land if we start counting the age of a star from the beginning of the cloud collapse that formed it we would have stars just reaching the main sequence than about 150 to 200 million years old how long any particular star currently on the main sequence will remain there depends on how much fuel it started with how fast it is burning that fuel and how long ago it started to burn the mass of a star gives us a measure of how much fuel it started with and its luminosity gives us a measure of how fast it is consuming this fuel but before we can make the lifetime calculation we need to understand how much energy we get from hydrogen when it fuses into helium here we are looking at the proton proton fusion chain the most common fusion reaction in our sun's core the mass of helium 4 at the end is a bit smaller than the mass of the four protons at the start the amount of energy generated is determined by Einstein's equation e equals MC squared we see that the production of each helium nucleus releases only a small amount of energy but by measurement we know that the Sun produces 3.9 times 10 to the 26 watts to produce this amount of energy it would take a tremendous number of these fusion events every second we calculate that 613 million metric tons of hydrogen fuse to form 609 million metric tons of helium converting 4 million metric tons of matter into energy every second to figure out how long it would take for our Sun to burn all the hydrogen is started with we simply divide the available hydrogen by the amount consumed per second the sun's mass is 2 times 10 to the 30th kilograms Fusion is only occurring in the core which represents about 10% of the sun's mass we see that once hydrogen burning began our Sun would take 10 billion years before it ran out of fuel that's the total amount of time the Sun will be a main-sequence star at the end it will expand and cool into a red giant star and consume the earth astronomers have empirically found that even though the more massive stars have more hydrogen fuel a corresponding dramatic increase in luminosity shows that they are consuming this fuel faster much faster a small change in mass leads to a small change in the core temperature but a very large change in the luminosity for example stars twice the mass of the Sun have over 10 times the luminosity and burnout in under two billion years in the other direction we see the stars with half the mass of the Sun have less than a tenth of the luminosity and remain on the main sequence five times longer at the extremes we have theta one Oh Ryan SC in a trapezium cluster at thirty three times the mass of the Sun its luminosity is over two hundred thousand times greater than the Sun and it won't last more than a few million years at the other end Wolf 359 is just under a tenth of the mass of the Sun and will remain a main-sequence star almost four hundred times longer than the Sun the dramatically shorter time on the main sequence for higher math stars an extremely extended time for lower mass stars is due to the proton fusion processes sensitivity to temperature to understand why this is the case we need to look at the proton proton activity in the core one level deeper deep inside a star's core protons are colliding at a tremendous rate but few of these collisions result in a fusion of the two protons that's because when protons collide they have to overcome a repulsive electric force due to the fact that they are both carrying a positive charge this is called the Coulomb of a barrier in order to understand why star luminosity is so sensitive to small increases in temperature we need to see how this barrier is breached we'll start with a look inside the Sun's core using the known relationship between temperature and kinetic energy we can calculate the average thermal energy and velocity of the protons in the Sun's core it depends entirely on the temperature 15 million degrees Kelvin we find that each proton has on average 2 kilo electron volts of kinetic energy and travel at just over 600,000 meters per second that's well over a million miles per hour with this we can calculate the number of times a proton will collide with another proton the number depends on the proton density cross-section and thermal velocity we cover cross section in the how small is a video book it represents the target area for determining a collision versus a miss we calculate that each proton experiences over 1 trillion collisions per second we can also calculate the sun's fusion rate per proton dividing the mass of the Sun's core by the mass of a proton gives us the number of protons in the core dividing the fusion rate calculated earlier by the number of protons in the core gives us the fusion rate per proton and if we divide this by the trillion collisions per second we get the number of fusions per collision we see that the probability that any particular collision will result in a Fusion is extremely small that's why if protons trillion collisions per second can go on for billions of years before one of them results in a fusion event [Music] to understand why the vast majority of proton-proton collisions in our Sun don't result in fusion even though they are colliding at incredible speeds we need to examine the strength of the electrostatic force separating them according to Coulomb's law two particles with the same charge with a force that is proportional to the product of their charge and inversely proportional to the square of their distance from each other very much like gravitational force in order to fuse the protons must get close enough for the attractive strong nuclear force to take over from the repulsive electric force the reach of the strong force is very small just over one centimetre or Fermi there are a million Burmese in a nanometer at this distance the electric repulsion force is overwhelming we see that the force is quite extreme given that it is working on a mass as small as a single proton if we look at it from a classical energy point of view we see that proton average energy at 15 million Kelvin is just not enough to overcome the potential energy barrier in fact the energy required to overcome the barrier is 300 times greater than the average energy of the protons to understand how often we can expect a proton to have the barriers energy at the sun's temperature we use probability distributions developed by James Maxwell and Ludwig Boltzmann in the mid-1800s this analysis shows that only one out of ten to the 200th collisions would cross the Coulomb barrier that's almost as good as none our Sun was simply not burn hydrogen if this was all there was but we know there is more because we have measured the fusion rate of a proton in our Sun's core and it's a hundred and eighty orders of magnitude more than this classical physics predicts at these extremely small distances quantum mechanics plays a dominating role a proton travels as a wave described by Schrodinger's equation as a wave its exact location is not completely knowable the square of the particles wavefunction gives us the probability for materializing as a particle at any particular location and time most of the time it will be found at the most probable location but we see that some of the wavefunction is on the far side of the barrier the wave amplitude is significantly smaller but the frequency is the same the small amplitude indicates that there is a small probability that it will materialize there but having the same frequency indicates that it will have the full particle energy the phenomenon is called tunneling at the sun's temperature the probability that a proton will tunnel through the barrier is quite small but a hundred and ninety three orders of magnitude more likely than classical physics would have it protons will cross the barrier and overlap many times before they actually trigger a fusion event to be exact converting probabilities to rates we see that on average there are a million successful tunnelling through the Coulomb barrier to get one fusion this is because in order to fuse the PI on transfer between the two protons or something similar has to occur in the extremely short period of time that the protons are in contact this particle sharing in the nucleus is similar to the electron sharing that binds molecules we cover how this works in the higgs-boson segment of the house small is a video book in addition one of the protons has to eject a neutrino and a positron to become a neutron all these nuclear processes are sensitive to proton energies that is temperature to conclude we see that small increases in the colliding protons energy driven by increases in temperature will increase collisions cross-sections Coulomb barrier penetration rates and fusion rates for overlapping protons the accumulative effect makes the hydrogen burn rate exponentially sensitive to temperature now given the mass of a star we know how long it will burn the core hydrogen it started with for the Sun that's around 10 billion years but how long the fuel will last from now depends on how long ago it started fusing let's take a look at one way we know how far long star is in a star cluster like m67 for example if we can find a star that is just starting to show the symptoms of having run out of its core hydrogen will know from its mass just how old it is that will then be the age of all 1100 plus stars in the cluster because they all formed at the same time we need to understand what observable effects we can use when a star's fuel runs out one is because core temperatures are not high enough to fuse helium once the hydrogen is used up fusion in the core ceases without fusion there is no nuclear energy source to supply heat to the central region of the star the long period of hydrostatic equilibrium ends gravity again takes over and the core begins to contract as the star's core shrinks the energy of the inward falling material is converted to heat the heat flows outward to cooler regions the added heat raises the temperature of the layer of hydrogen just outside the core once this shell becomes hot enough hydrogen fusion begins there the helium core continues to contract producing more heat all around it this leads to more fusion in additional shells of fresh hydrogen outside the core the additional fusion produces still more energy which also flows out into the upper layers of the star the first observable result is an increase in the star's luminosity with all the new energy pouring outward the outer layers of the star begin to expand the star eventually grows and grows until it reaches enormous proportions the expansion of a star's outer layers causes the temperature at the surface to cool here we have the second major observable result the star's surface temperature decreases the star becomes simultaneously more luminous and cooler on the HR diagram we see that the star leaves the main sequence and moves upward because it's brighter and to the right because it's cooler detecting this is then a matter of finding stars in a cluster leaving the main sequence here's a map of m67 stars to the HR diagram you can see where the stars are currently moving off the main line this is called the turn off point there are no longer any stars in the cluster higher on the main line the turn off point gives us the luminosity of the stars moving off the main sequence the mass luminosity relationship gives us the mass and with the mass we can calculate the age for m67 we find that it's around 4 billion years old we then know the age of all the stars in the cluster but this does not work for field stars like our Sun research into rotation rates and sunspots as age predictors called gyro chronology is ongoing but at this time we have no way to figure out the age of field stars by examination if we're going to figure out the age of the Sun we'll have to examine the material that formed around the Sun the comets asteroids moons planets and especially the earth [Music]

Channel: David Butler

Views: 140,894

Rating: 4.829514 out of 5

Keywords: STEM, Astronomy, stars, molecular cloud, star clusters, circumstellar, Protostar, Tauri, Field Star, collision, fusion, Coulomb

Id: eZcyBmnr-T4

Length: 31min 56sec (1916 seconds)

Published: Wed Oct 23 2019