How far away is it - 10 - The Milky Way (4K)

welcome to our final segment on the milky way in this segment we'll go over our current understanding about the structure and size of the Milky Way as a whole and our place in it will examine the galactic center with its supermassive black hole we'll go a little deeper into the nature of a black hole will explore the Galactic disk with its spiral arms and we'll cover the latest information on the Galactic halo and as usual we'll discuss how we came to know these things from our viewpoint deep inside the galaxy itself on January 1st 1990 from its orbit around the earth the good art Space Flight centers cosmic background Explorer created this edge on view of our Milky Way galaxy and infrared light here's a newer inside image of our galaxy in fact it's the most detailed map ever made it was released in 2018 by Gaia the European Space Agency spacecraft that recorded the position and brightness of 1.7 billion stars as well as the parallax proper motion and color of more than 1.3 billion stars the map shows the density of stars in each portion of the sky the galaxy has a center with a central bulge a disc of rotating stars and dust and a halo without dust clouds and peppered with globular star clusters the disc is at least 100,000 light-years in diameter and the halo is much larger than that we'll go into each of these galaxy components starting with the galactic center will cover how images like these are created from inside the galaxy and how impossible it is to get an image from outside the galaxy later on in this segment the world's great space observatories the Hubble Space Telescope the Spitzer Space Telescope and the Chandra x-ray Observatory have collaborated to produce this unprecedented look at the central region of our galaxy Hubble documented vast arcs of gas heated by stellar winds from very large stars Spitzer's infrared picked up the pervasive heat signals all these stars on Chandra detected x-ray sources from ultra dense neutron stars and small black holes together they produced the spectacular image the central object in the Milky Way is known as Sagittarius a star or sag a star for short it is surrounded by so many stars and gas and dust that it is almost impossible to see teams of astronomers and astrophysicists have been working on understanding Sagittarius a star for over 25 years the UCLA galactic center group along with the Keck Observatory on top of the Mauna Kea volcano in Hawaii and the European Southern Observatory in its array of very large telescopes in Chile and the Max Planck Institute for extraterrestrial physics in Germany and many others have made dramatic progress in advancing our understanding of this critically important part of our galaxy after decades of careful observations the speeds and orbits of around 45 stars around sag a-star have been calculated this enabled measuring the precise location of the point they are all orbiting around the measured orbits also identified the gravitational pull from this point which in turn gave us its mass at four million times the mass of our Sun but when we look at this point we don't see anything this was strong evidence that sag a-star was a black hole because stars are known to be unstable at much smaller masses the star s2 is a particular interest because it passes closer to Sag a star than any other it's a single main sequence star with 1015 times the mass of our Sun observations of the star showed that its orbit took it to within 20 light hours of Sagittarius a star in 2002 without bumping into anything that puts a gay stars four million Sun mass into a very small place for many astrophysicists this constituted proof that it was indeed a supermassive black hole but others pointed out that an extremely dense dim star cluster could produce these results but if sag a-star were a cluster s twos orbit would wobble it did not wobble this was the final proof point five hundred years after Copernicus with the Sun at the center of our solar system this team identified Sagittarius a star as a supermassive black hole at the center of our galaxy but we weren't done with s2 it's orbital period is 16 years following a 2002 passing a major effort was mounted to upgrade esos Very Large Telescope array to enable the precision needed to reveal the true geometry of space and time near this object and test Einstein's theory of general relativity these new instruments followed s2 very closely at the start of 2018 it was accelerating towards sag a-star reaching relativistic speeds on May 19th it reached the closest approach Peary Center at that point it was traveling at 7650 kilometers per second or 4750 3 miles per second that's almost three percent of the speed of light its distance from the black hole was just 18 billion kilometers or 11 billion miles that's only a hundred and twenty times our distance from the Sun the separation on the sky between the two points was just 15 mil of arc seconds it was also reddening in color as a black hole's gravitational field stretched its light to longer wavelengths the color change in this illustration is exaggerated for effect the actual revenue is quite small and would not be visible to the naked eye s2 s velocity changes close to the black hole or an excellent agreement with the predictions of general relativity in addition to change in the light wavelength agreed precisely with what Einstein's theory predicted but understanding what is happening this far away is always prone to errors I remember when we thought there was a gas cloud g2 that would be entering the black hole in 2014 this never materialized in our current case some astronomers point out that massive non luminous objects such as stellar-mass black holes might be present and could affect the orbital dynamics of s2 more research is needed to rule out this possibility here's a full dome illustration that shows how sag a-star might look to viewers on a planet orbiting s2 as it orbits the black hole we'll cover black holes and why our supermassive black hole might look like this but first we'll cover how the ESO Very Large Telescope actually measured the minut distance is associated with s2 and sag a-star 26,000 light-years away [Music] the Hubble Space Telescope can resolve angles on the sky as small as 50 milliseconds the angular distance between s2 and sag a star at Paris Center was just 15 mil arc seconds that's 42 billions of one degree and three times smaller than Hubble can resolve to follow s2 as closely as they did astronomers had to use a stellar interferometer these kinds of telescopes can resolve images 30 to 40 times smaller than optical telescopes this makes them extremely important tools for studying the galactic center as well as exoplanets they can even resolve sunspots on nearby stars so to understand how we know how close s2 got to sag a-star we need to understand how these stellar interferometers telescopes work in the speed of light chapter of the how fast is its video book we covered the Michelson interferometer used to measure minut distances in the lab interferometers can measure distances on the order of a few nanometers Michelson and Morley used it to show that the speed of light was a constant in order to create light interference Michelson illuminated the interferometer with fully coherent light coherent light has a common frequency and phase it always produces interference patterns on the far side of a double slit fully coherent light like the kind that lasers create will produce regions of fully destructive interference that is the dark regions have no light falling on them at all partially coherent light will produce regions of partially destructive interference meaning some light falls in the dark regions and incoherent light will not produce interference patterns at all we find in nature the waves can start out as incoherent and become partially coherent as they spread out watch how these ducts start in a chaotic mix of water waves as they enter the pond but as the waves move out they become quite orderly this is a geometrical effect the further away one travels from the source the less significant the distance between the individual wave generators becomes a point source for starlight would produce coherent light and at any distance from the source the light would create interference patterns but there are no point sources in nature stars have a diameter on the sky an extended thermal light source would start out with incoherent light but as the light moves away from the source his coherence increases just like with the ducks on the pond it is fascinating to note that incoherent light waves created by excited atoms and stars 20 billion kilometres apart can travel for 26,000 years and still carry the remnants of that starting condition they large enough stellar interferometer can use the visibility dimming of the interference pattern created by the light to detect the original star separation see how the amount the image fades is greater the further apart the two stars are the math involved was developed independently by Dutch physicist pH van sitter in 1934 and F Cernik in 1939 it's known as the van sitter to sir Nikki theorem from antiquity it was believed that the idea of empty space is a conceptual impossibility space is nothing but an abstraction we use to compare different arrangements of the objects concerning time it was believed that there can be no lapse of time without change occurring somewhere time is merely a measure of cycles of change within the world then in 1686 Isaac Newton founded classical mechanics on the view that space is real and distinct from objects and that time is real and passes uniformly without regard to whether anything moves in the world he spoke of absolute space an absolute time as a stage within which matter existed and moved as time flowed at a constant rate it was understood that space and time tell matter how to move but matter has no effect on space and time the idea that space and time act on matter but that matter does not act on space and time troubled Einstein noting that light curved in a gravitational field Einstein proposed that the mass of an object does indeed act on the space and time it exists in specifically he proposed that the presence of matter curves space-time this led Einstein to his theory of general relativity which predicts the existence of black holes as objects so massive that light itself cannot escape their gravity you'll recall explosions at the end of life for stars less than five times the mass of the Sun leave behind a white dwarf in these stars electron exclusion pressure is enough to counteract the inward force of gravity supernova explosions at the end of life of stars more than five times the mass of the Sun leave behind a neutron star in these stars electron exclusion pressure is insufficient to overcome the force of gravity but Neutron exclusion pressure is but if a star is greater than 30 times the mass of the Sun even Neutron exclusion pressure won't do the trick in fact there is no known force that will counteract the inward force of gravity for such a supernova or hypernova exploding star according to Albert Einstein's general theory of relativity the star will collapse into zero volume and infinite density this is called a singularity this defines a black hole it gets its name from the fact that such a singularity would create a gravitational pull that not even light could escape the object literally becomes invisible in 1916 Karl Schwarzschild contemporary of Einstein solved his equation for the special case of a non-rotating severe he found that although the diameter of the singularity is zero the radius at which light would be captured depends entirely on the mass of the black hole this is called the Schwarzschild radius and it defines the event horizon but it would be the rare black hole that doesn't spin in 1963 Roy Kerr developed a general solution for spinning black holes it showed that there is a second region beyond the event horizon that defines a volume around the black hole called the Ergo sphere in this region space itself is dragged around by the black hole's spin is called frame dragging also in this region light can enter stable orbits around the black hole this would produce a photon spherical shell encasing the black hole with the light from all the stars in the universe accumulated over the entire age of the black hole it would be a sight to see one of the things all rotating black holes have in common besides the fact that we can't see them is that matter flows in via an accretion disk the exact mechanism is not yet fully understood but we know that gamma-ray Jets shoot out at the poles carrying a percentage of the falling matter with it at speeds approaching the speed of light in late 2018 ESO s gravity instrument observed flares of infrared radiation coming from the accretion disk around sag a-star these flares came from clumps of gas swirling around at about 30% of the speed of light on a circular orbit just outside the event horizon they indicate that sag a star is spinning with a full rotation every 11 and 1/2 minutes this makes the four million solar mass sag a-star a supermassive kur black hole this new information also enabled calculating the distance from sag a star center to its event horizon at around 10 million kilometers or 15 times the radius of our Sun and the distance to the photon sphere at around 17 million kilometers to illustrate how a black hole might look we'll build sank a star here we are viewing it from the equatorial plane and the object is rotating in on the left and out on the right it's Center is dark out to the event horizon this thin ring around the black hole just outside the event horizon represents the cross-section of Sagi stars ergo sphere with a shell of orbiting light what we'd see is the light that leaks out in our direction the observed flares indicate that sag a star has the remnants of an accretion disk that is no longer feeding the black hole on a regular basis the massive amount of light rays emitted from the disks top face travel up and over the black hole and light rays emitted from the disks bottom face travel down and under the black hole this combination gives us the full image of how the black hole would actually look [Music] there are three classifications for black holes based on their mass stellar with masses up to ten times the mass of our Sun supermassive with millions or even billions of times the mass of our Sun and intermediate with masses somewhere in between sag a-star is a supermassive black hole in March 2018 the Japanese instrument ma X I aboard the International Space Station recorded an extremely strong x-ray outburst NASA's ni cer neutron star instrument also on the space station focused on the outburst for days and watched it fade in addition the Gaia mission was able to locate the x-ray source companion star and determine its distance at ten thousand light-years analysis showed that the x-ray object is a black hole with the mass of around ten Suns the x-rays are generated as matter from the star feeds the accretion disk around the black hole some astronomers calculate that there are as many as a hundred million stellar-mass black holes like this one in our galaxy most of these are invisible to us and only about a dozen have been identified for more information on black holes see the general relativity effects segment of the how fast is it video book the number of stars in the Milky Way is very difficult to determine but based on detailed analysis of star distances star motions hydrogen radiation from spiral arms galaxy rotation curves and mass including dark matter astronomers currently believe that the galaxy has a relatively flat rotating disc 100 to 120 thousand light-years wide and 1,000 light-years deep with some 100 to 400 billion stars this image out of the Spitzer Science Center and the University of Wisconsin represents an attempt to synthesize over a half-century of work on the Galactic disk structure based on data obtained from the literature at radio infrared and visible light wavelengths the galactic center itself with the supermassive black hole that we discussed earlier is shaped like a bar although most parts of the Milky Way galaxy are relatively uncrowded roughly 10 million stars are known to orbit within just a single Lightyear the galactic center in a region known as the central bulge recent surveys discovered the 2-3 kiloparsec arms named for their length they are now generally thought to be associated with gas flow roughly parallel to the central bar using infrared images from spitzer scientists have discovered that the Milky Way's elegant spiral structure is dominated by just two arms wrapping off the ends of the central bar one is named scutum Centaurus and the other is named Perseus each of these major arms consist of billions of young and old stars three thinner arms spiral out between the two giant arms these are called Sagittarius Norma and the outer arm these are primarily filled with gas and pockets of star-forming activity there is also a spur off to Sagittarius arm called the Orion spur it's 3,500 light years across and approximately 10,000 light years long we are located on the inner edge halfway along this spur around 26,000 light years from the galactic center when we fill the space between the arms we get the full picture it's interesting to note that the number of stars per unit volume of space in the region between arms is the same as the number in the arms themselves what distinguishes the arms is that they have a far greater number of younger stars in fact all the known h2 star forming regions in the galaxy exist inside the arms we don't see any in the area between the arms if we lay a grid over the galaxy we can locate some of the stars nebula and h2 regions we have seen in this chapter actually all the local neighborhood stars which fit into the red circle I use to locate our solar system that would be stars like Wolf 359 Altair Vega Polaris capella Aldebaran the Pleiades and Betelgeuse they are all with us in the Orion spur as is the Orion horse head cone which is head veil and many other nebula in Sagittarius we see the jewel-box star cluster and the trifid Omega Lagoon eagle and cat's paw nebulas among others in Perseus received the Rosetta heart and soul nebulas as well as the crab supernova to name just a few in fact except for the hypervelocity stars and a few of the supernova remnants everything we have seen in this chapter is within this red circle as vast an area as we have covered it is only a fraction of the Milky Way galaxy another point that ought to be covered is that we cannot see through the galactic core into the other side the core is simply too dense with stars and gas and dust to penetrate so this slice of the disk has not been seen or analyzed but our understanding of spiral galaxies is that they are symmetric so this picture makes that assumption and fills in the blanks accordingly here we see the sun's orbit around the galactic center our orbital speed is approximately 230 kilometers per second or 143 miles per second that's fast but it takes us around 213 million years to complete one orbit around the galactic center the last time we were in the same place in our orbit dinosaurs were just starting to appear on the earth and we have traveled around 110 thousandths of a revolution since the origin of humans here's a look at our solar systems the ecliptic plane with respect to the Galactic plane it's just over 60 degrees off we see that the solar system is quite out of alignment with the galaxy's disc Earth's 23-degree tilt to the solar plane puts us at an almost 63 degree tilt from the Galactic plane this is why the Milky Way appears at such a strange angle across the night sky also as the Sun orbits the galaxy it oscillates up and down relative to the plane of the galaxy it does this approximately 2.7 times each time around astronomers estimate that we are currently at around 75 to 100 light-years above the Galactic plane and moving down this estimate has us crossing the plane again in approximately 30 million years before we leave the galaxy's dusty disc we'll take a closer look at the dust itself it's critically important for calculating intrinsic star luminosity and it's the only galaxy content that we can see to accurately calculate the galaxy's rotation curve that star velocities as a function of how far from the center of the galaxy they are the Milky Way's rotation curve is one of the reasons scientists have proposed the existence of dark matter the dust is made of thin highly flattened flakes of graphite and silicon that's carbon and rock like minerals coated with water ice each dust flake is roughly the size with a wavelength of blue light more smaller the dust is probably formed in the cool outer layers of red giant stars and dispersed in the red giant winds and planetary nebulae the dust absorbs and scatters the light to pass through it the further the light has to travel the more of this dust it encounters and the dimmer it gets astronomers call this extinction due to this extinction effect stars in the Galactic disk can lose up to half their luminosity every 3,000 light years only the brightest stars can be seen more than 10,000 light years away these clouds are best viewed using radio astronomy this is because gas clouds radiate radio waves and radio waves pass through dust particles untouched because their wavelength is much larger than the size of these particles what's more the hydrogen in these regions emit a spectral line in the radio frequency band and this spectral line exhibits Doppler shifts enabling us to measure the clouds radio velocity relative to us in this line of sight reading we see a number of Peaks each one represents a cloud peaks have different frequencies because the clouds have different radial velocities the maximum peak is from a cloud that's radio velocity is close to its total orbital velocity the best way to map out the rotation curve for the galaxy's disc is to measure the orbital velocities and distances of gas clouds and star forming regions across the galaxy these are the h1 h2 and molecular clouds we covered in our segment on star birth nebula these are the best objects to analyze for three reasons one they trace out the spiral arms two we can see them clearly at great distances using radio astronomy and three there is a good way to calculate their distance for the inner part of the galaxy so for clouds closer to the center than we are we can scan the sky and by bit and create a map of the rotation velocity and distance or the inner galaxies this map can then be used to find distances to all the clouds and the stars they contain as long as they are closer to the center of the galaxy and we are for clouds further out there are no tangent points for these we have to use weaker methods for determining distance and rotational velocity we then do a best-fit line for the collected data here's a graphic superimposed on our Galactic curve that indicates the accuracy of methods used to provide the included data points the vertical lines through each point represent the range of possible velocities for any given distance notice that these lines are quite long rotation curves give us a measure of a systems mass and at the outer edge of the disk the star mass density drops off dramatically that's why in the 1970s everyone expected to see a rotation curve that looked like this but what we found is that where the velocities were expected to fall off they remained relatively constant if our current theory of gravity holds up for galactic distances then this curve tells us that our model of the Milky Way is missing something in order for objects far from the center of the galaxy to be moving faster than predicted there must be significant additional mass far from the galactic center exerting gravitational pulls on those stars not knowing what it is we call it dark matter and it extends way into the galaxy's halo at the turn of the 20th century astronomer Harlow Shapley studying a large number of RR Lyra stars inside globular clusters found that the center of the galaxy was far from the Sun he mapped 93 globular clusters they formed a spheroidal shape with their own center not in here at the Sun he concluded that these giant clusters formed the bony frame of the galaxy this area around the disk is called the Galactic halo or Corona it holds a large number of old stars and a hundred and fifty eight kilometers below itself has a diameter of at least six hundred thousand light-years based on the locations of the globular clusters although it may extend much further in 2007 using twenty thousand stars observed by the Sloan Digital Sky Survey an international team of astronomers discovered that the Milky Way halo is a mix of two distinct components rotating in opposite directions the outer halo and the inner halo then in 2018 a team of astronomers analyzed 7 million stars from the Gaia mission and found that 30 thousand of them were moving counter to the normal Milky Way flow star motions composition profiles indicated that they came from a different galaxy they call this new galaxy Gaia and Solaris using computer models for galaxy collisions they estimated that it collided with the Milky Way around ten billion years ago this is a computer simulation of the merger here we see the Gaia Enceladus is now our galaxy's inner halo on September 24 2012 Chandra found evidence that the Milky Way galaxy is embedded with a large amount of hot gas in the halo counting this vast amount of cast the mass of the halo is estimated to equal the mass of the stars in the galaxy but as massive as it is the amount of matter in this hot gas is not nearly enough to explain the galaxy's rotation curve dark matter or a new theory of gravity is still needed in 2018 using both Hubble and Gaia data on globular cluster sizes and velocities the mass of our galaxy was estimated to be at least 1.5 trillion times the mass of our Sun this is more than previous estimates and indicates that the Milky Way is among the universe's larger galaxies [Music] let's take a closer look at how an image like this is created from orbit we point the camera at the center of the galaxy and then turn it 180 degrees to face away from the center we're now looking through the plane of the galaxy away from the center then we scan the camera clockwise taking hundreds of pictures along the way we continue the rotation through the center and all the way back to the starting point note that the stars on the right edge of the image taken at the end of the rotation are adjacent to the Stars on the left edge of the image taken at the beginning in other words the entire right side of the image borders on the left now we rotate the camera up a bit and repeat the process we do this over and over until the entire northern sky is covered the last shot is taken with the camera pointing straight up perpendicular to the Galactic plane we then repeat the process for the southern sky and we have the entire picture once we have all the pictures covering the spherical surface of the sky all around us we map it to a flat surface there are a number of ways to do this astronomers use the elliptical projection method because it maintains the relative size and distance between celestial objects you may have seen maps of the earth that use this technique we started with an image of the Milky Way constructed within the galaxy whenever you see any picture of the whole Milky Way from outside the galaxy remember that it is an artist's drawing besides the galaxy is so large that the distance one must travel to see it all is way too far here's what I mean if we assume that our field of view is 140 degrees we can use trigonometry to find the distance to a point where such a picture could be taken that point is approximately 300 and 1000 trillion kilometers 187 thousand trillion miles from the sun's current location Voyager one left on its journey in 1977 and is traveling at 61 thousand kilometers per hour for 38,000 miles per hour it has already gone 20 1.2 billion kilometers or thirteen point two billion miles if we aim it at the photographic point at its current velocity Voyager won't reach this point for another 562 million years in our chapter on the Milky Way we studied the nearby stars where parallax told us how far away they were we developed the HR diagram as a way to calculate luminosity based on temperature and spectral analysis we covered key standard candles such as Sofia din RR Lyrae variables as well as type 1a supernovae and we examine star clusters planetary nebula and emission nebula for their beauty and value as standard candles this distance ladder took us all the way across the galaxy in our next chapter we'll use all these techniques to move out into intergalactic space

Channel: David Butler

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Keywords: STEM, Astronomy, howfarawayisit, \How far away is it\, \David Butler\, \Miky Way\, Galaxy, \galactic center\, \black hole\, \galactic disk\, \galactic halo\, \spiral arms\, Hubble, Spitzer, Chandra, \galactic buldge\, Sagittarius, UCLA, Keck, \Max Plank\, singularity, gravity, Schwarzschild, \event horizon\, \accretion disk\, gamma-ray, Perseus, Orion, Norma, Sharpley, \Dark Matter\, ESO, telescope

Id: uVxrsJ5lZlQ

Length: 41min 31sec (2491 seconds)

Published: Mon Apr 22 2019