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18. Galaxies

18. Galaxies

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Location of the Sun

Transcript: We view the Milky Way from a position within its enormous disk. The Milky Way disk is thirty thousand parsecs across, roughly four hundred parsecs thick, and it’s packed with young stars, gas clouds, obscuring dust, open clusters, and active star formation regions. The disk is imbedded within a spherical halo composed of galactic globular clusters and individual halo stars. The halo looks diffuse, but it actually contains most of the mass of the Milky Way galaxy. We’re located about eight thousand five hundred parsecs from the galactic center, and astronomers in talking about galactic distances use a different unit, the kiloparsec or a thousand parsecs. In these terms the galactic center is eight and a half kiloparsecs away, and the Milky Way is thirty kiloparsecs across. To give a sense of the scale of the Milky Way, if the galaxy were the United States, the stars would be individual microscopic specs separated by about a hundred yards each.

1min

26 Jul 2011

Rank #1

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The Milky Way

Transcript: The night sky blazes with light. Far from a city you can see six thousand stars, and long before the invention of the telescope people could plainly see a band of diffuse light that arches across the sky. Twenty-five hundred years ago Democritus, the Greek philosopher, attributed this glow to unresolved stars. It was called the Via Lactea or the Milky Way. Soon after the invention of the telescope Galileo confirmed Democritus’ idea and showed that the diffuse light is in fact comprised of the pinpoint light of many distant stars. A galaxy is a large collection of stars held together by gravity. The Milky Way galaxy contains the Sun, all the stars in the night sky, and billions more beyond.

26 Jul 2011

Rank #2

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Evolving HR Diagrams

Transcript: The HR diagram is a plot of stellar properties, luminosity and photospheric temperature. It’s a frozen snapshot in time, but over tens of millions to billions of years the main sequence population changes as stars exhaust their hydrogen and leave the main sequence to become giants, dwarfs, supernovae, and collapsed objects. This process can be used to measure age. Remember that when astronomers talk about stars, their position on the main sequence, and their movement on an HR diagram, they’re referring to a plot of stellar properties. The stars themselves are not moving in physical space. This is just the statistical way of discussing the properties of a set of stars.

26 Jul 2011

Rank #3

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Distance and Obscuration

Transcript: In the early part of the twentieth century, astronomers calculated the distances to stars by assuming that interstellar space was perfectly transparent. But eventually comparisons of distance to clusters in different directions in the sky yielded inconsistent results, and in 1930 Robert Trumpler showed that interstellar extinction or obscuration dims the light from all stars, groups, and clusters, that are larger than a distance of a few dozen parsecs. What this means is that the intensity of light falls off more rapidly than would be predicted by the inverse square law. We see a star as dimmer than it truly is, and we overestimate its distance. Without taking into account interstellar obscuration it’s impossible to correctly measure distances to stars, groups, and clusters, and map out the Milky Way galaxy.

26 Jul 2011

Rank #4

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Distances and Rare Stars

Transcript: Main sequence fitting can be applied in principle to any cluster. However, the rare variable stars like RR Lyraes and Cepheid variables are particularly valuable because the physics of their variations allows their luminosities to be estimated, and their luminosities allow them to be seen to large distances. RR Lyraes are a hundred times more luminous than the Sun, so they can be seen ten times further away than a Sun-like star could. Cepheid variables are ten thousand times more luminous than the Sun and so can be seen at distances a hundred times that of a Sun-like star. But we really need a large cluster to be able to detect even a few versions of these very rare stars, and that is a practical limitation.

26 Jul 2011

Rank #5

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Total Mass of the Galaxy

Transcript: The flat rotation curve of the Milky Way has profound implications for the mass distribution of our galaxy. In the solar system the circular orbits of the planet decline with increasing distance from the Sun in accordance with Kepler’s Law and with the idea that the Sun contains essentially all the mass in the solar system. In the Milky Way the situation is entirely different. The velocities are constant or rising with increasing distance, and the implied mass scales proportionally to distance. Out to the Sun’s radius the mass of the Milky Way is two times ten to the eleven solar masses. Out to the edge of the halo measured by halo stars, forty kiloparsecs, the mass is five times ten to the eleven, and from the motions of satellite dwarf companions to the Milky Way to a distance of a hundred kiloparsecs the mass is two times ten to the twelve solar masses, two trillion times the mass of the Sun. Thus most of the mass in the Milky Way galaxy is far out beyond the region of the disk, and most of it corresponds to material that does not emit light at any wavelength raising the issue of dark matter.

1min

26 Jul 2011

Rank #6

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Flat Rotation Curves

Transcript: Newton’s law of gravity gives astronomers a way of estimating the mass of something from the motions of objects within it. In the solar system or when an object has its mass concentrated in the center, the circular velocity declines with increasing distance from the center going as one over the square root of the distance. This is the characteristic of Keplerian orbits, but in the Milky Way there’s a flat rotation curve which means that the velocity is not declining but it’s flat or even rising with increasing distance. In this same formalism that means that the mass of the galaxy, estimated at different radii, would continue to increase. The high speed of stars in orbits in the disk of the galaxy is apparently driven by a large amount of mass in an extended halo of the Milky Way.

26 Jul 2011

Rank #7

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Mass of the Disk

Transcript: The motions of stars and gas within the disk of the galaxy can be used to estimate the mass of the Milky Way galaxy, but the Sun is one of billions of stars, some of which are interior to the Sun’s orbit and some of which are far beyond the Sun. So how is it possible to do this? Isaac Newton had a fundamental insight calculating that the motion of an orbiting object is controlled only by the mass within its orbit. This is obvious within the solar system where the planets’ motions are controlled by the Sun which sits at the center of the system. In the Milky Way it means that the Sun’s motions are only governed by the mass within the orbit of the Milky Way. All the regions outside the orbit of the Sun, the Sun does not feel those masses. Applying Newton’s calculation to the situation of the Milky Way disk yields a mass interior to the orbit of the Milky Way of two times ten to the forty-one kilograms, a hundred billion times the mass of the Sun.

1min

26 Jul 2011

Rank #8

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Rotation Curve of the Galaxy

Transcript: Maps of star and gas motions reveal the rotation curve of the Milky Way galaxy shown as a plot of orbital speed or circular velocity as a function of distance from the galactic center. In the Milky Way the speed is zero at the center and it rises rapidly to two hundred kilometers per second one kiloparsec out. Then there’s a slight decline and a steady rise to two hundred and twenty-five kilometers per second at the position of the Sun and a subsequent steady rise to two hundred and sixty kilometers per second fifteen kiloparsecs out from the center. Beyond that the motions are difficult to detect. This flat or slowly rising rotation curve is very different from Keplerian orbits which always decline steadily with distance from the center of mass.

26 Jul 2011

Rank #9

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Stochastic Star Formation

Transcript: Another idea to explain the existence of spiral arms is called the stochastic star formation theory. In this theory the star formation in one region triggers star formation in neighboring regions of the disk like a chain reaction. For up to a hundred million years a star formation region is lit up by young stars, and during this time differential rotation, the inner part of the region moving faster than the outer part, shears the star formation region into the segment of a spiral. Overall a spiral pattern is seen, but the pattern is transient because different stars are coming and going and being born and dying over the long period of time of the lifetime of the Milky Way galaxy.

26 Jul 2011

Rank #10