Episodes
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...
Published 07/26/11
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...
Published 07/26/11
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’...
Published 07/26/11
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...
Published 07/26/11
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...
Published 07/26/11
Transcript: Why does the galaxy have spiral arms? Some analogies just don’t work. A garden sprinkler sends out spiral patterns of water, but in this case the water is moving radially rather than the circular orbits within the Milky Way. If you stir cream into your coffee it can give the illusion of a spiral pattern, but in the case of the Milky Way stars have made fifty orbits in the history of the Milky Way and so the spiral pattern would be hopelessly scrambled after that many windings. ...
Published 07/26/11
Transcript: Doppler mapping of the disk of the Milky Way by radio astronomers has revealed spiral arms. Spiral arms are regions of enhanced star density, enhanced star formation, and large amounts of gas and obscuring dust. We live on the inner edge of what’s called the Orion arm. The arm beyond us is called the Perseus arm, and inside us is the Sagittarius and then the Centaurus arms. These spiral arms coil outward from the galactic center, trailing the direction of rotation.
Published 07/26/11
Transcript: If stars near the Sun share the same general motion around the center of our galaxy, and if visible light can only penetrate a kiloparsec or so which is a small fraction of the size of the disk, how do we know the overall motions? Astronomers use the twenty-one centimeter line of neutral hydrogen which reveals the cold gas clouds where stars are forming. They also use the carbon monoxide line at millimeter wavelengths. Both of these techniques use long wavelength emission that...
Published 07/26/11
Transcript: Stars near the Sun have radial velocities measured by the Doppler shift in the range of ten to twenty kilometers per second. Because us and the stars around us are all moving together at similar speeds around the center of our galaxy the differential speeds are small. Further from the Sun the orbital speeds do vary. The Milky Way does not rotate like a solid object with a constant angular velocity at every radius from the center. Nor does it follow Kepler’s third law because...
Published 07/26/11
Transcript: Stars near the Sun have radial velocities measured by the Doppler shift in the range of ten to twenty kilometers per second. Because us and the stars around us are all moving together at similar speeds around the center of our galaxy the differential speeds are small. Further from the Sun the orbital speeds do vary. The Milky Way does not rotate like a solid object with a constant angular velocity at every radius from the center. Nor does it follow Kepler’s third law because...
Published 07/26/11
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...
Published 07/26/11
Transcript: Astronomers use a special set of coordinates to define the position of objects within the Milky Way galaxy. The galactic equator runs along the center of the Milky Way band. Galactic longitude, abbreviated by the small letter L, is the angular distance along the Milky Way with zero at the galactic center in the Sagittarius region. L equals ninety degrees is in the constellation of Cygnus near the top of the Northern Cross. L equals a hundred and eighty degrees, opposite to the...
Published 07/26/11
Transcript: The third component of the Milky Way is the bulge of our galaxy. This concentration of stars is centered on the galactic center but is much smaller than the halo. The stars in the bulge are mostly old and red, but many are younger than the halo stars with ages of only a few billion years. The bulge is best seen with infrared observations that can penetrate the obscuring dust that lies in the plane of the disk of our galaxy.
Published 07/26/11
Transcript: The halo of the Milky Way galaxy is traced by the globular clusters and by individual stars that are dim and which contain a far lower metal abundance than the Sun or stars in the solar neighborhood. The distribution of globular clusters on the plane of the sky gives a clue both to the shape of the galaxy and to the Sun’s position within that distribution. For example, if we were at the center of a spherical cloud of globular clusters, we would count equal numbers in each...
Published 07/26/11
Transcript: The disk of our galaxy is traced by the band of stars in the Milky Way and also by the open star clusters which contain mostly young stars. The concentration of stars in the Milky Way is greatest in the direction of the Sagittarius constellation in the southern sky. This represents the direction towards the center of our galaxy. In visible light we cannot see much further than about a thousand parsecs in the plane of the Milky Way due to the obscuring effects of dust. This...
Published 07/26/11
Transcript: The sky is not the same in all directions. The Milky Way is a band of stars and gas and obscuring dust that encircles the entire sky. Away from the direction of the Milky Way the stars are more sparsely scattered. Even the Milky Way is not the same in every direction. There’s a greater concentration of stars in the southern sky near the constellation of Sagittarius. A long time ago it was realized that the distribution of stars might give a sign as to the shape of the galaxy...
Published 07/26/11
Transcript: William Herschel was born in 1738 in Germany to a musical family. He was a professional musician himself. He deserted the German army during the seven years war and made his way to England where he became the organist in the cathedral in Bath. While studying music and building his own musical instruments he read books by Robert Smith on musical harmony, then on mathematics, and then on astronomy which attracted his interest in that subject. Working with his sister Caroline he...
Published 07/26/11
Transcript: In the eighteenth century astronomers began using larger and larger telescopes to map the galaxy to begin to learn the distribution of stars far from the Sun. The astronomer William Herschel was prominent in this effort. The discoverer of Uranus used his telescope to sweep the sky in parallel strips night after night. This was in the days before telescopes had motorized drives so Herschel just used the rotation of the Earth to have stars scan across his visual field, and he...
Published 07/26/11
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...
Published 07/26/11
Transcript: The ages of stars are derived from stellar models. The physics is complex so computers are used to simulate energy transport mechanisms. The details depend on heavy element abundance and on the mechanism for helium diffusion in the atmosphere of the stars. Thus there are uncertainties attached to the prediction of luminosity from stellar models. There are also uncertainties attached to the determination of luminosity from observation of stars. This can include the effect of...
Published 07/26/11
Transcript: The ages of stars are derived from stellar models. The physics is complex so computers are used to simulate energy transport mechanisms. The details depend on heavy element abundance and on the mechanism for helium diffusion in the atmosphere of the stars. Thus there are uncertainties attached to the prediction of luminosity from stellar models. There are also uncertainties attached to the determination of luminosity from observation of stars. This can include the effect of...
Published 07/26/11
Transcript: Globular clusters are the largest, most massive, and oldest groups of stars we know of in the Milky Way. Typical ages are eight to ten billion years, but some globular clusters are as young as five billion years old and some are as old as twelve billion years. The stars are dim and red as might be expected from an old stellar system where there’s no ongoing star formation. Most of the massive stars have long ago left the main sequence. Heavy element abundances are far lower...
Published 07/26/11
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...
Published 07/26/11
Transcript: The properties of stars in a star cluster as measured in the HR diagram change with time, and this can be a chronometer for measuring the age of groups of stars. The main sequence for a young star cluster is fully populated all the way up to the most massive, most luminous, and hottest stars. Remember that the main sequence runs from high luminosity and high temperature and high mass down to low luminosity, low temperature, and low mass. After ten to the seven years stars more...
Published 07/26/11
Transcript: The ages of stars are derived from stellar models. The physics is complex so computers are used to simulate energy transport mechanisms. The details depend on heavy element abundance and on the mechanism for helium diffusion in the atmosphere of the stars. Thus there are uncertainties attached to the prediction of luminosity from stellar models. There are also uncertainties attached to the determination of luminosity from observation of stars. This can include the effect of...
Published 07/26/11