19. Galaxies 2

Transcript: Galaxy colors are indicative of their stellar populations, that is the distribution of colors of the stars in the galaxy based on their masses and luminosities. The stellar populations of elliptical galaxies are generally old, and so elliptical galaxies appear red. The bulges of spiral galaxies are also older stellar populations and appear red. But in the spiral arms many young stars are found, and so spiral galaxies in their spiral disks are blue or white in the color of their stars. Irregular galaxies mostly contain young, blue and white stars, although with very deep imaging it is possible in some cases to see an older, redder stellar population.

Transcript: It sounds simple to measure the size of a galaxy, but it is not because galaxies do not have sharp edges. As you can see in any deep image of a galaxy, the brightness of the stars that the galaxy contains just fades away gradually until it disappears into the darkness of the night sky. Galaxies are finite objects bound by gravity, but it requires a convention to measure their total size. Typically astronomers measure their brightness within a large aperture such as to include most of the light of the galaxy. This is not a good measure of the typical size because of the slow fading away of the light into the sky brightness. The more conventional measure of size is an aperture that contains fifty percent of the total light of the galaxy, or some other fixed fraction. As long as astronomers use a fixed convention for defining size, they can easily compare the sizes of different galaxy types.

Transcript: The distances to galaxies are measured by a range of indicators, and the most distant galaxies are only measured using redshift as the distance indicator. Thus we need a model for the expansion of the universe, the Hubble expansion, to estimate the distance to the most distant galaxies. For nearby galaxies we can use individual stellar types, especially the most luminous dying stars, supernovae, to estimate distances according to the inverse square law and an assumption that those stars are similar in the distant object as they are within our own galaxy. Without a distance to a galaxy, it’s impossible to get a true measure of its size, its mass, or its luminosity.

Transcript: The galaxies of three fundamental types have a range in properties. Spiral galaxies range in mass from a billion to about a trillion times the mass of the Sun. Their mass to light ratios are in the range of two to ten, and their diameters are in the range of ten to thirty kiloparsecs. Stellar populations range from typical A type stars for Sc galaxies to later K type stars for Sa galaxies. Elliptical galaxies range to the most massive systems known, up to ten to the thirteen solar masses, and as little as a million solar masses. Mass to light ratios are in the range of ten to thirty, and diameters range from one all the way up to two hundred kiloparsecs, more than the distance from the Milky Way to the Magellanic Clouds. Stellar populations are typical of G or K stars, and the irregular galaxies generally are small, masses in the range a hundred million to ten billion times the mass of the Sun. Mass to light ratios of one to three, and sizes are smaller than the Milky Way, one to ten kiloparsecs. Spectral properties of the stellar populations are in the range of A to F stars.

Transcript: The dwarf companions of the Milky Way give astronomers the best opportunity of weighing our galaxy or measuring its total mass. Remember that the rotation curve of the Milky Way galaxy out to the edge of the visible disk indicates that the mass enclosed continues to rise out to the edge of the stellar distribution. There’s a large amount of dark matter, matter that does not make visible light and cannot be composed of normal stars. The dwarf companions to the Milky Way in their orbits give evidence directly of the mass of the Milky Way on scales of a hundred kiloparsecs, far beyond the visible disk. A delicate mass modeling of these motions reveals that the total mass of our galaxy is two times ten to the twelve solar masses indicating that more than ninety percent of the Milky Way’s mass is in a large extended halo which must be composed of dark matter.

Transcript: The Milky Way has a dozen or so dwarf companions in elliptical orbits around the center of our galaxy, and every few years a new example is discovered. Some of the more recent discoveries are particularly interesting because they represent dwarfs that are either headed for a passage through the plane of our galaxy or have already been through the plane. These dwarfs get disrupted or ripped apart by the strong gravity in the plane of our galaxy. This connects with the separate observation of star streams in the halo of the Milky Way, spaghetti-like configurations of stars that may represent disrupted dwarf galaxies. Both of these pieces of evidence combine to point to the fact that the Milky Way ahs been assembled hierarchically from many small pieces and that this assemblage still continues today.

Transcript: The Small Magellanic Cloud is sixty-three kiloparsecs away and about eight kiloparsecs across. It’s an irregular galaxy with a bar-like configuration of blue stars, most of which are a few billion years old. The Small Magellanic Cloud is connected to the Large Magellanic Cloud by a bridge of cold, diffuse hydrogen gas, originally detected by radio astronomers, called the Magellanic Stream. This extends from the Small Magellanic Cloud in an arc that loops behind the south galactic pole and in the other direction reaches down into the plane of the Milky Way. Both Magellanic Clouds are gravitationally bound to the Milky Way, and their orbits take them through the disk of the galaxy. So the Magellanic Stream is a trail of gas drawn out during such a passage about five hundred million years ago.

Transcript: When Magellan traveled round the world in the early sixteenth century, there was no bright star near the southern celestial pole, so for navigation he used two glowing patches of light which became known the Magellanic Clouds. But of course, they must have been known throughout prehistory and were undoubtedly the subject of myth and legend. They’re companions to the Milky Way galaxies, and they are extremely important in astronomy because their stellar nurseries are close enough to get a detailed view and identify individual examples of rare stellar populations such as RR Lyraes, Cepheids, novae, planetary nebulae, and more exotic variable stars.

Transcript: When Magellan traveled round the world in the early sixteenth century, there was no bright star near the southern celestial pole, so for navigation he used two glowing patches of light which became known the Magellanic Clouds. But of course, they must have been known throughout prehistory and were undoubtedly the subject of myth and legend. They’re companions to the Milky Way galaxies, and they are extremely important in astronomy because their stellar nurseries are close enough to get a detailed view and identify individual examples of rare stellar populations such as RR Lyraes, Cepheids, novae, planetary nebulae, and more exotic variable stars.

Transcript: Most of the dwarf galaxies in the Local Group are dwarf elliptical or dwarf spheroidal galaxies. They are in many ways like giant globular clusters, and they may in fact be related to globular clusters. They are found in swarms around the Milky Way, M31, and other luminous galaxies. Dwarf spheroidals have very little gas and dust and mostly old or intermediate aged stellar populations in the range of five to ten billion years, but some have had star formation more recently in the range of one to three billion years ago. Dwarf spheroidals have been located in the Local Group, and there are examples known beyond the local group. But they become very difficult to detect far from the Milky Way because they’re compact, and they tend to look just like red stars.

Transcript: We are very familiar with spiral galaxies because the Milky Way is a spiral, and M31 is a prominent nearby example. The main components of spiral galaxies are disks, bulges, and halos. Spirals may also have a bright nucleus or stellar bar. When viewed face-on, the spiral arms are clearly visible and outlined by young, hot, blue stars. When viewed edge-on, there’s an obscuring band of dust in the plane of the disk. On average spiral galaxies are scattered at random orientations through the universe, so purely face-on, purely edge-on, and intermediate inclinations are seen. There’s a gradual transition of properties in the classification sequence going from Sa to Sb to Sc corresponding to less tight spiral arms and less prominent bulges. The analogous sequence for a barred spiral galaxy is called SBa going to SBb going to SBc.

Transcript: A handful of galaxies in the Local Group are dwarf irregular galaxies. These are substantially smaller even than the Magellanic clouds which are about ten percent of the stellar mass of the Milky Way. The smallest dwarf irregulars need only be a few percent of the stellar mass of the Milky Way, but dwarf irregulars have a large fraction of their mass in gas, as much as fifty to eighty percent compared to five or ten percent for larger galaxies. Dwarf irregulars also have a large relative amount of dark matter. Dwarf irregulars are often lit up only by a single complex of star formation, and if that region had faded or was not active it would be hard to detect at all. And dwarf irregulars are essentially impossible to detect in the universe much more distant than the Local Group.

Transcript: Let’s explore a region of space centered on the Milky Way galaxy, a cube a million parsecs or a megaparsec on a side. That’s equivalent to three and a quarter million lightyears. We would find a collection of galaxies called the local group. There are two large galaxies in the Local Group, the Milky Way and Andromeda, M31, separated by three quarters of a million parsecs. M33 is near M31, and the remainder of the space contains only two dozen or so dwarfs clumped around the Milky Way and M31. In the analogy of a living room, the Milky Way and M31 would be like dinner plates twenty feet apart. The Magellanic Clouds would be two cotton balls within a foot of the Milky Way, and all the other dwarf galaxies would be other cotton balls clumped around the Milky Way and Andromeda.

Transcript: Peculiar galaxies do not form a neat class but rather are a ragtag collection of galaxies that do not fit into any of the standard morphological categories. Peculiar galaxies are usually highly disturbed with extended loops and tails that are rarely seen in irregular galaxies. Also, unlike irregular galaxies they can be very large and luminous. In general they show signs of interaction, and often they have nearby companions indicating to astronomers that galaxies should not always be considered in isolation.

Transcript: Irregular galaxies are non-symmetric. They have ragged or irregular shapes and are generally small, much smaller than the Milky Way in size. Some have spiral arms but without the overall degree of symmetry of normal spiral galaxies. Most irregulars have intense regions of star formation and substantial populations of young and hot blue stars. Two galaxies visible to the naked eye if you live in the southern hemisphere are irregular galaxies: the Large and Small Magellanic Clouds. The only other galaxy visible to the naked eye is the Andromeda galaxy, M31.

Transcript: Elliptical galaxies are smooth concentrations of mostly old, red stars. They have a large range in size from tiny dwarfs to huge galaxies three or four times the Milky Way’s size. Their true shapes range from spherical to highly squashed spheres, almost cigar shaped, and this sequence corresponds to the range from E0 to E7. Since they are three dimensional objects viewed in space and we only see a two dimensional projection, an E0 galaxy may look round but it need not be spherical. It could be an elongated object seen end-on. By contrast, an E7 galaxy must be a flat galaxy seen edge-on. The rotation of elliptical galaxies is too small to explain their flattening.

Transcript: Lenticular or S0 galaxies are a type of galaxy that emerged after Hubble did his work. They’re called lenticular after their lens-like appearance, and they’re intermediate in properties between spiral and elliptical galaxies. Lenticular galaxies have prominent bulges and disks without spiral arms. Their star distribution is essentially smooth. The halo is usually invisible, although as with all massive galaxies the halo contains most of the mass.

Transcript: Hubble used his careful photographic observations with the hundred-inch telescope at Mount Wilson to draw up a system of the classification for galaxies. He drew it as a tuning fork diagram. At the base of the tuning fork were the smooth, red, elliptical galaxies branching out into spirals either with or without bars. In addition to the presence or absence of bars, the spirals are distinguished by the prominence of the bulge and by the tightness and prominence of spiral arms. Hubble hoped that his sequence of galaxies in the tuning fork diagram would correspond to an evolutionary sequence. That has turned out not to be the case. But his morphological classification is still used today, and it’s a useful way of separating out the fundamental properties of galaxies.

Transcript: Galaxy morphology is the study of the shape and structure or just the appearance of galaxies. It’s fairly simple information; we don’t need to know the distance, the mass, the age, the size, or the redshift to do morphological work. Morphology cannot tell us everything about an object or about a class of objects, but it is a good starting point for classification and understanding. In the history of the species, the morphology of species and of fossils we used to develop the theory of natural selection, so the hope is that studies of galaxy morphology will lead to a physical underpinning that will explain the differences between galaxies.

Transcript: The enormous distances between stars and between the Milky Way and other galaxies give a sign of exactly how difficult it will be to travel between the stars or even beyond our galaxy. Light travel time corresponds to the time it takes for light, traveling at three hundred thousand kilometers per second, to go between objects in the Milky Way, but the best spacecraft technology that we have at the moment or can project would accelerate a probe only to about one percent of the velocity of light. So light travel times have to be multiplied by a hundred. Using this conversion it would take about four hundred years using current technology to reach Alpha Centauri, the nearest star. It would take twenty-six hundred years to reach Vega, thirteen thousand years to reach the Hyades cluster, and forty thousand years to reach the Pleiades cluster. Orion would take a hundred and fifty thousand years with current spacecraft technology. Until we develop means to travel at close to the speed of light with our probes, it will be very difficult to travel to the stars.

Transcript: In terrestrial scales the speed of light is so fast, three hundred thousand kilometers per second, that light appears to travel instantaneously. The light travel time to the Moon is just over a second. Light reaches us from the Sun in about eight minutes and crosses the solar system in five hours, but on the distance of the stars light travel times begin to become significant. To the nearest star Alpha Centauri light has taken four and a bit years to reach us, to Vega, twenty-six years. Light from the Hyades we see having traveled a hundred and thirty-four years, and from the Pleiades, four hundred and eleven years. Light from the Orion star formation region travels for fifteen hundred years before reaching us, and from the center of our galaxy, twenty-nine thousand years. The far edge of our galaxy is twenty-four thousand parsecs away or seventy-eight thousand lightyears. Beyond the Milky Way light travel times amount to millions of years. The Andromeda nebula is six hundred and seventy kiloparsecs away or two and a quarter million years ago. We see light from the Andromeda nebula which left before humans had even be come a separate species.

Transcript: As an analogy for the difficulty of measuring distances in the universe, consider a terrestrial situation. You’re standing on the roof of a building. You can measure the roof with a tape measure. That’s as direct as measuring the distance to planets with radar. To measure distances in the nearby streets you roughly know what the size of people are, so you use the people as distance indicators for measurements within a mile or so. Over distances of some miles you can probably use the size of buildings as a distance indicator, and going to the horizon, the appearance of the mountains gives you a rough idea of distance although you cannot resolve the individual trees on those mountains. And the presence of smog in the atmosphere, like obscuration in the Milky Way, could lead you to misestimate the distance. In all of these situations, systematic errors can arise, yet distance is fundamental to astronomy because without distance we cannot calculate the size, the mass, the luminosity, or the age of anything in the universe.

Transcript: Random errors are errors which improve with the quality or amount of data, so if we want to beat down random errors to a smaller value we simply acquire more observations or build a larger telescope. Systematic errors in astronomy are insidious, because they do not improve with more data or better data. Systematic errors are usually caused by an incomplete or imperfect physical understanding, in this case of our distance indicators themselves. Since the distance scale is a ladder of overlapping indicators, an error in a lower part of the ladder leads to a larger error when we move far from the Milky Way. The history of systematic errors in astronomy is sobering because people have made large mistakes in their calculation of distances. Shapley severely overestimated the size of the Milky Way because he miscalculated obscuration which tends to make an object look fainter than it really is. Hubble made a large error in his estimate of the distance to Andromeda nebula because he did not realize that there were two fundamentally different types of Cepheid variables, and even to the present day our understanding and our physical basis for distance indicators is not always as good as we would want.

Transcript: The role of errors is always important in astronomy but in no case more than in the distance scale. Random errors tend to grow as we move away from the Milky Way and even from the solar system. Within the solar system radar gives us accurate measure of the distance to nearby planets with a precision of ten to the minus four percent. The distance to the nearby stars using the parallax technique is accurate to about one percent. However, when we move beyond the Milky Way we are using distance indicators whose physical basis is not always totally reliable and where the data is limited by the brightness of the objects. So the error to the distances of nearby galaxies is usually in the range of five to ten percent, and for most distant galaxies it can be ten or twenty percent.

Transcript: The distance scale in astronomy is a set of measurements that define distances all the way from the solar system to the most remote galaxies. Conceptually it’s a pyramid with nearby methods being direct and fairly accurate while the errors accumulate and grow to the point where the measurement of the distance to galaxies is rarely more accurate than ten percent. Why are many techniques needed to establish a distance scale? In part, it’s due to the vastness of space. Any distance indicator that can be found near the Sun must also be found ten thousand times further away to be seen on the other side of the galaxy at which point it’s ten to the eight or a hundred million times fainter. Similarly, a distance indicator that can be seen on the other side of the galaxy must also be seen a thousand times further away to be seen in distant galaxies at which point it’s a million times fainter. Thus nearby distance indicators become too faint to be useful at some point, whereas the most luminous distance indicators are very rare locally, for example supernovae which only occur once every fifty years or so in the entire Milky Way.

Transcript: Any property of a star or galaxy that can be used to measure distance is called a distance indicator. In astronomy the best distance indicators have a clear physical or astrophysical basis and are not purely empirically determined. Within the solar system, the most direct technique possible is radar which uses our own measurement and knowledge of the speed of light and electromagnetic radiation. Also, Kepler’s laws are used within the solar system. To measure nearby stars the geometric method of parallax is used. For more distant stars main sequence fitting can be used. To go to the edge of the Milky Way and beyond we use luminous variable stars whose properties are well understood and related to their luminosities, the RR Lyrae stars and the Cepheid variables. Finally to go to the largest distances beyond the Milky Way the most luminous possible stellar moments are used, the peak brightness of a supernova, in particular Type I supernovae which represent the well regulated process whereby a white dwarf detonates when mass is transferred on it from a more massive member of a binary system.

Transcript: Hubble’s use of Cepheid variables to measure the distance to the Andromeda nebula is sufficiently important in the history of astronomy to study his logic carefully. He started by taking sequential observations on photographic plates over a period of months allowing him to identify variable stars in the Andromeda nebula. He knew that there was a universal period-luminosity relationship for Cepheid variables in the Milky Way. He then identified Cepheids with the same periods near the Sun whose distances were measured by other means and in the Andromeda nebula. These therefore have the same luminosity or absolute magnitude. He measured the apparent brightness difference between the Cepheid variables and the ones in the Milky Way. They were typically a million times fainter. By the inverse square law the M31 Cepheids must be the square root of a million, a thousand times further away. So if the local Cepheid is at a distance of two thousand light years the M31 Cepheid must be a thousand times further away, two million light years.

Transcript: The first step in understanding the nature of the nebulae involve resolving the nebulae. As newer, larger telescopes were built near the beginning of the twentieth century astronomers gained powerful tools to break up the diffuse light of the nebulae into the pinpoint light of many stars. Remember that the resolution of a telescope increases with its size, subject to the limitation of the Earth’s atmosphere. In the early twentieth century Edward Fath and other astronomers had noted the pinpoint light that composed the nebulae and, using a similar logic to the idea that the stars are much more distant than the Sun as given by their relative brightnesses and the inverse square law, deduced that if the pinpoint light in the nebulae were related to the starlight of the Milky Way, those stars must be extremely distant, far more than the distance to the edge of the Milky Way galaxy. This evidence, however impressive, is still indirect, and it took Edwin Hubble and his use of Cepheid variables to clench the fact that at least one of the nebulae, the Andromeda nebula, was remote from the Milky Way itself.

Transcript: Edwin Hubble was highly accomplished. A boxer, a Rhodes Scholar, a lawyer, all before turning his attention to astronomy. He joined the Mount Wilson Observatory just before the first World War where he was able to use their new hundred-inch telescope, the Hooker reflector, then the world’s largest. Hubble’s two main achievements in his career were monumental. First he demonstrated that many of the nebulae were distant systems of stars remote from the Milky Way galaxy. At a stroke he expanded the size of the known universe by a factor of hundreds. Second, he established that galaxies had redshifts that indicated the universal expansion, and this fact leads to the idea of the big bang model of the universe. The premier research facility in astronomy, the Hubble Space Telescope, is named after him.