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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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. ...

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...

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...

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,...

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...

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...

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...

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...