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Physics LibreTexts

15.19 The Local Group



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The local group of galaxies surrounding the Milky Way. Click here for original source URL.

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Andromeda, our largest neighbor galaxy. Click here for original source URL.

To understand the neighborhood around the Milky Way, let's take a reconnaissance journey, describing the nearby galaxies in order of distance. Our journey will span a distance of 1000 kpc, or 3.25 million light years. The galaxies in this volume of space make up the Local Group. Our cosmic neighborhood teaches us a very important feature of the universe: galaxies are not randomly distributed through space; they tend to cluster. Most of the galaxies in the Local Group are clumped into two subgroups, those around the Milky Way and those around the Andromeda Galaxy. To visualize the distances, imagine the volume containing the Local Group to be a medium sized room. On this scale, the Milky Way would be the size of a dinner plate. The Magellanic Clouds would be like crumpled balls of cotton 8 or 10 centimeters across within a meter of the plate. A dozen or so galaxies of various shapes, from 1 to 15 centimeters across, would be scattered across the room. Andromeda, the nearest galaxy resembling the Milky Way, would be another dinner plate 7 meters away.

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The Large and Small Magellanic Clouds. Click here for original source URL.

Since prehistoric times, humans must have been aware of two glowing patches in the southern sky. Since there is no bright south polar star, the clouds helped navigators to mark the pole. Europeans heard them described during Magellan's around-the-world expedition in the early 16th century, so they came to be called the Magellanic Clouds. The clouds are in fact small galaxies moving in orbits around the Milky Way. The Large Magellanic Cloud is 50 kpc away, and the Small Magellanic Cloud 61 kpc, less than three times the distance to the far edge of our own galaxy.

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Central portion of the Tarantula Nebula. Click here for original source URL.

What kind of galaxies are the companions of ours, the Magellanic Clouds? They are several times smaller than the Milky Way, judging by star counts and measures of neutral hydrogen that show that they are only a few percent as massive as the Milky Way. The clouds do not show the Milky Way's beautiful spiral structure; they are irregular galaxies. Each contains a softly glowing, bar-like structure composed of stars. Somewhat off the end of the bar in the Large Magellanic Cloud is the spectacular Tarantula Nebula, also known as 30 Doradus. This luminous nebula can be seen with the naked eye. In fact, if it were moved to the distance of the Orion Nebula, it would fill the whole constellation of Orion and be bright enough to cast shadows on Earth! In its center is a cluster 60 pc in diameter containing thousands of massive, bluish super giant stars.

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Large Magellanic Cloud. Click here for original source URL.

We can best understand other galaxies and their evolution by considering their stellar populations. The Large Magellanic Cloud has a disk population similar to that found in our own spiral arm of the Milky Way. Faint-star photometry has also identified red giants and main-sequence stars. The bulk of the star formation in both clouds occurred 1 to 3 billion years ago, but it continues to the present day. There is little dust in the two clouds, except in the prominent young nebulae. The stellar populations are younger and more deficient in heavy elements than the solar neighborhood.

The two Magellanic Clouds are connected by a bridge of diffuse hydrogen gas called the Magellanic Stream. Australian radio astronomers showed that this long filament of HI also extends from the small cloud in an arc beyond the south galactic pole, and in the other direction it reaches into the plane of the Milky Way. This filament resembles the bridge between the two clouds themselves. The Magellanic Clouds are satellites of our own galaxy, gravitationally bound to the Milky Way. Their orbits are likely to take them through the Milky Way disk, and astronomers speculate that the Magellanic stream is a tail of gas drawn out during such an encounter about half a billion years ago.

The Magellanic Clouds are important to modern astronomers because they provide a cornucopia of stellar types at essentially identical distances. Use of the Magellanic Clouds as a stellar laboratory was given a boost by the explosion of Supernova 1987a in the Large Magellanic Cloud. The Hubble Space Telescope has been used to study the ring of hot gas that was thrown off by the death of this star, deriving an accurate distance of 50 kpc using simple geometric arguments. The door is now open to calibrate many distance indicators, using the rich stellar nursery of the Large Magellanic Cloud.

The Local Group contains a number of small stellar systems. Most of these are dwarf elliptical galaxies, and a few are irregulars. The elliptical or spheroidal galaxies resemble giant globular clusters. Dwarf elliptical are dominated by old halo stars and have little gas or dust. In most respects they are less impressive than our own giant spiral disk, with its chaotic clouds of gas and dust and regions of continuing star formation. However, they appear to be more active than globular clusters, whose stars are all around 10 to 13 billion years old. Analysis of H-R diagrams of the individual stars in dwarf elliptical indicates that some of their stars are relatively young, only 3 to 9 billion years old. Dwarf elliptical are the most common type of galaxy, but their diffuse light makes them difficult to detect. The satellite companions of the Milky Way are 5 to 30 times smaller and 1000 to 100,000 times less luminous than large spirals like M 31 and the Milky Way.

At 670 kpc, we encounter the first spiral galaxy truly comparable to the Milky Way, along with several of its smaller satellite galaxies. This galaxy must have been known since prehistoric times, since it’s visible to the naked eye as a hazy patch on a clear, dark night. It was first recorded in a star catalog by Arab astronomer al-Sufi in 964 A.D. Edwin Hubble used Cepheid variables in the Andromeda galaxy (M 31) to finally settle the debate over the nature of the so-called spiral nebulae. The Andromeda galaxy is slightly larger than the Milky Way and similar in stellar content. The naked eye sees it as a faint patch of light, but this is really only the brightest, innermost region, a few kilo parsecs across. Images made with large telescopes show that the spiral arms form a disk at least 30 kpc across. As with the Milky Way, there are globular clusters and a halo of HI gas reaching perhaps 100 kpc in diameter. The Andromeda Galaxy played an important role in the discovery of the two main stellar populations.

In the Local Group, galaxies are clearly made up of the light of many individual stars. Recall that stars in the neighborhood of the Sun are typically separated by about 1 parsec. That is a separation in three dimensions; when a galaxy disk is viewed face-on, projection effects will make the typical separation in the plane of the sky several times smaller, around 0.2 to 0.3 parsecs. We can predict the distance out to which individual stars in a galaxy could be resolved. The best telescope on the ground can resolve stars with a separation of about 1/2 second of arc. The small angle equation gives D = 206,265 d/a = 206,265 × 0.3/0.5 = 1.2 x 105pc, or 120 kpc. Hubble was able to resolve Cepheids at the larger distance of M 31 by working on the periphery of the galaxy and by using the fact that Cepheids are much brighter than the surrounding stars. The Hubble Space Telescope — named after the father of modern cosmology — has angular resolution that's ten times better than ground-based telescopes. This enables individual stars to be resolved in galaxies out to ten times the distance, which encompasses the entire Local Group.

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A parsec is the distance from the Sun to an astronomical object which has aparallax angle of one arcsecond. (1 AU and 1 pc are not to scale (1 pc = 206265 AU)). Click here for original source URL.

An imaginary voyage through the galactic neighborhood brings to mind the vast distances that separate galaxies. When astronomers gather information about the universe, they use electromagnetic radiation that travels at 300,000 km/s. This enormous number — the speed of light and all other forms of electromagnetic radiation — is nature's ultimate speed limit. Nothing can move faster. So far we have used parsecs and kilo parsecs to measure distances beyond the Solar System. The parsec is a unit based on the geometry of stellar parallax. We could equally well use the time it takes light to travel a particular distance. The natural unit is a light-year; the distance that light travels in a year. Remember that 1 parsec = 3.26 light-years (remember also that a light-year is a unit of distance, not time). The distance to Andromeda is therefore 670,000 × 3.26 = 2.8 x 106 light-years. When you go out on a dark night and look at Andromeda through a small telescope, the light you are seeing has taken nearly 3 million years to reach Earth!

The time that light takes to traverse the vast distances of the universe is called light travel time. As we explore regions more distant from the Milky Way, the light travel time increases. The further out in space we look, the further back in time we look. Large telescopes can be used as "time machines" to view parts of the universe that are remote in both space and time.