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  • Historical survey of the study of galaxies
    • Early observations and conceptions
  • Types of galaxies
    • Principal schemes of classification
      • Spiral galaxies
  • The external galaxies
    • Physical properties of external galaxies
    • Structure
    • Clusters of galaxies
      • Types of clusters
    • Extragalactic radio and X-ray sources
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Articles from Britannica Encyclopedias for elementary and high school students.
Paul W. Hodge
Professor Emeritus, Department of Astronomy, University of Washington, Seattle. Author of The Andromeda Galaxy; Higher then Everest: An Adventurer's Guide to the Solar System; and others.

Galaxy, any of the systems of stars and interstellar matter that make up the universe. Many such assemblages are so enormous that they contain hundreds of billions of stars.

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Nature has provided an immensely varied array of galaxies, ranging from faint, diffuse dwarf objects to brilliant spiral-shaped giants. Virtually all galaxies appear to have been formed soon after the universe began, and they pervade space, even into the depths of the farthest reaches penetrated by powerful modern telescopes. Galaxies usually exist in clusters, some of which in turn are grouped into larger clusters that measure hundreds of millions of light-years across. (A light-year is the distance traversed by light in one year, traveling at a velocity of 300,000 km per second [km/sec], or 650,000,000 miles per hour.) These so-called superclusters are separated by nearly empty voids, and this causes the gross structure of the universe to look somewhat like a network of sheets and chains of galaxies.

Galaxies differ from one another in shape, with variations resulting from the way in which the systems were formed and subsequently evolved. Galaxies are extremely varied not only in structure but also in the amount of activity observed. Some are the sites of vigorous star formation, with its attendant glowing gas and clouds of dust and molecular complexes. Others, by contrast, are quiescent, having long ago ceased to form new stars. Perhaps the most conspicuous activity in galaxies occurs in their nuclei, where evidence suggests that in many cases supermassive objects—probably black holes—lurk. These central black holes apparently formed several billion years ago; they are now observed forming in galaxies at large distances (and, therefore, because of the time it takes light to travel to Earth, at times in the far distant past) as brilliant objects called quasars.


The existence of galaxies was not recognized until the early 20th century. Since then, however, galaxies have become one of the focal points of astronomical investigation. The notable developments and achievements in the study of galaxies are surveyed here. Included in the discussion are the external galaxies (i.e., those lying outside the Milky Way Galaxy, the local galaxy to which the Sun and Earth belong), their distribution in clusters and superclusters, and the evolution of galaxies and quasars. For further details on the Milky Way Galaxy, seeMilky Way Galaxy. For specifics about the components of galaxies, seestar and nebula.

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Historical survey of the study of galaxies

Early observations and conceptions

The dispute over the nature of what were once termed spiral nebulae stands as one of the most significant in the development of astronomy. On this dispute hinged the question of the magnitude of the universe: were we confined to a single, limited stellar system that lay embedded alone in empty space, or was our Milky Way Galaxy just one of millions of galaxies that pervaded space, stretching beyond the vast distances probed by our most powerful telescopes? How this question arose, and how it was resolved, is an important element in the development of our prevailing view of the universe.

Up until 1925, spiral nebulae and their related forms had uncertain status. Some scientists, notably Heber D. Curtis of the United States and Knut Lundmark of Sweden, argued that they might be remote aggregates of stars similar in size to the Milky Way Galaxy. Centuries earlier the German philosopher Immanuel Kant, among others, had suggested much the same idea, but that was long before the tools were available to actually measure distances and thus prove it. During the early 1920s astronomers were divided. Although some deduced that spiral nebulae were actually extragalactic star systems, there was evidence that convinced many that such nebulae were local clouds of material, possibly new solar systems in the process of forming.

The problem of the Magellanic Clouds

It is now known that the nearest external galaxies are the Magellanic Clouds, two patchy irregular objects visible in the skies of the Southern Hemisphere. For years, most experts who regarded the Magellanic Clouds as portions of the Milky Way Galaxy system separated from the main stream could not study them because of their position. (Both Magellanic Clouds are too far south to be seen from most northern latitudes.) Moreover, the irregular shapes of the objects and their numerous hot blue stars, star clusters, and gas clouds did indeed make them resemble the southern Milky Way Galaxy.

The American astronomer Harlow Shapley, noted for his far-reaching work on the size and structure of the Milky Way Galaxy, was one of the first to appreciate the importance of the Magellanic Clouds in terms of the nature of spiral nebulae. To gauge the distance of the Clouds, he made use of the period-luminosity (P-L) relation discovered by Henrietta Leavitt of the Harvard College Observatory. In 1912 Leavitt had found that there was a close correlation between the periods of pulsation (variations in light) and the luminosities (intrinsic, or absolute, brightnesses) of a class of stars called Cepheid variables in the Small Magellanic Cloud. Leavitt’s discovery, however, was of little practical value until Shapley worked out a calibration of the absolute brightnesses of pulsating stars closely analogous to the Cepheids, the so-called RR Lyrae variables. With this quantified form of the P-L relation, he was able to calculate the distances to the Magellanic Clouds, determining that they were about 75,000 light-years from Earth. The significance of the Clouds, however, continued to elude scientists of the time. For them, these objects still seemed to be anomalous, irregular patches of the Milky Way Galaxy, farther away than initially thought but not sufficient to settle the question of the nature of the universe.

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Marketing share of iPhone is increasing sharply, but Android is still the commonest cell phone system. The Android layout design tool is, of course, still the most important thing in product design. In fact, the layout design has already been decided when the wireframe design is decided. Well, which are the best wireframe tools that we should notice now?

1. Balsamiq

If you are after the static sketch product wireframe design, Balsamiq should be one of your choices. Everything in this wireframing tool is in sketch style. I believe this kind of style would give you more ideas and inspirations while designing. As a great Android layout design tool, it is the one you won't miss out.

2. Mockplus

Mockplus is made for prototyping'faster, smarter and easier'. But, you may have no idea that Mockplus also has the sketch style, although that makes the interactions look really strange. After all, Mockplus is more about using solid line to make a clear wireframe. It tries to keep it as simple as it could, when it presents what designers are thinking about. What's more, at the first step of creating a wireframe design in Mockplus, it will require you to choose the project type of your design. This is convenient and useful as an Android layout design tool. And, you can find many Android layout design template in their tutorial.

3. Adobe XD

The market of prototyping and wireframing is getting bigger, Adobe, as a great company in the design industry, is not going to wait. Adobe XD is the best example. You can import your design to this tool, and create your new wireframe. But, the layout design in Android of Adobe XD looks simpler, because it doesn't provide packaged components such as 'Tree' like Mockplus.

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4. Sketch

Speaking of Android layout design tool, we must mention this one, Sketch. Since Sketch launched on Mac, it took the market share of design and wireframing tools aggressively and rapidly. Sketch keeps trying to be the best at what it can do, and, at the same time, it supports many kinds of plug-ins. This makes it more functional.

5. PS

One of the important points of Android layout design is image design. Hence, we must list PS here as an option. Because, sometimes, PS is not just a simple tool, it has already become a skill that companies requirewhen it comes to layout design in Android. In terms of the user's number, PS is the champion, even though you don't count the users who are using the pirates.

There are way more than 5 Android layout design tools, but in my opinion, these five tools are the most typical ones. They have almost covered every aspect of an Android layout design tool, and they are relatively easy to use. And, what do you think? Which one is the best in your opinion?

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