He had found that these spherically symmetrical groups of densely packed stars, as compared with the much closer open clusters , were unusual in their distribution. While the known open clusters are concentrated heavily in the bright belt of the Milky Way Galaxy, the globular clusters are for the most part absent from those areas except in the general direction of the constellation Sagittarius , where there is a concentration of faint globular clusters. Shapley assumed that this centre must also be the centre of the Milky Way Galaxy.
The globular clusters, he argued, form a giant skeleton around the disk of the Milky Way Galaxy, and the system is thus immensely larger than was previously thought, its total extent measuring nearly , light-years. Shapley succeeded in making the first reliable determination of the size of the Milky Way Galaxy largely by using Cepheids and RR Lyrae stars as distance indicators.
His approach was based on the P-L relation discovered by Leavitt and on the assumption that all these variables have the same P-L relation. As he saw it, this assumption was most likely true in the case of the RR Lyrae stars, because all variables of this type in any given globular cluster have the same apparent brightness. If all RR Lyrae variables have the same intrinsic brightness, then it follows that differences in apparent brightness must be due to different distances from Earth.
The final step in developing a procedure for determining the distances of variables was to calculate the distances of a handful of such stars by an independent method so as to enable calibration.
How do galaxies form?
Shapley could not make use of the trigonometric parallax method, since there are no variables close enough for direct distance measurement. However, he had recourse to a technique devised by the Danish astronomer Ejnar Hertzsprung that could determine distances to certain nearby field variables i. Shapley applied the zero point of the Cepheid distance scale to the globular clusters he had studied with the cm inch telescope at Mount Wilson. Some of these clusters contained RR Lyrae variables, and for these Shapley could calculate distances in a straightforward manner from the P-L relation.
For other globular clusters he made distance determinations, using a relationship that he discovered between the brightnesses of the RR Lyrae stars and the brightness of the brightest red stars.
For still others he made use of apparent diameters, which he found to be relatively uniform for clusters of known distance. The final result was a catalog of distances for 69 globular clusters, from which Shapley deduced his revolutionary model of the Milky Way Galaxy—one that not only significantly extended the limits of the galactic system but that also displaced the Sun from its centre to a location nearer its edge.
How could the existing stellar data be so wrong? The reason for the incorrectness of the star count methods was not learned until , when Lick Observatory astronomer Robert J. Trumpler , while studying open clusters, discovered that interstellar dust pervades the plane of the Milky Way Galaxy and obscures objects beyond only a few thousand light-years. This dust thus renders the centre of the system invisible optically and makes it appear that globular clusters and spiral nebulae avoid the band of the Milky Way.
He thought that, if the Milky Way Galaxy was so immense, then the spiral nebulae must lie within it. His conviction was reinforced by two lines of evidence. One of these has already been mentioned—the nova S Andromeda was so bright as to suggest that the Andromeda Nebula most certainly was only a few hundred light-years away.
During the early 20th century, one of the most important branches of astronomy was astrometry, the precise measurements of stellar positions and motions. Van Maanen was one of the leading experts in this field. Most of his determinations of stellar positions were accurate and have stood the test of time, but he made one serious and still poorly understood error when he pursued a problem tangential to his main interests.
In a series of papers published in the early s, van Maanen reported on his discovery and measurement of the rotation of spiral nebulae.
Forming the present-day spiral galaxies | ESA/Hubble
Using early plates taken by others at the cm inch Mount Wilson telescope as well as more recent ones taken about 10 years later, van Maanen measured the positions of several knotlike, nearly stellar images in the spiral arms of some of the largest-known spiral nebulae e. Comparing the positions, he found distinct changes indicative of a rotation of the spiral pattern against the background of surrounding field stars.
In each case, the rotation occurs in the sense that the spiral arms trail. The periods of rotation were all approximately , years. Angular motions were about 0. Shapley seized the van Maanen results as evidence that the spirals had to be nearby; otherwise, their true space velocities of rotation would have to be impossibly large. For example, if M51 is rotating at an apparent rate of 0. It is unclear just why such a crucial measurement went wrong. Van Maanen repeated the measures and obtained the same answer even after Hubble demonstrated the truth about the distances to the spirals.
However, subsequent workers, using the same plates, failed to find any rotation. Among the various hypotheses that science historians have proposed as an explanation for the error are two particularly reasonable ideas: Many of these plates had been taken in an unconventional manner by Ritchey, who swung the plate holder out of the field whenever the quality of the images was temporarily poor because of atmospheric turbulence.
The resulting plates appeared excellent, having been exposed only during times of very fine seeing; however, according to some interpretations, the images had a slight asymmetry that led to a very small displacement of star images compared with nonstellar images. Such an error could look like rotation if not recognized for what it really was.
In any case, the van Maanen rotation was accepted by many astronomers, including Shapley, and temporarily sidetracked progress toward recognizing the truth about galaxies. The nature of galaxies and scale of the universe were the subject of the Great Debate, a public program arranged in by the National Academy of Sciences at the Smithsonian Institution in Washington, D.
Featured were talks by Shapley and the aforementioned Heber Curtis , who were recognized as spokesmen for opposite views on the nature of spiral nebulae and the Milky Way Galaxy. This so-called debate has often been cited as an illustration of how revolutionary new concepts are assimilated by science. A careful reading of the documents involved suggests that, on the broader topic of the scale of the universe, both men were making incorrect conclusions but for the same reasons—namely, for being unable to accept and comprehend the incredibly large scale of things.
Forming the present-day spiral galaxies
Shapley correctly argued for an enormous Milky Way Galaxy on the basis of the P-L relation and the globular clusters, while Curtis incorrectly rejected these lines of evidence, advocating instead a small galactic system. Given a Milky Way Galaxy system of limited scale, Curtis could argue for and consider plausible the extragalactic nature of the spiral nebulae. Shapley, on the other hand, incorrectly rejected the island universe theory of the spirals i.
Furthermore, he put aside the apparent faint novae in M31, preferring to interpret S Andromeda as an ordinary nova, for otherwise that object would have been unbelievably luminous.
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Unfortunately for him, such phenomena—called supernovae —do in fact exist, as was realized a few years later. Curtis was willing to concede that there might be two classes of novae, yet, because he considered the Milky Way Galaxy to be small, he underestimated their differences. For Curtis, however, the matter provided less of a problem: The Shapley-Curtis debate took place near the end of the era of the single-galaxy universe.
During the early s Hubble detected 15 stars in the small, irregular cloudlike object NGC that varied in luminosity, and he suspected that they might include Cepheids. After considerable effort, he determined that 11 of them were in fact Cepheid variables, with properties indistinguishable from those of normal Cepheids in the Milky Way Galaxy and in the Magellanic Clouds. Their periods ranged from 12 to 64 days, and they were all very faint, much fainter than their Magellanic counterparts.
Nevertheless, they fit a P-L relation of the same nature as had been discovered by Leavitt. This calibration was wrong, as is now known, because of the confusion at that time over the nature of Cepheids. Today it is recognized that the actual distance is closer to 2,, light-years. Technically, then, this faint nebula can be considered the first recognized external galaxy. The Magellanic Clouds continued to be regarded simply as appendages to the Milky Way Galaxy, and the other bright nebulae, M31 and M33, were still being studied at the Mount Wilson Observatory.
Although Hubble announced his discovery of Cepheids in M31 at a meeting in , he did not complete his research and publish the results for this conspicuous spiral galaxy until five years later. While the Cepheids made it possible to determine the distance and nature of NGC , some of its other features corroborated the conclusion that it was a separate, distant galaxy. Hubble discovered within it five diffuse nebulae, which are glowing gaseous clouds composed mostly of ionized hydrogen, designated H II regions.
H stands for hydrogen and II indicates that most of it is ionized; H I, by contrast, signifies neutral hydrogen. Calculating their diameters, Hubble ascertained that the sizes of the diffuse nebulae were normal, similar to those of local examples of giant H II regions.
Five other diffuse objects discerned by Hubble were definitely not gaseous nebulae. It also includes Python docstrings from the code. You can generate your own copy of the documentation. Contribution of documentation is very welcome. To generate the documentation:. Galaxy Development This page collects resources that are helpful to development of various aspects of the Galaxy software.
Source code Source Code - Where is the source code and how to contribute. Contributing - Describes how to contribute to the core galaxy project. Documentation We use Sphinx to generate documentation on classes and methods and much more in the code base. To generate the documentation: Install Sphinx Go to the doc directory and run Sphinx with make html Install missing dependencies, and rerun Sphinx until you get working output. The API source code itself can be found here and is by its nature the most up-to-date and complete source of information.
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Historical survey of the study of galaxies
Refinery builds and runs workflows using bioblend The Galaxy IPython Docker Runtime leverages bioblend to interface with Galaxy's history. This school of thought argues that the resulting clumps were each the size of multiple galaxies, which in turn broke down into individual galaxies. These latter theories would explain why galaxies occur in clusters.
Either way -- bottom-up or top-down -- the resulting clumps then collapsed into protogalaxies consisting of dark matter and hydrogen gas. The hydrogen then fell toward the center of the protogalaxy while the dark matter remained as an outer halo surrounding it. Astronomers recognize two main galaxy types: These differences in shape, according to one theory, are due to star formation. Stars develop inside a protogalaxy when clouds of gas mix and collide. If the stars in a protogalaxy form all at once, then the mature galaxy essentially retains the spherical shape of the protogalaxy and becomes an elliptical galaxy.
Spiral galaxies occur when the stars inside the protogalaxy arise at different intervals. The gas between developing stars continues to collapse and the resulting gravitational differences manhandle the protogalaxy's stars, dust and gas.