Why do virtually all the galaxies

The exact value of the Hubble constant is still somewhat uncertain, but is generally believed to be around 65 kilometers per second for every megaparsec in distance. So essentially, the Hubble constant reflects the rate at which the universe is expanding.

So to determine an object's distance, we only need to know its velocity. Velocity is measurable thanks to the Doppler shift. By taking the spectrum of a distant object, such as a galaxy, astronomers can see a shift in the lines of its spectrum and from this shift determine its velocity.

Putting this velocity into the Hubble equation, they determine the distance. Note that this method of determining distances is based on observation the shift in the spectrum and on a theory Hubble's Law. However, as the universe continued to expand, the regions of higher density acquired still more mass because they exerted a slightly larger than average gravitational force on surrounding material. If the inward pull of gravity was high enough, the denser individual regions ultimately stopped expanding.

In many regions the collapse was more rapid in one direction, so the concentrations of matter were not spherical but came to resemble giant clumps, pancakes, and rope-like filaments—each much larger than individual galaxies. These elongated clumps existed throughout the early universe, oriented in different directions and collapsing at different rates.

The clumps provided the framework for the large-scale filamentary and bubble-like structures that we see preserved in the universe today. Within the clumps, smaller structures formed first, then merged to build larger ones, like Lego pieces being put together one by one to create a giant Lego metropolis.

The first dense concentrations of matter that collapsed were the size of small dwarf galaxies or globular clusters—which helps explain why globular clusters are the oldest things in the Milky Way and most other galaxies.

These fragments then gradually assembled to build galaxies, galaxy clusters, and, ultimately, superclusters of galaxies. According to this picture, small galaxies and large star clusters first formed in the highest density regions of all—the filaments and nodes where the pancakes intersect—when the universe was about two percent of its current age.

Some stars may have formed even before the first star clusters and galaxies came into existence. Some galaxy-galaxy collisions triggered massive bursts of star formation, and some of these led to the formation of black holes. In that rich, crowded environment, black holes found constant food and grew in mass.

The development of massive black holes then triggered quasars and other active galactic nuclei whose powerful outflows of energy and matter shut off the star formation in their host galaxies.

The early universe must have been an exciting place! Clusters of galaxies then formed as individual galaxies congregated, drawn together by their mutual gravitational attraction Figure 4. First, a few galaxies came together to form groups, much like our own Local Group. Then the groups began combining to form clusters and, eventually, superclusters.

This model predicts that clusters and superclusters should still be in the process of gathering together, and observations do in fact suggest that clusters are still gathering up their flocks of galaxies and collecting more gas as it flows in along filaments.

In some instances we even see entire clusters of galaxies merging together. It appears that active galactic nuclei in general, and certainly quasars, were much more common during the early history of the universe than they are at present. See also: Quasar. The energy source that drives all of these diverse phenomena is released when matter falls into a supermassive black hole occupying the center of a galaxy Fig. These black holes are found to have masses in the rough range of a million to 10 billion solar masses.

The origin of supermassive black holes continues to be explored, though part of their aggregate bulk owes to the high density of material expected to accumulate at the center of a galaxy due to its gravitational field. Such a black hole will continue to accrete any gas that finds its way into the vicinity.

See also: Black hole. As this gas falls toward the black hole, its angular momentum causes it to take up a nearly circular orbit in a disk of material surrounding the black hole. This disk called an accretion disk slowly injects gas into the black hole. The black hole's enormous gravitational field compresses and heats the gas to very high temperatures, causing it to radiate. Intense jet radio emission, powered by energy released during infall onto the central black hole, is ejected along the minor axis of the accretion disk.

Depending on the viewing angle of the observer, the resulting morphology can account for a wide variety of active galactic nuclei, radio galaxies, and quasars.

See also: Angular momentum. A given mass of gas can release 10 or more times as much energy in this way as it could if it were used as nuclear fuel in a star or a reactor. A gas infall rate onto the central engine of several solar masses per year suffices to power the most luminous active galactic nuclei in the universe.

There is ample gas available in the interstellar medium to act as a fuel supply. Gas may be driven into the central regions of the galaxy, perhaps following a merger, where it can supply and activate the nucleus.

The mechanisms that convert the thermal radiation generated in this way into the nonthermal radiation and relativistic plasmas observed in active galactic nuclei are not well-known. See also: High-energy astrophysics. Virtually all large galaxies have supermassive black holes in their centers, though most are not active enough to qualify as active nuclei.

The feedback from these black holes into their wider galactic hosts has a profound influence on a galaxy's overall starmaking and evolution over cosmic history, a mechanism that remains the subject of significant ongoing research. To learn more about subscribing to AccessScience, or to request a no-risk trial of this award-winning scientific reference for your institution, fill in your information and a member of our Sales Team will contact you as soon as possible.

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Contributors include more than 10, highly qualified scientists and 46 Nobel Prize winners. Key Concepts Hide Galaxies are large aggregates of stars, gas, dust, planets, rocks, and dark matter, existing by the hundreds of billions throughout the observable universe.

Three forms of galaxies are spiral, elliptical, and irregular. Our home galaxy, the Milky Way, is a spiral. Other forms outside of these three also exist.

Galaxies typically occur in associations such as clusters and filaments that are separated by large voids, forming an overall universal architecture called the Cosmic Web. Virtually all large galaxies have supermassive black holes in their centers, the activity of which have a profound influence on the evolution of their galactic hosts. Cosmologists study galaxies to understand the large-scale structure and evolution of the universe.

Jee and H. Ford Johns Hopkins University ]. Composition Galaxies consist of stars, gas, dust, planets, rocks, and a mysterious substance called dark matter. See also: Dark matter Modern space telescopes and giant telescopes on the ground make it possible to determine the kinds of stars in a galaxy, the amount and composition of its gas, and the optical properties of its dust. See also: Gravitational lens ; Stellar evolution The mixture of star types varies depending on the evolutionary history of a particular galaxy.

See also: Star ; Stellar population Galaxies contain gas mostly un-ionized hydrogen in amounts varying from essentially zero up to a considerable fraction of their total mass.

See also: Interstellar matter Most, if not all, galaxies are dominated by dark matter, a form of matter whose nature is still unclear and whose existence has been confirmed only by gravitational effects on the surrounding visible matter. Form and size Scientists recognize three broad varieties of galaxies: spiral, elliptical, and irregular. Spiral galaxies A common form for a galaxy is to have is a disk with a central bulge.

See also: Milky Way Galaxy Fig. Credit: R. Elliptical galaxies Another common type of galaxy is an ellipsoid with radially decreasing brightness. This galaxy is a source of radio emission, and it has an active nucleus.

Credit: Canada-France-Hawaii Telescope. Irregular galaxies Other, rarer forms of galaxies include a transition class called S0 that has a disk superimposed on an otherwise elliptical type of light distribution, and an irregular Irr class composed of galaxies with chaotic forms Fig. Gas and dust streams surround it. Exotic galaxy types Some galaxies lie outside the normal range of morphologies.

Starbursters One of the more spectacular examples of exotic galaxies is the starbursters, galaxies that are presently manufacturing stars at an unusually vigorous rate. Low-surface-brightness galaxies Another type of exotic galaxy is the low-surface-brightness galaxy, which has such a low spatial density of stars that it is almost invisible. They are categorised by a single number derived from the equation:. In the Hubble classification , the roundest galaxies are labelled E0 and the flattest, E7.

In the Hubble Classification scheme, elliptical galaxies are allocated a number from 0 to 7 indicating their ellipticity. Unlike spiral galaxies , elliptical galaxies are not supported by rotation.

The orbits of the constituent stars are random and often very elongated, leading to a shape for the galaxy determined by the speed of the stars in each direction.