How did galaxies form?
How did galaxies form?
While observational tests on the details of cosmology proceed apace, astronomers are focusing on the mechanics of how matter came together in the early universe.The
key question is: Did galaxies, stars, or black holes come first?
The infant universe was a relatively uni* form sea of severai-thousand-degree gas and dark matter — the unseen, mysterious, and much more predominant form of matter that is indirectly known to exist because of its huge gravitational influence on galaxies, But how galaxies, stars, and black holes came together is the key to understanding the puzzle of the early universe.
Based on the microwave background data, astronomers think matter coalesced when the universe cooled and became "transparent" 380,000 years after the Big Bang, Structures like stars and galaxies formed about 1 billion years after the Big Bang. But exactly how matter clumped is open to future research.
Deciphering galaxy formation goes back to Walter Baade, who studied stars in galaxies
and tried to interpret how the galaxies formed. One of the premier researchers at California's Mt. Wilson Observatory in the 1950s, Baade discovered a group of stars around the Milky Way with few metals (elements heavier than hydrogen and helium). These stars are ancient, probably 11 billion years old. Metals thrown out into interstellar space by supernovae and other processes were eventually incorporated into younger stars in our galaxy.
Baade's discovery led to a model of galaxy formation in the 1960s nicknamed ELS, after Olin Eggen, Donald Lynden-Bell, and Allan Sandage.The ELS model says galaxies collapsed as single objects out of gas clouds. As the gas fell in by gravity, it first formed a spherical halo. As more gas coalesced, it began spinning and was enriched with metals, creating disks inside galaxies.
WHEEL IN THE SKY. Hoag's Object is a ring of hot, blue stars wheeling around a cooler, yellow nucleus. The whole galaxy measures about 120,000 light-years across, slightly larger than the Milky Way.
A different idea proposed recently is the merger theory. It could have been hatched on Wall Street when the merger buzz was about AOL with Time-Warner and Exxon with Mobil. But those mergers are minuscule compared with the unions of protogalaxies — blobs of gas without stars that gravitated together and merged to form galaxies in the early universe — and galaxies of various sizes merging later with other galaxies.
Indeed, over the past few years it has become increasingly clear that many galaxies, perhaps the vast majority, formed when small gas clouds came together, merging into larger and larger structures as time went on. This is called the bottom-up path.
"We don't really know which is the dominant path yet,"says John S.
A STAR-LADEN SOMBRERO. Beautifully formed spiral galaxies like the Sombrero Galaxy, seen from our line of sight as edge-on, coalesced as clumps of matter aggregated in the early universe.
WWW.ASTRONOMY.COM 15
NASA/THE HUBBLE HERITAGE TEAM
LOOPS AND BLOBS.
The disturbed galaxy NGC 1316 hints at its chaotic past. Probably the result of a head-on collision between two galaxies, NGC 1316 exhibits great turbulence in its core.
Gallagher III of the University of Wisconsin. "There's a strong theoretical prejudice to make small things and have them grow bigger, by having gas fall into them or capturing their neighbors.
But astronomers haven't yet proven that this is the main way it happens." However, circumstantial evidence is accumulating that mergers are the primary form of making galaxies.
Two "deep fields" imaged by Hubble show distant galaxies and reveal numerous blob-like objects that appear to be protogalaxies. These are likely the fragments that clung together to form the larger"normal” galaxies we see around us. Some galaxy
experts believe the Milky Way may have formed from the mergers of 100 or more small galaxies over time.
The question of whether galaxies came together as gas, then commenced forming stars, or whether stars formed from little pockets of gas and then aggregated into galaxies, is unclear. A third possibility is that black holes formed initially as dense pockets of matter.
They then swept up material around themselves, and galaxies formed from the surrounding gas that didn't get sucked in by the black holes.
Just 3 decades ago, astronomers thought black holes, regions of intense gravity from which no matter or light can escape, were mathematical oddities. But now, astronomers armed with large telescopes infer their presence in the centers of most large and medium-sized galaxies. They are the driving engines in distant quasars, highly energetic infant galaxies. Astronomers are now leaning toward a consensus that black holes inhabit the cores of most galaxies, but perhaps not the small ones. EE
While observational tests on the details of cosmology proceed apace, astronomers are focusing on the mechanics of how matter came together in the early universe.The
key question is: Did galaxies, stars, or black holes come first?
The infant universe was a relatively uni* form sea of severai-thousand-degree gas and dark matter — the unseen, mysterious, and much more predominant form of matter that is indirectly known to exist because of its huge gravitational influence on galaxies, But how galaxies, stars, and black holes came together is the key to understanding the puzzle of the early universe.
Based on the microwave background data, astronomers think matter coalesced when the universe cooled and became "transparent" 380,000 years after the Big Bang, Structures like stars and galaxies formed about 1 billion years after the Big Bang. But exactly how matter clumped is open to future research.
Deciphering galaxy formation goes back to Walter Baade, who studied stars in galaxies
and tried to interpret how the galaxies formed. One of the premier researchers at California's Mt. Wilson Observatory in the 1950s, Baade discovered a group of stars around the Milky Way with few metals (elements heavier than hydrogen and helium). These stars are ancient, probably 11 billion years old. Metals thrown out into interstellar space by supernovae and other processes were eventually incorporated into younger stars in our galaxy.
Baade's discovery led to a model of galaxy formation in the 1960s nicknamed ELS, after Olin Eggen, Donald Lynden-Bell, and Allan Sandage.The ELS model says galaxies collapsed as single objects out of gas clouds. As the gas fell in by gravity, it first formed a spherical halo. As more gas coalesced, it began spinning and was enriched with metals, creating disks inside galaxies.
WHEEL IN THE SKY. Hoag's Object is a ring of hot, blue stars wheeling around a cooler, yellow nucleus. The whole galaxy measures about 120,000 light-years across, slightly larger than the Milky Way.
A different idea proposed recently is the merger theory. It could have been hatched on Wall Street when the merger buzz was about AOL with Time-Warner and Exxon with Mobil. But those mergers are minuscule compared with the unions of protogalaxies — blobs of gas without stars that gravitated together and merged to form galaxies in the early universe — and galaxies of various sizes merging later with other galaxies.
Indeed, over the past few years it has become increasingly clear that many galaxies, perhaps the vast majority, formed when small gas clouds came together, merging into larger and larger structures as time went on. This is called the bottom-up path.
"We don't really know which is the dominant path yet,"says John S.
A STAR-LADEN SOMBRERO. Beautifully formed spiral galaxies like the Sombrero Galaxy, seen from our line of sight as edge-on, coalesced as clumps of matter aggregated in the early universe.
WWW.ASTRONOMY.COM 15
NASA/THE HUBBLE HERITAGE TEAM
LOOPS AND BLOBS.
The disturbed galaxy NGC 1316 hints at its chaotic past. Probably the result of a head-on collision between two galaxies, NGC 1316 exhibits great turbulence in its core.
Gallagher III of the University of Wisconsin. "There's a strong theoretical prejudice to make small things and have them grow bigger, by having gas fall into them or capturing their neighbors.
But astronomers haven't yet proven that this is the main way it happens." However, circumstantial evidence is accumulating that mergers are the primary form of making galaxies.
Two "deep fields" imaged by Hubble show distant galaxies and reveal numerous blob-like objects that appear to be protogalaxies. These are likely the fragments that clung together to form the larger"normal” galaxies we see around us. Some galaxy
experts believe the Milky Way may have formed from the mergers of 100 or more small galaxies over time.
The question of whether galaxies came together as gas, then commenced forming stars, or whether stars formed from little pockets of gas and then aggregated into galaxies, is unclear. A third possibility is that black holes formed initially as dense pockets of matter.
They then swept up material around themselves, and galaxies formed from the surrounding gas that didn't get sucked in by the black holes.
Just 3 decades ago, astronomers thought black holes, regions of intense gravity from which no matter or light can escape, were mathematical oddities. But now, astronomers armed with large telescopes infer their presence in the centers of most large and medium-sized galaxies. They are the driving engines in distant quasars, highly energetic infant galaxies. Astronomers are now leaning toward a consensus that black holes inhabit the cores of most galaxies, but perhaps not the small ones. EE
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