Galaxies
Galaxies are vast cosmic islands of stars, gas, dust, and dark matter held together by gravity. Hubble’s keen eye has revealed intricate details of the shapes, structures, and histories of galaxies — whether alone, as part of small groups, or within immense clusters. From supermassive black holes at galactic centers to giant bursts of star formation to titanic collisions between galaxies, these discoveries allow astronomers to probe the current properties of galaxies as well as examine how they formed and developed over time.
Classification of Galaxies
Astronomers classify galaxies into three major categories: elliptical, spiral and irregular. These galaxies span a wide range of sizes, from dwarf galaxies containing as few as 100 million stars to giant galaxies with more than a trillion stars.
Ellipticals, which account for about one-third of all galaxies, vary from nearly circular to very elongated. They possess comparatively little gas and dust, contain older stars and are not actively forming stars anymore. The largest and rarest of these, called giant ellipticals, are about 300,000 light-years across. Astronomers theorize that these are formed by the mergers of smaller galaxies. Much more common are dwarf ellipticals, which are only a few thousand light-years wide.
Spiral galaxies appear as flat, blue-white disks of stars, gas and dust with yellowish bulges in their centers. These galaxies are divided into two groups: normal spirals and barred spirals. In barred spirals, the bar of stars runs through the central bulge. The arms of barred spirals usually start at the end of the bar instead of from the bulge. Spirals are actively forming stars and comprise a large fraction of all the galaxies in the local universe.
Irregular galaxies, which have very little dust, are neither disk-like nor elliptical. Astronomers often see irregular galaxies as they peer deeply into the universe, which is equivalent to looking back in time. These galaxies are abundant in the early universe, before spirals and ellipticals developed.
Aside from these three classic categories, astronomers have also identified many unusually shaped galaxies that seem to be in a transitory phase of galactic development. These include those in the process of colliding or interacting, and those with active nuclei ejecting jets of gas.
Ellipticals, which account for about one-third of all galaxies, vary from nearly circular to very elongated. They possess comparatively little gas and dust, contain older stars and are not actively forming stars anymore. The largest and rarest of these, called giant ellipticals, are about 300,000 light-years across. Astronomers theorize that these are formed by the mergers of smaller galaxies. Much more common are dwarf ellipticals, which are only a few thousand light-years wide.
Spiral galaxies appear as flat, blue-white disks of stars, gas and dust with yellowish bulges in their centers. These galaxies are divided into two groups: normal spirals and barred spirals. In barred spirals, the bar of stars runs through the central bulge. The arms of barred spirals usually start at the end of the bar instead of from the bulge. Spirals are actively forming stars and comprise a large fraction of all the galaxies in the local universe.
Irregular galaxies, which have very little dust, are neither disk-like nor elliptical. Astronomers often see irregular galaxies as they peer deeply into the universe, which is equivalent to looking back in time. These galaxies are abundant in the early universe, before spirals and ellipticals developed.
Aside from these three classic categories, astronomers have also identified many unusually shaped galaxies that seem to be in a transitory phase of galactic development. These include those in the process of colliding or interacting, and those with active nuclei ejecting jets of gas.
How do Galaxies Form?
The appearance and make-up of galaxies are shaped over billions of years by interactions with groups of stars and other galaxies. While we don't know for certain how galaxies formed and took the many shapes that we presently see, we have some ideas about their origins and evolution. Using supercomputers, scientists can look back in time and simulate how a galaxy may have formed in the early universe and grown into what we see today.
Astronomer Edwin Hubble’s observations led to the idea that the universe is expanding. Scientists estimate the age of the universe at 13.8 billion years based on the rate of expansion. Because the deeper you look into space, the further you see back in time, we can conclude that galaxies several billions of light-years away formed fairly soon after the big bang. While most galaxies formed early, data indicates that some galaxies have formed within the past few billion years — relatively recently in cosmic terms.
The early universe was filled mainly with hydrogen and helium, with some areas slightly denser than others. These dense areas slightly slowed the universe’s expansion, allowing the hydrogen and helium to accumulate into small clouds swirling through space. Gravity caused the gas in these clouds to collapse and form the first generation of stars. These first stars rapidly burned out.
Gravity continued to collapse the clouds. As other clouds came close to each other, gravity sent them careening into one another and knitted the clouds into larger, spinning packs. As the clouds further collapsed, they became rotating disks, which amassed more gas and dust. New stars formed, creating extensive spiral arms filled with colonies of stars. Sprinkled along the periphery were globular clusters, along with a halo of gas, dust and dark matter.
While Hubble is unable to see the very first galaxies, it can track the development of galaxies over much of cosmic time. The series of Hubble Deep Field images and other deep surveys have uncovered galaxies at many different distances out in the universe, and therefore at many different times in their development. Farther galaxies, seen earlier in time, have less developed structure. Nearer galaxies, seen later in time, grow to resemble the familiar galaxy shapes we see in the local universe.
Astronomer Edwin Hubble’s observations led to the idea that the universe is expanding. Scientists estimate the age of the universe at 13.8 billion years based on the rate of expansion. Because the deeper you look into space, the further you see back in time, we can conclude that galaxies several billions of light-years away formed fairly soon after the big bang. While most galaxies formed early, data indicates that some galaxies have formed within the past few billion years — relatively recently in cosmic terms.
The early universe was filled mainly with hydrogen and helium, with some areas slightly denser than others. These dense areas slightly slowed the universe’s expansion, allowing the hydrogen and helium to accumulate into small clouds swirling through space. Gravity caused the gas in these clouds to collapse and form the first generation of stars. These first stars rapidly burned out.
Gravity continued to collapse the clouds. As other clouds came close to each other, gravity sent them careening into one another and knitted the clouds into larger, spinning packs. As the clouds further collapsed, they became rotating disks, which amassed more gas and dust. New stars formed, creating extensive spiral arms filled with colonies of stars. Sprinkled along the periphery were globular clusters, along with a halo of gas, dust and dark matter.
While Hubble is unable to see the very first galaxies, it can track the development of galaxies over much of cosmic time. The series of Hubble Deep Field images and other deep surveys have uncovered galaxies at many different distances out in the universe, and therefore at many different times in their development. Farther galaxies, seen earlier in time, have less developed structure. Nearer galaxies, seen later in time, grow to resemble the familiar galaxy shapes we see in the local universe.
Can Galaxies Collide?
While the distances between galaxies seem large, so too are galaxies’ diameters. Compared to stars, galaxies are relatively close to one another. They interact and even collide. When galaxies collide, they pass through each other; their stars don’t crash into each other because of the immense distances between them. However, gravitational interactions between colliding galaxies could create new waves of star formation, supernovas and even black holes. Collisions do distort a galaxy’s shape and computer models show that collisions between spiral galaxies can eventually make ellipticals.
Four billion years from now, our own Milky Way galaxy is destined for a collision with the neighboring spiral Andromeda galaxy. The Sun will likely be flung into a new region of our galaxy, but our Earth and solar system are in no danger of being destroyed. Andromeda, also known as M31, is now 2.5 million light-years away, but it is inexorably falling toward the Milky Way under the mutual pull of gravity between the two galaxies and the invisible dark matter that surrounds them both.
Computer simulations derived from Hubble data show that it will take an additional two billion years or more after the encounter for the interacting galaxies to completely merge under the tug of gravity. They will reshape into a single elliptical galaxy similar to the kind commonly seen in the local universe. Simulations show that our solar system will probably be tossed much farther from the galactic core than it is today.
To make matters more complicated, the third largest galaxy in the Local Group, the Triangulum galaxy or M33, will join in the collision and perhaps later merge with the Andromeda/Milky Way pair. There is a small chance that M33 will hit the Milky Way first.
Four billion years from now, our own Milky Way galaxy is destined for a collision with the neighboring spiral Andromeda galaxy. The Sun will likely be flung into a new region of our galaxy, but our Earth and solar system are in no danger of being destroyed. Andromeda, also known as M31, is now 2.5 million light-years away, but it is inexorably falling toward the Milky Way under the mutual pull of gravity between the two galaxies and the invisible dark matter that surrounds them both.
Computer simulations derived from Hubble data show that it will take an additional two billion years or more after the encounter for the interacting galaxies to completely merge under the tug of gravity. They will reshape into a single elliptical galaxy similar to the kind commonly seen in the local universe. Simulations show that our solar system will probably be tossed much farther from the galactic core than it is today.
To make matters more complicated, the third largest galaxy in the Local Group, the Triangulum galaxy or M33, will join in the collision and perhaps later merge with the Andromeda/Milky Way pair. There is a small chance that M33 will hit the Milky Way first.