Let's look at the history of galaxies in astronomy.
The Greeks coined the term "galaxies kuklos" for "milky circle" when describing the Milky Way. The Milky Way was a faint band of light, but they had no idea what it was composed of.
When Galileo looked at the Milky Way with the first telescope, he determined that it was made up of numerous stars.
We've known for centuries that our solar system was located within the Milky Way because the Milky Way surrounds us. We can see it throughout the year in all parts of the sky, but it's brighter during the summer, when we're looking at the center of the galaxy. However, to astronomers in the 18th century and earlier, it wasn't clear that the Milky Way was a galaxy and not just a distribution of stars.
In the late 18th century, astronomers William and Caroline Herschel mapped the distances to stars in many directions. They determined that the Milky Way was a disk-like cloud of stars with the sunnear the center.
In 1781, Charles Messier cataloged various nebulae (faint patches of light) throughout the sky and classified several of them as spiral nebulae.
In the early 20th century, astronomer Harlow Shapely measured the distributions and locations of globular star clusters. He determined that the center of the Milky Way was 28,000 light years fromEarth, near the constellations of Sagittarius and Scorpio, and that the center was a bulge, rather than a flat area.
Shapely later argued that the spiral nebulae discovered by Messier were "island universes" or galaxies (retaining the Greek wording). However, another astronomer named Heber Curtis argued that spiral nebulae were merely part of the Milky Way. The debate raged on for years because astronomers needed larger, more powerful, telescopes to resolve the details.
In 1924, Edwin Hubble settled the debate. He used a large telescope (100-inch diameter, larger than ones that were available to Shapely and Curtis) at Mount Wilson in California and resolved that the spiral nebulae had structure and stars called Cepheid variables, like those in the Milky Way. (These stars change their brightness regularly, and the luminosity is directly related to the period of their brightness cycle.) Hubble used the light curves of the Cepheid variables to measure their distances from Earth and found that they were much farther away than the known limits of the Milky Way. Therefore, these spiral nebulae were indeed other galaxies outside our own.
There are still many mysteries surrounding galaxy formation, but on the next page we'll explain some of the best theories about it.
1923: Other galaxies exist
In the early 1900s, astronomers were debating the makeup of spiral nebulae — cloudy, spiral-shaped objects found throughout the night sky. Were they gas clouds located within our Milky Way galaxy, or were they vast groups of stars located far beyond our galaxy?
In 1919, American astronomer Edwin Hubble tackled the question. His keen astronomical knowledge was combined with a powerful tool – the Hooker telescope with its 100-inch mirror, on top of Mount Wilson in California. Hubble used the telescope’s resolution and light-gathering power to take a series of photographs of the great nebula in Andromeda. For the first time, the images revealed faint stars in the nebula.
Hubble now knew the Andromeda nebula was a collection of stars, but how far away was it? To find out, he used a known method for calculating distance based on very brightnvariable stars.
In 1923, Hubble found dozens of these variable stars in Andromeda, and determined their distance. He calculated that Andromeda must be at least 10 times farther away than the farthest stars in the Milky Way. The Andromeda nebula was really the Andromeda galaxy. This discovery implied that the other, even fainter, spirals were probably also galaxies even farther away.
Hubble published his work in 1929 and changed forever our view of the universe. Astronomers no longer thought our galaxy was the entire universe. Now they knew that the universe was composed of many, many galaxies.
Seven decades later, a telescope named in Hubble’s honor helped discover that the observable universe was truly vast, and contained nearly 100 billion galaxies.
Galaxy Formation
One of the greatest challenges facing astronomers today is understanding how galaxies form.
Observations by Hubble Space Telescope and ground-based instruments show that the first galaxies took shape as little as one billion years after the Big Bang, which probably took place about 13 billion to 14 billion years ago.
There are two leading theories to explain how the first galaxies formed. The truth may involve a bit of both ideas.
One says that galaxies were born when vast clouds of gas and dust collapsed under their own gravitational pull, allowing stars to form.
The other, which has gained strength in recent years, says the young universe contained many small "lumps" of matter, which clumped together to form galaxies. Hubble Space Telescope has photographed many such lumps, which may be the precursors to modern galaxies. According to this theory, most of the early large galaxies were spirals. But over time, many spirals merged to form ellipticals.
The galaxy-formation process has not stopped. Our universe continues to evolve. Small galaxies are frequently gobbled up by larger ones. The Milky Way may contain the remains of several smaller galaxies that it has swallowed during its long lifetime. The Milky Way is digesting at least two small galaxies even now, and may pull in others over the next few billion years.
Galaxy mergers happen fairly often. A large portion of the bright galaxies that we see today may have formed from the mergers of two or more smaller galaxies.
Mergers are common because the universe is crowded on the galactic distance scale. The disk of the Milky Way, for example, spans about 100,000 light-years; the nearest major galaxy, the great spiral in Andromeda, which is a little bigger than the Milky Way, is about 2.5 million light-years away. That means the distance between these two galaxies is only about 25 times greater than the sizes of the galaxies themselves. That doesn't leave a lot of "elbow room" for galaxies.
Galaxies are very massive, too, so their gravity is strong. When you crowd them together, the attraction can be so strong that two galaxies latch on to each other and don't let go. Eventually they merge, forming a single giant city of stars.
The largest galaxies are giant ellipticals. They look like eggs or footballs. They can be 10 times the Milky Way's size and contain more than a trillion stars. Such galaxies probably formed when two or more spirals, like the Milky Way, merged to form a single galaxy.
One bit of evidence supporting the merger theory is the large number of ellipticals in dense clusters of galaxies, where mergers must be common. Two giant ellipticals dominate the center of the densely packed Coma Cluster, for example. And the heart of the Virgo cluster contains three giant ellipticals that each span almost one million light-years.
Mergers can take anywhere from a few hundred million to a few billion years to complete. They can trigger intense bursts of new star formation, and even create gigantic black holes.
Stars Remain Unscathed
Galactic collisions rarely produce head-on wrecks between individual stars. Even when two galaxies ram together, the distance between stars is enormous. Yet stars can suffer ill effects from the collisions. They can be thrown into new orbits, or even thrown clear of their parent galaxies into intergalactic space.
While galactic collisions rarely destroy stars, they often create them. As vast clouds of gas and dust in merging galaxies slam together, they can create thousands or even millions of new stars.
• Are there any connections between the three types of galaxies?
• How do galaxies form? How do galaxies evolve?
P.S. You can find all the pictures/ movies either on NASA’s website, or in the textbook.
Elliptical Galaxies Spiral Galaxies Irregular Galaxies
First Galaxies Born Sooner After Big Bang Than Thought
Using the galaxy cluster Abell 383 (center) as a "gravitational lens," astronomers identified a galaxy so far away we see it as it was 950 million years after the Big Bang. It is visible as two tiny dots on either side of Abell 383. Distant objects seen through gravitational lenses are typically multiply imaged and heavily distorted.
Credit: NASA, ESA, J. Richard (CRAL), J.-P. Kneib (LAM). Acknowledgement: Marc Postman (STScI)
The first galaxies may have formed much earlier than thought, a new study suggests — just 200 million years or so after the universe's birth.
Using several different telescopes, astronomers have discovered a distant galaxy whose stars appear to have formed 200 million years after the Big Bang, the explosive event that brought the universe into being.
That's about 300 million years earlier than the oldest previously known galaxies. The universe itself is estimated to be 13.7 billion years old.
The finding could force astronomers to rethink what they seem to know about the cosmos and its early days, researchers said.[Photo of the newfound galaxy]
"This challenges theories of how soon galaxies formed and evolved in the first years of the universe," study lead author Johan Richard, of France's Center of Astronomical Research of Lyon, said in a statement. "It could even help solve the mystery of how the hydrogen fog that filled the early universe was cleared."
In a galaxy far, far away
The newfound galaxy is not the farthest-flung galaxy ever detected; several with younger stars have been spotted at greater distances, the researchers said.
They detected the galaxy through a cluster of galaxies called Abell 383, whose powerful gravity bends light rays almost as a magnifying glass would. The alignment of the newfound galaxy, Abell 383 and Earth amplified the galaxy's light, allowing the researchers to make detailed observations.
“Without this big lens in space, we could not study galaxies this faint with currently available observing facilities,” said study co-author Eiichi Egami of the University of Arizona. “Thanks to nature, we have this great opportunity to see our universe as it was eons ago.”
Using the Keck-2 telescope in Hawaii, the team then analyzed the galaxy's light, determining its redshift. "Redshift" measures the distance that an object has moved away from Earth as space expands, through observations of the stretching of the object's light to longer (or redder) wavelengths.
Light from objects moving away from us shifts to the red end of the spectrum as its wavelengths are stretched. The shift, known as the Doppler phenomenon, is experienced on Earth when sound waves from an ambulance change pitch when the ambulance moves toward you versus away from you.
Astronomers use redshift measurements to determine an object's distance, and by extension its age. The bigger the redshift, the greater the distance.
A diagram (not to scale) of how gravitational lensing works.
Credit: NASA, ESA & L. Calçada
The universe’s first galaxies
The newfound galaxy’s redshift turned out to be 6.027, which indicates that astronomers are viewing it the way it appeared when the universe was around 950 million years old.
However, the stars in the galaxy appear to be at least 750 million years old, meaning that they must have formed just 200 million years or so after the Big Bang.That’s a few hundred million years earlier than astronomers had thought galaxy formation first started. Other studies had detected farther-flung galaxies that seem to have formed about 500 million years after the universe’s birth.
"Our work confirms some earlier observations that had hinted at the presence of old stars in early galaxies," said co-author Dan Stark, of the University of Cambridge in the United Kingdom. "This suggests that the first galaxies have been around for a lot longer than previously thought.”
Richard and his team publish their results in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.
Early universe explained?
The discovery has implications beyond the question of when galaxies first formed, researchers said. For example, it may help explain how the universe became "reionized."
About 300,000 years after the Big Bang, the hydrogen in the universe was neutral, meaning it carried no charge. Over the course of the next1 billion years, however, something threw off enough radiation to ionize most of this hydrogen, splitting it into its constituent electrons and protons. This reionization made the hydrogen transparent to ultraviolet light, clearing the "fog" of the early universe.
Astronomers suspected that the radiation that powered this reionization must have come from galaxies. But researchers had not found enough old, distant candidate galaxies to provide the necessary radiation.
The new study may help solve this enigma, researchers said.
“It seems probable that there are in fact far more galaxies out there in the early universe than we previously estimated — it’s just that many galaxies are older and fainter, like the one we have just discovered,” said co-author Jean-Paul Kneib, of the Laboratoire d’Astrophysique de Marseille in France.
As of today, we can discover these galaxies only by observing them through massive clusters that act as cosmic telescopes, as Abell 383 does. NASA's next-generation James Webb Space Telescope, however, will specialize in high-resolution observations of distant, highly redshifted objects.
The James Webb Space Telescope, an infrared space observatory slated to launch no earlier than autumn 2015, could help solve this and other cosmic mysteries, researchers said.
Observations by Hubble Space Telescope and ground-based instruments show that the first galaxies took shape as little as one billion years after the Big Bang, which probably took place about 13 billion to 14 billion years ago.
There are two leading theories to explain how the first galaxies formed. The truth may involve a bit of both ideas.
One says that galaxies were born when vast clouds of gas and dust collapsed under their own gravitational pull, allowing stars to form.
The other, which has gained strength in recent years, says the young universe contained many small "lumps" of matter, which clumped together to form galaxies. Hubble Space Telescope has photographed many such lumps, which may be the precursors to modern galaxies. According to this theory, most of the early large galaxies were spirals. But over time, many spirals merged to form ellipticals.
The galaxy-formation process has not stopped. Our universe continues to evolve. Small galaxies are frequently gobbled up by larger ones. The Milky Way may contain the remains of several smaller galaxies that it has swallowed during its long lifetime. The Milky Way is digesting at least two small galaxies even now, and may pull in others over the next few billion years.
Galaxy mergers happen fairly often. A large portion of the bright galaxies that we see today may have formed from the mergers of two or more smaller galaxies.
Mergers are common because the universe is crowded on the galactic distance scale. The disk of the Milky Way, for example, spans about 100,000 light-years; the nearest major galaxy, the great spiral in Andromeda, which is a little bigger than the Milky Way, is about 2.5 million light-years away. That means the distance between these two galaxies is only about 25 times greater than the sizes of the galaxies themselves. That doesn't leave a lot of "elbow room" for galaxies.
Galaxies are very massive, too, so their gravity is strong. When you crowd them together, the attraction can be so strong that two galaxies latch on to each other and don't let go. Eventually they merge, forming a single giant city of stars.
The largest galaxies are giant ellipticals. They look like eggs or footballs. They can be 10 times the Milky Way's size and contain more than a trillion stars. Such galaxies probably formed when two or more spirals, like the Milky Way, merged to form a single galaxy.
One bit of evidence supporting the merger theory is the large number of ellipticals in dense clusters of galaxies, where mergers must be common. Two giant ellipticals dominate the center of the densely packed Coma Cluster, for example. And the heart of the Virgo cluster contains three giant ellipticals that each span almost one million light-years.
Mergers can take anywhere from a few hundred million to a few billion years to complete. They can trigger intense bursts of new star formation, and even create gigantic black holes.
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