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. 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. 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. 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. 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, 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.
The appearance and make-up of galaxies are shaped over billions of years by interactions with groups of stars and other galaxies. Astronomers' 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. 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.
Question 1: How are spiral galaxies described in the passage, and what distinguishes them from other types of galaxies?
Answer: Spiral galaxies are described as flat, blue-white disks of stars, gas, and dust with yellowish bulges in their centers. They are distinguished by their actively forming stars and comprise a large fraction of all the galaxies in the local universe.
Question 2: What are the two groups into which spiral galaxies are divided, and what characterizes each group?
Answer: Spiral galaxies are divided into two groups: normal spirals and barred spirals. Normal spirals have no central bar structure, while barred spirals have a central bar-shaped structure.
Question 3: How are irregular galaxies described, and why are they significant in astronomy?
Answer: Irregular galaxies are described as having very little dust and neither disk-like nor elliptical shapes. They are significant because they are abundant in the early universe and offer insights into galactic development before the formation of spirals and ellipticals.
Question 4: What are some examples of unusually shaped galaxies mentioned in the passage, and why are they considered significant?
Answer: Examples of unusually shaped galaxies mentioned in the passage include those in the process of colliding or interacting, and those with active nuclei ejecting jets of gas. They are considered significant because they represent transitional phases of galactic development and provide insights into galactic evolution.
Question 5: How do galaxies interact with each other, and what are the potential consequences of these interactions?
Answer: Galaxies interact and even collide due to their relatively close proximity in the universe. These interactions can distort a galaxy's shape and lead to new waves of star formation, supernovas, and even the creation of black holes.
Question 6: What is the fate of our Milky Way galaxy in the distant future, according to the passage?
Answer: Four billion years from now, our Milky Way galaxy is destined for a collision with the neighboring spiral Andromeda galaxy. Despite this collision, our Earth and solar system are not in danger of being destroyed.
