https://youtu.be/siE-DM-vmno?si=RmmfjQtpw_YeaWBU
我們迄今，連飛出太陽系，都還做不到，要想飛出銀河系，則要六點四萬光年 （如果可以以光速飛行），至於其他星系，則根本不可能飛到，因為宇宙膨脹，它們離開我們的速度，大于光速，意思是說，有朝一日，據說三十億年後，如果地球還存在，地球上居然還有人，那個人，舉頭看著夜空，除了銀河系，或銀河系所屬的超星系團，什麼都看不到，
The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which form part of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster （拉尼亞凱亞超星系團).
Why the observable universe is only a portion of the entire cosmos? (ChatGPT, 2024-10-24)
The observable universe is only a portion of the entire cosmos because the universe has a finite age, and light travels at a finite speed. This limits the distance from which we can receive information, effectively creating a boundary for what we can observe.
Reasons for the Observable Universe’s Limits
1. Finite Age of the Universe:
• The universe is about 13.8 billion years old, which means we can only see objects whose light has had enough time to reach us since the beginning of the universe. This creates a “horizon” beyond which we cannot observe anything because light from those regions has not yet reached Earth.
2. Expansion of the Universe:
• The universe has been expanding since the Big Bang, meaning that the most distant objects are now much farther away than their original positions when they emitted the light we currently observe. The current radius of the observable universe is about 46.5 billion light-years, despite the universe being 13.8 billion years old, due to the ongoing expansion.
3. Cosmic Inflation:
• During the early moments of the universe, an incredibly rapid expansion known as inflation occurred. This expansion stretched space so quickly that regions of the universe became widely separated, placing some areas far beyond our observational reach.
4. Event Horizons:
• Even though space extends beyond the observable limit, we cannot see beyond this horizon because any light or signal emitted from those regions will never reach us due to the universe’s accelerating expansion.
Beyond the Observable Universe
The actual cosmos could be vastly larger than the observable universe, potentially infinite. While we can only observe a finite region, there may be countless other regions, galaxies, and structures that exist beyond our observational horizon, which remain permanently inaccessible to us.
How many superclusters are there in the cosmos? (ChatGPT, 2024-10-24)
The observable universe contains about 10 million superclusters, based on current cosmological estimates. Superclusters are the largest known structures in the universe, consisting of groups of galaxy clusters and galaxy groups bound together by gravity.
Understanding Superclusters
• Superclusters are composed of many smaller structures, such as galaxy clusters, groups, and individual galaxies. They form a “cosmic web” structure, with galaxies arranged along filaments and separated by vast, empty voids.
• Some well-known superclusters besides the Laniakea Supercluster (which includes the Milky Way) are the Shapley Supercluster, Hercules Supercluster, and Coma Supercluster.
Mapping Challenges
While there are estimates of millions of superclusters, mapping them is challenging because their boundaries are not always well-defined, and they are interconnected by the larger cosmic web. Advanced surveys like the Sloan Digital Sky Survey (SDSS) and upcoming projects aim to provide more detailed maps of supercluster distribution in the universe.
Given that the observable universe is only a portion of the entire cosmos, the actual number of superclusters beyond what we can observe might be significantly higher.
How huge is the Laniakea Supercluster? （ChatGPT, 2024-10-24)
The Laniakea Supercluster, which includes our own Milky Way galaxy, is an enormous cosmic structure that spans approximately 520 million light-years across. It contains about 100,000 galaxies and has a total mass of roughly 100 million billion times the mass of the Sun (10^17 solar masses).
Structure and Significance
• The Laniakea Supercluster is part of a network of interconnected superclusters that form a “cosmic web” structure, with galaxies arranged along filaments and separated by vast voids.
• It was identified in 2014 by astronomers who mapped the motion of galaxies and found that Laniakea encompasses various previously recognized structures, such as the Virgo Cluster (where the Milky Way is located) and the Hydra-Centaurus Supercluster.
• The supercluster is named “Laniakea,” which means “immense heaven” in Hawaiian, reflecting its vast scale.
The gravitational center of Laniakea, which influences the motion of its galaxies, is known as the “Great Attractor,” a region with significant mass concentration that draws galaxies toward it.
How huge is the Virgo Supercluster? (ChatGPT, 2024-10-24)
The Virgo Supercluster is about 110 million light-years in diameter and contains roughly 1,500 to 2,000 galaxies. It is centered around the Virgo Cluster, a dense collection of galaxies located about 54 million light-years from the Milky Way. The Milky Way itself is situated on the outskirts of the Virgo Supercluster, within a smaller grouping of galaxies known as the Local Group.
Structure and Significance
• The Virgo Supercluster is just one part of a larger structure, which, as more detailed mapping has revealed, is actually part of the Laniakea Supercluster.
• Its core, the Virgo Cluster, has several thousand galaxies and serves as a gravitational focal point for galaxies within the supercluster.
The Virgo Supercluster’s large-scale structure plays an important role in shaping the motion and distribution of galaxies in the local universe, influencing the dynamics within Laniakea.
How huge the Local Group, to which the Milky Way galaxy belongs? (ChatGPT, 2024-10-24)
The Local Group, which includes the Milky Way, is about 10 million light-years in diameter and consists of more than 80 galaxies. It is a relatively small galaxy cluster compared to superclusters like the Virgo or Laniakea Superclusters.
Structure of the Local Group
• The Local Group is dominated by three large spiral galaxies: the Milky Way, the Andromeda Galaxy (M31), and the Triangulum Galaxy (M33). The Milky Way and Andromeda are the two largest members and are on a collision course, expected to merge in about 4.5 billion years.
• In addition to these large galaxies, the Local Group contains many dwarf galaxies that orbit the larger members. Some well-known examples are the Magellanic Clouds, which orbit the Milky Way, and M32 and M110, which are satellites of Andromeda.
Gravitational Dynamics
The Local Group is gravitationally bound, meaning the galaxies within it are not receding from each other due to cosmic expansion but are instead moving under their mutual gravitational attraction. It is part of the larger Virgo Supercluster, which in turn belongs to the Laniakea Supercluster.
How huge is the Milky Way galaxy? (ChatGPT, 2024-10-24)
The Milky Way galaxy is about 100,000 to 120,000 light-years in diameter and has a thickness of roughly 1,000 light-years in the disk region. It contains an estimated 100 to 400 billion stars, along with vast amounts of gas, dust, and dark matter.
Structure
• Galactic Disk: The Milky Way is a barred spiral galaxy, featuring a central bar-shaped region of stars surrounded by a disk that contains spiral arms where star formation is active.
• Galactic Bulge: At the center, there is a dense, spherical bulge of stars and a supermassive black hole known as Sagittarius A*, which has a mass of about 4 million times the mass of the Sun.
• Halo: Surrounding the disk is a roughly spherical halo that contains older stars, globular clusters, and dark matter, extending to about 300,000 light-years in diameter.
The Milky Way’s size and mass make it one of the two largest galaxies in the Local Group, along with the Andromeda Galaxy.
How huge is the Sun system, with its circling planets? (ChatGPT, 2024-10-24)
The Solar System, with the Sun at its center, spans a vast area, extending far beyond the orbits of the known planets. Its size can be considered at different scales depending on what outer boundary is used:
Planetary Scale
• The farthest major planet from the Sun is Neptune, whose orbit has an average radius of about 4.5 billion kilometers (2.8 billion miles), or roughly 30 astronomical units (AU), where 1 AU is the average distance from Earth to the Sun (about 150 million kilometers or 93 million miles).
Extending to the Kuiper Belt
• The Kuiper Belt, a region of icy objects that includes dwarf planets like Pluto, extends from about 30 AU to 50 AU from the Sun. This marks the outer edge of the region where most short-period comets originate.
The Heliosphere and Oort Cloud
• The heliosphere, the region dominated by the solar wind, extends out to about 100 to 120 AU, where the solar wind meets the interstellar medium at a boundary called the heliopause.
• Farther out lies the Oort Cloud, a hypothetical spherical shell of icy bodies that is believed to extend from about 2,000 AU to possibly 100,000 AU (0.3 to 1.5 light-years) from the Sun. This marks the true outer boundary of the Solar System’s influence, where objects are still loosely bound to the Sun’s gravity.
In total, the Solar System’s reach could be considered to extend up to 1 to 2 light-years in diameter if including the Oort Cloud, though most familiar objects lie within the inner few dozen AU.
How many Sun systems exist in the Milky Way galaxy? (ChatGPT, 2024-10-24)
The Milky Way galaxy is estimated to contain 100 to 400 billion star systems, with each star potentially having its own system of planets, moons, asteroids, and comets, similar to our Solar System.
Exoplanetary Systems
• Exoplanet surveys have shown that most stars in the Milky Way likely host planets. Studies suggest that, on average, each star could have at least one planet orbiting it. This means there could be hundreds of billions of planetary systems in the galaxy.
• Some star systems may have multiple planets, just like our Solar System, while others might have just a few or none.
Types of Stars and Systems
• The Milky Way contains a variety of stars, including Sun-like stars, red dwarfs, giants, and binary systems. The most common type is the red dwarf, which makes up about 70-80% of the stars in the galaxy. Many of these red dwarfs are also believed to host planets.
Given these estimates, it’s likely that the number of “Sun systems” or star systems with planets in the Milky Way reaches into the hundreds of billions.
Have we so far flied out of the Sun system? (ChatGPT, 2024-10-24)
Yes, we have sent spacecraft beyond the main region of the Solar System, though not entirely out of its influence. The most notable examples are Voyager 1 and Voyager 2, launched in 1977.
Voyager 1
• Voyager 1 became the first human-made object to enter interstellar space in 2012, when it crossed the heliopause, the boundary where the solar wind gives way to the interstellar medium. The heliopause is located roughly 120 astronomical units (AU) from the Sun.
• As of now, Voyager 1 is over 160 AU from the Sun, traveling farther into interstellar space.
Voyager 2
• Voyager 2 also reached interstellar space, crossing the heliopause in 2018, at a distance of about 119 AU from the Sun.
• It is currently over 135 AU from the Sun, still heading outward.
Still Inside the Solar System
Although both Voyager spacecraft have left the heliosphere, they are still technically within the Solar System. The Oort Cloud, a distant shell of icy objects that marks the outermost boundary of the Sun’s gravitational influence, extends up to 100,000 AU (1.5 light-years) from the Sun. It will take tens of thousands of years for the Voyagers to pass through this region.
In summary, while we have sent probes beyond the main planetary region and the heliosphere, they have not yet fully exited the Solar System, as defined by the extent of the Oort Cloud.
