Cosmic galaxies: a journey of exploration from the visible to the invisible
In the vast universe, galaxies are like scattered pearls, each with its own unique characteristics, and together constitute a magnificent picture of the universe. The diversity of these galaxies is not only reflected in their morphology, but also in their composition and evolution.
The diversity of galaxies
Astronomers broadly divide galaxies into several main types: elliptical galaxies, spiral galaxies, barred spiral galaxies, and irregular galaxies. Among them, spiral galaxies are probably the most familiar type, because the Milky Way galaxy we are in belongs to this category.
Spiral galaxies usually consist of a central nuclear sphere and a disk that rotates around it, dotted with graceful spiral arms, which often harbor large amounts of stars and interstellar matter.
Elliptical galaxies have a more rounded appearance, they often lack a clear structure, and the stars inside move in different directions. The stars in these galaxies are usually older and have less interstellar gas and dust, so the rate of new star formation is lower.
Irregular galaxies, as the name suggests, do not have a fixed shape. They may be formed as a result of interactions or mergers between galaxies, taking on complex and unique morphologies.
A striking example is the Wheel Galaxy, which formed as a result of the interaction of two galaxies. Located 5 billion light-years away from Earth, this galaxy looks like a giant cosmic wheel, with a bright core at its center, surrounded by a blue outer ring.
This unique structure is caused by a small galaxy passing through the center of a large galaxy, triggering an effect similar to ripples on the surface of the water, resulting in the peculiar morphology we see now.
The dynamic nature of the universe
However, galaxies do not exist statically. In fact, the entire universe is constantly moving and expanding. This phenomenon of cosmic expansion was first discovered by American astronomer Edwin Hubble, who observed that distant galaxies are moving away from us at an alarming rate.
This expansion not only affects the distance between galaxies, but also our ability to observe the universe. As the universe continues to expand, more and more galaxies are outfitting our "light cone" – a concept that describes the region of space-time where light is able to reach an observer.
When a galaxy extends beyond the light cone, it means that the light it emits will never reach the Earth, and we will never be able to observe it.
Astronomers estimate that about 98% of galaxies in the universe are now beyond Earth's light cone. This staggering number means that the universe we can observe is only a small part of the entire universe. Even more worrying is that over time, more and more galaxies will be beyond our observations, making the visible universe smaller and smaller.
The universe in numbers
So, how many galaxies are there in the universe? The answer to this question is constantly updated as our observation technology advances. Early estimates were relatively conservative, but with the use of more advanced telescopes, this number became larger.
In 2003, astronomers adopted an innovative method to estimate the number of galaxies. They divided the entire sky into 12.7 million small regions, then looked closely at one of the regions and counted the number of galaxies in it.
Assuming that the universe is homogeneous on a large scale, they multiplied this number by 12.7 million to get a preliminary estimate: there are about 2 trillion galaxies in the observable universe.
However, this figure may still be a far underestimate of the actual situation. Considering the expansion of the universe and the limitation of the speed of light, scientists believe that the universe we can observe may be only a small part of the real universe, perhaps only one in 250. If this assumption is true, then the total number of galaxies in the entire universe could be as high as 500 trillion.
This number is almost beyond human imagination. You know, even our own galaxy already contains about 200 billion stars. And in the entire universe, there may be 500 trillion such galaxies, each containing hundreds of millions of stars. This means that there may be more stars in the universe than all the grains of sand on Earth.
A Window to the Universe: The Evolution of Telescopes
Human understanding of the universe is inseparable from the advancement of observation tools. From the earliest optical telescopes to modern space telescopes, every technological leap has opened a new window for us to understand the universe.
Since its launch in 1990, the Hubble Space Telescope has been an important tool for human exploration of the universe. It is not disturbed by the Earth's atmosphere and is able to capture extremely distant and faint celestial bodies.
In 2015, the Hubble telescope took a stunning photo of the Andromeda Galaxy, a 1.5 billion pixel image showing hundreds of millions of stars in the galaxy, allowing us to see an outer galaxy in detail so clearly for the first time.
In 2021, the more advanced James Webb Space Telescope was launched, marking a new era in astronomical observation. The Webb telescope is located at Earth's second Lagrangian point, a position that avoids light pollution from the Earth and the Moon while maintaining a stable orbit. Its primary mirror is up to 6.5 meters in diameter, much larger than the Hubble telescope, and is able to capture fainter light.
The Webb telescope mainly observes light in the mid-infrared band, which allows it to "see through" dust clouds in the universe and observe objects that were previously invisible.
In 2022, it managed to photograph galaxies 13.1 billion light-years from Earth, almost touching the early post-Big Bang era. These observations not only allow us to see more galaxies, but also help us understand the state and evolution of the early universe.
With the continuous advancement of observation technology, our understanding of the universe is also deepening. However, each major discovery seems to remind us that we don't know enough about the universe. The vastness and complexity of the universe is far beyond our imagination, and this is what motivates us to continue to explore.
In this process of exploration, we are not only searching for the mysteries of the universe, but also thinking about the place of human beings in the universe. In the face of such a vast universe, we seem insignificant, but at the same time, we are incomparably precious as intelligent beings who can understand and explore this universe.
In the future, we will continue to challenge the limits of science and technology, expand our cognitive boundaries, and perhaps one day, we will be able to uncover more mysteries of the universe and understand our role and meaning in this vast universe.