All About Galaxies
By Ningthoujam Dipak Singh
Cosmofluencer (Season 2)
A galaxy is a vast cosmic assembly of stars, dust, gas, and dark matter held together by gravity. Galaxies range in size from small dwarf galaxies to massive ones with trillions of stars, exhibiting diverse shapes like spirals and ellipticals.
Some galaxies have supermassive black holes at their centre. They form groups and clusters, creating a cosmic neighbourhood. Most galaxies are ancient, with ages between 10 and 13.6 billion years, serving as a window into the universe’s past.
Formation & Evolution
Galaxies formed after the Big Bang as matter spread out, and gravity pulled them together. Two theories explain this process. The old “top-down” idea suggests big gas clouds collapsed into galaxies with early stars forming the core. The newer “bottom-up” theory includes dark matter and mergers. It postulates that small proto-galaxies, not necessarily galaxy-sized at first, grew by merging over time. This seems more in line with what we observe today, where diverse galaxies result from smaller ones joining forces and dark matter playing a crucial role in the cosmic dance of creation.
Galaxies changed over time through mergers, like cosmic collisions. Telescopes like the Hubble show us these mergers happening now. When we look at really far galaxies, they look smaller and less organised than closer ones. This means galaxies evolved from their early days to what we see today. It’s like looking at old pictures and noticing how things have changed over the years. The ongoing mergers and the differences in faraway galaxies suggest that the cosmic story of galaxies involves a lot of transformations, with collisions and mergers playing a big role in shaping the galaxies we see in the universe.
Primary Types of Galaxies
Spiral Galaxies
A spiral galaxy, like our Milky Way, features a spinning disk with arms extending from the centre. They are of two types: normal spirals and barred spirals (which feature a straight bar of stars connecting to the arms). How we see them depends on our angle of viewing from a face-on to a side-on perspective. Spiral galaxies, constituting about 60% of all galaxies, host the majority of stars. Despite their beauty, the mystery remains about how their arms stay in shape.
Elliptical Galaxies
Elliptical galaxies look smooth and round, and consist mostly of old stars. Unlike spirals with their arms, ellipticals lack detailed structures. They’re often found in groups of galaxies. People used to think they evolved into spiral galaxies, but now scientists believe ellipticals form when spiral galaxies collide and merge. The stars in elliptical galaxies are older and redder than those in spirals. Ellipticals have used up most of their star-forming material, so they don’t make many new stars. Scientists call them “red and dead” because of their low star formation and abundance of old stars.
Irregular Galaxies
Irregular galaxies come in unusual shapes like toothpicks or rings and vary in size. Astronomers believe their odd shapes are often caused by interactions with other galaxies, like when one galaxy’s gravity affects another. This can lead to distorted shapes or even the creation of new galaxies. Some irregular galaxies might be in between stages when spiral and elliptical galaxies collide. These galaxies have a mix of old and young stars and contain a lot of gas and dust, which are crucial for forming new stars. So, irregular galaxies are like cosmic mashups formed from galactic interactions, creating a mix of shapes and stellar generations.
Conclusion
There are billions of galaxies in the universe, each with its own tale as narrated by stars, gas, and dust. Powerful telescopes like the Hubble offer us a glimpse into faraway galaxies that may hold answers to the origin of the universe. The process of galaxy formation and mergers is ever-evolving and continues to date.
Even with the most sophisticated instruments we have only managed to scratch the surface. There’s much more to learn about in the fascinating world of galaxies. With every observation and discovery in the scientific realms, we inch closer to a better understanding of these enchanting behemoths. Do you think we will ever be able to know all the secrets they hold?
References
- Space.com | How galaxies form: Theories, variants and growth
- ESA Hubble | Spiral Galaxy
- NASA | Galaxies