Some galaxies are like the Milky Way, but some are quite different.
A galaxy is a vast system of dust, gas, dark matter, and a million to trillion stars that are held together by gravity. Supermassive black holes are also thought to be at the center of almost all large galaxies. In our own galaxy, the Milky Way, the Sun is one of about 100 to 400 billion stars revolving around Sagittarius A *, a supermassive black hole with a mass equal to four million suns.
The deeper we look at the universe, the more galaxies we see. A 2016 study estimated that there are two trillion or two million galaxies in the observable universe. Some of these remote systems are like our own Milky Way galaxy, others are quite different.
Before the 20th century, we did not know that galaxies other than the Milky Way existed; Earlier astronomers classified them as “nebulae” because they looked like obscure clouds. But in the 1920s, astronomer Edwin Hubble showed that the Andromeda “nebula” was a galaxy in its own right. Since it is so far away from us, light from Andromeda takes more than 2.5 million years to fill the gap. Despite the immense distance, Andromeda is the largest galaxy closest to our Milky Way, and it is so bright in the night sky that it is visible to the naked eye in the Northern Hemisphere.
In 1936, Hubble introduced a way to classify galaxies, dividing them into four main types: spiral galaxies, lenticular galaxies, elliptical galaxies, and irregular galaxies.
More than two-thirds of all observed galaxies are spiral galaxies. A spiral galaxy consists of a flat, rotating disk with a central bulge surrounded by a spiral arm. This rotational speed of hundreds of kilometers per second can cause disk material to take on a distinct spiral shape, such as a cosmic pinwheel. Our Milky Way, like other spiral galaxies, has a linear, starred bar at its center.
Elliptical galaxies are shaped according to their names: they are usually round but can extend one axis longer than the other so that some look like a cigar. The largest known galaxy in the universe – the giant elliptical galaxy – could hold one trillion stars and spread over two million light-years. Elliptical galaxies can also be small, in which case they are called dwarf elliptical galaxies.
The elliptical galaxy contains many old stars, but also a small amount of dust and other interstellar matter. Their stars orbit the galactic center, as in the disk of a spiral galaxy, but they do so in a more random direction. Some new stars are known to form in elliptical galaxies. They are common in galaxy clusters.
Lenticular galaxies, such as the iconic Sombrero galaxy, sit between elliptical and spiral galaxies. They are called “lenticular” because they are similar to lenses: like spiral galaxies, stars have a thin, rotating star disk and a central bulge, but they do not have a spiral arm. Like elliptical galaxies, they have little dust and interstellar matter, and they often appear to form in densely populated regions of space.
Galaxies that are not spiral, lenticular, or elliptical are called irregular galaxies. Irregular galaxies – such as the large and small Magellanic clouds that surround our Milky Way – are misplaced and do not have a distinct form, often under the gravitational influence of other nearby galaxies. They are full of gas and dust, which makes them a great nursery for new star formation.
Galactic clusters and aggregation
Some galaxies are seen alone or in pairs, but they are often part of larger groups known as clusters, clusters, and superclusters. Our Milky Way, for example, consists of a local group, a galaxy group that spans about 10 million light-years, including the Andromeda galaxy and its satellites. Both the local cluster and its neighboring galaxy cluster, the Virgo cluster, contain larger Virgo superclusters, densities of galaxies that span approximately 100 million light-years across. The virgin supercluster, instead, is a part of the Laniakia, an even larger supercluster of 100,000 galaxies that was defined by astronomers in 2014.
Clusters of galaxies often interact and even merge into a dynamic cosmic dance of gravitational interaction. When two galaxies collide and merge, gases can flow toward the galactic center, which can quickly trigger events such as star formation. Our own Milky Way will merge with the Andromeda Galaxy in about 4.5 billion years.
Astronomers have predicted that our home galaxy will merge with our neighboring galaxy Andromeda.
Since elliptical galaxies have older stars and fewer gases than spiral galaxies, the types of galaxies seem to represent part of the natural evolution: spiral galaxies interact and coalesce with age, losing their familiar shape and becoming elliptical galaxies. But astronomers are still working on specific issues, such as why elliptical galaxies follow certain patterns in brightness, size, and chemical composition.
The first stars in the universe ignited about 180 million years after the Big Bang, the explosive moment 13.8 billion years ago that marked the origin of the universe as we know it. Gravity sculpted the first galaxies in shape when the universe was 400 million years old or less than 3 percent of its current age.
Astronomers now think that almost all galaxies – with possible exceptions – are embedded in a huge halo of dark matter. Theoretical models further suggest that early in the universe, the huge tendrils of Dark Matter provided the gravitational scaffold normal matter needed to first merge into the galaxy.
However, there are still open questions about how galaxies form. Some believe that galaxies are made up of small clusters of about one million stars, known as globular clusters, while others believe that galaxies were formed first and then globular clusters. It is also difficult to determine how many stars in a given galaxy are formed from their own gas, formed in another galaxy, and later join the group.
By letting astronomers see the farthest boundaries of the universe এবং and early moments, instruments like NASA’s James Webb Space Telescope will help solve long-standing questions.