A galaxy is a vast island of gas, dust and stars in an ocean of space. Typically, galaxies are millions of light-years apart. Galaxies are the building blocks of our universe. Their distribution isn’t random, as one might suppose. Instead, galaxies are along unimaginably long filaments across the universe, forming a cosmic web of star cities.
A galaxy can contain hundreds of billions of stars and be many thousands of light-years across. Our own galaxy, the Milky Way, is around 100,000 light-years in diameter. That’s about 587,900 trillion miles, or nearly a million trillion kilometers.
You’ve probably heard that our Sun is located in the Milky Way galaxy. But what is a galaxy anyway? The simple answer is that a galaxy is a collection of stars held together by their mutual galaxy. In other words, all the stars in a galaxy are kept together by the gravity of all the other stars (as well as the invisible, mysterious dark matter).
We know the Milky Way pretty well, so let’s consider it as a good example of a galaxy. The Milky Way is a spiral galaxy. It has a bright central core with a high density of stars, and then a flattened disk surrounding it – like a spinning record. Two spiral arms start just outside the core, and then spiral outward like a pinwheel to the outer edges of the galaxy. The Milky Way measures about 100,000 light-years across, and is thought to contain 200-400 billion stars.
Before Hubble’s study of galaxies, we believed that our galaxy was the only one in the universe. Astronomers thought that the smudges of light they saw through their telescopes were in fact nebulae within our own galaxy. However, Hubble discovered that these nebulae were galaxies. Additionally, it was Hubble who demonstrated, by measuring their velocities, that they lie at vast distances from us.
But the stars we can see are just a tiny fraction of the complete galaxy. It’s also surrounded by a vast halo of dark matter. This material is invisible, and doesn’t interact with regular matter or give off any kind of radiation that we can detect. But astronomers can measure its effects because it does exert a gravitational force on other matter. In fact, the Milky Way is made up of mostly dark matter. The stars account for about 580 billion solar masses, and the dark matter could be another 6 trillion solar masses.
The three types of galaxies are spiral, elliptical or irregular.
Galaxy sizes vary widely, ranging from very small to unbelievably enormous. Small dwarf galaxies contain about 100 million stars and giant galaxies contain more than a trillion stars.
These galaxies lie millions of light-years beyond the Milky Way, at distances so huge they appear tiny in all but the largest telescopes. Moreover, he demonstrated that, wherever he looked, galaxies were receding from us in all directions, and the further away they are, the faster they are receding. Thus, Hubble had discovered that the universe is expanding.
Our Milky Way is just an example of a galaxy, though. There is another type of galaxy called elliptical, and they’re even more common. The smallest galaxies in the Universe, the ultra-compact dwarf galaxies are only a little larger than a globular star cluster. But then the largest galaxies in the Universe also have this elliptical (egg-like) shape. A good example is the galaxy M87. It’s thought to have 2.7 trillion stars.
Stars are collected together into galaxies. Galaxies are collected together into groups of galaxies, and these groups are collected into clusters. The largest structures in the Universe are galaxy superclusters, which contain millions of galaxies and can measure hundreds of millions of light-years across.
The most common type of galaxy is a spiral galaxy. The Milky Way is a spiral galaxy. Spiral galaxies have majestic, sweeping arms, thousands of light years long, made up of millions upon millions of stars. Their spiral arms stand out because of bright stars, glowing gas and dust. Spiral galaxies are active with star formation.
Also, spiral galaxies have a bright center, made up of a dense concentration of stars, so tightly packed that from a distance the galaxy’s center looks like a solid ball. This ball of stars is known as the galactic bulge.
Also, there are two types of spiral galaxies. There are regular spirals and barred spirals. If the spiral has bars, they extend off the central bulge. Then, the spiral arms start at the end of the bar. Elliptical galaxies are the universe’s largest galaxies. In fact, giant elliptical galaxies can be about 300,000 light-years across. While, the dwarf elliptical galaxies – the most common elliptical – are only a few thousand light-years across. There are several shapes of elliptical galaxies, ranging from circular to football-shaped.
Overall, 1/3 of all galaxies are elliptical galaxies. Elliptical galaxies contain very little gas and dust – compared to a spiral or irregular galaxy – and they are no longer actively forming stars. The stars in elliptical galaxies are older stars and contain very few heavier elements.
Irregular shaped galaxies have all sorts of different shapes but they don’t look like a spiral or elliptical galaxy.
Galaxy are flying
Although most galaxies are flying apart from each other, those astronomically close to each other will be gravitationally bound to each other. Caught in an inexorable gravitational dance, eventually they merge, passing through each other over millions of years, eventually forming a single, amorphous elliptical galaxy. Gravity shockwaves compress huge clouds of interstellar gas and dust during such mergers, giving rise to new generations of stars.
The Milky Way is caught in such a gravitational embrace with M31, aka the Andromeda galaxy, which is 2 1/2 million light-years distant. Both galaxies are moving toward each other because of gravitational attraction: they will merge in about 6 billion years. However, both galaxies are surrounded by huge halos of gas which may extend for millions of light-years, and it was discovered that the halos of the Milky Way and M31 have already started to touch.