What are galaxies?
Galaxies are huge collections of stars, dust and gas. They usually contain several million to over a trillion stars and can range in size from a few thousand to several hundred thousand light-years across. There are hundreds of billions of galaxies in the Universe. Galaxies come in many different sizes, shapes and brightnesses and, like stars, are found alone, in pairs, or in larger groups called clusters. Galaxies are divided into three basic types: spirals, ellipticals and irregulars.
How many galaxies are in the Universe?
The number of galaxies in the universe is unknown, but it is thought to be somewhere between hundreds of billions and two trillion.
In one study in 2016, astronomers examined the Hubble eXtreme Deep Field (XDF) – a small patch of sky imaged very deeply in ultraviolet, visible and infrared light – and a total of approximately 5,500 galaxies were found. If you scale this number by the number of such images it would take to cover the entire sky, you find that there would be about 176 billion galaxies visible in observations like these. However this is only a lower limit on the number of galaxies in the universe as there may be even more that are too small, too faint, too distant and/or are hidden behind gas and dust. Astronomers used computer simulations to both try and match what is seen with telescopes, and estimate how many more galaxies there may be. In this way they came up with the estimate of two trillion.
A more recent study in 2021 used data from NASA’s New Horizon’s spacecraft to measure the total light seen in seven patches of sky. According to their analysis, the total number of galaxies contributing to this light is more likely in the hundreds of billions, rather than trillions.
Estimates of the total number of galaxies will continue to be refined as more powerful telescopes study the universe. NASA’s JWST observatory, launched December 25, 2021, will be able see even further back and image smaller and fainter distant galaxies. This will lead to a new estimate. Whatever the final number may be, it is worth noting that it will be incredibly large! There are a few hundred billion stars in our Milky Way galaxy so there’s at least one galaxy in the universe for every star in the Milky Way. It’s a big universe…
What is the Milky Way?
The Milky Way is the Galaxy in which we live. It is a spiral shaped galaxy that contains several hundred billion stars, including our Sun. It is about 100,000 light-years across and about 10,000 light-years thick. If you are at a place which has a very dark night sky, you can sometimes see the Milky Way as a thick band of stars in the sky. We live out in the suburbs of the Milky Way – not near the center, but not near the edge either.
What is an irregular galaxy?
Irregular galaxies have no particular shape. They are among the smallest galaxies and are full of gas and dust. Having a lot of gas and dust means that these galaxies have a lot of star formation going on within them. This can make them very bright. The Large and Small Magellanic Clouds are examples of irregular galaxies. They are two small galaxies which orbit around our own Milky Way Galaxy. About 20% of all galaxies are irregulars.
What is an elliptical galaxy?
Elliptical galaxies are shaped like ellipses (stretched circles). They are divided into eight types: E0-E7 depending on how elliptical they are. E0 ellipticals are nearly circular, while E7s are very stretched out. Elliptical galaxies are made up of mostly old stars, and do not have much gas and dust. There is very little new star formation in these galaxies. Elliptical galaxies also come in many sizes. The largest galaxies we see are ellipticals, but, elliptical galaxies can also be small. About 60% of all galaxies are ellipticals.
What is a spiral galaxy?
Spiral galaxies get their name from the shape of their disks. In a spiral galaxy, the stars, gas and dust are gathered in spiral arms that spread outward from the galaxy’s center. Spiral galaxys are divided into three main types depending on how tightly wound their spiral arms are: Sa, Sb and Sc. Sa galaxies have very tightly wound arms around a larger central nucleus. Sc galaxies have very loosely wound arms around a smaller nucleus. Sb’s are between, having moderately wound arms around an average sized nucleus. Spiral galaxies have a lot of gas, dust and newly forming stars. Since they have a lot of hot, young stars, they are often among the brightest galaxies in the universe. About 20% of all galaxies are spirals. We live in a spiral galaxy called the Milky Way.