- There are four types of galaxies: elliptical, spiral, barred spiral, and irregular.
- Most galaxies are between 1 to 100 parsecs away and 3 to 300 light-years in diameter.
- Once estimated at 200 billion, the number of known galaxies is now believed to be 2 trillion.
The universe is vast, with new evidence arising attesting to its size as technology continues to evolve over time. Studies of the early universe begin with the Big Bang theory and assert that during recombination, the formation of hydrogen and helium some 300,000 years after the Big Bang, fluctuations in density created larger structures, causing baryonic matter to condense into cold, dark matter halos, which would then turn into galaxies.
Today, the observable universe is estimated to contain more than two trillion galaxies, an increase from the previously-believed 200 billion thought to exist based on images from the mid-1990s data reported from the Hubble Ultra Deep Field, which had revealed a host of faint galaxies. The change in number is based on data studied by a team at the University of Nottingham, in the United Kingdom, which discovered there were likely ten times more galaxies in the early universe than exist now. Many of these were small and dim, and were absorbed into larger nearby galaxies.
The new data was derived from deep-space Hubble images, which were transformed into 3-D images to allow for accurate measurements of galaxies as they existed throughout the history of the universe. Some observations were made based on mathematical models that allowed the team to conclude where galaxies may exist that are unable to be seen with modern technology. With this knowledge, the team determined that there must be up to 90% more galaxies that may be too faint or too far away to be seen with telescopes available today.
What are the different kinds of galaxies?
There are four main categories of galaxies, which are classified by their visual morphology: elliptical, spiral, spiral barred, or irregular. In some cases, galaxies are further separated into subcategories.
The most common type is the spiral galaxy, with as many as 77%, including the Andromeda galaxy. About two-thirds of these also have a bar structure and are therefore categorized as barred spiral galaxies, such as the Milky Way. Spiral galaxies are made of a flat, spinning disc and spiral arms, which can move at speeds of hundreds of miles per second. They can be subcategorized according to the tightness of their spirals and the size of the bulge at their center, often believed to contain a black hole.
Elliptical double-ringed are the rarest type of galaxy in the universe, with only about 0.1% of galaxies estimated to share this structure, often referred to as the Hoag-type galaxy. Regular elliptical galaxies are more common: about 10 to 15% of galaxies in the Virgo Supercluster, in which the Milky Way exists, are elliptical, notable for their dim appearance as compared to spiral galaxies. Elliptical galaxies are more elongated and spherical by nature and do not have the same central nucleus as the spiral-type. However, they tend to be brighter in the center and more dim toward the outer edges. The largest of all galaxies tend to be giant ellipticals, which can measure up to one million light years across and contain one trillion stars or more.
For the most part, irregular galaxies are quite small and all together account for about 25% of galaxies in the known universe. These are considered irregular due to their lack of distinct shape: no definitive central bulge, spiral arms or discs, but like other types of galaxies they are constantly moving. They tend to have been either elliptical or spiral but were distorted by the gravitational pull of larger, neighboring galaxies.
Despite the technological advancements of the last 30 years, there is still so much we do not know about the universe. Is there life on one of the many, many galaxies with whom we share this vast, expansive state? Perhaps we will learn the answer to that question within our lifetime.