Galaxies are collections of stars, gas, dust and dark matter throughout the universe. Their size and scale can vary greatly. The Milky Way we live in has 200 billion stars, which puts it about halfway between the largest galaxies and the smaller types, such as dwarf galaxies.
Like people, these galaxies are shaped by their environment, which explains the different shapes we see, says Karen Masters. She is an astrophysicist at Haverford College and a project scientist at Galaxy Zoo, a citizen science project that classifies different types of galaxies.
“There’s a lot of diversity in the universe,” Masters tells Inverse. Spiral galaxies have a round shape like a Blu-Ray, while elliptical galaxies look like a pigskin American football. Dwarf galaxies, which are much less massive than spirals or ellipticals, tend to be much more irregular, she says.
A key step to thinking about the shapes of galaxies is understanding how they fit into the environment around them. For the Masters, these galaxies are like building blocks. “Sometimes I think of them as fairy lights illuminating the branches of the tree, where the branches are sort of the basic infrastructure that we have,” she says.
The infrastructure she is talking about is dark matter, which is a concentration of mass that is invisible to the telescopes we use.
However, we can infer the presence of dark matter because of the gravitational effects it produces on the matter we can see. One common effect that dark matter produces is the distortion of light, as NASA’s Jet Propulsion Laboratory explained on a webpage.
“As light from distant galaxies travels to us, it must pass through the gravitational fields of other galaxies. Consequently, we see distorted images of distant galaxies,” JPL said. “Ordinary matter does not account for the amount of distortion that astronomers observe.”
The three main types of galaxies – with many dwarf galaxies belonging to irregular. A. Field (STScI)
Galaxies have huge concentrations of dark matter, which also causes them to be gravitationally attracted to each other, Masters says. “There will be galaxies that gobble up small galaxies,” she says, citing the long history of the Milky Way as an example. The Milky Way is also in a possible collision with a large neighbor, the Andromeda galaxy.
Another important element about the shape of a galaxy is when it formed. When the universe was smaller and at an earlier stage of expansion after the Big Bang, mergers were much more common because galaxies interacted more often, Masters says. Gas was also quite abundant in these younger galaxies, leading to more possible materials for the galaxies to use during these mergers.
Nowadays, about 13.8 billion years after the formation of the universe, most galaxies are changing more slowly. Internal changes lead to the emergence of structures such as band shapes. However, mergers still occur, albeit less frequently than before.
The stars’ orbits can also affect galaxy shape through various “density waves,” which occur when material compacts inside galaxies into something equivalent to a traffic jam, Masters says. This is because the orbits do not occur regularly and the stars can collide together due to their mutual gravity influencing each other.
In contrast, the supermassive black holes embedded in the centers of many galaxies do not appear to have a significant effect on how the galaxy is shaped because the black holes are so small relative to the galaxy’s mass, Masters adds.
Because galaxies have so many different varieties of shapes within the big three (elliptical, spiral and dwarf), Masters says Galaxy Zoo continues to be important in helping to classify these different types. While machine learning makes computers more adept at picking out galaxy types, human intervention is good for putting discoveries into the big picture.
As observatories like the James Webb Space Telescope accumulate observations, Masters says, the number and types of galaxies we see will only increase. The community, she added, is very excited about the high-resolution views of early galaxies that Webb will provide when it completes commissioning this summer.
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