How do galaxies get their shape?  The Science of the Webb Telescope's Key Objectives, Explained

How do galaxies get their shape? The Science of the Webb Telescope’s Key Objectives, Explained

Galaxies are clusters of stars, gas, dust, and dark matter throughout the universe. Their size and scope can vary significantly. The Milky Way we live in has 200 billion stars, which puts it roughly midway between the very largest galaxies and smaller types like dwarf galaxies.

Like humans, these galaxies are shaped by their surroundings, which explains the different shapes we see, says Karen Masters. She is an astrophysicist at Haverford College and a project scientist at the Galaxy Zoo, a citizen science project classifying different types of galaxies.

“There’s a lot of variety out there in the universe,” says Masters Turning back. Spiral galaxies are circular in shape like a Blu-ray, while elliptical galaxies are shaped like an American pigskin football. Dwarf galaxies, which are much less massive than spiral or ellipse galaxies, tend to be much more irregular, she says.

A crucial step in thinking about the shapes of galaxies is understanding how they fit into the environment around them. To the masters, these galaxies are like building blocks. “Sometimes I see them as strings of lights illuminating the branches of the tree, where the branches are sort of like the underlying infrastructure that we have,” she says.

The infrastructure it refers to is dark matter, that is, mass concentrations invisible to the telescopes we use.

However, we can infer the presence of dark matter because of the gravitational effect it exerts on visible matter. A common effect dark matter produces is light distortion, NASA’s Jet Propulsion Laboratory explains on a website.

“When light from distant galaxies travels to us, it has to pass through the gravitational fields of other galaxies. Hence, we see distorted images of the distant galaxies,” explained JPL. “Ordinary matter does not explain the amount of distortion that astronomers observe.”

The three main types of galaxies – with many dwarf galaxies belonging to irregular ones. A. Field (STScI)

Galaxies have enormous concentrations of dark matter, which also causes them to attract each other, says Masters. “There will be galaxies that devour small galaxies,” she says, citing the long history of the Milky Way as an example. The Milky Way is also on an eventual collision course with a large neighbor, the Andromeda galaxy.

Another important element in the shape of a galaxy is when it was formed. When the universe was smaller and at an earlier stage of expansion after the Big Bang, mergers were much more common because galaxies were interacting more often, says Masters. Gas was also quite abundant in these younger galaxies, leading to more possible materials that galaxies could use during these mergers.

Today, about 13.8 billion years after the universe formed, most galaxies are changing more slowly. Internal changes lead to structures such as bar shapes. However, mergers still occur, albeit less frequently than before.

The orbits of stars can also affect a galaxy’s shape through various “density waves” that occur when material in galaxies is condensed in a kind of jam, says Masters. That’s because orbits aren’t regular and stars may jam together due to their mutual gravity affecting each other.

Conversely, because the black holes are so small relative to a galaxy’s mass, supermassive black holes embedded in the centers of many galaxies don’t appear to have a significant impact on a galaxy’s shape, Masters adds.

Because galaxies come in so many different shapes within the big three (elliptical, spiral, and dwarf), Masters says Galaxy Zoo remains important to help classify these different types. While machine learning is making computers more adept at recognizing types of galaxies, human intervention is good for putting the results into the bigger picture.

As observatories like the James Webb Space Telescope make more 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 once commissioning is complete this summer.

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