When you look at the night sky through a telescope, you see more than just stars. Many of those points of light are actually galaxies. Galaxies are vast collections of stars, dust, and dark matter, all bound by gravity.
Astronomers are still unsure how galaxies formed. After the Big Bang, the universe was mostly hydrogen and helium. Some believe gravity pulled gas and dust together to create stars. These stars then clustered into galaxies. Others think mass was drawn together before stars were formed. Techniques to measure galaxy mass are improving, such as a 2018 study that used 3D movements of galaxies to refine the Milky Way’s mass.
Discovering Island Universes
In the early 1900s, many astronomers thought the universe was just the Milky Way. Harlow Shapley, however, proposed that spiral-shaped blobs were separate galaxies, calling them “island universes.” In 1924, Edwin Hubble identified Cepheid variable stars outside the Milky Way. He measured their distances and discovered that galaxies are moving away from us. The farther away they are, the faster they move. This led to the conclusion that the universe is expanding, and later, that this expansion is accelerating.
Galaxy Characteristics
Most galaxies contain black holes at their centers, which can emit huge amounts of energy. Some black holes are extremely large, even in small galaxies. Material around these black holes can be pushed outwards by jets. Quasars, the universe’s most energetic objects, can also be found at galaxy centers.
Galaxies are classified by shape, which reflects their characteristics and evolution.
Spiral galaxies, like the Milky Way, have arms that spiral outward. They are the most common type of galaxy. Gas and dust in these galaxies rotate at high speeds, fueling new star formation.
Elliptical galaxies lack spiral arms and range from circular to elongated shapes. They contain less dust and have mostly older stars, as star formation has largely ceased. Though they are less visible, astronomers believe more than half of all galaxies are elliptical.
Irregular galaxies make up the remaining 3 percent. They have no specific shape and are often distorted by the gravity of other galaxies. Collisions with other galaxies can also affect their forms.
When Galaxies Collide
Galaxies do not exist alone but are grouped in clusters. Some clusters contain over a thousand galaxies, while others are smaller. The Milky Way is part of the Local Group, which has about 50 galaxies.
Occasionally, galaxies collide, merging stars and dust. This process is crucial for their growth. In late 2018, astronomers observed two supermassive black holes merging during a galaxy collision.
While individual stars rarely collide, the merging of galaxies increases star formation. The Milky Way is on a collision course with the Andromeda galaxy in about 5 billion years. Both galaxies have likely merged with others in the past.
Emerging Galaxy Research
Recent studies have focused on how dark matter influences galaxy evolution. Dark matter and dark energy make up most of the universe’s mass and energy, but they are hard to detect directly.
In 2017, astronomers found two massive galaxies formed in a sea of dark matter. Their large size raised questions about how they evolved. Shortly after, a group of galaxies was found moving together in a pattern possibly explained by dark matter.
However, a 2018 study challenged the dark matter theory. It discovered that the galaxy NGC 1052-DF2 has far less dark matter than expected. This finding could change our understanding of galaxy evolution, but it remains controversial.
Ongoing surveys with advanced technology are helping scientists find previously invisible galaxies. For instance, the Multi Unit Spectroscopic Explorer revealed a group of 72 hidden galaxies in 2017.
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