Universe
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Galaxies. (2023). In Q-files Encyclopedia, Space, Universe. Retrieved from
https://www.q-files.com/space/universe/galaxies
"Galaxies." Space, Universe, Q-files Encyclopedia, 20 Feb. 2023.
https://www.q-files.com/space/universe/galaxies.
Accessed 19 Mar. 2024.
Galaxies 2023. Space, Universe. Retrieved 19 March 2024, from
https://www.q-files.com/space/universe/galaxies
Space, Universe, s.v. "Galaxies," accessed March 19, 2024.
https://www.q-files.com/space/universe/galaxies
Galaxies
Stars do not exist in isolation but are clustered together in vast "cities" in space called galaxies. The galaxy in which the Sun is situated, the Milky Way Galaxy, is a colossal barred spiral of about 200 billion stars that measures approximately 100,000 light years across. There are reckoned to be roughly 200 billion other galaxies in the Universe, each of them containing billions of stars. Most galaxies are elliptical (oval) in shape. Many are spirals or barred spirals (having a bar-shaped core) like the Milky Way. There are also other galaxies that have irregular shapes.
Composition
A galaxy is composed of stars and the remnants of old, dying stars. Lying between the stars is what is known as the interstellar medium: a region of gas, dust and cosmic rays (showers of high-energy subatomic particles). Dark matter, matter that gives out neither light nor any other form of electromagnetic radiation, makes up about 90% of the mass of most galaxies. Supermassive black holes may exist at the centre of many galaxies.
The Local Group
Galaxies are not normally found on their own. Most are clumped together in clusters. Our own galaxy, the Milky Way, belongs to a cluster called the Local Group. It consists of more than 50 galaxies. The Local Group contains different types of galaxies: spirals (like Andromeda), barred spirals (like the Milky Way), ellipticals and irregulars—those with no obvious shape. The nearest galaxy to the Milky Way is thought to be the Canis Major Dwarf, an irregular galaxy lying about 25,000 light years away. The Andromeda Galaxy, the largest in the Local Group, is the nearest spiral, lying 2.25 million light years distant.
Edwin Hubble
Edwin Hubble (1889–1953) was an American astronomer. He is best known for his discovery of galaxies other than our own Milky Way Galaxy, and for his part in discovering that the Universe is expanding. Between 1925 and 1929, Hubble discovered the existence of isolated masses of stars called galaxies that were too distant to be part of our own galaxy. Up until then, most astronomers had believed that the Universe consisted of the Milky Way alone.
Hubble also devised a method for classifying galaxies, using the terms “ellipticals”, “spirals” and “barred spirals” to describe their basic shapes. In 1929 he announced what has become known as Hubble’s Law: all distant galaxies are moving away from us and from each other, and the farther away they are, the faster they move. In other words, the Universe is expanding.
We now know that it is only galaxy clusters that move apart as the Universe expands. Held together by gravity, the galaxies within the Local Group (along with other clusters) do not move away from each other.
Consultant: Mike Goldsmith