Galaxy

A galaxy is a group of many stars, with gas, dust, and dark matter.^[1][2][3][4] The name 'galaxy' is taken from the Greek word galaxia meaning milky, a reference to our own galaxy, the Milky Way.

Gravity holds galaxies together against the general expansion of the universe.^[3] In effect, the expansion of the universe takes place between groups of galaxies, not inside those groups. Gravity holds the galaxy together. The same applies to groups and clusters of galaxies, such as our Local Group where the Milky Way is, and the Virgo Cluster, a collection of more than 1,000 (might even be 2,000) galaxies. The gravitation is produced by the matter and energy in a galaxy or group of galaxies. Everything in a galaxy moves around a centre of mass, which is also an effect of gravity.

There are various types of galaxies: elliptical, spiral and lenticular galaxies, which can all be with or without bars. There are also irregular galaxies.

All galaxies exist inside the universe. The observable Universe contains more than 2 trillion (10¹²) galaxies^[5] and, overall, as many as an estimated 1×10²⁴ stars^[6][7] (more stars than all the grains of sand on planet Earth).^[8]

1. ↑ Sparke L.S. & Gallagher III J.S. 2000. Galaxies in the Universe: an introduction. Cambridge: Cambridge University Press. ISBN 0521671868.
2. ↑ Hupp E; Roy S. & Watzke M. 2006.[1] Archived 2020-03-28 at the Wayback Machine
3. ↑ ^{3.0 3.1} Cox, Brian; Cohen, Andrew (2011). Wonders of the Universe. HarperCollins. p. 24. ISBN 9780007395828.
4. ↑ "What is a Galaxy? | NASA Space Place – NASA Science for Kids".
5. ↑ Fountain, Henry (17 October 2016). "Two Trillion Galaxies, at the Very Least". The New York Times. Retrieved 17 October 2016.
6. ↑ "How many stars are there in the Universe?". European Space Agency. 2019. Retrieved 21 September 2019.
7. ↑ Marov, Mikhail Ya. (2015). "The Structure of the Universe". The Fundamentals of Modern Astrophysics. pp. 279–294. doi:10.1007/978-1-4614-8730-2_10. ISBN 978-1-4614-8729-6.
8. ↑ Mackie, Glen (1 February 2002). "To see the Universe in a Grain of Taranaki Sand". w:Centre for Astrophysics and Supercomputing. Retrieved 28 January 2017.