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- For the automobile, see Ford Galaxy.
Gravity holds galaxies together, against the general expansion of the universe. In effect, the expansion of the universe takes place between groups of galaxies, not inside those groups. This is because the mass within a galaxy holds the galaxy together, and the same applies to the groups of galaxies, such as our Local Group. The gravitation is produced by the matter and energy in a galaxy or group of galaxies. Everything in a galaxy moves around a centre, and that 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. Then there are irregular galaxies. All galaxies exist inside the universe. There are probably over 170 billion (1.7x1011) galaxies within distance we can see or the observable universe.
There are galaxies of different sizes. Typical galaxies range from dwarfs with as few as ten million (107) stars up to giants with a hundred trillion (1014) stars, all orbiting the galaxy's center of mass. Galaxies may contain many multiple star systems, star clusters, and various interstellar clouds. The Sun is one of the stars in the Milky Way galaxy; the Solar System includes the Earth and all the other objects that orbit the Sun.
Star clusters are not galaxies, they are inside galaxies. Globular clusters are spherical tightly knit balls of stars which are part of the outer halo of the Milky Way. One of the largest (and oldest) known star clusters, Messier 15, has several million stars, packed closely together, with a black hole at its centre. The stars are too closely packed to get an accurate count, but it certainly has more stars than some of the smaller galaxies.
Within galaxy clusters, galaxies move relative to other galaxies. They can and do collide. When this happens, the stars generally move past each other, but gas clouds and dust interact, and can form a burst of new stars. Gravity pulls both galaxies into somewhat new shapes, formng bars, rings or tail-like structures.
Many galaxies also continue to form new generations of stars. The Milky Way, and all spiral shaped galaxies like it (see above image of NGC 2997), produce new stars at a rate of one or two stars per year. These stars are formed in the vast interstellar clouds that account for about 1% to 10% of the mass of these galaxies. Globular star clusters, on the other hand, are not currently forming stars because this activity happened billions of years ago and then stopped once all of the gas and dust clouds were used up.
In the astronomical literature, the word 'Galaxy' with a capital G is used for our galaxy, the Milky Way. The billions of other galaxies are just written as 'galaxy' with a lowercase g. The term Milky Way first came out in the English language in a poem by Chaucer.
When William Herschel wrote his catalogue of deep sky objects, he used the name spiral nebula for objects like the Andromeda Galaxy, but nebula is now only used for diffuse structures within a galaxy.
In Greek mythology, Zeus places his son (the baby Heracles) whose mother was a mortal woman on Hera's breast while she is sleeping so that the baby will drink her divine milk and become immortal. However, Hera wakes up while she is breastfeeding the baby and realizes she is nursing a baby she does not know. According to Greek mythology, she then pushes the baby away and a stream of her milk sprays the night sky, making a faint band of light known as the Milky Way.
- List of galaxies
- List of nearest galaxies
- Most distant things
- Local Group
- IC 1101: the largest known galaxy, with about 100 trillion stars.
- Sparke L.S. & Gallagher III J.S. 2000. Galaxies in the Universe: an introduction. Cambridge: Cambridge University Press. ISBN 0521671868.
- Hupp E.; Roy S.; Watzke M. 2006.
- Cox, Brian; Cohen, Andrew (2011). Wonders of the Universe. HarperCollins. p. 24. .
- Gott, J. Richard, III; et al. 2005. A map of the Universe. The Astrophysical Journal 624 (2): 463–484.
- Mackie, Glen 2002. 
- ESO 2000: 
- Wilford, John Noble 1990. "Sighting of largest galaxy hints at clues on the clustering of matter". New York Times. Retrieved 2010-05-06
- Geoffrey Chaucer. The House of Fame, ~1380."Online Etymology Dictionary". http://www.etymonline.com/index.php?term=galaxy. Retrieved 2007-01-03.
- "Explore the Archer's Realm". space.com. 2005-09-02. http://www.space.com/spacewatch/050902_teapot.html. Retrieved 2007-01-03.
- Koneãn˘, Lubomír. "Emblematics, agriculture, and mythography in the origin of the Milky Way" (PDF). Academy of Sciences of the Czech Republic. Archived from the original on July 20, 2006. http://web.archive.org/web/20060720204104/http://www.udu.cas.cz/collegium/tintoretto.pdf. Retrieved 2007-01-05.
- Uson, Juan M. et al (October 1990). "The central galaxy in Abell 2029 – an old supergiant". Science 250 (4980): 539–540. .
- Galaxies, SEDS Messier pages
- An Atlas of The Universe
- Galaxies — Information and amateur observations
- The Oldest Galaxy Yet Found
- Galaxies — discussed on BBC Radio 4's "In Our Time" programme
- Galaxy classification project, harnessing the power of the internet and the human brain
- How many galaxies are in our universe?