This giant spiral disk of stars, dust and gas is 170,000 light-years across, or nearly twice the diameter of our Milky Way galaxy. (Credit for Hubble Image: NASA, ESA, K. Kuntz (JHU), F. Bresolin (University of Hawaii), J. Trauger (Jet Propulsion Lab), J. Mould (NOAO), Y.-H. Chu (University of Illinois, Urbana), and STScI)

Galaxies are gravitationally bound collections of stars, planets, gas, dust, dark matter and other objects, including black holes. They represent the standard "building blocks" of large-scale structure in the universe. Galaxies group hierarchically into clusters of galaxies and superclusters. These groupings in turn comprise larger structures of long, luminous filaments and sheets of galaxies, separated by gaping voids containing few galaxies. In general, galaxies come in three broad shapes and sizes: giant ellipticals, spirals, and small dwarfs. Astrophysicists study phenomena such as the mergers of galaxies, star formation rates and the influences from supernovae and black holes to understand the development of these and other galactic types over cosmic history. Cosmologists, meanwhile, use the evolution of the large-scale structures composed of galaxies to probe how the universe has changed since the Big Bang 13.8 billion years ago. A topic of major interest is the distribution of dark matter, which corresponds to the location of superclusters and filaments. Another is the effect of dark energy, which is accelerating the universe's expansion as evidenced by the speed of distant galaxies receding from our own galaxy, the Milky Way.

Building the Universe Pixel by Pixel

Aug 20, 2014

Three 'mapmakers' of the universe – Ralf Kaehler, Stuart Levy and Dylan Nelson – discuss how their dramatically intricate 3-D universes can tell important stories about the cosmos.

Spotlight Live: Black Holes & Our Cosmic Evolution

Sep 05, 2013
Black hole

Billions of years ago, a subset of galaxies in the relatively young universe were producing new stars at a rate of 1,000 per year. 

Black Holes Revisited

Sep 05, 2013
Black hole

Andrea Merloni, Priyamvada Natarajan, Tommaso Treu and John Wise discuss the latest findings about black holes, including new questions about how supermassive black holes at the centers of galaxies came into existence.

Spotlight Live: Dark Energy – On the Brink of Discovery?

Aug 08, 2013

On August 22, three leading members of the new dark energy collaboration answered your questions about the dark energy and the expansion of the universe.

Revealing Dark Energy's Hold on the Universe

Aug 08, 2013
Artist's conception of dark energy, gravity

A new collaboration aims to learn how dark energy is driving the accelerated expansion of the universe.

Spotlight Live: Starbursts in the Early Universe

Mar 29, 2013

The milky way today may fire up one new star every year, but billions of years ago,some galaxies were producing new stars at a rate of 1,000 per year. 

Spotlight Roundtable: Witnessing Starbursts in Young Galaxies

Mar 13, 2013
Artist impression of starburst source

John Carlstrom, Dan Marrone and Joaquin Vieira discuss how the world’s most powerful radio telescope revealed that the most vigorous bursts of star birth in the cosmos took place much earlier than previously thought.

Are We Closing In On Dark Matter?

Dec 05, 2012

As the search for dark matter intensifies, a colloquium brought together cosmologists, particle physicists and observational astrophysicists --- three fields now united in the hunt to determine what is dark matter.

Afterglow: Dispatches from the Birth of the Universe

Oct 31, 2012

Lawrence M. Krauss, John C. Mather, Amber Miller, Lyman Page and David Spergel discuss how the cosmic background radiation is poised to reveal a host of bold questions about the cosmos.

Shining a Light on Dark Galaxies

Oct 05, 2012
The sky around the quasar HE0109-3518

Dark galaxies – galaxies with few if any stars and made predominately of dense gas – have been impossible to detect directly until now. Members of an international team of astronomers discuss their discovery and the place these galaxies hold in the universe.


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