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looking back some 13 billion years

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nasa's hubble space telescope offers a

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glimpse of the early universe revealing

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countless galaxies in a tiny area of sky

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galaxies are the visible foundation of

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the universe

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each one a collection of stars planets

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gas dust and dark matter held together

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by gravity

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hubble's observations give us insight

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into how galaxies form grow and evolve

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through time

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hubble's namesake astronomer edwin

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hubble pioneered the study of galaxies

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based simply on their appearance

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he divided galaxies into three basic

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forms

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ellipticals spirals

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and irregulars

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labeled the tuning fork diagram edwin

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hubble's basic arrangement is still in

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use today

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elliptical galaxies are nearly spherical

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to egg-shaped groups of old stars that

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lack the gas and dust needed to form new

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stars

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rotation doesn't play a big part in

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their shape the movements of their stars

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often in long oval orbits determines an

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elliptical shape elliptical galaxies are

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often near the center of galaxy clusters

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suggesting they may form when galaxies

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merge

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[Music]

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the best known galaxies are spirals

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the center of a spiral galaxy has a

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large roughly spherical swarm of stars

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called a bulge

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this bulge looks similar to an

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elliptical galaxy but spirals differ

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from ellipticals because they rotate

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rotation gives spiral galaxies the flat

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disc that holds their spiral shaped arms

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unlike ellipticals spirals have a mix of

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young and old stars

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star formation in spirals is similar to

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a traffic jam on the interstate

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like cars on the highway slower moving

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matter in the spiral's disk creates a

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bottleneck concentrating star-forming

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gas and dust along the inner part of

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their spiral arms

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this traffic jam of matter can get so

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dense that it gravitationally collapses

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creating new stars

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spiral galaxies are subdivided into

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unbarred and barred and organized by the

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size of their central bulge and how

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tightly their arms are wound

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bars form in spiral galaxies when star

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orbits become unstable and stretched out

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as their orbits lengthen they create a

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bar the bar grows as gravity captures

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more nearby stars

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irregular galaxies don't fit into one of

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the other categories they are shapeless

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and have no symmetry or ordered

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structure

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irregulars may hold old and young stars

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and often have knots of gas and dust

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forming new stars

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astronomers have expanded edwin hubble's

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basic tuning fork diagram to include

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galaxies that fall between his three

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categories

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intermediate spiral galaxies sit between

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unbarred and barred spirals and have a

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small bar

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lenticular galaxies sit between

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elliptical galaxies and spirals

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they have a central bulge of stars and a

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flattened disc but no spiral arms

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like ellipticals lenticular galaxies

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don't have much gas and dust and also

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have mainly old stars

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when viewed edge on their shape

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resembles a lens which is why they're

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called lenticular

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edwin hubble's tuning fork was a first

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step in understanding galaxies and how

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they evolve

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observations by the hubble space

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telescope will continue to improve our