4.2 : Galaxies

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How are galaxies classified?
Into four main categories (spiral, barred spiral, elliptical and irregular) based on their appearance from Earth.
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Describe a spiral (S) galaxy.
A flattened, disc-shaped system.
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Describe a barred spiral (BS) galaxy.
A spiral galaxy with a bright 'bar' running through its nucleus from which its spiral arms emerge.
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Describe an elliptical (E) galaxy.
A relatively smooth, spheroidal distribution of stars that is brighter at its centre.
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Describe an irregular (Irr) galaxy.
A galaxy with very little structure of symmetry.
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How are spiral and barred spiral galaxies sub-classified?
With the letter a, b or c, which corresponds to the strength of its nuclear bulge and the 'openness' of its spiral arms (Sa galaxies have a predominant nucleus and tightly wound arms).
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How are elliptical galaxies sub-classified?
With a number from 0 to 7, depending on its shape (E0 is nearly circular while E7 is highly elongated).
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What is a lenticular galaxy (S0)?
A class of galaxy considered to be between ellipticals and spirals. It differs from ellipticals because it has a bulge and a thin disk, but is different from Sa because it has no spiral structure.
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How is Hubble's galaxy classification scheme most often shown?
On a Tuning Fork Diagram.
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What galaxy type is the Milky Way?
SBb.
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What galaxies are included in the Local Group?
The Milky Way, the Adromeda Galaxy (a giant spiral galaxy and the largest in the group), the SMC and LMC (both irregular satellite galaxies of the Milky Way), the Triangulum Galaxy (a spiral galaxy) and several satellite and dwarf galaxies.
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What is a cluster?
A gravitational agglomeration of galaxies that often contain a giant elliptical galaxy in its central region.
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What are some examples of clusters close to us?
The Virgo Cluster (the nearest large cluster to us, its centre situated 15-20Mpc away), the Fornax Cluster and the Coma Cluster.
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What is a supercluster?
A collection of clusters bound together by gravity.
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What is an active galaxy?
Galaxies that emit radiation at wavelengths other than visible light, particularly radiowaves and X-rays.
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What is an active galactic nuclei (AGN)?
An extremely small nucleus believed to be the source of an active galaxy's energy.
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Describe a radio galaxy.
A (typically) elliptical galaxy that emits more than a million times more energy at radio wavelengths than normal galaxies, this radio emission often originating from symmetrical pairs of 'lobes' that lie on opposite sides of the galaxy.
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Describe a Seyfert galaxy.
A spiral galaxy with an extremely compact but bright nucleus and strong emission lines in its spectrum, with strong and often variable emission of IR and X-rays.
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Describe a quasar galaxy (also called a quasi-stellar radio source or a quasi-stellar object).
A compact galaxy that emits huge quantities of energy in all regions from X-rays to (sometimes, but not always) radio waves, with high redshift.
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Describe a blazar galaxy.
A compact galaxy (much closer to us than quasars) that emits strongly in all regions of the spectrum from radio waves to gamma rays. They can vary dramatically in optical brightness in a timescale of one or two days.
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Other cards in this set

Card 2

Front

Describe a spiral (S) galaxy.

Back

A flattened, disc-shaped system.

Card 3

Front

Describe a barred spiral (BS) galaxy.

Back

Preview of the front of card 3

Card 4

Front

Describe an elliptical (E) galaxy.

Back

Preview of the front of card 4

Card 5

Front

Describe an irregular (Irr) galaxy.

Back

Preview of the front of card 5
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