Galaxy - Revision history

Evilscientist: add wikilinks

2010-06-16T23:54:53Z

add wikilinks

Evilscientist: add telescope link

2009-11-19T01:33:23Z

add telescope link

Evilscientist at 19:57, 9 February 2009

2009-02-09T19:57:05Z

Evilscientist at 13:19, 30 January 2009

2009-01-30T13:19:16Z

Evilscientist at 05:21, 27 January 2009

2009-01-27T05:21:23Z

Evilscientist at 04:44, 27 January 2009

2009-01-27T04:44:35Z

New page

Galaxies are collections of billions of stars all bound together by their mutual gravitational attraction. The Milky Way, which can be seen across part of the night time sky from a dark site, is actually our own galaxy as seen from our vantage point in it. To a large telescope there are millions of these "island universes" to be seen. To smaller telescopes, hundreds are visible from a dark site. In fact from a dark site, two galaxies similar to our own are visible to the unaided eye as faint fuzzy patches of light. Much fewer, of course, are visible from inside a light polluted city.

When they were first discovered, galaxies were called nebulae. In fact, anything that was faint and fuzzy through a telescope was called a nebula, which is simply Latin for cloud. Nebulae as we understand them today are clouds of gas, of which we can really only see those in our own galaxy. However, owing to uncertainty as to their distance, galaxies were thought to be in our own galaxy until the 1920's when Edwin Hubble, who having discovered Cepheid variables in [[M31]] was able to determine that the distance to it was far greater than the estimated size of our own galaxy. This meant the galaxies were islands of stars in space like our own Milky Way.<ref name="hubble1">Hubble, Edwin, ''Cepheids in Spiral Nebulae'', The Observatory Volume 1925, V48 pp 139-142</ref>

==Classification of Galaxies==

Galaxies come in three basic shapes, elliptical, spiral and irregular. Spiral galaxies are further divided into normal spirals and bared spirals. Normal spirals look like simple spirals of varying tightness. Barred spirals are identified by having a "bar" of stars across their nuclei. The most common classification system used today was developed in the 1920's by Edwin Hubble himself<ref name="hubble2">Hubble, Edwin, ''Extra-Galactic Nebulae'', Astrophysical Journal, 1926 V64 pp 321-369</ref>. The table below shows the classifications of galaxies developed by Hubble and the examples he provided in his paper.

{| cellspacing="0" cellpadding="20" border="1"
|+Hubble Classification of Galaxies
!Class
!Description
|-
|E0..E7
|Elliptical Galaxies: The number provides the oblateness: 0=almost spherical, 7=almost flat
|-
|S
|Normal Spirals 
*Sa - tightly spiraled, bright nucleus (Hubble called this early spiral)
*Sb - moderately spiraled, medium nucleus (Hubble called this intermediate spiral)
*Sc - openly spiraled, small nucleus (Hubble called this late spiral)
|-
|SB
|Barred Spirals 
*SBa - tightly spiraled, prominent nucleus (Early)
*SBb - moderately spiraled, less prominent nucleus (intermediate)
*SBc - openly spiraled, almost no nucleus (late)
|-
|Irr
|Irregulars: Galaxies with no defined shape.
|}

Modern taxonomy adds an additional class - S0, lenticular galaxies. These are galaxies that exist between E7 and Sa/SBa.
===Hubble sequence of galaxies===

It is important to note that this is not an evolutionary view. Galaxies do not progress from one type to another.

[[image:Hubble_sequence_photo.png]]

===Hubble's Exemplars of Class===
{| cellspacing="0" border="1"
|+Ellipticals
|-
!E0
!E2
!E5
!E7
|-
|[[Image:ngc3379.jpg|thumb|NGC3379]]
|[[Image:ngc221.jpg|thumb|NGC221]]
|[[Image:ngc4621.jpg|thumb|NGC4621]]
|[[Image:ngc2117.jpg|thumb|NGC2117]]
|}

{| cellspacing="0" border="1"
|+Normal Spirals
|-
!Sa
!Sb
!Sc
|-
|[[Image:ngc4594.jpg|thumb|NGC4594]]
|[[Image:ngc2841.jpg|thumb|NGC2841]]
|[[Image:ngc5457.jpg|thumb|NGC5457]]
|}

{| cellspacing="0" border="1"
|+Barred Spirals
|-
!SBa
!SBb
!SBc
|-
|[[Image:ngc2859.jpg|thumb|NGC2859]]
|[[Image:ngc3351.jpg|thumb|NGC3351]]
|[[Image:ngc7479.jpg|thumb|NGC7479]]
|}

{| cellspacing="0" border="1"
|+Irregulars
|-
|[[Image:ngc4449.jpg|thumb|NGC4449]]
|}

==References==
<references/>

[[Category:Astronomical]]

@@ Line 1: / Line 1: @@
 Galaxies are collections of billions of stars all bound together by their mutual gravitational attraction. The Milky Way, which can be seen across part of the night time sky from a dark site, is actually our own galaxy as seen from our vantage point in it. To a large [[telescope]] there are millions of these "island universes" to be seen. To smaller telescopes, hundreds are visible from a dark site. In fact from a dark site, two galaxies similar to our own are visible to the unaided eye as faint fuzzy patches of light. Much fewer, of course, are visible from inside a light polluted city.
-When they were first discovered, galaxies were called nebulae. In fact, anything that was faint and fuzzy through a telescope was called a nebula, which is simply Latin for cloud. Nebulae as we understand them today are clouds of gas, of which we can really only see those in our own galaxy. However, owing to uncertainty as to their distance, galaxies were thought to be in our own galaxy until the 1920's when Edwin Hubble, who having discovered Cepheid variables in [[M31]] was able to determine that the distance to it was far greater than the estimated size of our own galaxy. This meant the galaxies were islands of stars in space like our own Milky Way.<ref name="hubble1">Hubble, Edwin, ''Cepheids in Spiral Nebulae'', <u>The Observatory</U> , 1925, V48 pp 139-142</ref>
+When they were first discovered, galaxies were called [[Nebula|nebulae]]. In fact, anything that was faint and fuzzy through a telescope was called a nebula, which is simply Latin for cloud. [[Nebula|Nebulae]] as we understand them today are clouds of gas, of which we can really only see those in our own galaxy. However, owing to uncertainty as to their distance, galaxies were thought to be in our own galaxy until the 1920's when Edwin Hubble, who having discovered Cepheid variables in [[M31]] was able to determine that the distance to it was far greater than the estimated size of our own galaxy. This meant the galaxies were islands of stars in space like our own Milky Way.<ref name="hubble1">Hubble, Edwin, ''Cepheids in Spiral Nebulae'', <u>The Observatory</U> , 1925, V48 pp 139-142</ref>
 ==Classification of Galaxies==

← Older revision		Revision as of 01:33, 19 November 2009
Line 1:		Line 1:
−	Galaxies are collections of billions of stars all bound together by their mutual gravitational attraction. The Milky Way, which can be seen across part of the night time sky from a dark site, is actually our own galaxy as seen from our vantage point in it. To a large telescope there are millions of these "island universes" to be seen. To smaller telescopes, hundreds are visible from a dark site. In fact from a dark site, two galaxies similar to our own are visible to the unaided eye as faint fuzzy patches of light. Much fewer, of course, are visible from inside a light polluted city.	+	Galaxies are collections of billions of stars all bound together by their mutual gravitational attraction. The Milky Way, which can be seen across part of the night time sky from a dark site, is actually our own galaxy as seen from our vantage point in it. To a large [[telescope]] there are millions of these "island universes" to be seen. To smaller telescopes, hundreds are visible from a dark site. In fact from a dark site, two galaxies similar to our own are visible to the unaided eye as faint fuzzy patches of light. Much fewer, of course, are visible from inside a light polluted city.

	When they were first discovered, galaxies were called nebulae. In fact, anything that was faint and fuzzy through a telescope was called a nebula, which is simply Latin for cloud. Nebulae as we understand them today are clouds of gas, of which we can really only see those in our own galaxy. However, owing to uncertainty as to their distance, galaxies were thought to be in our own galaxy until the 1920's when Edwin Hubble, who having discovered Cepheid variables in [[M31]] was able to determine that the distance to it was far greater than the estimated size of our own galaxy. This meant the galaxies were islands of stars in space like our own Milky Way.<ref name="hubble1">Hubble, Edwin, ''Cepheids in Spiral Nebulae'', <u>The Observatory</U> , 1925, V48 pp 139-142</ref>		When they were first discovered, galaxies were called nebulae. In fact, anything that was faint and fuzzy through a telescope was called a nebula, which is simply Latin for cloud. Nebulae as we understand them today are clouds of gas, of which we can really only see those in our own galaxy. However, owing to uncertainty as to their distance, galaxies were thought to be in our own galaxy until the 1920's when Edwin Hubble, who having discovered Cepheid variables in [[M31]] was able to determine that the distance to it was far greater than the estimated size of our own galaxy. This meant the galaxies were islands of stars in space like our own Milky Way.<ref name="hubble1">Hubble, Edwin, ''Cepheids in Spiral Nebulae'', <u>The Observatory</U> , 1925, V48 pp 139-142</ref>

← Older revision		Revision as of 19:57, 9 February 2009
Line 87:		Line 87:
	<references/>		<references/>

−	[[Category:Astronomical]]	+	[[Category:Astronomical object]]

@@ Line 1: / Line 1: @@
 Galaxies are collections of billions of stars all bound together by their mutual gravitational attraction. The Milky Way, which can be seen across part of the night time sky from a dark site, is actually our own galaxy as seen from our vantage point in it. To a large telescope there are millions of these "island universes" to be seen. To smaller telescopes, hundreds are visible from a dark site. In fact from a dark site, two galaxies similar to our own are visible to the unaided eye as faint fuzzy patches of light. Much fewer, of course, are visible from inside a light polluted city.
-When they were first discovered, galaxies were called nebulae. In fact, anything that was faint and fuzzy through a telescope was called a nebula, which is simply Latin for cloud. Nebulae as we understand them today are clouds of gas, of which we can really only see those in our own galaxy. However, owing to uncertainty as to their distance, galaxies were thought to be in our own galaxy until the 1920's when Edwin Hubble, who having discovered Cepheid variables in [[M31]] was able to determine that the distance to it was far greater than the estimated size of our own galaxy. This meant the galaxies were islands of stars in space like our own Milky Way.<ref name="hubble1">Hubble, Edwin, ''Cepheids in Spiral Nebulae'', <u>The Observatory</U> Volume 1925, V48 pp 139-142</u></ref>
+When they were first discovered, galaxies were called nebulae. In fact, anything that was faint and fuzzy through a telescope was called a nebula, which is simply Latin for cloud. Nebulae as we understand them today are clouds of gas, of which we can really only see those in our own galaxy. However, owing to uncertainty as to their distance, galaxies were thought to be in our own galaxy until the 1920's when Edwin Hubble, who having discovered Cepheid variables in [[M31]] was able to determine that the distance to it was far greater than the estimated size of our own galaxy. This meant the galaxies were islands of stars in space like our own Milky Way.<ref name="hubble1">Hubble, Edwin, ''Cepheids in Spiral Nebulae'', <u>The Observatory</U> , 1925, V48 pp 139-142</ref>
 ==Classification of Galaxies==

@@ Line 50: / Line 50: @@
 |[[Image:ngc221.jpg|thumb|NGC221]]
 |[[Image:ngc4621.jpg|thumb|NGC4621]]
-|[[Image:ngc2117.jpg|thumb|NGC2117]]
+|[[Image:ngc2119.jpg|thumb|NGC2119]] <ref>Hubble erroneously lists this as the open cluster NGC 2117.</ref>
 |}
@@ Line 83: / Line 83: @@
 |}
 ==References==
 <references/>
 [[Category:Astronomical]]