GALAXIES Beyond the confines
of the Milky Way, space is filled with a myriad more galaxies, many in clusters with hundreds
or even thousands of members.
The most widely used scheme of classification was devised by Hubble during the 1920s and revised
by Sandage in 1961. It defines three major classes of galaxy: elliptical, spiral and irregular. There
are also several sub-classes as illustrated in the famous "tuning fork" diagram below.
On the left of the diagram are the Elliptical galaxies, named by E0 to E7. The E stands for elliptical,
obviously, and the number indicates how egg-shaped the ellipse is - 0 means a ball shape, and 7 looks rather
like a thowing discus.
S0 galaxies - called lenticular galaxies because they are shaped like a lens in a
magnfiying glass. The description is made up of the "S", meaning lenticular, the "0", meaning no arms.
Continuing along that branch the next 3 types are all have spiral arms, and they are grouped by how
tightly those arms are wound and how large the central bulge is - the two happen to be closely related.
The name is defined by the "S" and the lower case letter after which indicates how wound up the arms are:
from "a" to "d". a - note the tightly-wound, smooth arms, and the central bright disc; b - better defined
spiral arms; c - much more loosely wound spiral arms, both of which are clearly defined; d - very loose
arms, with much of the luminosity in the arms, not the disc.
The lower branch of the tuning fork diagram is largely a copy of the upper branch, but its occupants
all have a line of stars through the centre - a bar.These are named SBx, where x is the lower case letter
as in the unbarred cases above.
NEBULAE Of all the sights in the sky, the delicate
clouds of gas and dust known as nebulae - the birthplaces and graveyards of the stars -
are among the most stunning.
Emission Nebulae
Emission nebulae are clouds of high temperature gas. The atoms in the cloud are
energized by ultraviolet light from a nearby star and emit radiation as they fall
back into lower energy states (in much the same way as a neon light). These nebulae
are usually red because the predominant emission line of hydrogen happens to be red
(other colors are produced by other atoms, but hydrogen is by far the most abundant).
Emission nebulae are usually the sites of recent and ongoing star formation.
Trifid Nebula (M20),gets its name from the three dark lanes the divide it.
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Reflection nebulae are clouds of dust which are simply reflecting the light
of a nearby star or stars. Reflection nebulae are also usually sites of star formation.
They are usually blue because the scattering is more efficient for blue light. Reflection
nebulae and emission nebulae are often seen together and are sometimes both referred to as
diffuse nebulae.
Dark nebulae are clouds of dust which are simply blocking the light from whatever is
behind. They are physically very similar to reflection nebulae; they look different only
because of the geometry of the light source, the cloud and the Earth. Dark nebulae are also
often seen in conjunction with reflection and emission nebulae. A typical diffuse nebula is
a few hundred light-years across.
Planetary nebulae are shells of gas thrown out by some stars near the end of their lives.
Our Sun will probably produce a planetary nebula in about 5 billion years. They have nothing
at all to do with planets; the terminology was invented because they often look a little like
planets in small telescopes. A typical planetary nebula is less than one light-year across.
Helix Nebula (NGC 7293), the closest planetary to the Earth.
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Supernovae occur when a massive star ends its life in an amazing blaze of glory.
For a few days a supernova emits as much energy as a whole galaxy. When it's all over,
a large fraction of the star is blown into space as a supernova remnant. A typical supernova
remnant is at most few light-years across.
