EDUCATION FIRST

1. GALAXIES
RAINNE ADRIELLE CATILO
GRADE 6 JADE
SAN PASCUAL CENTRAL SCHOOL

2. A galaxy is a gravitationally bound
system of stars, stellar remnants,
interstellar gas, dust, and dark
matter.The word galaxy is derived
from the Greek galaxias , literally
"milky", a reference to the Milky Way.
Galaxies range in size from dwarfs
with just a few hundred million (108)
stars to giants with one hundred
trillion stars,(1014)each orbiting its
galaxy's center of mass.

3. Galaxies are categorized according to
their visual morphology as elliptical,
spiral, or irregular. Many galaxies are
thought to have black holes at their
active centers. The Milky Way's central
black hole, known as Sagittarius A*,
has a mass four million times greater
than the Sun.As of March 2016, GN-z11
is the oldest and most distant observed
galaxy with a comoving distance of 32
billion light-years from Earth, and
observed as it existed just 400 million
years after the Big Bang.

4. Recent estimates of the number of
galaxies in the observable universe range
from 200 billion (2×1011) to 2 trillion
(2×1012) or more,[8][9] containing more
stars than all the grains of sand on planet
Earth.[10] Most of the galaxies are 1,000
to 100,000 parsecs in diameter and
separated by distances on the order of
millions of parsecs (or megaparsecs).

5. The space between galaxies is filled
with a tenuous gas having an average
density of less than one atom per
cubic meter. The majority of galaxies
are gravitationally organized into
groups, clusters, and superclusters.
At the largest scale, these
associations are generally arranged
into sheets and filaments surrounded
by immense voids. The largest
structure of galaxies yet recognised
is a cluster of superclusters that has
been named Laniakea.

6.  An elliptical galaxy is a type of galaxy
having an approximately ellipsoidal
shape and a smooth, nearly featureless
brightness profile. Unlike flat spiral
galaxies with organization and
structure, they are more three-
dimensional, without much structure,
and their stars are in somewhat
random orbits around the center.

7.  They are one of the three main classes of galaxy
originally described by Edwin Hubble in his 1936
work The Realm of the Nebulae, along with spiral
and lenticular galaxies. Elliptical galaxies range in
shape from nearly spherical to highly flat and in
size from tens of millions to over one hundred
trillion stars. Originally Edwin Hubble
hypothesized that elliptical galaxies evolved into
spiral galaxies, which was later discovered to be
false.Stars found inside of elliptical galaxies are on
average much older than stars found in spiral
galaxies.

8.  Spiral galaxies form a class of
galaxy originally described by
Edwin Hubble in his 1936 work
The Realm of the Nebulae and,
as such, form part of the
Hubble sequence. Most spiral
galaxies consist of a flat,
rotating disk containing stars,
gas and dust, and a central
concentration of stars known as
the bulge. These are often
surrounded by a much fainter
halo of stars, many of which
reside in globular clusters.

9.  Spiral galaxies are named by their spiral structures that
extend from the center into the galactic disc. The spiral arms
are sites of ongoing star formation and are brighter than the
surrounding disc because of the young, hot OB stars that
inhabit them.
 Roughly two-thirds of all spirals are observed to have an
additional component in the form of a bar-like
structure,extending from the central bulge, at the ends of
which the spiral arms begin. The proportion of barred
spirals relative to their barless cousins has likely changed
over the history of the Universe, with only about 10%
containing bars about 8 billion years ago, to roughly a
quarter 2.5 billion years ago, until present, where over two-
thirds of the galaxies in the visible universe (Hubble
volume) have bars.

10.  In the 1970s, our own Milky Way was confirmed to
be a barred spiral, although the bar itself is difficult
to observe from the Earth's current position within
the galactic disc. The most convincing evidence for
the stars forming a bar in the galactic center comes
from several recent surveys, including the Spitzer
Space Telescope .
 Together with irregular galaxies, spiral galaxies
make up approximately 60% of galaxies in today's
universe.They are mostly found in low-density
regions and are rare in the centers of galaxy
clusters.

11.  An irregular galaxy is a
galaxy that does not have a
distinct regular shape,
unlike a spiral or an
elliptical galaxy. Irregular
galaxies do not fall into
any of the regular classes
of the Hubble sequence,
and they are often chaotic
in appearance, with
neither a nuclear bulge nor
any trace of spiral arm
structure.

12.  Collectively they are thought to make up about a
quarter of all galaxies. Some irregular galaxies
were once spiral or elliptical galaxies but were
deformed by an uneven external gravitational
force. Irregular galaxies may contain abundant
amounts of gas and dust.This is not necessarily
true for dwarf irregulars.
 Irregular galaxies are commonly small, about one
tenth the mass of the Milky Way galaxy. Due to
their small sizes, they are prone to environmental
effects like crashing with large galaxies and
intergalactic clouds.

13.  A barred spiral galaxy is a spiral
galaxy with a central bar-shaped
structure composed of stars. Bars are
found in approximately two-thirds of
all spiral galaxies. Bars generally
affect both the motions of stars and
interstellar gas within spiral galaxies
and can affect spiral arms as well.The
Milky Way Galaxy, where our own
Solar System is located, is classified as
a barred spiral galaxy.

14.  Edwin Hubble classified spiral galaxies of this type as
"SB" (spiral, barred) in his Hubble sequence and
arranged them into sub-categories based on how open
the arms of the spiral are. SBa types feature tightly
bound arms, while SBc types are at the other extreme
and have loosely bound arms. SBb-type galaxies lie in
between the two. SB0 is a barred lenticular galaxy. A
new type, SBm, was subsequently created to describe
somewhat irregular barred spirals, such as the
Magellanic Cloud galaxies, which were once classified
as irregular galaxies, but have since been found to
contain barred spiral structures. Among other types in
Hubble's classifications for the galaxies are the spiral
galaxy, elliptical galaxy and irregular galaxy.

15.  A lenticular galaxy is a type of
galaxy intermediate between an
elliptical and a spiral galaxy in
galaxy morphological classification
schemes. Lenticular galaxies are
disc galaxies (like spiral galaxies)
that have used up or lost most of
their interstellar matter and
therefore have very little ongoing
star formation. They may,
however, retain significant dust in
their disks.

16.  As a result, they consist mainly of aging stars
(like elliptical galaxies). Because of their ill-
defined spiral arms, if they are inclined face-on
it is often difficult to distinguish between them
and elliptical galaxies. Despite the
morphological differences, lenticular and
elliptical galaxies share common properties like
spectral features and scaling relations. Both can
be considered early-type galaxies that are
passively evolving, at least in the local part of
the Universe.

17.  A peculiar galaxy is a galaxy of
unusual size, shape, or
composition. Between five and ten
percent of known galaxies are
categorized as peculiar.
Astronomers have identified two
types of peculiar
galaxies: interacting
galaxies and active galactic

18.  When two galaxies come close to each other,
their mutual gravitational forces can cause
them to acquire highly irregular shapes The
terms 'peculiar galaxy' and 'interacting galaxy'
have now become synonymous because the
majority of peculiar galaxies attribute their
forms to such gravitational forces.