Download Class 4 Galaxies Galaxy Classification Formation of Galaxies

Class 1 Introduction, Background History of Modern Astronomy
The Night Sky, Eclipses and the Seasons
Kepler's Laws
Newtonian Gravity
General Relativity
Matter and Light
Telescopes
Class 2 Solar System
Characteristics
Formation
Exosolar Planets
Class 3 Stars
The Sun
Stellar Evolution of Low and High Mass Stars
Deaths of Stars
Class 4
Galaxies Galaxy Classification
Formation of Galaxies
Galactic Evolution
Class 5 Cosmology
Class 6 Special Topics
Large-Scale Structure of the Universe
Big Bang Cosmology
Why is Pluto no longer a planet? IAU planet classification
Observing with a Telescope
Galaxy Classification
Discovery of our Milky Way galaxy
●Identification of types of galaxies
Spiral, Elliptical, Irregular
●
Map of the Milky Way circa turn of last century
William Herschel’s, Eighteenth-century, constructed this “map” of the
Galaxy by counting the numbers of stars he saw in different directions
in the sky. Our Sun appears to lie near the center of the distribution,
and the long axis of the diagram lies in the plane of the Galactic disk.
Sun
Assuming all stars are as luminous as one another, “he
concluded that the Galaxy was a somewhat flattened, roughly
disk-shaped collection of stars lying in the plane of the Milky
Way, with the Sun at its center “
But Herschel failed to take into account the absorption of visible light
by the then unknown interstellar gas and dust.
Photo credit: John Swierzbin, AAA: the Milky Way from Santa Fe, New Mexico.
The center of the galaxy is located within the bright white region to the right of
and just below the middle of the image.
The entire image width covers about one-half a degree, about the same
angular width as the full moon.
In this spectacular image, observations using infrared light and X-ray light see
through the obscuring dust and reveal the intense activity near the galactic core.
Poor Herschel did not
have the modern
benefit of multiwavelength astronomy.
These images are the
breakdown of the
previous page's
composite image.
The systematic
cataloging of stars
enabled the discovery
of pulsating variable
stars, paving the way
for a true 3-D map of
our galaxy.
RR Lyrae and
Cepheid variable
stars revolutionized
distance calculations.
The double image
shows the star's
varying luminosity.
Brightness ~ Luminosity/Distance
Luminosity is
determined by
the nature and
period of the
light curves.
This was
discovered in
1908 by
Henrietta
Leavitt.
2
Harlow Shapley then mapped RR Lyrae stars
in globular clusters and revealed the true
extent of stars in the Milky Way Galaxy—the
region that we now call the Galactic halo.
The hub of the distribution
Shapley saw, 8 kpc from
the Sun, is the Galactic
center.
The parsec is equal to just
under 31 million million
kilometres (about 19 million
million miles), or about 3.26
light-years.
Structure of the Milky Way
Disk
●Central bulge
●Halo
●Dark Matter distribution
●
The Disk
The disk is characterized by nearly circular orbits
of stars and gas and dust. Its thickness compared
to its diameter is about the same relative
dimensions as an index card: about 1 to 150.
The density-wave theory
is the current most viable
model for the spiral arms
observed in spiral
galaxies.
This theory holds that the
spiral arms are waves of
gas compression and star
formation moving through
the material of the
Galactic disk.
Gas motion is indicated
by red arrows, and arm
motion by white arrows.
Gas enters an arm from
behind, is compressed,
and forms stars.
The spiral pattern is
traced by high density
dust lanes and new, bright
The Central Bulge and Halo
Halo stars
have orbits
with largely
random
orientations
and
eccentricities.
The oldest
stars of our
galaxy are
found in the
Halo.
The orbital
properties of
bulge stars are
intermediate
between those
of disk stars
and those of
halo stars:
eccentric with
random
inclinations.
This Chandra image of the supermassive black hole
at our Galaxy's center, Sagittarius A* or Sgr A*, was
made from the longest X-ray exposure of that region
to date.
The
supermassive
black hole at the
center of our
galaxy is
estimated to be
a few million
times the mass
of the sun
contained within
a region less
than the size of
Jupiter's orbit.
The central
bulge of our
galaxy has a
stellar
density of
about one
million times
greater than
our location
in the Milky
Way. Think
of what the
“night sky” is
there.
8.4 x 8.4 arcminutes
Dark matter distribution
The rotation curve for the Milky Way Galaxy plots rotation speed against
distance from the Galactic center. We can use this curve to compute the
mass of the Galaxy. The dashed curve is the expectation if the Galaxy
“ended” at a radius of 15 kpc, the limit of most of the known spiral
structure and the globular cluster distribution. Since the red curve does
not follow this dashed line, but instead stays well above it, implies there
must be additional unseen matter beyond that radius.
In the late 1920's
Edwin Hubble
observed
Cepheids in
Andromeda and
other galaxies,
demonstrating that
there are many
vast,
gravitationally
bound
assemblages of
stars, gas, dust,
dark matter, and
radiation
separated from us
by almost
incomprehensibly
large distances.
Spiral Galaxies, Type S
Variation in shape among spiral galaxies
determines their type. As we progress from
type Sa to Sb to Sc, the bulges become
smaller while the spiral arms tend to
become less tightly wound.
Barred Spiral Galaxies, Type SB
Roughly half of all spirals, including the Milky Way, have a
bar-like structure, extending from the central bulge.
The variation from SBa to SBc is similar to that for normal
spiral galaxies, except that now the spiral arms begin at either
end of a bar through the galactic center.
Elliptical Galaxies, Type E
(a) The E1 galaxy M49 is nearly circular in appearance. (b) M84 is a slightly more elongated
elliptical galaxy, classified as E3. Both of these galaxies lack spiral structure, and neither shows
evidence of cool interstellar dust or gas, although each has an extensive X-ray halo of hot gas that
extends far beyond the visible portion of the galaxy. (c) This false-color X-ray image of the giant
elliptical galaxy 3C295 displays hot X-ray-emitting gas (red) within and well beyond the galaxy itself
(white), (AURA; SAO)
Irregular Galaxies, Type Irr
Irregular Galaxy Shapes Some irregular (Irr II) galaxies. (a) The oddly shaped galaxies NGC 4485
and NGC 4490 may be close to one another and interacting gravitationally. (b) The galaxy M82
seems to show an explosive appearance, although interpretations remain uncertain. (AURA;
Subaru)
Full spectrum and masses of galaxies
* Active galaxy – Seyfert,
quasar, blazars. Have huge
luminosity from central region
and sometimes jets.
* Dwarf galaxy – most
galaxies in the universe are
dwarf galaxies with just a few
billion stars: dwarf irregulars,
dwarf ellipticals, ultracompact dwarf galaxies, etc.
* Lenticular galaxy
* Ring galaxy
* Starburst galaxy
Formation and evolution of galaxies
First,
Hydrogen and helium are ionized, i. e., no electrons are bounded to
the nuclei, when formed initially within the first fifteen minutes after the
beginning of spacetime. As the expanding universe cools down, the
electrons get captured by the ions, making them neutral. This process
is known as recombination. This is thought to have occurred about
377,000 years after the Big Bang.
At the end of recombination, with most of the atoms in the universe are
neutral, the photons can now travel freely: the universe has become
transparent. The photons emitted right after the recombination can now
travel undisturbed. These photons are the cosmic microwave
background (CMB) radiation. Therefore the CMB is a picture of the
universe at the end of this epoch.
Then,
Large irregular clouds of hydrogen and helium contained regions
that were slightly more dense than others. The higher density
allowed gravity to trigger collapse. As the large cloud collapsed, it
cooled. On an even smaller scale, pieces of the collapsing cloud,
also collapsed into even smaller pieces. These smaller denser
regions created the first stars.
Structure formation in the big bang model proceeds hierarchically, with
smaller structures forming before larger ones. The first structures to form
are quasars, which are thought to be bright, early active galaxies, and
population III stars.
Quasars
Population III stars
Artist renditions
Infrared images
When the first stars reached the end of their life cycle, they
exploded, heating the surrounding gas and slowing the
collapse of the galaxy cloud. These explosions also
introduced heavier metals, such as carbon and nitrogen, into
the galactic cloud. Eventually, this process of collapse, star
formation, and slowing, balanced, giving us stable galaxies.
The oldest star:
HE 1523-0901 is a red giant star located in the Milky Way galaxy. It
is thought to be a second generation Population II star. The star's
age, as measured by ESO's Very Large Telescope, is 13.2 billion
years. This makes it the oldest object yet discovered in the galaxy.
HE 1523-0901 is the first star whose age was determined using the
decay of the radioactive elements uranium and thorium in tandem
with measurements of several neutron capture elements.
The oldest galaxy:
In 2007 the Keck telescope, a team from California Institute of
Technology found six star forming galaxies about 13.2 billion light
years (light travel distance) away and therefore created when the
universe was only 500 million years old.
The first galaxies
The HUDF
(Hubble Ultradeep field) is
the deepest
image of the
universe ever
taken. It
identifies
galaxies that
existed
between 400
and 800 million
years after the
Big Bang
(redshifts
between 7 and
12).
In the HUDF we learn that:
* High rates of star formation exist during the very early stages of
galaxy formation, under a billion years after the Big Bang. 100's –
1000's of stars form in these galaxies each year. Our Milky Way
produces about 4 stars per year.
* Galaxies at high redshifts are smaller and less symmetrical than
ones at lower redshifts, demonstrating the rapid evolution of galaxies
in the first couple of billion years after the Big Bang.
This high-resolution image of
the HUDF includes the
smallest, reddest galaxies,
about 9000, and are some of
the most distant galaxies to
have been taken by an optical
telescope, existing at the time
shortly after the big bang.
After galaxies form, they evolve primarily
by mergers and interactions
A spiral galaxy, ESO 510-G13, was warped as a
result of colliding with another galaxy. After the
other galaxy is completely absorbed, the distortion
will disappear. The process typically takes millions
if not billions of years.
The Antennae Galaxies are a dramatic pair of colliding
galaxies. In such a collision, the stars within each galaxy will
pass by each other (virtually) without incident. This is due to
the relatively large interstellar distances. Diffuse gas clouds
readily collide to produce shocks which in turn stimulate
bursts of star formation. The bright, blue knots indicate the
hot, young stars that have recently ignited as a result of the
merger.
An image of NGC 4676 (also called the Mice
Galaxies) is an example of a present merger.
About 10 to13.2 billion years ago ...
(a)The Milky Way galaxy possibly formed through the merger of
several smaller systems (tidal tails presently observed support this).
(b) Early on, our galaxy was irregularly shaped, with gas distributed
throughout its volume. When stars formed during this stage, there
was no preferred direction in which they moved and no preferred
location in which they were found and formed the halo.
(c) In time, the gas and dust fell to the galactic plane and formed a
spinning disk. The stars that had already formed were left behind, in
the halo.
(d) New stars forming in the disk inherit its overall rotation and so
orbit the galactic center on ordered, circular orbits.
Future evolution
The Andromeda Galaxy and the Milky Way are approaching one another at a speed of 100 to
140 kilometers per second (62 to 87 mi/s).The collision will occur in about 2.5 -7.3 billion years.
Andromeda's tangential velocity with respect to the Milky Way is only known to within about a
factor of two, which creates uncertainty about the details of the collision. Such events are
frequent among the galaxies in galaxy groups.
The Earth and
the Solar
System in the
event of a
collision are
presently
unknown, but
there is a small
chance that the
Solar System
could be
ejected from the
Milky Way or
join
Andromeda.
The two
galaxies will
likely merge to
form a giant
elliptical galaxy.
Our Local Group of Galaxies