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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
Large-Scale Structure of the Universe Big Bang Cosmology Why is Pluto no longer a planet? IAU planet classification Observing with a Telescope

Class 5 Cosmology Class 6 Special Topics


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, Galaxy by counting in the sky. Our Sun and the long axis of Eighteenth-century, constructed this "map" of the the numbers of stars he saw in different directions appears to lie near the center of the distribution, 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

2

Luminosity is determined by the nature and period of the light curves. This was discovered in 1908 by Henrietta Leavitt.


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 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 Disk

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 t o dat e.
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

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.


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
Artist renditions

Population III stars

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