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The Pinwheel Galaxy (M101)
I Zwicky 18 - A Very Young Galaxy
I Zwicky 18, was once thought to be the youngest galaxy ever detected at between 500 million and one billion years old, and about 59 million light years away. A dwarf irregular galaxy, it is typical of the types of galaxy that formed in the early days of the Universe.  It is producing many new stars in the areas that show as intensely blue/white in the picture.  The outer tendrils are gas that has been heated by the radiation produced by star formation.  Recent Hubble images of I Zwicky 18 show a population of old faint stars intermixed  with the  bright stars.  Therefore it appears that the galaxy is about the same age as its neighbors, including our Milky Way, at about 10 billion years old.  The recent formation of many new stars is probably the result of gravitational disruption caused by I Zwicky 18's smaller companion galaxy, in the upper right of the image.
The Tadpole Galaxy (ARP 188)
The Tadpole Galaxy is a disrupted barred spiral galaxy.  It is located about 130 Mpc (420 million light years) from us in the direction of the constellation Draco.  Astronomers believe that a smaller galaxy, the Intruder, crossed in front of the Tadpole Galaxy, from left to right as viewed from Earth.  The Intruder was slung around behind the Tadpole by their mutual gravitational attraction.  During this close encounter, tidal forces drew out the spiral galaxy's stars, gas, and dust, forming  the  long, conspicuous  tail, which  is  about  280,000 light-years long.  The intruder galaxy itself, estimated to lie about 300 thousand light-years behind the Tadpole, can be seen through the foreground spiral arms at the upper left.  The Tadpole Galaxy will probably lose its tail as it ages, and the clusters of massive blue stars will become smaller satellites of the Tadpole galaxy.     
ARP 220 is, in fact, two galaxies that collided, and are merging. It is about 250 million light-years away, and is classified as an Ultraluminous Infrared Galaxy as most of the energy it emits is in the far infrared.  The collision resulted in the formation of a huge number of new stars, several hundred million, located in a large number of star clusters in the central region of the galaxy.  There is an X-ray source at what was the center of one of the two pre-merger galaxies, with another less luminous source near to it.  These are probably supermassive black holes that were present in the two galaxies prior to the collision.  Over time, several hundred million years, the two black holes will probably merge into a larger supermassive black hole in the center of the combined galaxy.  The full size image includes  a second image for comparison taken by a ground based telescope.  It shows the phenomenal clarity and resolving power of the Hubble, which is equivalent to reading the date on a quarter from a mile away. 
Galaxy ARP 220
The Antennae galaxies, NGC 4038 and NGC 4039, are a pair of spiral galaxies that started to interact and merge a few hundred million years ago.  They are the nearest and, perhaps, youngest examples of a pair of colliding galaxies.  During the course of the collision, billions of stars will be formed. The brightest and most compact of these star birth regions are called super star clusters.  Nearly half of the faint objects in the Antennae image are young clusters containing tens of thousands of stars. The orange blobs to the left and right of image center are the two cores of the original galaxies and consist mainly of old stars criss-crossed by filaments of dust, which appears brown in the image. The two galaxies are dotted with brilliant blue star-forming regions surrounded by glowing hydrogen gas, appearing in the image in pink.

By age dating the clusters in the image, astronomers find that only about 10 percent of the newly formed super star clusters in the Antennae will survive beyond the first 10 million years. The vast majority of the super star clusters formed during this interaction will disperse, with the individual stars becoming part of the smooth background of the galaxy. It is however believed that about a hundred of the most massive clusters will survive to form regular globular clusters, similar to the globular clusters found in our own Milky Way galaxy.  They are approximately 14 Mpc (about 45 million light-years) away. 

The Antennae galaxies take their name from the long antenna-like "arms" extending far out from the nuclei of the two galaxies, as shown in the lower image. These "tidal tails" were formed during the initial encounter of the galaxies some 200 to 300 million years ago. They give us a preview of what may happen when our Milky Way galaxy collides with the neighboring Andromeda galaxy several billion years in the future.
The Antennae Galaxies
M101, called the Pinwheel Galaxy, is a giant spiral galaxy, approximately 170,000 light-years across.  Our Milky Way galaxy is approximately 100,000 light-years across.  It is estimated to contain at least one trillion (1012) stars. Look at the spiral arms for the very bright blue/white patches that represent areas where new stars are forming, and bursting into life.  In the cosmologically recent past, it is likely that M101 experienced a close encounter with another galaxy.  This may have caused the asymmetry seen in the photograph.  It would also amplify the density waves in the spiral arms leading to increased compression of the hydrogen causing the increase in star formation.  M101 is about 7.2 Mpc or 23.4 million light-years from Earth.  There is video of the Pinwheel Galaxy that shows its location in the sky, and zooms in to a close up view. 
WILLIAM & DEBORAH HILLYARD
Astronomy & Cosmology

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Hubble Space Telescope

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