Who are we? We find that we live on an insignificant planet of a humdrum star lost in a galaxy tucked away in some forgotten corner of a universe in which there are far more galaxies than people. ~ Carl Sagan

Galaxies

Veteran night sky photographer Brad Goldpaint took the amazing photo of the Milky Way (below right) over Mount Shasta, California, during three years of astronomical photo sessions. The image is featured in Goldpaint's night sky observing video "Within Two Worlds." Copyright 2012 Goldpaint Photography.

The Milky Way galaxy is most significant to humans because it is home sweet home. But when it comes down to it, our galaxy is a typical barred spiral, much like billions of other galaxies in the universe. A glance up at the night sky reveals a broad swath of light. Described by the ancients as a river, as milk, and as a path, among other things, the band has been visible in the heavens since Earth first formed. In reality, this intriguing line of light is the center of our galaxy, as seen from one of its outer arms. The Milky Way is a barred spiral galaxy, about 100,000 light-years across, contains over 200 billion stars, and enough dust and gas to make billions more.


The solar system lies about 30,000 light-years from the galactic center, and about 20 light-years above the plane of the galaxy. More than half the stars found in the Milky Way are older than the 4.5 billion year old sun.

The most common stars in the galaxy are red dwarfs, a cool star about a tenth the mass of the sun. Once thought unsuitable for potential life-bearing planets because such bodies would have to be too close to meet the criteria, red dwarfs are now considered potential suspects.

As late as the 1920s, astronomers thought all of the stars in the universe were contained inside of the Milky Way. It wasn't until Edwin Hubble discovered a special star known as a Cepheid variable, which allowed him to precisely measure distances, that astronomers realized that the fuzzy patches once classified as nebula were actually separate galaxies.

 
If you could look down on it from the top, you would see a central bulge surrounded by four large spiral arms that wrap around it. Spiral galaxies make up about two-third of the galaxies in the universe. Unlike a regular spiral, a barred spiral contains a bar across its center region, and has two major arms. The Milky Way also contains two significant minor arms, as well as two smaller spurs. One of the spurs, known as the Orion Arm, contains the sun and the solar system. The Orion arm is located between two major arms, Perseus and Sagittarius.

The Milky Way does not sit still, but is constantly rotating. As such, the arms are moving through space. The sun and the solar system travel with them. The solar system travels at an average speed of 515,000 miles per hour (828,000 kilometers per hour). Even at this rapid speed, the solar system would take about 230 million years to travel all the way around the Milky Way.

Curled around the center of the galaxy, the spiral arms contain a high amount of dust and gas. New stars are constantly formed within the arms. These arms are contained in what is called the disk of the galaxy. It is only about 1,000 light-years thick. At the center of the galaxy is the galactic bulge.

The heart of the Milky Way is crammed full of gas, dust, and stars. The bulge is the reason that you can only see a small percentage of the total stars in the galaxy. Dust and gas within it are so thick that you can't even peer into the bulge of the Milky Way, much less see out the other side. Tucked inside the very center of the galaxy is a monstrous black hole, billions of times as massive as the sun. This super-massive black hole may have started off smaller, but the ample supply of dust and gas allowed it to gorge itself and grow into a giant. The greedy glutton also consumes whatever stars it can get a grip on. Although black holes cannot be directly viewed, scientists can see their gravitational effects as they change and distort the paths of the material around it, or as they fire off jets. Most galaxies are thought to have a black hole in their heart.

The bulge and the arms are the most obvious components of the Milky Way, but they are not the only pieces. The galaxy is surrounded by a spherical halo of hot gas, old stars and globular clusters. Although the halo stretches for hundreds of thousands of light-years, it only contains about two percent as many stars as are found within the disk. Dust, gas, and stars are the most visible ingredients in the galaxy, but the Milky Way is also made up of dark matter. Scientists can't directly detect the material, but like black holes, they can measure it based on its effect on the objects around it. As such, dark matter is estimated to make up 90 percent of the mass of the galaxy.

Not only is the Milky Way spinning, it is also moving through the universe. Despite how empty space might appear in the movies, it is filled with dust and gas and other galaxies. The massive collections of stars are constantly crashing into one another, and the Milky Way is not immune.

In about four billion years, the Milky Way will collide with its nearest neighbor, the Andromeda Galaxy. The two are rushing towards each other at about 70 miles per second (112 km per second). When they collide, they will provide a fresh influx of material that will kick of star formation anew.

The Andromeda Galaxy is obviously not the most careful of drivers. It shows signs of having already crashed into another galaxy in the past. Although it is the same age as the Milky Way, it hosts a large ring of dust in its center, and several older stars. Of course, the imminent collision shouldn't be a problem for inhabitants of Earth. By the time the two galaxies ram headlong, the sun will already have ballooned into a red giant, making our planet uninhabitable.
Other Galaxies
Arp 81 - Two merging galaxies. Distance: 280,000,000 light years. Diameter: 200,000 light years.

Centaurus A - Elliptical galaxy with a prominent dust lane. Possesses radio jets of a similar size to those of Hercules A. Distance: 13,000,000 light years. Diameter: 97,000 light years.

Cygnus A - Elliptical galaxy with bright radio jets. Distance: 760,000,000 light years. Diameter: 500,000 light years (of jets).

ESO 350-40 - Cartwheel - a ring galaxy. Distance: 500,000,000 light years. Diameter: 150,000 light years.

ESO 507-070 - Merger remnant of two galaxies. Distance: 300,000,000 light years. Diameter: 240,000 light years.

Hercules A - Giant elliptical galaxy with powerful radio jets powered by a super massive black hole at the galaxy's center.
Distance: 2,100,000,000 light years. Diameter: 1,500,000 light years.
 

Hoag's Object - Elliptical galaxy surrounded by a ring of blue stars. Distance: 600,000,000 light years. Diameter: 120,000 light years (of outer ring).

M31 Andromeda - Nearby spiral in our Local Group. About as massive as the Milky Way. Distance: 2,500,000 light years. Diameter: 220,000 light years.

M33 Triangulum - Smaller spiral in our Local Group. Distance: 2,700,000 light years. Diameter: 50,000 light years.

Malin 1 - Arguably the largest spiral galaxy. Normal stellar disc embedded in a huge, very faint halo. Distance: 1,400,000,000 light years. Diameter: 30,000 light years (inner disc). Diameter: 650,000 light years (outer disc).

Messier 87 - A giant elliptical at the center of the Virgo Cluster. At its center, material falling onto a super massive black hole is emitting powerful jets. Distance: 53,000,000 light years. Diameter: 980,000 light years.

Messier 100 - Spiral galaxy in the Virgo Cluster. Distance: 55,000,000 light years. Diameter 160,000 light years.

M104 Sombrero - Spiral galaxy with a prominent bulge and dust ring. Distance: 28,000,000 light years. Diameter: 50,000 light years.

NGC 1316 - Fornax A - Dusty elliptical galaxy. Distance: 62,000,000 light years. Diameter 220,000 light years.

NGC 1365 - Barred spiral galaxy in the Fornax Cluster. Distance: 61,000,000 light years. Diameter: 200,000 light years.

NGC 4921 - A spiral galaxy in the Coma Cluster, it has lost much of its gas and can no longer form many new, blue stars, giving it an unusually pale appearance. Distance: 320,000,000 light years.

NGC 6670 - Two interacting galaxies seen edge-on. Distance: 400,000,000 light years. Diameter: 120,000 light years.
NGC 6872 - Claimant for title of largest spiral galaxy, disturbed by recent interactions with IC 4970. Distance: 220,000,000 light years. Diameter: 520,000 light years.

NGC 7049 - Elliptical galaxy with an unusual dust ring. Distance: 90,000,000 light years.
Diameter: 150,000 light years.

Starburst NGC 908 - Starbursting galaxy with disturbed spiral arms. Distance: 65,000,000 light years. Diameter: 75,000 light years.

UGC 10214 Tadpole - Disturbed spiral galaxy with a very long tail. Distance: 420,000,000 light years. Total length: 390,000 light years.
Formation of Galaxies

After the Big Bang, the Universe was composed of radiation and subatomic particles. What happened next is up for debate - did small particles slowly team up and gradually form stars, star clusters, and eventually galaxies? Or did the Universe first organize as immense clumps of matter that later subdivided into galaxies?

Theoretical investigations indicate that galaxies formed from a diluted but lumpy mixture of hydrogen and helium gas - the primordial elements forged in the Big Bang.

The investigations also indicate that two vastly different scales of mass prevailed less than 100 million years after the Big Bang, which ultimately affected the formation of galaxies. Matter either was clumped into vast collections more than a million times the mass of the Milky Way, or into small clumps one million times smaller than the mass of our Milky Way. Superclusters of galaxies may have evolved from the former. Globular clusters may have evolved from the latter.

As we look deeper into the Universe and therefore back in time, galaxies appear to emit more of their light in the blue part of the visible spectrum. This blue light is a sign that very young, massive and luminous stars are forming. Since we see these galaxies as they were between 5 and 10 billion years ago, we appear to be witnessing events that occurred within a few billion years after these galaxies were formed.

Astronomers also have noticed that as they examine the images of these distant blue galaxies, the images are frequently distorted or contain what appear to be multiple nuclei. The Milky Way seen at a similar great distance would look like a uniformed flattened disk, with a single bright nucleus -- the galactic center. Nearby "multiple-nuclei" galaxies that have been studied show the cores of individual galaxies colliding and merging into one single system of stars and gas. These collisions are violent, and take millions of years to play out. But in at least some instances, such as NGC 1275, recently observed with the Hubble Space Telescope, galaxy collisions can actually trigger the formation of massive stars.

In the depths of space, we may be witnessing collisions between smaller galaxies triggering the formation of massive luminous stars. The images, rich in blue light, gives tantalizing evidence that "environment" may have been more important than cosmic "genetics." Galactic cannibalism was far more common in the ancient past. Galaxies may have grown to their current size by consuming their neighbors. The ultimate building blocks may indeed have been the paltry million-solar-mass clumps that theoreticians believe were abundant before the Universe was a few million years old.

Source: NASA Space Short, NASA Headquarters, Washington, D. C.