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Galaxies in Space

GalaxiesStars beyond counting populate the universe. Most reside in dense groups known as galaxies. These 'island universes' come in many shapes and sizes, and contain anywhere from a few million stars to a trillion or more. Some are still churning out lots of new stars, while others are quietly living out their lives. And some galaxies are merging to form even bigger cities of stars. Even the smallest galaxies contain only a million stars or so. The Milky Way is home to several hundred billion stars. And the largest galaxies contain more than one trillion stars. Galaxies also contain vast clouds of gas and dust, which are the raw materials for new stars. Galaxies also contain vast quantities of "dark matter" -- matter that produces no detectable light or other form of energy, but that reveals its presence through its gravitational pull on the visible stars and gas. In the Milky Way, dark matter appears to account for more than 90 percent of the galaxy's total mass. Most of the dark matter resides in a "halo" that surrounds the galaxy's bright disk and extends hundreds of thousands of light-years into space. Galaxies are sprinkled throughout the universe. Only three galaxies outside the Milky Way are easily visible to the unaided eye -- the great galaxy in Andromeda (the Andromeda Nebula) and the Large and Small Magellanic Clouds. These are some of our nearest galactic neighbors. The farthest galaxies ever observed are more than 10 billion light-years away. These galaxies formed soon after the universe itself was born. In theory, if the universe lasts long enough, the galaxies will die. Their stars will burn out. Some of the stars will drift away, but some will fall into giant "black holes" that lurk in the hearts of most galaxies. Eventually, all galaxies will disappear from sight.Galaxies can be classified in several ways. The most common is a system developed by Edwin Hubble, which is based on the shapes of galaxies. The most beautiful galaxies are called spirals. The Milky Way is a spiral, and so is the Andromeda galaxy.

The Milky Way:In summer, in a clear place with dark night skies, an irregular glowing band arcs high overhead. The ancients likened it to a stripe of milk spilled across the sky. Today, we know that this band of light is the combined glow of countless millions of stars in the flat disk of the Milky Way galaxy, our galactic home. The Milky Way is a spiral galaxy, so seen from above it would look like a pinwheel. The Milky Way consists of a bulge of stars in the core, probably a thick bar of stars flanking the core, and bright spiral arms wrapping around the core. This pinwheel is about 100,000 light-years in diameter, but only about 2,000 light-years thick, so it forms a thin disk. Our solar system is about 27,000 light-years from its center. The vast Milky Way contains a myriad of features, including star clusters, stellar nurseries, and a jumbled region of stars, gas, and magnetic fields in the core. In the 18th century, William Herschel suggested that the Sun resides in a rotating disk of stars. Since the band of the Milky Way is roughly equally bright all around the sky, Herschel suggested that the Sun is in the middle of it. What Herschel didn't know is that our galaxy is full of dust. Elements like silicon, carbon, and iron are forged in the cores of stars and released into space late in the stars' lives. The dust obscures our view. The problem is like being dropped into a forest on a foggy day. You can see many trees in all directions, but you can't see very far in any direction. Fortunately, the dust is concentrated in the Milky Way's disk. The disk is surrounded by a roughly spherical halo that is relatively free of dust. But the halo contains about 200 globular star clusters, which are ball-shaped groups of hundreds of thousands of stars. In 1917, Harlow Shapley noted that most of the globular clusters appeared on one side of the sky. Based on this, he proposed that the Sun is near the edge of the galaxy's disk. He reasoned that the spherical halo of globulars is centered on the core of the Milky Way's disk. This meant that, from our off-centered vantage point, we see more globular clusters on one side of the sky. In spite of this breakthrough, the dimensions of the Milky Way and any detailed idea of its structure were poorly known. Most modern information came after the beginnings of radio and infrared astronomy. The reason is simple. Like visible light, radio and infrared are forms of energy, but with longer wavelengths. These wavelengths pass through the dust in the Milky Way, so they can reach radio and infrared telescopes on Earth. Recall that on a foggy day, your car headlights have a tough time penetrating the fog, but your car's radio works just fine. Radio astronomy provided the first new key to studying the disk of the Milky Way. Everything in the galaxy orbits the center of the disk. Objects nearer the center orbit faster than objects orbiting farther out. So by measuring the motions of many clouds of gas and dust, radio astronomers gave us our first murky insight into the structure of the galaxy's disk. Today, we know that the disk contains between 200 billion and 400 billion stars. A black hole perhaps four million times as massive as the Sun sits in the middle of the Milky Way. It's surrounded by giant stars, clouds of dust, and magnetic fields that make the core a dynamic place. The galaxy's main constituent is invisible "dark matter" that permeates the halo, extending several hundred thousand light-years in all directions. This material reveals its presence only through its gravitational pull on the Milky Way's visible stars and gas clouds. Dark matter may account for 90 percent of the Milky Way's total mass.

Antennae Galaxies:This image of the Antennae galaxies is the sharpest yet of this merging pair of 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. The two spiral galaxies started to interact a few hundred million years ago, making the Antennae galaxies one of the nearest and youngest examples of a pair of colliding galaxies. 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.The new image allows astronomers to better distinguish between the stars and super star clusters created in the collision of two spiral galaxies. 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. The Antennae galaxies take their name from the long antenna-like "arms" extending far out from the nuclei of the two galaxies, best seen by ground-based telescopes. 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 in several billion years.

Irregular Galaxies:Most galaxies can be categorized by their shape. Our own Milky Way is a spiral galaxy, for example, and the largest galaxies in the Universe are elliptical galaxies. But some galaxies defy categorization. These are the irregular galaxies, and each one is unique in shape, age and structure.Irregular galaxies are often chaotic in shape, with no central bulge or spiral arms. Although they used to have a more familiar shape, a dramatic collision with another galaxy has distorted their shape.Astronomers maintain two classifications of irregular galaxies. Irr-I galaxies have some structure, but they're still distorted enough that they can't be classified as spiral, elliptical or lenticular shaped. Irr-II galaxies don't have any structure at all.The nearby Magellanic Clouds were once thought to be irregular galaxies. Although astronomers have detected a faint barred spiral shape.There's only one irregular galaxy in the Messier catalog of objects, and that's M82; also known as the Cigar Galaxy. It's located in the constellation Ursa Major about 12 million light-years away, and is famous for its heavy amounts of star formation. In fact, in infrared light, M82 is the brightest galaxy in the sky. Even in visible light, it's 5 times brighter than the Milky Way. yourself


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