Andromeda Galaxy (M31)
"Space is big. Really big. You just won't believe how vastly hugely mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space." - The Hitchhiker's Guide to the Galaxy
Galaxies are large systems of stars and interstellar matter, typically containing several million to some trillion stars. We live in a large spiral galaxy called the Milky Way, which contains more than 2 billion stars. The Andromeda Galaxy is the nearest large galaxy to our own Milky Way Galaxy. Even so, there are approximately 2.2 million light years of void in between, an almost inconceivable distance.
Galaxies in the universe are not uniformly distributed as lonely "islands" among the void, but tend to form into groups ranging from a few to several thousand individual galaxies.
The Milky Way is a member of a smaller group of galaxies called the Local Group. Along with the Milky Way, the Local Group is dominated by two other large galaxies, the Andromeda Galaxy (M31) and the Triangulum Galaxy (M33). There are over 30 small galaxies in the Local Group, and the best known of these small galaxies are the Large and Small Magellanic Clouds, which can be seen by the naked eye in the Southern Hemisphere. The nearest of these is the Large Magellanic Cloud and is only 179,000 light years distant. The Magellanic Clouds are gravitationally bound to the Milky Way along with several recently discovered dwarf galaxies. The Andromeda Galaxy also has its satellite galaxies, the most well known being M32 and M110, which can be seen in the picture at the top of this page.
The Local Group itself is part of a large group of galaxies called the Virgo Supercluster, which is dominated by the Virgo Cluster of galaxies, familiar to amateur astronomers who have pointed their telescopes toward the constellation of Virgo. Messier discovered the Virgo Cluster, although at the time the nature of galaxies was not understood by astronomers, so he called it a concentration of nebulae. He cataloged 16, but the Virgo Cluster contains several dozens of large and thousands of small galaxies.
Large clusters of galaxies like the Virgo Cluster tend to attract and incorporate small groups and individual galaxies in their neighborhood. It is not yet known whether or not our Local Group will be swallowed up by the Virgo Cluster at some time.
By "zooming-out" by a factor of 10 from the previous map and showing the universe within 1 billion light years, we can appreciate the nonuniform distribution of the clusters and superclusters. Three true intergalactic voids are shown in the image.
Quasars: Ancient Infernos in the Deep Beyond
Quasar is an acronym for Quasi-Stellar Radio Source. In the 1950s astronomers began point a new sort of telescope at the heavens, radio telescopes that could observe celestial emissions of radio waves rather than visible light. At the time they were just experimenting and had no idea whether or not listening to radio waves from the heavens would provide any useful scientific knowledge. A decade later in the 1963 the first quasar was discovered using radio telescopes. When astronomer pointed their telescopes at the object, it had the appearance of a single star, and given these two facts, they called it a Quasi-Stellar Radio Source, or Quasar for short. Later, however, it was discovered that these quasars were not stars in our Galaxy. In fact they were distant objects, very distant. They determined that the quasars were farther away than any of the distant galaxies seen.
The first quasar discovered by radio astronomers in 1963 was 3C 273, a radio source that appeared to be a star-like object. They expected it to be a new type of radio-emitting star, and so they split its light into a spectrum looking for the familiar dark absorption lines telling them what elements were present in the star. The lines turned out to make no sense at all to them. Pondering this, they soon realized that they were looking at the typical absorption lines of hydrogen, the most abundant element in the universe, only shifted towards the red end of the spectrum. With the Doppler effect, light from an approaching object is blue-shifted and light from a receding object is red-shifted. Working out the red shift of the hydrogen lines in 3C 273, astronomers were able to discover how fast it was going away from us, and therefore how distant it must be. Quasar 3C 273 was found to be an astonishing 2 billion light years away, making it the most distant object known in universe at that time! At this great distance it was too amazingly bright to be a star. In fact it was giving out 100 times more light than the most brilliant galaxies.
The Brightest Quasar
Distance: 2,000,000,000 (ly) | Visual Brightness: 12.8 variable (mag)
Many more quasars have been discovered since the discovery of 3C 273, and they all are the most distant objects in the visible universe. When we look at light from the heavens, we are looking back into time as well as into space. When you look at light from the Proxima Centauri star, for example, the closet star to our Sun, the light you are seeing left the star over 4 years ago, since Proxima Centauri is 4.22 light years away. Since the quasars are billions of light years away, the light we are seeing is truly ancient, meaning that quasars must have been among the first visible objects to form after the Big Bang.
Astronomers now believe that quasars are a special sort of galaxies. Since they are billions of light years away, astronomers had to come up with an explanation for their incredible brightness. The best theory so far says that a quasar's energy is generated by a giant black hole at its center, up to a billion times more massive than the Sun. This black hole is voraciously sucking matter from a nearby galaxy. Even though nothing including light can escape from the black hole itself, the region around it has become very hot as gravity pulls and compresses matter into the black hole. The super-heated matter surrounding the black hole is actually what gives the quasar its brilliant light. Some quasars can change the amount of light they give out over the course of days, meaning the region producing all the energy must be as small as a few light-days in diameter! No large object can change its light output faster than the time it takes light to pass the change through the object.
Above is a gallery of portraits of six quasars taken by the Hubble Space Telescope. The quasars in the two columns on the right show associated galaxies breaking up because they are merging with the quasars. The galaxies should provide plenty of debris to feed the hungry black hole in the center of the quasar.
Incidentally, pulsars, like quasars, are radio sources but the similarity ends there. First of all, pulsars are located right here in the Milky Way Galaxy. They are a very special type of dead star, spinning rapidly and sending out flashes of radiation including radio waves in tight beams. Many, if not all dead neutron stars are pulsars, but we can only see the pulsing effect if the beam of radiation travels toward the Earth. A neutron star is the aftermath of a giant stellar explosion called a supernova; it is the core of the original star collapsed and compressed to a sphere only a few kilometers across. During this collapse the the core begins to spin more and more rapidly, generating pulses of either radio waves or X-rays. When they were first discovered, pulsars were thought to be beacons from an extraterrestrial intelligence. The original pulsar was even nicknamed LGM-1 (LGM standing for Little Green Men), but its proper name is PSR 1919+21. More than 500 pulsars have been since discovered, some in regions that are very obviously the remains of supernovas. The most famous of these pulsars is the Crab Pulsar at the heart of the Crab Nebula (M1) supernova remnant.
Listen to the sounds of pulsars:
Please contact Adam if you have questions or comments about this page. Research and image sources are provided when possible. Adam's Page
Images shown here are for noncommercial educational purposes.
The photograph of Andromeda and the 3C 273 quasar are from the SEDS Messier Database.
The maps of the galaxies on this page were created by Richard Powell and are on his site of star maps.
The gallery of quasars image if from the NASA Astronomy Picture of the Day site.