Astronomy Magazine is an excellent resource for amateur astronomers and enthusiasts of the discipline (such as myself). Founded in 1973, the magazine is celebrating its 35th anniversary this year. In commemoration it has produced a special issue that has as one of several articles a breakdown of the Top 10 Astronomical Discoveries from 1973 to the present time.
1. Dark Energy - In 1998 it was reasoned based on discoveries by Saul Perlmutter and Brian Schmidt that the expansion of the universe is accelerating. What causes this acceleration is believed to be a force produced by Dark Energy. What exactly Dark Energy is, is any one's guess right now but data from the Wilkinson Microwave Anistropy Probe (one of our most important Eyes in the Sky) seems to indicate that it comprises 72% of the mass of the universe (wow!). Dark Matter (another mystery) is believed to make up 23% of the mass of the universe with visible matter making up the remaining 5%.
2. Exoplanets - These are planets that orbit stars other than that of our sun. A variety of techniques are used to locate them of which gravitational lensing is one of the most important. At present we have identified almost 300 Exoplanets since the first one was revealed in 1990. The smallest exoplanet has a mass five times that of Earth and orbits the red-dwarf star Gliese 581.
3. The Missions of the Voyager spacecraft - Launched in 1977, Voyager 1 and 2, have discovered numerous moons around the outer planets. They also showed that the Giant Red Spot on Jupiter is a high-pressure system, that the planet has an auroral glow near its poles and has a complex ring system (made up of ringlets) encircling it. Data about the volcanic nature of the Jovian moon - Io, has also come to light while discoveries have been made about Europa (it is now believed to be a very young moon with few impact craters), Callisto (has many craters) and Gannymede (has a rough terrain).
The Voyagers showed that Saturn's large moon Titan has a nitrogen-rich atmosphere with a pressure level equivalency of fifty times that present on the Earth's surface. Uranus' strong magnetic field has also been documented thanks to the Voyagers as have revelations regarding Uranus' ring system. Neptune is now believed to be a site for large storms, with its moon Triton (which has a thin atmosphere) demonstrating geyser-like eruptions.
Both spacecraft have crossed the solar system boundary and are presently 13 billion kilometers away from the Sun.
4. Inflation during the Early Big Bang - Why is there almost no temperature variation in the cosmos despite the fact the ancient cosmos did not have time to reach thermal equilibrium? The best answer to this question resides with a model developed by MIT physicist Alan Guth in 1981. Guth argues that within in a small fraction of a second of the Big Bang the cosmos underwent a rapid inflation or hyper-expansion. The end result is the system of thermal equilibrium and a flat universe. Any evidence for inflation? Yes according to the Wilkinson Microwave Anisotropy Probe (the same device that provided evidence that the universe is 13.7 billion years old) which revealed support for this idea from data obtained in 2006. Now theorists are on work expanding (no pun here) the inflation theory to include such notions as 'eternal inflation' and multiverses.
5. Black Holes exist - Stephen Hawking can sleep better at night knowing this fact courtesy of discoveries made by the Hubble Telescope. It appears as though Black Holes exist at the center of most if not all galaxies). What is the mass of one of these black holes? Somewhere in the order of several million to a billion Sun's (and you thought you had a weight problem).
6. Gamma Ray Bursts - The bursts originate from the distant universe, occur at a frequency of one burst per day (as evidenced by NASA's Compton Gamma-Ray Observatory) and produce more energy in a few seconds than the sun will over its 10 billion year lifespan. The cause of these bursts are thought to be intense supernovae. Short burst often arise from neutron stars or black holes.
7. Galaxies eat one another for fun - Believe it or not but they do with our own Milky Way and the neighbouring Andromeda galaxy being two of the worst predatory culprits within the Local Group. Once such victim is the Sagittaurius Dwarf Galaxy that resides about 100,000 light years from Earth - another is Omega Centauri. Cannibalized galaxies generally appear as massive globular clusters.
8. Supernova 1987A - The closest supernova to Earth since the discovery of the Telescope - Supernova 1987A's stellar blast occured on February 24th 1987 (it is located about 160,000 light years from Earth in the Large Magellanic Cloud). One of its gifts to Earth were trillions and trillions of neutrinos of which two dozen were captured by detectors on our home planet. The region is currently glowing as shock waves run into the gaseous rings. To see some photographs of this go to Supernova1987A.
9. The Great Attractor - Our galaxy is part of a Local Group that is moving toward the Virgo Supercluster but in the 1970's observations indicated that this supercluster is in itself moving towards a Great Attractor's Center that contains up to 100,000 galaxies and is located 250 million light years from Earth.
10. The Milky Way is a Barred Spiral - Yes our home galaxy is not a true spiral but contains a bar that extends 27,000 light years and tilts at an angle of 45 degrees to a line from the sun to the galaxy's center.