MILKY WAY GALAXY
The Milky Way is our own galaxy viewed from the inside.
It is a vast collection of more than 200 billion stars, planets, nebulae, clusters, dust and gas. Our own sun and solar system are also part of the Milky Way galaxy.
Seen as a luminous band of light and star clouds that stretch across the night sky, the brighter star clouds of the Milky Way are frequently mistaken for real Earth-bound clouds by observers who are unfamiliar with their appearance. Sometimes interrupted by dark nebulae and rifts, these star clouds are actually innumerable unresolved faint stars. From a really dark location, the Milky Way is even bright enough to cast shadows.
The central bulge of the Milky Way, the brightest and most spectacular portion, is visible here in the Sagittarius - Scorpius region. Vega and Lyra are at upper left in this photo, and Alpha and Beta Centauri (Rigel-Kentaurus and Hadar respectively) are the two brightest stars at the right at the end of a dark rift in the Milky Way.
The brightest star cloud in the photo is the large Sagittarius star cloud, seen almost in the center of the photo. Numerous red emission nebula, clouds of glowing gas in space are seen sprinkled throughout the image, as well as dark nebula, clouds of non-luminous dust that obscure the stars behind them, such as the great rift in the Milky Way at left. Also visible in this photo is the Zodiacal Light, seen as a faint, low contrast, triangle of light at the lower left pointing towards the center of the frame. It is more easily seen here if you do not look directly at the photo, sort of like using averted vision while observing with a telescope.
Halley's Comet is also visible above the dark rift that ends at Alpha Centauri. It is not spectacular here, but a faint dust tail can be seen by keen observers.
Recent observations indicate that the Milky Way is a giant as spiral galaxies go, with a mass of possibly more than 750 billion solar masses, and a diameter of 100,000 light years. M31, the Andromeda Galaxy, appears to be larger, but the Milky Way is more massive.
Our Milky Way is a spiral galaxy of Hubble type Sb or Sc and a member of the local group, a group of galaxies which contain the Milky Way, Magellanic Clouds, M33, and the M31 - M32 - M110 system, and many other small galaxies.
Our own solar system is located about 28,000 light years from galactic center, in a relatively quiet section of an outer arm.
The Milky Way is a Spiral Galaxy and is where our Solar System is located. Our Solar System is more on the west side of the Milky Way. The Milky Way is about about 100,000 light years wide and 3,000 light years tall. There is a bulge in the center of it. The Milky Way also has a halo that has a radius of 50,000 light years, which is above the nucleus. The halo is made up of very old stars called globular clusters.
The Milky Way, it turns out, is no ordinary spiral galaxy. According to a massive new survey of stars at the heart of the galaxy by Wisconsin astronomers, including professor of astronomy Edward Churchwell and professor of physics Robert Benjamin, the Milky Way has a definitive bar feature -- some 27,000 light years in length -- that distinguishes it from pedestrian spiral galaxies, as shown in this artist's rendering. The survey, conducted using NASA's Spitzer Space Telescope, sampled light from an estimated 30 million stars in the plane of the galaxy in an effort to build a detailed portrait of the inner regions of the Milky Way.
Explanation: Inspired during a visit to Fort Davis, Texas, home of McDonald Observatory and dark night skies, photographer Larry Landolfi created this tantalizing fantasy view. The composited image suggests the Milky Way is a heavenly extension of a deserted country road. Of course, the name for our galaxy, the Milky Way (in Latin, Via Lactea), does refer to its appearance as a milky band or path in the sky. In fact, the word galaxy itself derives from the Greek for milk. Visible on moonless nights from dark sky areas, though not so colorful as in this image, the glowing celestial band is due to the collective light of myriad stars along the plane of our galaxy, too faint to be distinguished individually. The diffuse starlight is cut by dark swaths of obscuring galactic dust clouds. At the beginning of the 17th century, Galileo turned his telescope on the Milky Way and announced it to be composed of innumerable stars.
If our solar system was the size of a coffee cup, the Milky Way Galaxy would be the size of the North American Continent.
Since ancient times people have speculated about the nature of the hazy band of light that stretches around the entire sky. It is widest and brightest in the summer sky, especially in Sagittarius. There is a long twisty dark lane through Cygnus known as the Great Rift. In autumn the path winds north past Cassiopeia and Perseus, in winter past Orion, and in Spring it reaches down to the Southern Cross.
Our galaxy appears to be in the shape of a big pancake with a bulge in the middle. Our solar system is embedded inside the pancake about half way between the edge and the middle. When we try to look out along the edges we see the combined light of billions of stars. Most of those stars are too far away to pick out individually but together they added up to a milky haze.
Our galaxy is about 100,000 light years across. The central bulge is about 16,000 light years thick. The thinner region where our solar system resides is about 3000 light years thick. Our solar system orbits around the core once every 200 million years. The total number of stars in the Milky Way is probably several hundred billion.
The core of our galaxy lies in the direction of Sagittarius. We have detected that stars in that region are circling the center at very high speed. The simplest explanation for why those stars can travel so fast without flying completely out of the galaxy is that there is a super massive black hole in the core. The mass of the black hole is estimated at 2.7 million times the mass of the Sun.
The disk and central bulge are only the obvious parts of the galaxy; the parts that glow in the dark. There is also a part that we can't see with our eyes but can be detected by other means, directly and indirectly. We can directly measure light outside the range of human eyes, such as infrared and ultraviolet. We can also deduce where mass exists by its gravitational effect on other objects. We have concluded that the visible disk of the galaxy is surrounded by a huge sphere of material we call the halo.
One of the very visible populations in the halo is the globular clusters. These are the oldest objects made out of stars in the Universe. The globulars formed long before the birth of galaxies. When the galaxies came along, the globulars were caught by the gravity and have been orbiting around them ever since. There are about 200 globular clusters orbiting in the halo of the Milky Way.
The Milky Way Galaxy is just one of hundreds of billions of galaxies that populate our universe. Our galaxy's name - the Milky Way - is a translation from the Latin "Via Lactea" which is derived from the Greek word "Kiklios Galaxios" which means milky circle. If you go to a dark sky location, the plane of the Milky Way appears as a swath of light across the night sky. There is a Greek myth for the origin of the Milky Way. That is that the baby Heracles is brought by Zeus to Hera to drink her milk so he could become immortal and the Milky Way is nothing more than spilt milk.
It was the Greek Democritus (460-370BC) who first claimed that the Milky Way consisted of distant stars. William Herschel in 1785 made the first map of the Milky Way. Herschel was the first to study and measure the distribution of stars in space. He counted the stars he could see and concluded that the stars were grouped into a huge disk formation and he was right.
In the above polar view of the Milky Way we can see the spiral arms and the dense collection of stars that make up core of the galaxy. The entire galaxy is rotating in a clockwise direction. As to classification, the Milky Way is a SBc barred spiral galaxy. And that red dot above the Orion Arm is the location of the Solar System.
If we look at the Milky Way edge-on from outside our galaxy, We would see a large, flat disk of stars, dust and gas with a dense central ball of stars. Above and below the plane of the Milky Way we would see globular clusters whose distribution is centered about the center of the galaxy. We can't clearly see the galactic center from our location because the interstellar dust clouds between us and the galactic nucleus reduce the amount of light that reaches us from the nucleus by a factor of about 1013.
The Age of the Milky Way Galaxy
With estimates of the age of the universe centered around 14 or so billion years old, estimates for the age of the Milky Way galaxy range from 800 million to 13.5 billion years old.
The Size of the Milky Way Galaxy
Estimates of the size of the Milky Way galaxy vary due to a degree of uncertainty in the observations used to make these determinations. Estimates for the diameter of the Milky Way galaxy range from 100,000 light years up to 120,000 light years.
As to the size of the galactic bulge that makes up the core of the galaxy, it's diameter is estimated at around 30,000 light years in the north-south direction. The diameter in the equatorial plane is estimated to be 40,000 light years.
Out in the disk of the galaxy, which is where our solar system is located, the thickness of the disk is estimated to be 1,000 light years.
The Mass of the Milky Way Galaxy
One method that astronomers have used to calculate the mass of the Milky Way galaxy is to use Kepler's 3rd law that relates orbit radius and period with central mass. We figure out the radius of our orbit relative to the center of the galaxy and the period (the time it takes to complete one complete orbit). Plugging those numbers into Kepler's 3rd law we can estimate the mass inside our orbit. Doing the math we come up with a number that is just shy of 100 billion solar masses, with one solar mass being equal to the mass of our Sun. Overall mass estimates of the Milky Way are on the order of 750 billion to 1 trillion solar masses.
The Number of Stars in the Milky Way Galaxy
If we assume that the average star has a mass equal to the Sun's, we can use the number we arrived at for the galaxy's mass to come up with an estimate for the number of stars. Given that our calculation in the previous paragraph yielded a mass of 100 billion solar masses, we could guess that there are approximately 100 billion stars in the galaxy. Of course somewhere between 2.5 and 3 million of the solar masses are locked up in a super massive black hole named Sagittarius A that's at the center of the galaxy. Other more thorough estimates yield numbers of stars that range up to a high of 400 billion.
Chronology of Space Exploration
The USSR's Sputnik electrified the world in 1957 and launched the age of space exploration. Sputnik showed that it could happen and that it was an important thing to do. With its catch-up satellite Explorer in 1958, America made an important leap forward in the vital discovery of the Van Allen Radiation Belts.
Both nations leaped wholeheartedly into exploration:
The first American satellite was Explorer 1.
The U.S. Army Ballistic Missile Agency blasted Explorer 1 into orbit from the Air Force Missile and Test Center at Cape Canaveral, Florida, on January 31, 1958. The Army used a modified Jupiter-C military rocket to send the tiny satellite aloft.
With Explorer 1, the United States showed it could compete with the Soviet Union, which had launched two artificial satellites to orbit within the previous three months. The Soviets had electrified the public when it launched the world's first human-made satellite, Sputnik 1, on October 4, 1957.
The rocket project was headed by Wernher Von Braun at the U.S. Army Ballistic Missile Agency headquarters at Huntsville, Alabama.
The Explorer 1 satellite was built by William Pickering and a team at the Jet Propulsion Laboratory, Pasadena, California. At the State University of Iowa, physicist James Van Allen and graduate student Wei Ching Lin built the cosmic ray Geiger counters that were the science instruments inside Explorer.
Explorer 1 was the first satellite to carry science instruments. It followed a looping flight path that orbited Earth once every 114 minutes. The satellite went as high as 1,594 miles (2,565 kilometers ) and as low as 225 miles (362 kilometers) above Earth.
Explorer 1 weighed 18 lbs., which compared with a weight of 1,200-lbs. for Sputnik 2, which also carried a live dog to orbit. However, Van Allen's science package inside Explorer discovered a previously-unknown radiation belt around our planet. It was named the Van Allen Belt and was recognized as the greatest science contribution of the International Geophysical Year (1958).
The results of the launches of Sputnik-1 and Explorer-1 were the Space Age, public interest in technology, growth of a high-tech economy, and an increased emphasis on science education.
Chronology of Space Exploration
The USSR's Sputnik electrified the world in 1957 and launched the age of space exploration. Sputnik showed that it could happen and that it was an important thing to do. With its catch-up satellite Explorer in 1958, America made an important leap forward in the vital discovery of the Van Allen Radiation Belts.
Both nations leaped wholeheartedly into exploration:
1959: The USSR probe Luna 1 was the first human-made object to leave Earth's gravity.
1959: The USSR probe Lunik 2 was the first human-made object to reach another world when it crashed on the Moon.
1959: The USSR probe Lunik 3 was the first spacecraft to go behind the Moon.
1960: Echo, the first communication satellite, was launched by the U.S.
1960: The U.S. launched TIROS, the first weather satellite.
1962: NASA launched Telstar, the first active real-time communications satellite.
1962: The U.S. probe Mariner 2 completed the first successful flyby of another planet, Venus.
1965: The USSR spacecraft Venera 3 was the first to reach another planet when it crashed into Venus.
1965: The U.S. probe Mariner 4 completed the first successful flyby of Mars.
1970: The USSR's spacecraft Venera 7 transmitted data from the surface of Venus.
1971: The U.S. spacecraft Pioneer 10 was launched to become the first human-made object to leave the Solar System.
1971: The USSR spacecraft Mars 3 made a soft landing on Mars.
1971: The U.S. spacecraft Mariner 9 flew into orbit around Mars.
1973: The U.S. spacecraft Pioneer 10 flew past Jupiter.
1974: The U.S. spacecraft Mariner 10 flew past Mercury.
1976: The U.S. spacecraft Viking 1 and 2 landed on Mars.
1977: Voyager 1 and 2 left Earth for Jupiter, Saturn, Uranus and Neptune.
1979: The U.S. spacecraft Voyager 1 and 2 flew past Jupiter.
1979: The U.S. spacecraft Pioneer 11 flew past Saturn.
1980: The U.S. spacecraft Voyager 1 flew past Saturn.
1981: The U.S. spacecraft Voyager 2 flew past Saturn.
1983: The U.S. spacecraft Pioneer 10 left the Solar System.
1986: The U.S. spacecraft Voyager 2 spacecraft flew past Uranus.
1986: Halley's Comet returned to the inner Solar System and was visited by spacecraft from the U.S., USSR, Japan and Europe.
1989: The U.S. spacecraft Voyager 2 flew by Neptune,
1990: The Hubble Space Telescope was launched.
1990: The Magellan spacecraft arrived at Venus.
1991: The Compton Gamma Ray Observatory telescope was launched.
1990s: Cosmic Background Explorer (COBE) satellite seemed to support the Big Bang theory.
1993: Mars Observer launched, but failed three days before arriving there.
1996: Mars Global Surveyor launched.
1997: Pathfinder, with its rover Sojourner, landed on Mars.
1997: Cassini probe, carrying Huygens, launched to Saturn.
1998: Japan launched its unsuccessful Nozomi probe toward Mars.
1999: NASA lost the Climate Orbiter and the Polar Lander spacecraft as they arrived at Mars.
1999: The Chandra X-Ray Observatory telescope was launched.
2001: The U.S. probe Mars Odyssey was launcxhed
2003: Spitzer Infrared Telescope launched.
2003: The European Space Agency launched its successful probe Mars Express carrying the Beagle 2 lander, which failed.
2003: The U.S. sent the successful Mars exploration rovers Spirit and Opportunity.
2003: The European probe Mars Express arrived at Mars.
2004: The Mars exploration rovers Spirit and Opportunity landed.
2004: Cassini probe arrived at Saturn.
2004: NASA launched MESSENGER to fly by Venus and on to Mercury.
2005: Huygens probe landed on Saturn's big moon Titan.
2005: NASA launched Mars Reconnaissance Orbiter
2005: The European Space Agency launched its Venus Express spacecraft.
2006: Mars Reconnaissance Orbiter flew into Mars orbit.
2006: The European Space Agency's Venus Express arrived at Venus.
2006: NASA launched New Horizons probe to Pluto and Solar System bodies beyond.
2007: NASA launched the robot explorer Phoenix to Mars. (August)
2007: Japan launched Kaguya Moon probe. (September)
2007: China launched Chang'e Moon probe. (October)
2008: India plans launch of Chandrayaan Moon probe. (April)
2008: U.S. plans launch of Lunar Reconnaissance Orbiter Moon probe. (October)
2012: Russia plans launch of Moon probe.
2020: South Korea plans launch of Moon probe.
