The most widely used classification scheme for galaxies is based on one devised by Edwin P. Hubble and further refined by astronomer Gerard de Vaucouleurs. It uses the three main types, and then further breaks them down by specific characteristics (openness of spirals, size and extent of bars, size of galactic bulges). In this age of multi-wavelength observing, the subclassifications also include markers for such characteristics as a galaxy’s star-formation rate and age spectrum of its stars.
Spiral galaxies are the most common type in the universe. Our Milky Way is a spiral, as the rather close-by Andromeda Galaxy. Spirals are large rotating disks of stars and nebulae, surrounded by a shell of dark matter. The central bright region at the core of a galaxy is called the “galactic bulge”. Many spirals have a halo of stars and star clusters arrayed above and below the disk. Spirals that have large, bright bars of stars and material cutting across their central sections are called “barred spirals”. A large majority of galaxies have these bars, and astronomers study them to understand what function they play within the galaxy. In addition to bars, many spirals may also contain supermassive black holes in their cores. Subgroups of spirals are defined by the characteristics of their bulges, spiral arms, and how tightly wound those arms are.
Elliptical galaxies are roughly egg-shaped (ellipsoidal or ovoid) found largely in galaxy clusters and smaller compact groups. Most ellipticals contain older, low-mass stars, and because they lack a great deal of star-making gas and dust clouds, there is little new star formation occurring in them. Ellipticals can have as few as a hundred million to perhaps a hundred trillion stars, and they can range in size from a few thousand light-years across to more than a few hundred thousand. Astronomers now suspect that every elliptical has a central supermassive black hole that is related to the mass of the galaxy itself. Messier 87 is an example of an elliptical galaxy. There are some subgroups of ellipticals, including “dwarf ellipticals” with properties that put them somewhere between regular ellipticals and the tightly knit groups of stars called globular clusters.
Irregular galaxies are as their name suggests: irregular in shape. The best example of an irregular that can be seen from Earth is the Small Magellanic Cloud. Irregulars usually do not have enough structure to characterize them as spirals or ellipticals. They may show some bar structure, they may have active regions of star formation, and some smaller ones are listed as “dwarf irregulars”, very similar to the very earliest galaxies that formed about 13.5 billion years ago. Irregulars are characterized by their structures (or lack of them).
What is a Solar Eclipse?
A solar eclipse is a natural event that takes place on Earth when the Moon moves in its orbit between Earth and the Sun (this is also known as an occultation). It happens at New Moon, when the Sun and Moon are in conjunction with each other. If the Moon was only slightly closer to Earth, and orbited in the same plane and its orbit was circular, we would see eclipses each month. The lunar orbit is elliptical and tilted with respect to Earth’s orbit, so we can only see up to 5 eclipses per year. Depending on the geometry of the Sun, Moon and Earth, the Sun can be totally blocked, or it can be partially blocked.
During an eclipse, the Moon’s shadow (which is divided into two parts: the dark umbra and the lighter penumbra) moves across Earth’s surface. Safety note: do NOT ever look at the Sun directly during an eclipse unless it is during a total solar eclipse. The bright light of the Sun can damage your eyes very quickly.
Total Solar Eclipse
A total solar eclipse occurs when the Moon completely blocks the solar disk. In a total solar eclipse, the narrowest part of the path (where the Sun is completely blocked and the Moon casts its darkest shadow (called the umbra)) is called the “zone of totality”.
Observers in this path see a darkened Sun (often described as a “hole in the sky”) with the ghostly glow of the solar corona extending out to space. A phenomenon called “Bailey’s Beads” often appears as sunlight shines out through valleys on the lunar surface. If the Sun is active, observers can also see solar prominences, loops, and flares during totality. A total solar eclipse is the ONLY time when it is safe to look directly at the Sun. ALL other solar observations (even in partial phases) require special solar filters so that you do not harm your eyes.
Total solar eclipses have not always been visible from Earth. In the past, the Moon was too close to Earth and during eclipses it completely blotted out the Sun’s disk. Over time, the lunar orbit has changed at the rate of just over 2 cm per year and in the current epoch, the alignment is nearly perfect at times. However, the Moon’s orbit will continue to widen, and in perhaps 600 million years, total solar eclipses will no longer occur. Instead, future observers will see partial and annular eclipses only.
Annular Solar Eclipse
Not every solar eclipse is a total one. When the Moon is farther away in its orbit than usual, it appears too small to completely cover the Sun’s disk. During such an event, a bright ring of sunlight shines around the Moon. This type of eclipse is a called an “annular” eclipse. It comes from the Latin word “annulus” which means “ring”.
The period of annularity during such an eclipse can last anywhere from 5 or 6 minutes to up to 12 minutes. However, even though the Sun is mostly covered by the Moon, enough bright sunlight escapes during annularity that observers cannot ever look at the Sun directly. These events require eye protection throughout the entire eclipse.
Partial Solar Eclipse
A partial solar eclipse occurs when Earth moves through the lunar penumbra (the lighter part of the Moon’s shadow) as the Moon moves between Earth and the Sun. The Moon does not block the entire solar disk, as seen from Earth. Depending on your location during a partial eclipse, you might see anything from a small sliver of the Sun being blotted out to a nearly total eclipse.
To view any eclipse safely, use approved filters or use an indirect method of viewing, such as projecting sunlight through a telescope and onto a white piece of paper or cardboard. NEVER look at the Sun through a telescope unless it has the appropriate filter. Blindness and severe eye damage can result due to improper observation technique.
When is the next Solar Eclipse?
Eclipses can occur each year, and they are predictable. There are several places online where you can get up-to-date calendars for all the types of solar eclipses.
Facts about Solar Eclipses
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V (also known as V: The Series) is a one-hour weekly television series that aired in the United States on NBC in 1984-85. It is a continuation of the science fiction franchise about an alien invasion of Earth by a carnivorous race of reptilians known as "The Visitors"
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Apollo 11 was the spaceflight that landed the first humans, Americans Neil Armstrong and Buzz Aldrin, on the Moon on July 20, 1969, at 20:18 UTC. Armstrong became the first to step onto the lunar surface 6 hours later on July 21 at 02:56 UTC.
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A Mars landing is a landing of a spacecraft on the surface of Mars. Of multiple attempted Mars landings by robotic, unmanned spacecraft, seven were successful.There have also been studies for a possible manned mission to Mars, including a landing, but none have been attempted
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The web's most comprehensive and up to date UFO information source Provided by America's foremost UFO Reporting Agency
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|THE BEST UFO LINKS
WORLD UFO PHOTOS
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