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Astronomy Classroom

Astronomy is the scientific study of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earth's atmosphere (such as the cosmic background radiation). It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the formation and development of the universe.

Astronomy is one of the oldest sciences. Astronomers of early civilizations performed methodical observations of the night sky, and astronomical artifacts have been found from much earlier periods. However, the invention of the telescope was required before astronomy was able to develop into a modern science. Historically, astronomy has included disciplines as diverse as astrometry, celestial navigation, observational astronomy, the making of calendars, and even astrology, but professional astronomy is nowadays often considered to be synonymous with astrophysics.

Since the 20th century, the field of professional astronomy split into observational and theoretical branches. Observational astronomy is focused on acquiring and analyzing data, mainly using basic principles of physics. Theoretical astronomy is oriented towards the development of computer or analytical models to describe astronomical objects and phenomena. The two fields complement each other, with theoretical astronomy seeking to explain the observational results, and observations being used to confirm theoretical results. (Couristy of The Wikimedia Foundation)

Dec. 14, 2007: The solar physics community is abuzz this week. No, there haven't been any great eruptions or solar storms. The source of the excitement is a modest knot of magnetism that popped over the sun's eastern limb on Dec. 11th, pictured below in a pair of images from the orbiting Solar and Heliospheric Observatory (SOHO).

It may not look like much, but "this patch of magnetism could be a sign of the next solar cycle," says solar physicist David Hathaway of the Marshall Space Flight Center.

Above: From SOHO, a UV-wavelength image of the sun and a map showing positive (white) and negative (black) magnetic polarities. The new high-latitude active region is magnetically reversed, marking it as a harbinger of a new solar cycle.

For more than a year, the sun has been experiencing a lull in activity, marking the end of Solar Cycle 23, which peaked with many furious storms in 2000--2003. "Solar minimum is upon us," he says.

The big question now is, when will the next solar cycle begin?

It could be starting now.

"New solar cycles always begin with a high-latitude, reversed polarity sunspot," explains Hathaway. "Reversed polarity " means a sunspot with opposite magnetic polarity compared to sunspots from the previous solar cycle. "High-latitude" refers to the sun's grid of latitude and longitude. Old cycle spots congregate near the sun's equator. New cycle spots appear higher, around 25 or 30 degrees latitude.

The region that appeared on Dec. 11th fits both these criteria. It is high latitude (24 degrees N) and magnetically reversed. Just one problem: There is no sunspot. So far the region is just a bright knot of magnetic fields. If, however, these fields coalesce into a dark sunspot, scientists are ready to announce that Solar Cycle 24 has officially begun.

Below: Solar Cycle 23 is coming to an end. What's next? Image credit: NOAA/Space Weather Prediction Center.

Many forecasters believe Solar Cycle 24 will be big and intense. Peaking in 2011 or 2012, the cycle to come could have significant impacts on telecommunications, air traffic, power grids and GPS systems. (And don't forget the Northern Lights!) In this age of satellites and cell phones, the next solar cycle could make itself felt as never before.

The furious storms won't start right away, however. Solar cycles usually take a few years to build to a frenzy and Cycle 24 will be no exception. "We still have some quiet times ahead," says Hathaway.

Meanwhile, all eyes are on a promising little active region. Will it become the first sunspot of a new solar cycle? (story and photo credit Science@NASA)

Our Solar System:

From our small world we have gazed upon the cosmic ocean for thousands of years. Ancient astronomers observed points of light that appeared to move among the stars. They called these objects planets, meaning wanderers, and named them after Roman deities - Jupiter, king of the gods; Mars, the god of war; Mercury, messenger of the gods; Venus, the god of love and beauty, and Saturn, father of Jupiter and god of agriculture. The stargazers also observed comets with sparkling tails, and meteors or shooting stars apparently falling from the sky.

Since the invention of the telescope, three more planets have been discovered in our solar system: Uranus (1781), Neptune (1846), and Pluto (1930). In addition, there are thousands of small bodies such as asteroids and comets. Most of the asteroids orbit in a region between the orbits of Mars and Jupiter, while the home of comets lies far beyond the orbit of Pluto, in the Oort Cloud.

The four planets closest to the Sun - Mercury, Venus, Earth, and Mars - are called the terrestrial planets because they have solid rocky surfaces. The four large planets beyond the orbit of Mars - Jupiter, Saturn, Uranus, and Neptune - are called gas giants. Tiny, distant, Pluto has a solid but icier surface than the terrestrial planets. (Story and photo credit NASA)

What is a planet?
The International Astronomical Union (IAU) said that the definition for a planet is now officially known as "a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape and (c) has cleared the neighborhood around its orbit." At the same time, new moons are also being discovered, both around existing planets and within these mysterious new worlds. Once the existence of a moon is confirmed and its orbit determined, the moon is given a final name by the International Astronomical Union (IAU), the organization that assumed this task since 1919.
(Story and photo credit NASA)

How Did the Solar System Form?

Virtually everywhere we look in the Solar System we find continual change - predictable or chaotic, physical or chemical, subtle or catastrophic. Only by observing Solar System bodies under different conditions and from a variety of vantage points can we begin to understand the processes by which they evolved from their initial formative states to the wide diversity we see today, and how they are changing as these processes continue.

Orion nebula

Photo: Stellar Nursery - The Orion Nebula is birthing new stars.

Planetary processes such as impacts, volcanism, tectonics, climate change, and greenhouse-gas warming are difficult to comprehend when their study is confined to just one body – Earth for example – but by comparing how these processes operate and interact in a variety of planetary settings, we can gain insight into their variations and effects. As we move into the era of discovery and study of extrasolar planets, our efforts in our own neighborhood provide context for our observations of these newly found, distant solar systems.

What we know about the formation of the Solar System comes both from what we observe elsewhere in the universe of other nascent solar systems and from the evidence of our own past that we can still find today. Some tantalizing clues may emerge from the analysis of results from Genesis mission that gathered solar wind, the Stardust mission that captured samples from a comet's tail, and the Huygens probe to which examined the relatively unchanged atmosphere of Titan.

Our Solar System began as a rotating cloud of gas and dust about 4.6 billion years ago. Something, perhaps a shockwave from a nearby supernove, caused that cloud to begin to coalesce. Smaller particles clumped together into increasingly larger objects, with the greatest density in the center. The spinning motion caused the cloud to flatten into a pancake called an accretion disk.

Over millions of years, the center of this disk accumulated mass, and as the mass increased, so did the temperature. Eventually, the core reached a critical point when it ignited – nuclear fusion began. The Sun was born.

Meanwhile the disk continued to spin, containing all of the gas and dust that hadn't been sucked into the Sun. Within this material were the building blocks of every bit of matter we have on Earth today, including minerals, water and organic molecules like methane. The density of material was greatest at the middle of the disk. Dust and pebble-sized objects collided into larger and larger clumps. Eventually these clumps became the terrestrial planets: Mercury, Venus, Earth, and Mars.

The terrestrial planets formed at about the same time, in the same general region of space, and experienced similar forces and processes during their development. Yet today they are different in very fundamental ways. What did our neighbor planets experience to result in their vastly different atmospheres, and what are the implications for our home planet? Ongoing research like the MESSENGER mission to Mercury will help to answer this question.

Farther from the Sun, it was cool enough that water could freeze. Tiny chunks of ice collided, swept up gas and dust, and became the gas giants: Jupiter, Saturn, Uranus, and Neptune. Uranus and Neptune, being farther from the dense center of the disk, ended up smaller.

Beyond Neptune, matter got scarce, and the objects stayed small. These distant regions are known as the Kuiper Belt and the Oort Cloud. Our picture of the Solar System continues to expand as we look deeper into these regions and find objects like Sedna, the most distant object of the Solar System.


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