Astronomy fanatica

last SG · 2005-02-27T00:18:37+00:00

u'll find it on page 4 of this thread

poon cho tang SG

Stellar evolution

In astronomy, stellar evolution is the sequence of changes that a star undergoes during its "lifetime", the millions or billions of years during which it emits light and heat. Over the course of that time, the star will change radically.

Stellar evolution is not studied by observing the life cycle of a single star - most stellar changes occur too slowly to be detected even over many centuries. Instead, astronomers come to understand how stars evolve by observing numerous stars, each at a different point in its life cycle, and running computer models that simulate stellar structure.

Birth

Stellar evolution begins with a Giant Molecular Cloud (GMC). Most of the empty space inside a galaxy actually contains around 0.1 to 1 particles per cm³; within a GMC, a typical density is a few million particles per cm³. A GMC contains 100,000 to 10,000,000 times as much mass as our sun by virtue of its size: 50 to 300 light years across.

Emission nebula NGC 604 lies in a spiral arm of galaxy M33, 2.7 million light-years away in the constellation Triangulum. This is a site where stars are being born. Though such nebulae are common in galaxies, this one is particularly large, nearly 1,500 light-years across. At the heart of NGC 604 are over 200 hot stars, much more massive than our Sun (15 to 60 solar masses). They heat the gaseous walls of the nebula making the gas fluoresce. Their light also highlights the nebula's three-dimensional shape, like a lantern in a cavern. The image was taken on January 17, 1995 with Hubble's Wide Field and Planetary Camera 2.

As GMCs orbit the galaxy, one of several events might occur to cause them to collapse. GMCs may collide with each other, or pass through dense regions of spiral arms. The explosion of a nearby supernova can also be a trigger, sending shocked matter into the GMC at very high speeds. Finally, galactic collisions can trigger massive bursts of star formation as the gas clouds in each galaxy are compressed and agitated by the collision.

A collapsing GMC fragments as it collapses, breaking into smaller and smaller chunks. Fragments with masses of less than about 50 solar masses are able to form into stars. In these fragments, the gas is heated as it collapses due to the release of gravitational potential energy, and the cloud becomes a protostar as it forms into a spherical rotating object.

This initial stage of stellar existence is almost invariably hidden away deep inside dense clouds of gas and dust. Often, these star-forming cocoons can be seen in silhouette against bright emission from surrounding gas, and are then known as Bok globules.

In some protostars, the mass is too low for temperatures to reach those required for nuclear fusion to begin; these are brown dwarfs, and they have masses less than a tenth of the sun's mass. They die away slowly, cooling gradually over hundreds of billions of years. However, in protostars with greater than this mass, the temperature at the centre of the protostar will eventually reach over 10 million K, at which point fusion reactions begin, fusing hydrogen into helium. The star begins to shine. The onset of nuclear fusion sets up a hydrostatic equilibrium in which energy released by the core prevents the outer layers from collapsing under their own gravity. The star then exists in a stable state.

Issue #25

poon cho tang SG

Astronomy Picture of the Day

2005 March 18

Moon, Mercury, Monaco

Explanation: Low on the western horizon after sunset, a slender crescent Moon and wandering planet Mercury join the lights of Menton and Monaco along the French Riviera. Astronomer Vincent Jacques took advantage of this gorgeous photo opportunity a week ago on March 11, when the Moon and Mercury were separated in the sky by just three degrees. Of course, the Moon in a slender crescent phase is always seen near the horizon, as is Mercury - a bright planet which can be difficult to glimpse as it never strays far from the Sun in Earth's sky. In the coming days good views of Mercury will indeed be fleeting as the solar system's innermost planet is rapidly dropping closer to the glare of the setting Sun. But tonight a waxing Moon will join another bright planet wandering overhead through the evening sky, Saturn.

Credit & Copyright: Vincent Jacques

poon cho tang SG

Stellar evolution - Cont'd

Maturity

New stars come in a variety of sizes and colors. They range from blue (hot) to red (cool) and from less than half a solar mass to more than twenty. The brightness and color of a star depends on its surface temperature, which in turn depends on its mass.

A new star will fall at a specific point on the main sequence of the H-R diagram. Small, cool stars may remain on the main sequence for hundreds of billions of years, while supermassive, hot stars will only be on the main sequence for a million or several million years. Mid-sized stars, like the Sun, will remain on the main sequence for several billion years. Once a star expends most of the hydrogen in its core, it moves off the main sequence.

The middle years of a star's life

After millions to billions of years, depending on its initial mass, a star exhausts all the hydrogen in its core. Larger and hotter stars consume their hydrogen much more rapidly than cooler and less massive ones. Once the core's ready supply of hydrogen is gone, nuclear processes there cease.

A dense starfield in Sagittarius

Without the outward pressure generated by these reactions to counteract the force of gravity, the outer layers of the star begin to collapse inward toward the core. The temperature and pressure increase as during formation, but now to even higher levels, until helium fusion begins at core temperatures of around 100 billion K.

The very hot core causes the outer layers of the star to expand enormously; the star becomes as much as 100 times larger than it was during its main sequence lifetime. It is now a red giant, and the helium burning phase lasts for a few million years. Almost all red giant stars are variable.

What happens next depends, once more, on the star's mass.

Issue #26

poon cho tang SG

Astronomy Picture of the Day

2005 March 19

NGC 2266: Old Cluster in the New General Catalog

Explanation: The New General Catalog of star clusters and nebulae really isn't so new. In fact, it was published in 1888 - an attempt by J. L. E. Dreyer to consolidate the work of astronomers William, Caroline, and John Herschel along with others into a useful single, complete catalog of astronomical discoveries and measurements. Dreyer's work was successful and is still important today as this famous catalog continues to lend its "NGC" to bright clusters, galaxies, and nebulae. Take for example this star cluster known as NGC 2266 (item number 2,266 in the NGC compilation). It lies about 10,000 light-years distant in the constellation Gemini and represents an open or galactic cluster. With an age of about 1 billion years, NGC 2266 is old for a galactic cluster. Its evolved red giant stars are readily apparent in this gorgeous three-color image.

Credit & Copyright: Till Credner & Sven Kohle, Bonn University

poon cho tang SG

Stellar evolution - Cont'd

The later years and death of stars

Geriatric low-mass stars

What happens when a low mass star exhausts its fuel is not directly known: the universe is around 13.7 billion years old, which is less time (by several orders of magnitude, in some cases) than it takes for the fuel to be exhausted. Current theory is based on computer modelling.

They may fuse helium in core hot-spots causing an unstable and uneven reaction as well as a heavy solar wind. In this case, the star will form no planetary nebula but simply evaporate, leaving little more than a brown dwarf.

But a star with less than about half a solar mass will never be able to fuse helium even after the core ceases hydrogen fusion. There simply isn't a stellar envelope massive enough to bear down enough pressure on the core. These are the red dwarf stars, such as Proxima Centauri, which live for hundreds of billions of years. When nuclear reactions eventually cease in their cores, they will continue to glow weakly in the infrared and microwave part of the spectrum for many billions of years.

Mid-sized stars

Once a medium-size star (0.4 to 3.4 times the mass of our Sun) has reached the red giant phase, its outer layers continue to expand, the core contracts inward, and helium nuclei fuse into carbon. The fusion releases energy, granting the star a temporary reprieve. In a Sun-sized star, this process will take approximately one billion years.

Helium burning reactions are extremely sensitive to temperature, which causes great instability. Huge pulsations build up, which eventually give the outer layers of the star enough kinetic energy to be ejected entirely. The star becomes a planetary nebula. At the center of the nebular remains the core of the star, which will cool to become a white dwarf, typically weighing about 0.6 solar masses, but only the size of the Earth.

The Cat's Eye Nebula, a planetary nebula formed by the death of a star with about the same mass as the sun.

White dwarfs

White dwarfs are stable because the inward pull of gravity is balanced by the degeneracy pressure of the star's electrons. (This should not be confused with the electrical repulsion of electrons, but is a consequence of the Pauli exclusion principle.) With no fuel left to burn, the star radiates its remaining heat into icy space for many millions of years.

In the end, all that remains is a cold dark mass sometimes called a black dwarf. However, the universe is not old enough for any black dwarf stars to exist.

If the white dwarf's mass is increased to beyond the Chandrasekhar limit (1.4 solar masses; named for the physicist who discovered it) then electron degeneracy pressure fails and the star collapses. Mass transfer in a binary system may cause such an increase in mass. This causes the white dwarf to be blasted apart in a supernova event known as a Type-Ia supernova. These supernovae may be many times more powerful than the death of a massive star (a Type-II supernova). Hence, no white dwarf more massive than 1.4 solar masses can exist; electron degeneracy pressure isn't strong enough.

If a white dwarf forms in a binary system close to another star, hydrogen from the larger companion may accrete around and onto a white dwarf until it gets hot enough to fuse in a runaway reaction, although the white dwarf remains below the Chandrasekhar limit. This explosion is termed a Nova.

Issue #27

poon cho tang SG

Astronomy Picture of the Day

2005 March 20

The Equal Night

Explanation: Today, the Sun crosses the celestial equator heading north, marking the Vernal Equinox -- the first day of spring in the Northern Hemisphere and autumn in the south. Equinox means equal night and with the Sun on the celestial equator, Earthlings will experience 12 hours of daylight and 12 hours of darkness. For those in the Northern Hemisphere, the days will continue to grow longer with the Sun marching higher in the sky as summer approaches. A few weeks after the northern Autumnal Equinox of 1994, the Crew of the Shuttle Endeavor recorded this image of the Sun poised above the Earth's limb. Glare illuminates Endeavor's vertical tail (pointing toward the Earth) along with radar equipment in the payload bay. The space shuttle is expected to return to flight later this year with the launch of STS-114.

Credit: STS-68 Crew, NASA

poon cho tang SG

Stellar evolution - Cont'd

The later years and death of stars - Cont'd

Supermassive stars

After the outer layers of a star greater than five solar masses have swollen into a gigantic red supergiant, the core begins to yield to gravity and starts to shrink. As it shrinks, it grows hotter and denser, and a new series of nuclear reactions begin to occur. These reactions begin creating and expending progressively heavier elements, temporarily halting the collapse of the core.

The Crab Nebula, the shattered remnants of a star which exploded as a supernova almost 1,000 years ago.

Eventually, as the star progresses through heavier elements on the periodic table, silicon fuses to iron-56. Until now, the star has been maintained by these energy-liberating fusion reactions, but iron cannot release energy through fusion; instead, iron fusion absorbs energy. Once this occurs, there is no further energy outflow to counteract the enormous force of gravity, and the interior of the star collapses nearly instantly.

What happens next is not clearly understood. But whatever it is can cause a tremendous supernova explosion in less than a fraction of a second.

The accompanying surge of neutrinos starts a shock wave while the continuing jets of neutrinos blast much of the star's accumulated materialthe so-called seed elements, lighter than and including ironinto space. As some of the escaping mass is bombarded by the neutrinos, its atoms capture them, creating a spectrum of heavier-than-iron material including the radioactive elements up to uranium. Without supernovae, no elements heavier than iron would exist.

The shock wave and jets of neutrinos continue to propel the material away from the dying star and off into interstellar space. Then, streaming through space, the material from the supernova may collide with other cosmic debris, perhaps to form new stars, planets or moons, or to serve as raw materials for a vast variety of living things.

Modern science does not have a clear understanding of the actual explosion mechanism, nor what exactly remains of the original star. There are, however, two possiblities: neutron stars and black holes.

Neutron stars

It is known that in some supernovae, the intense gravity inside the supergiant forces the electrons into the atomic nuclei, where they combine with the protons to form neutrons. The electromagnetic forces keeping separate nuclei apart are gone (proportionally, if nuclei were the size of dust motes, atoms would be as large as football stadiums), and the entire core of the star becomes nothing but a dense ball of contiguous neutrons or a single atomic nucleus.

Bubble-like shock wave still expanding from a supernova explosion 15,000 years ago.

These stars, known as neutron stars, are extremely smallno bigger than the size of a large cityand are phenomenally dense. Their period of revolution can be extremely rapid, with some spinning at forty revolutions per second. When these rapidly rotating stars' northern or southern magnetic poles are aligned with the Earth, a pulse of radio activity is received each revolution. These neutron stars are called pulsars, and were the first of the neutron stars to be discovered.

Black holes

It is widely believed that not all supernovae form neutron stars. If the stellar mass is high enough, the neutrons themselves will be crushed and the star will collapse until its radius is smaller than the Schwarzschild radius and becomes a black hole.

Black holes are predicted by Albert Einstein's theory of general relativity. According to classical general relativity, no matter or information can flow from the interior of a black hole to an outside observer, although quantum mechanics may allow deviations from this strict rule. The existence of black holes in the universe is well supported both theoretically and by astronomical observation.

Questions remain, however. Understanding of stellar collapse is not good enough to tell whether it is possible to collapse directly to a black hole without a supernova, if there are supernovae which then form black holes, or what the exact relationship is between the initial mass of the star and the final object that remains.

Issue #28

poon cho tang SG

Astronomy Picture of the Day

2005 March 21

Orion's Horsehead Nebula

Explanation: The Horsehead Nebula is one of the most famous nebulae on the sky. It is visible as the dark indentation to the red emission nebula seen above and to the right of center in the above photograph. The bright star on the left is located in the belt of the familiar constellation of Orion. The horse-head feature is dark because it is really an opaque dust cloud which lies in front of the bright red emission nebula. Like clouds in Earth's atmosphere, this cosmic cloud has assumed a recognizable shape by chance. After many thousands of years, the internal motions of the cloud will alter its appearance. The emission nebula's red color is caused by electrons recombining with protons to form hydrogen atoms. Also visible in the picture are blue reflection nebulae, which preferentially reflect the blue light from nearby stars.

Credit & Copyright: Ryan Steinberg & Family, Adam Block, NOAO, AURA, NSF

poon cho tang SG

Star formation

Star formation is the process by which gas in molecular clouds gets transformed into stars.

In the current paradigm of star formation, cores of molecular clouds (regions of especially high density) become gravitationally unstable, fragment and begin to collapse. Part of the gravitational energy lost in this collapse is radiated in the infrared, with the remainder increasing the temperature of the core of the object. The accretion of material happens partially through a circumstellar disc. When the density and temperature are high enough, deuterium fusion ignition occurs, and the outward pressure of the resultant radiation slows (but does not stop) the collapse. Material from the cloud continues to "rain" onto the protostar. In this stage bipolar flows are produced, probably an effect of the angular momentum of the infalling material. Finally, hydrogen begins to fuse in the core of the star, and the rest of the enveloping material is cleared away.

The stages of the process are well defined in stars with masses around one solar mass or less. In high mass stars, the length of the star formation process is comparable to the other timescales of their evolution, much shorter, and the process is not so well defined. The later evolution of stars are studied in stellar evolution.

Observations

Key elements of star formation are only available by observing in wavelengths other than the optical. The structure of the molecular cloud and the effects of the protostar are best observed in rotational transitions of CO and other molecules; these are observed in the millimeter and submillimeter range. The radiation from the protostar and early star has to be observed in infrared astronomy wavelengths, the extinction caused by the rest of the cloud where it is being formed is usually too big to allow us to observe it in the visual part of the spectrum.

The formation of individual stars can only be directly observed in our Galaxy, but in distant galaxies star formation has been detected through its unique spectral signature.

References & Further reading

The theory of collapse of gas under its own gravity was developed by Jeans and although the theory does not treat many phenomena known to be important it is a very useful and widely used first approximation. The original paper is available free of charge:

- J. H. Jeans. Philosophical Transactions of the Royal Society of London. Series A, Containing Papers of a Mathematical or Physical Character, Vol. 199. (1902), pp. 1-53.

Issue #29

poon cho tang SG

Astronomy Picture of the Day

2005 March 22

To Fly Free in Space

Explanation: At about 100 meters from the cargo bay of the space shuttle Challenger, Bruce McCandless II was further out than anyone had ever been before. Guided by a Manned Maneuvering Unit (MMU), astronaut McCandless, pictured above, was floating free in space. McCandless and fellow NASA astronaut Robert Stewart were the first to experience such an "untethered space walk" during Space Shuttle mission 41-B in 1984. The MMU works by shooting jets of nitrogen and has since been used to help deploy and retrieve satellites. With a mass over 140 kilograms, an MMU is heavy on Earth, but, like everything, is weightless in space. The MMU was replaced in 2001 with the SAFER backpack propulsion unit.

Credit: STS-41B, NASA

poon cho tang SG

Planetary science

Planetary science, also known as planetology or planetary astronomy, is the science of planets and the solar system, and incorporates an interdisciplinary approach drawing from diverse sciences. Planetary science may be considered a part of the Earth sciences, or more logically, as its parent field. Research tends to be done by a combination of astronomy, space exploration (particularly unmanned space missions), and comparative, experimental and meteorite work based on Earth. There is also an important theoretical component and considerable use of computer simulation.

Planetary science studies objects ranging in size from micrometeoroids to gas giants, their composition, dynamics and history.

When the discipline concerns itself with a celestial body in particular, a specialised term is used, as shown in the table below (only Heliology, Geology, Selenology, and Areology are currently in common use):

Body - Term
Sun - Heliology
Mercury - Hermeology
Venus - Cytherology
Earth - Geology
Moon - Selenology
Mars - Areology
Jupiter - Zenology
Saturn - Kronology
Uranus - Uranology
Neptune - Poseidology
Pluto - Hadeology

Issue #30

poon cho tang SG

Astronomy Picture of the Day

2005 March 23

A Dust Devil Swirling on Mars

Explanation: What is that wisp on the horizon? Scientists think that the slight white apparition is actually a Martian dust devil that was caught swirling across Mars. The above image was taken earlier this month by the robotic rover Spirit. The swirling cloud was found by comparing the above image to a previous image of the same area. Fresh dust devil tracks have been seen on Mars before, but actually seeing one up close was a surprise. The most similar phenomena to Martian dust devils on Earth are tornadoes and waterspouts. The ultimate cause of Martian dust devils remains unknown, but might be related to rising air heated by sun-warmed rocks and soil. Just the previous day, Spirit's power acquisition increased unexpectedly, possibly the result of a dust devil passing near or over the Spirit rover and effectively cleaning its solar panels.

Credit: Mars Exploration Rover Mission, JPL, NASA

poon cho tang SG

Planet

A planet (from Greek, meaning planetes or "wanderers") is a body of considerable mass that orbits a star and that produces no energy through nuclear fusion. Prior to the 1990s only nine were known (all of them in our own solar system); as of 3 November 2004, 133 are known, with all of the new discoveries being extrasolar planets, sometimes known as "exoplanets".

Planets are thought to form from the collapsing nebula that a planet's star formed out of, aggregating from gas and dust that orbits the protostar in a dense protostellar disk before the star's core ignites and its solar wind blows the remaining material away.

Within the solar system

Except for Earth (which was not perceived as being a planet by the ancients), all of the accepted planets in the solar system are named after Greek or Roman gods (however, some non-European languages such as Chinese use different names). Moons are also named after gods and characters from classical mythology or (in the case of Uranus) from the plays of Shakespeare. Asteroids can be named, at the discretion of their discoverers, after anybody or anything (subject to approval by the International Astronomical Union's panel on nomenclature). The act of naming planets and their features is known as planetary nomenclature.

Accepted planets

According to the authority of the International Astronomical Union, there are nine planets in our solar system (in increasing distance from the Sun):

Mercury
Venus
Earth - with the Moon
Mars - 2 satellites (Deimos, Phobos)
Jupiter - 63 confirmed natural satellites
Saturn - 30 confirmed satellites
Uranus - 27 moons
Neptune - 13 moons
Pluto (many astronomers think it should be classified only as a Kuiper belt object and not also as a planet) - 1 satellite Charon

The nine planets in our solar system

Other objects

Recently an object, 90377 Sedna, has been discovered orbiting the Sun 90 AU (13 billion kilometres) away, three times farther than Pluto. Sedna, after the Inuit goddess of the sea, is a 11802360 km (730-1470 miles) diameter object. The diameter is still uncertain but believed to be between 1/2 and 3/4 of Pluto's. Several news sources have already reported Sedna as the tenth planet, but that is not generally accepted by astronomers. Another possible planet is 90482 Orcus, an object with an orbit and mass similar to Pluto's. Other candidates include 50000 Quaoar and 20000 Varuna.

Several hypothetical planets, like Planet X (supposedly beyond the orbit of Pluto) or Vulcan (thought to orbit inside the orbit of Mercury), were posited at various historical times, and were subjects of intense searches that found nothing.

Classification

Astronomers distinguish between minor planets, such as asteroids, comets, and trans-Neptunian objects; and major (or true) planets.

Planets within Earth's solar system can be divided into categories according to composition.

-- Terrestrial or rocky: Planets that are similar to Earth with bodies largely composed of rock: Mercury, Venus, Earth, Mars

-- Jovian or gas giant: Those with a composition largely made up of gaseous material: Jupiter, Saturn, Uranus, Neptune. Uranian planets are a sub-class of gas giants, distinguished from true Jovians by their depletion in hydrogen and helium and a significant composition of rock and ice.

-- Icy: Sometimes a third category is added to include bodies like Pluto, whose composition is primarily ice; this category of "icy" bodies also includes many non-planetary bodies such as the icy moons of the outer planets of our solar system (e.g. Triton).

The eight rocky and gaseous planets are universally recognized as major planets. Ceres was called a planet when first discovered, but was reclassified as an asteroid when many similar objects were found. Given recent discoveries of many trans-Neptunian objects which are very similar to Pluto in orbit, size and composition, many people think it should be similarly redefined as a minor planet. For example, Mike Brown of Caltech defines a planet to be: any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit (link). Using this definition, neither Pluto nor Sedna would be a major planet.

Many consider the Earth and its Moon to be a double planet, for several reasons:

-- The Moon, as measured by its diameter, is 1.5 times larger than Pluto.

-- The gravitational force of the Sun on the Moon is larger than the gravitational force of the Earth on the Moon (by about a factor of 2.2)

The latter fact is not unique in the solar system, but is unusual for such a large satellite. Other satellites for which the Sun's gravity is actually stronger than the primary's:

-- The outermost moon of Jupiter (S/2003 J 2; factor of 1.5)

-- The outermost moon of Uranus (S/2001 U 2; factor of 1.2)

-- The two outermost moons of Neptune (S/2002 N 4 and S/2003 N 1; factor of 2.1)

-- Several asteroid moons (e.g. S/2001 (22) 1 Linus by a factor of 1.6; S/1998 (45) 1 Petit-Prince by a factor of 2.8; S/1993 243 (1) Dactyl by a factor of 1.3; and, most remarkably, S/2001 (66391) 1 by a large factor of 625.)

Extrasolar planets

Most extrasolar planets (those outside our solar system) discovered to date have masses which are about the same or larger than Jupiter's.

Exceptions include three planets discovered orbiting PSR B1257+12 a burned-out star, or supernova remnant, called a pulsar, comparable in size to the terrestrial planets; and planets orbiting the stars Mu Arae, 55 Cancri and GJ 436 which are approximately neptune sized (link).

It is far from clear if the newly discovered large planets would resemble gas giants in our solar system or if they are of an entirely different type or types which are unknown in our solar system. In particular, some of the newly discovered planets, known as hot Jupiters, orbit extremely close to their parent star, in nearly circular orbits. They therefore receive much more stellar radiation than the gas giants in our solar system, which makes it questionable whether they are the same type of planet at all.

The National Aeronautics and Space Administration of the United States has a program underway to develop a Terrestrial Planet Finder artificial satellite, which would be capable of detecting the planets with masses comparable to terrestrial planets. The frequency of occurrence of these planets is one of the variables in the Drake equation which estimates the number of intelligent, communicating civilizations that exist in our galaxy.

Interstellar planets are rogues in interstellar space, not gravitationally linked to any given solar system. No interstellar planet is known to date, but their existence is considered a plausible hypothesis on the grounds that the results of computer simulations of the origin and evolution of planetary systems often include the formation and subsequent ejection of bodies of significant mass.

Current technology is not sensitive enough to detect planets of relatively small mass and orbiting far because such planets cause very small "wobble effect" on their star. Discoveries of smaller planets will require radical improvements in telescopes.

Issue #31

poon cho tang SG

Astronomy Picture of the Day

2005 March 24

Simeis 147: Supernova Remnant

Explanation: It's easy to get lost following the intricate filaments in this detailed image of faint supernova remnant Simeis 147. Seen towards the constellation Taurus it covers nearly 3 degrees (6 full moons) on the sky corresponding to a width of 150 light-years at the stellar debris cloud's estimated distance of 3,000 light-years. The color composite image includes eight hours of exposure time with an H-alpha filter, transmiting only the light from recombining hydrogen atoms in the expanding nebulosity and tracing the regions of shocked, glowing gas. This supernova remnant has an apparent age of about 100,000 years - meaning light from the massive stellar explosion first reached Earth 100,000 years ago - but this expanding remnant is not the only aftermath. The cosmic catastrophe also left behind a spinning neutron star or pulsar, all that remains of the original star's core.

Credit & Copyright: Robert Gendler

poon cho tang SG

Definition of Planet

Although planets are the principle component of the solar system other than the sun, a definition of what a planet actually is, is surprisingly elusive. This article details the questions that may arise when trying to formulate a strict definition of the word.

History and etymology

What defines a planet has always been somewhat arbitrary. When the word was originally coined by the ancient Greeks, a planet was any object that appeared to wander against the field of fixed stars that made up the night sky; hence, "planetes", meaning "wanderers." This included not only the five "classical" planets, that is, Mercury, Venus, Mars, Jupiter and Saturn, but also the Sun and the Moon. Eventually, when the heliocentric model was accepted over the geocentric, Earth was placed among their number and the Sun was dropped, and, after Galileo discovered his four satellites of Jupiter, the Moon was also eventually reclassified. A "planet" could then be defined as "any object that orbited the Sun, rather than another object."

Asteroids: minor planets

The word did not need to be rethought again until 1801, when the first asteroid, Ceres, was discovered. Bode's Law, a mathematical function which generates the size of the semimajor axis of planetary orbits, predicts a body between Mars and Jupiter, and Ceres was found to lie at almost exactly the required distance.

Then in 1802, Heinrich Olbers discovered 2 Pallas, a second "planet" at roughly the same distance from the Sun as the recently discovered "planet" 1 Ceres. The idea that two planets could occupy the same orbit was an affront to thousands of years of thinking. Eventually these "planets" numbered in their thousands, and were given their own separate classification, asteroids, letting the concept of planet survive with little modification. Asteroids are often referred to as minor planets.

The Pluto controversy

The discovery of Pluto by Clyde W. Tombaugh in 1930 had little initial effect on the idea of what a planet should be, since, despite being smaller than Earth's Moon, and in a more eccentric orbit than any other planet, it still appeared to broadly fit the contemporary definition of a planet.

The relative sizes of the Earth on the left, with (from top to bottom) the Moon, Pluto and its moon Charon, Sedna and Quaoar on the right.

Then, beginning in 1992, astronomers began to notice large numbers of icy bodies beyond the orbit of Neptune that were similar in composition and size to Pluto. They concluded that they had discovered the long-rumoured Kuiper Belt, the ring of icy debris that is the source of all short-period comets including Halley's comet. Pluto's planetary status was thrown into question, with many scientists claiming that it should be demoted to the largest object in the Kuiper Belt and that a "planet" should be redifined along the lines of "an object that orbits the Sun directly and in isolation, rather than as part of a larger group of objects." There has been no small outcry at this suggestion, and the International Astronomical Union has not officially done so as of yet.

The continuing discovery of objects not much smaller than Pluto, such as Quaoar and Sedna, seems certain to re-ignite the debate over Pluto's planetary status in the near future.

Problems with strict definitions

However, even with the Pluto controversy excluded, there is no agreed consensus on what a strict definition of a planet should be. A definition such as "a body of considerable mass that orbits a star and that doesn't produce energy through nuclear fusion" seems fairly unambiguous, but is actually frought with uncertainties. Do planets necessarily orbit stars? Many astronomers have claimed to have spotted "rogue planets" drifting in space unattended by any star. "Of considerable mass" should distinguish planets from minor solar system bodies such as comets and asteroids, but it is rather vague. What is "considerable mass?" Mercury has avoided any dispute to its planetary status for thousands of years, yet there are moons in the solar system that exceed it in size. And finally, "does not produce energy from nuclear fusion" certainly separates a planet such as the Earth from a star like our Sun, yet what about brown dwarfs, stars too small to commence fusion in their cores? Many of them are likely to orbit other stars. Does that make them planets? And what about objects that don't produce energy through nuclear fusion, but did at one time? Would a white dwarf orbiting a star be reclassified as a planet?

An arbitrary definition

Ultimately, there is probably no real need for a strict definition of what a planet is, with convention and history instead playing the major role in determining what is considered a planet and what is not. The situation is somewhat similar to the consideration of what is a continent - a consideration determined partly by geography and partly by history and culture, with no strict definition possible.

Issue #32

poon cho tang SG

Astronomy Picture of the Day

2005 March 25

Huygens Discovers Luna Saturni

Explanation: In 1655, three hundred fifty years ago on this date, Dutch astronomer Christiaan Huygens discovered Luna Saturni - now known as Saturn's moon Titan. To celebrate, consider this intriguing picture of his telescope lens, all that remains of the instrument he used, designed and constructed in collaboration with his brother, Constantijn Huygens. The lens itself measures 57 millimeters (just over 2 inches) in diameter and is inscribed along the border "X 3 FEBR. MDCLV": its focal length (10 Rhineland feet) and the date of its final polishing, 3 February 1655. It also bears a verse from the Roman poet Ovid, "Admovere Oculis Distantia Sidera Nostris" (They brought the distant stars closer to our eyes). Huygens used the verse as part of an anagram announcing his discovery. The use of an anagram, a practice common in his time, established a date for his discovery but kept its details secret until he wished to reveal them. Decoded and translated, his anagram reads "A moon revolves around Saturn in 16 days and 4 hours.", a good agreement with the modern value for Titan's orbital period.

Credit & Copyright: Utrecht Univ. Museum, Inst. for History and Foundations of Science
Courtesy: Rob van Gent, Tiemen Cocquyt, and Carl Koppeschaar

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Prograde and retrograde motion

Retrograde motion is the orbital motion of a body in a direction opposite that which is normal to spatial bodies within a given system. 'Retrograde' derives from the Latin words retro, backwards, and gradus, step.

In the Solar system, almost everything rotates in the same sense: all major planets orbit the Sun counterclockwise as seen from the pole star (Polaris). Most planets spin in the same sense, including Earth. The same happens with the orbital motions of the Moon, Mars' moons, and the biggest moons of Jupiter and Saturn around their planets. All these motions are called "direct" or "prograde."

Rotation in the opposite sense is called retrograde. Venus, Uranus and Pluto spin clockwise, so they have a retrograde rotation. Some small moons orbit clockwise around their planet, and are called retrograde satellites. Some comets and small asteroids orbit the Sun in retrograde orbits. All these are the exception rather than the rule.

When we observe the sky, we expect most objects to appear to move in a particular direction with the passing of time (diurnal motion). The apparent motion of most bodies in the sky is from east to west. However it is possible to observe a body moving west to east, such as an artificial satellite or Space Shuttle that is orbiting eastward (the preferred direction, because the rotation of the Earth assists in acquiring the required orbital speed). This orbit might be considered retrograde motion in this sense. However, as the Space Shuttle and such satellites you see going eastward would be seen orbiting the Earth counterclockwise if seen from the Pole Star, they are considered direct satellites. There are also artificial satellites which go clockwise as seen from the pole star; they are called retrograde satellites and you can see them in the sky going westward.

Retrograde motion should not be confused with retrogradation. The latter term is used in reference to the motion of the outer planets (Mars, Jupiter, Saturn, and so forth). Though these planets appear to move from east to west on a nightly basis in response to the spin of Earth, they are most of the time drifting slowly eastward with respect to the background of stars, which can be observed by noting the position of these planets for several nights in a row. This motion is normal for these planets, so it is called direct motion (not retrograde). However, since Earth completes its orbit in a shorter period of time than these outer planets, we occasionally overtake an outer planet, like a faster car on a multiple-lane highway. When this occurs, the planet we are passing will first appear to stop its eastward drift, and it will then appear to drift back toward the west. This is retrogradation, since the planet seems to be moving in a direction opposite to that which is typical for planets. Finally as Earth swings past the planet in its orbit, it appears to resume its normal west-to-east drift on successive nights.

Mars goes through retrogradation about every 25.7 months. The more distant outer planets retrograde more frequently. The period between such retrogradations is the synodic period of the planet.

This retrogradation puzzled ancient astronomers, and was one reason why they named these bodies 'planets' which in Greek means 'wanderers'. In the geocentric model of the solar system, this motion was accounted for by having the planets travel in deferents and epicycles.

In modern astronomy, the term retrograde motion refers to objects which are actually moving in a direction opposite that which is normal to spatial bodies within a given system, as opposed to merely observed phenomena (retrogradation) such as that described above.

Some significant examples of retrograde motion in the solar system:

-- Venus rotates slowly in the retrograde direction.
-- The moons Ananke, Carme, Pasiphaë and Sinope all orbit Jupiter in a retrograde direction, and are thought to be fragments of a single body that Jupiter captured long ago. Many other minor moons of Jupiter orbit retrograde.
-- The moon Phoebe orbits Saturn in a retrograde direction, and is thought to be a captured Kuiper belt object.
-- The moon Triton orbits Neptune in a retrograde direction, and is also thought to be a captured Kuiper belt object.
-- The planet Uranus has an axial tilt which is very near to 90°, and can be considered to be rotating in a retrograde direction depending on one's interpretation.

Issue #33

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Astronomy Picture of the Day

2005 March 26

Composite Crab

Explanation: The Crab Pulsar, a city-sized, magnetized neutron star spinning 30 times a second, lies at the center of this composite image of the inner region of the well-known Crab Nebula. The spectacular picture combines optical data (red) from the Hubble Space Telescope and x-ray images (blue) from the Chandra Observatory, also used in the popular Crab Pulsar movies. Like a cosmic dynamo the pulsar powers the x-ray and optical emission from the nebula, accelerating charged particles and producing the eerie, glowing x-ray jets. Ring-like structures are x-ray emitting regions where the high energy particles slam into the nebular material. The innermost ring is about a light-year across. With more mass than the Sun and the density of an atomic nucleus, the spinning pulsar is the collapsed core of a massive star that exploded, while the nebula is the expanding remnant of the star's outer layers. The supernova explosion was witnessed in the year 1054.

Credit: J. Hester (ASU) et al., CXC, HST, NASA

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Asteroid

An asteroid is a small, solid object in our Solar System, orbiting the Sun. An asteroid is an example of a minor planet (or planetoid), which are much smaller than planets. Most asteroids are believed to be remnants of the protoplanetary disc which were not incorporated into planets during the system's formation. Some asteroids have moons. The vast majority of the asteroids are within the asteroid belt, with elliptical orbits between those of Mars and Jupiter.

This picture of Eros, taken on February 14, 2000, shows the view looking from one end of the asteroid across the gouge on its underside and toward the opposite end. In this mosaic, constructed from two images taken after the NEAR spacecraft was inserted into orbit, features as small as 120 feet (35 meters) across can be seen. House-sized boulders are present in several places; one lies on the edge of the giant crater separating the two ends of the asteroid. A bright patch is visible on the asteroid in the top left-hand part of this image, and shallow troughs can be see just below this patch. The troughs run parallel to the asteroid's long dimension.

Definition

The term "asteroid", meaning star-like (from the Greek asteroeides, aster "star" + -eidos "form, shape"), was coined in 1802 by Sir William Herschel shortly after Olbers discovered the second one, 2 Pallas, in late March of the same year, to describe their star-like appearance; the other then-known planets all show discs, by comparison. He also applied that term to the small moons of the giant planets. The first scientific paper to use the word in its title was published in 1840 by Erman.

The exact definition of an asteroid is unsettled. The term "minor planet" (or "planetoid") carries no strong suggestion about the composition of the object or its general location in the solar system, and some argue that not every minor planet should be called an "asteroid".

One way to classify asteroids is in terms of size. A working definition is that asteroids are larger than 50 m in diameter, distinguishing them from meteoroids, which are typically boulder-sized or smaller. The distinction is made because asteroids are large enough to survive passage through Earth's atmosphere and strike Earth largely intact while the smaller meteoroids generally break up high in Earth's atmosphere.

Thus, it would be safest to use the term "asteroid" for Solar System objects that are bigger than meteoroids, smaller than planets, and made out of rock, not ice.

The term artificial asteroid is sometimes used to designate man-made objects which have ended up in solar orbits, such as the Mariner IV probe.

Asteroids in Earth's Solar System

Hundreds of thousands of asteroids have been discovered within Earth's solar system. As of February 24, 2005, from a total of 277,090 minor planets with calculated orbits 99,906 asteroids had been calculated well enough to be given official numbers and 12,198 of these had been officially named (598 of which have names requiring diacriticals). The first numbered but unnamed minor planet was (3360) 1981 VA; the last numbered and named minor planet was 95959 Covadonga.

Current estimates put the total number of asteroids in the solar system at several million. The largest asteroid in our inner solar system is 1 Ceres, with a diameter of 900-1000 km. Two other large asteroids are 2 Pallas and 4 Vesta; both have diameters of ~500 km. 4 Vesta is the only main belt asteroid that is sometimes visible to the naked eye (in some very rare occasions, a near-Earth asteroid may be visible without technical aid; see 2004 MN4).

4 Vesta, one of the largest asteroids. NASA Hubble Space Telescope image of the asteroid 4 Vesta, taken in May 1996 when it was 177 Gm from Earth.

Issue #34

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Astronomy Picture of the Day

2005 March 27

The Einstein Cross Gravitational Lens

Explanation: Most galaxies have a single nucleus -- does this galaxy have four? The strange answer leads astronomers to conclude that the nucleus of the surrounding galaxy is not even visible in this image. The central cloverleaf is rather light emitted from a background quasar. The gravitational field of the visible foreground galaxy breaks light from this distant quasar into four distinct images. The quasar must be properly aligned behind the center of a massive galaxy for a mirage like this to be evident. The general effect is known as gravitational lensing, and this specific case is known as the Einstein Cross. Stranger still, the images of the Einstein Cross vary in relative brightness, enhanced occasionally by the additional gravitational microlensing effect of specific stars in the foreground galaxy.

Credit & Copyright: J. Rhoads (STScI) et al., WIYN, AURA, NOAO, NSF

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