Stars twinkle when there appear to be variations in their brightness. Astronomers call this phenomenon atmospheric scintillation, and it’s caused by motion in the atmosphere.
Specifically, changes in atmospheric temperature cause small fluctuations in the air’s density. As starlight passes through the atmosphere, it’s refracted or slightly alters direction, creating a twinkling effect.
This is more obvious when viewing stars closer to the horizon because there’s a thicker layer of atmosphere. Astronomers compensate for atmospheric scintillation by using special adaptive optics on the most sophisticated telescopes. Space-based observatories like the Hubble also allow us to view stars and other objects without atmospheric scintillation.
Earth’s rivers, lakes, seas, and oceans teem with life. More than two-thirds of the planet’s surface is covered in water. In deep oceans, most marine life exists in the top 100 m (330 ft) or so, as this is where light and vast quantities of microscopic organisms, called plankton, are most abundant. Plankton is crucial as it forms the basis of marine life diets. All underwater life either feeds directly on plankton or preys on creatures that do.
A FISH Fish live in water and have a protective covering of scales on their outer bodies. They breathe by absorbing dissolved oxygen in the water using organs called gills located at the back of their heads. Most fish have an array of blades projecting on their body called fins. These give the fish stability in water and help control the fish’s direction of movement.
BONY FISH Every species of bony fish has a skeleton of bones with a flexible spine, or backbone, running the length of the body. An organ called the swim bladder enables a constant buoyancy, regardless of the changing water pressure. This ocean sunfish is the heaviest bony fish – some weigh as much as 2,000 kg (4,400 lbs).
CARTILAGINOUS FISH Skates, rays, and sharks, such as this great white, are all cartilaginous fish. They lack a true backbone, but have a structure made of gristly cartilage. They also lack swim bladders and must maintain their depth by swimming. Most species of shark are predators. Rays are flattened fish that live on the seabed where they hunt smaller fish.
MARINE MAMMALS Some mammals spend most or all of their lives in water. These include many species of whale, porpoise, and dolphin, and plant-eating dugongs and manatees. All of these creatures that stay in the water must surface to breathe. Other marine mammals, such as this sea lion, along with walruses and seals, are skilled swimmers but must return to land to breed and reproduce.
ECHINODERMS Starfish, sea urchins, and star dollars are all echinoderms. Their skeletons are made of chalky plates that are sometimes covered in small spines. All echinoderms’ bodies are divided into five parts. Starfish, for example, all have five arms and five sets of digestive and reproductive organs. If an arm breaks off, it will grow back.
CRUSTACEANS This group of more than 40,000 species of invertebrates ranges from tiny water fleas no more than 0.1 mm in size up to the Japanese spider crab with a leg span in excess of 3.7 m (13 ft). Crustaceans include shrimps, lobsters, and crabs. They all have hard-jointed shells as well as gills, eyes on stalks, and four or more pairs of jointed legs.
CNIDARIANS Jellyfish, sea anemones, and coral are all cnidarians. These creatures have no brains or central nervous system. They tend to have a bell-shaped or hollow body and a mouth ringed with tentacles. The mouth contains stinging cells, called nematocysts, which can disable prey. Some jellyfish are harmful to humans – for example, an Australian box jellyfish sting can kill within minutes.
A CEPHALOPODS Molluscs that lack a complete hard outer shell but have tentacles are known as cephalopods. These include cuttlefish, squid, and species of octopus. An octopus has eight arms, a well-developed brain, and good eyesight. It can react quickly to danger, squirting out a jet of water to propel itself away.
MOLLUSCS Most molluscs are soft-bodied creatures covered in a shell. These shells are made from calcium carbonate secreted by a body part called the mantle. The giant clam can live for up to 100 years. It can grow to more than 1.2 m (4 ft) across. However, most molluscs (including snails, oysters, and mussels) are much smaller.
SPONGES There are as many as 10,000 species of sponge. Sponges are simple, invertebrate creatures. They are found mostly in saltwater habitats although some freshwater species exist. Sponges attach to rocks and feed on tiny food particles that flow through openings called ostia.
The wax that makes up a candle’s body is made of a carbon and hydrogen compound called paraffin. As a solid, it isn’t actually very flammable and if you were to hold a flame against it, it would melt and then evaporate rather than burn.
This is where the wick comes in When you light the string at the top of the candle, the liquid wax is drawn up the wick by capillary action (the ability of a liquid to flow upwards against gravity in a small tube). It becomes so hot that it turns into a gas, which mixes with oxygen in the air and combusts.
We take for granted that a candle burns with a yellow, cone shaped flame, providing light. The reason this happens is due to a process called incomplete combustion, which produces bits of soot. This soot gets extremely hot, causing each particle to glow and produce the characteristic yellow colour.
Billions of fat cells exist in all body types, sandwiched between the skin and muscle tissue, and it’s the size of these fat cells that determine a person’s weight.
So how does this build up of fatty deposits get broken down when you’re working on losing weight? Put simply, it involves a biochemical process that converts these space demanding molecules in fat cells into usable energy.
The entire process begins when you increase activity levels and reduce calorie intake. By consuming fewer calories than you’re burning, the body will react to the reduction of available energy by producing fat-mobilising hormones, which in turn signal important enzymes, which help break down fat reserves for more energy.
The key enzyme in this process is lipase. Lipase stimulates fat cells so that they release triglycerides (the form of fat within the fat cell). Each triglyceride molecule is then broken down into glycerol and three fatty acids.
The glycerol is broken down further by the liver to release energy, while the free fatty acids are transported directly to muscles via the bloodstream. The enzyme lipoprotein lipase helps the muscle cells absorb the fatty acids, which can be burned for extra energy.
For centuries, people have wondered whether distant stars had planets orbiting around them. Unfortunately, most stars are so far away that it was impossible to spot any planets. But modern instruments have now made it possible to detect planets, and more than 400 of them have already been found.
BABY PLANETARY SYSTEMS Out in space, new solar systems are still forming. This is the Orion Nebula, where many stars are being born. Around each new star is a spinning disk of gas and dust. If material in this disk starts to clump together, it eventually forms planets that orbit the star.
Pulling power The first exoplanet in orbit around a Sun-like star was discovered in 1995. The planet was detected from a tiny wobble in the motion of the star 51 Pegasi. As the planet, called 51 Pegasi b, orbited the star, its gravity sometimes pulled the star toward Earth and sometimes away from it. This wobble showed up as slight shifts in the spectrum of the starlight. Since then, hundreds of exoplanets have been found from the wobbles they create in nearby stars.
DUSTY DISKS Planets form inside huge rotating disks of dust and gas. Even before the first exoplanets were spotted, dust disks were found around many young stars. The first was the disk around a star called Beta Pictoris. In 2008, scientists discovered an object very close to this star. They think it is a giant planet, located somewhere inside the disk.
A planet like Earth? As planetary systems are fairly common, there may be many exoplanets similar to Earth scattered across the universe. We have not yet found one, but space observatories are expected to do so in the next few years. The system below, called HR 8799, was one of the first multiplanet systems to be recorded. Images like this prove that complex planetary systems do exist—systems that might just contain an Earth-like planet.
Asteroids and comets both orbit the Sun, and are the remains of objects formed in our Solar System. Sometimes their unusual orbits bring them close to planets and moons.
The difference is in their composition. Asteroids typically comprise rocks and metals, while comets also have ice, dust and organic matter in addition to the rocky material. Asteroids stay stable and solid, but if a comet gets close to the Sun, some of its ice melts off.
That’s what gives comets their characteristic tails’ – fuzzy trails pointing away from the Sun that contain ice and compounds such as ammonia It’s likely that Asteroids would have formed closer to the Sun, while comets formed further away in the Solar System, making them able to retain ice.
Some astronomers theorise that comets could have formed closer to the Sun, before being flung out by gravitational forces from gas giants like Jupiter.
Another difference between comets and asteroids is that the former can have huge, elliptical orbits, while asteroids usually have circular, shorter orbits.
A blue whale have weigh the maximum amount as twenty elephants, however it’s no drawback floating within the ocean. The large ships that ferry product round the world will hold thousands of big containers while not sinking. Things float if they’re less dense than the fabric around them-that is, if an exact volume of the thing weighs but a similar volume of its surroundings.
WHY DO THINGS SINK? Gravity pulls things down, even in water. However water pushes upward against things floating in it. If there’s enough water pressure at a lower place one thing to balance its weight, it floats. Wood is a smaller amount dense than water. The load of a block of wood is a smaller amount than the pressure of the water beneath, therefore it floats. Stone is a lot of dense than water, therefore its weight pulls it down in spite of the pressure of the water beneath.
GOING UP Heavy things will fly if they will produce enough upward force to elevate their weight. Before planes were made-up, individuals took to the skies in balloons. These soar into the sky once hot air is pumped up within the large material dome. A balloon choked with hot air weighs but a similar balloon choked with cold air. It’s less dense than the air around it, therefore it soars into the sky. huge balloons will produce enough force to hold individuals with them.
CRAFTY RAFTS The world’s biggest load ships carry up to 18,000 immense containers, every as huge as a truck. Ships work by spreading their weight over a large space. Most of a ship is simply empty house, therefore it weighs but a similar volume of water. Though it’s implausibly significant, it still floats.
Most of the rock and ice from the Solar Nebula that didn’t fall into the young Sun eventually went into building Jupiter and, to a lesser extent, the other big planets of our solar system. But there were still some leftover building blocks that never got incorporated into planets, like the small (0.6–6 miles [1- 10 kilometers]) rocky planetesimals in the main asteroid belt that Jupiter’s gravity prevented from growing into full fledged planets, and similar icy ones beyond Neptune that were simply too far apart and that collided too rarely to grow into large planets.
This latter class of so-called trans-Neptunian objects is of special interest, partly because the first one ever discovered is also the most famous: Pluto.
Pluto is a small, icy, rocky world in an elliptical orbit between about 30 to 50 astronomical units (AU). It is only about 20 percent the mass and 35 percent the volume of our Moon, and yet it has a large icy moon of its own, Charon, along with at least four other smaller icy moons, and a thin comet-like atmosphere of nitrogen, methane, and carbon monoxide.
Since the early 1990s, astronomers have discovered many more “Plutos” out beyond Neptune, orbiting within a doughnut-shaped disk called the Kuiper belt, named after the Dutch-American astronomer Gerard P. Kuiper. Beyond the Kuiper belt, which contains small icy bodies that formed in the zone between about 30 and 55 AU, another “scattered disk” consists of icy bodies that formed closer to the Sun but were flung out there by gravitational encounters with Jupiter to between 30 and 100 AU. More than 1,100 trans-Neptunian objects are now known. As it became clear that there are huge numbers of Pluto-like objects in the Kuiper belt and scattered disk, the International Astronomical Union demoted Pluto and other similar objects in 2006 to dwarf-planet status.
Pluto is the last well-known body in our solar system not yet visited by a space mission. That will change in 2015, when the New Horizons mission flies past Pluto and its moons and reveals them as new worlds rather than just fuzzy points of light.
If Earth and Venus can be considered fraternal twin planets, Uranus and Neptune are more like identical twins. Both are denizens of the deep outer solar system, with Neptune, out at 30 astronomical units, taking approximately 165 Earth years to orbit the Sun once. Both ice giants are about the same size and mass (Neptune is slightly heavier, at 17 Earth masses), and both have a similar composition: about 80 percent hydrogen, 19 percent helium, and trace amounts of methane and other hydrocarbons.
As it does on Uranus, methane gives Neptune its beautiful azure color. Neptune, like Uranus, is another ice giant world with a modest number of icy satellites (13) and a system of dark icy rings. From telescopic measurements, data from the 1989 Voyager 2 flyby, and laboratory studies, astronomers have deduced that Neptune’s gaseous atmosphere extends about 10 to 20 percent of the way to the center of the planet. Then, as pressure and temperature increase, higher concentrations of water, ammonia, and methane form a hot liquid mantle. Astronomers refer to this zone as “icy” because the molecules there are thought to have originally come from the mostly icy outer Solar Nebula planetesimals that were part of Neptune’s original building blocks. Some astronomers even think of this zone as a water-ammonia ocean, and computer simulations suggest that a rain of diamonds fall through this ocean to the planet’s Earthlike core of rock, iron, and nickel.
It’s puzzling to astronomers that the ice giants reside in the far outer solar system, because there may not have been enough solar nebula material at those distances to form them. One explanation may be that they formed closer to the Sun and slowly migrated out, perhaps gravitationally nudged by Jupiter and/or Saturn. We think of the solar system today as stable, running like clockwork, but when the planets were forming, it was much more violent and chaotic.
Our solar system’s seventh planet, unlike the first six, was not known to the ancients. Uranus was discovered in 1781 by telescopic observations of the English astronomer Sir William Herschel. Indeed, it had been observed by many other astronomers as early as 1690, but because of its extremely slow motion across the sky (an 84-year orbit period), it was mistaken for a star.
Because Uranus has an average orbital distance of about 19 astronomical units (Saturn, the next closest planet to the Sun, has an average orbital distance of about 9.5 astronomical units), its discovery instantly doubled the size of the solar system.
At 4 times the diameter and 15 times the mass of Earth, Uranus is classified as a giant planet, but it is much smaller than planetary cousins Jupiter and Saturn. Still, the atmosphere of Uranus contains mostly hydrogen and helium, and the planet’s distinctive blue-green color is caused by methane clouds and hazes in the upper atmosphere. Storms on Uranus are rare, and the cloud and haze bands are usually quite faint.
Uranus has a different overall planetary composition from Jupiter and Saturn, however, with significant amounts of ice and rock in the deep interior. In fact, the ratio of ice and rock to gas is so much higher in Uranus (and Neptune), as compared to Jupiter and Saturn, that the planet is more appropriately called an ice giant instead of a gas giant.
As discovered by telescopic observations and the Voyager 2 flyby in 1986, Uranus has 5 large moons and 22 smaller moons, all of them dark and icy. The planet also sports a series of about a dozen thin, dark, icy rings, possibly formed from a relatively recent breakup of one or more small moons.
Perhaps the strangest thing about Uranus is that its spin axis is tilted on its side by about 98 degrees relative to the ecliptic (the plane of Earth’s orbit around the Sun). The unusual axial tilt of Uranus may be a result of a grazing giant impact or a close encounter with Jupiter long ago. Whatever the case may be, it is yet another of our solar system’s many unsolved mysteries.