Embryogenesis is the step in the life cycle after fertilisation – the development of the embryo, starting from the zygote (fertilised egg). Organisms can differ drastically in the how embryo develops, especially when they belong to different phyla.
Growth is the enlargement of a tissue or organism. Growth continues after the embryonic stage, and occurs through cell division, enlargement of cells or accumulation of extracellular material. In plants, growth results in an adult organism that is strikingly different from the embryo. The dividing cells tend to be distinct from differentiated cells (see stem cell).
In some tissues dividing cells are restricted to special areas, such as the growth plates of bones. But some stem cells move to where they are needed, from the bone marrow to form muscle, bone or adipose (fat) tissue.
Many animals have a larval stage, with a body plan different from that of the adult organism. The larva abrubtly develops into an adult in a process called metamorphosis. For example, caterpillars (butterfly larvae) are specialized for feeding whereas adult butterflies (imagos) are specialised for flight and reproduction. When the caterpillar has grown enough, it turns into an immobile pupa. Here, the imago develops from imaginal discs found inside the larva.
Regeneration is the reactivation of development so that a missing body part grows back. This phenomenon has been studied particularly in salamanders, where the adults can reconstruct a whole limb after it has been amputated. Researchers hope to one day be able to induce regeneration in humans. There is little spontaneous regeneration in adult humans, although the liver is a notable exception. Like for salamanders, the regeneration of the liver involves reversing some cells to an earlier state.
Solidarity
Solidarity can refer to:1924
1924 was a leap year starting on Tuesday.
1929
Year 1929 (MCMXXIX) was a common year starting on Tuesday in the Gregorian calendar. By January 1 of this year, every state in the entire world had adopted the Gregorian calendar, having abandoned the Julian calendar.
1920s
The 1920s was the decade that started on January 1, 1920 and ended on December 31, 1929. This decade was known as the Roaring Twenties or the Jazz Age.
Outer space
Space, also known as outer space, is the near-vacuum between celestial bodies. It is where everything (all of the planets, stars, galaxies and other objects) is found.
On Earth, space begins at the Kármán line (100 km above sea level). This is where Earth's atmosphere is said to stop and outer space begins. This is not a natural boundary but is a convention used by scientists and diplomats.
However, the space near Earth is quite crowded by astronomical standards. A list of spaces goes like this:
Exploring space is very difficult because it contains no air and is so large that even the fastest ships can only explore a tiny part of it. It takes 3 days of traveling to reach the Moon and, depending on speed, it would take a long time to reach the closest star Proxima Centauri.
Manned spacecraft are designed to keep good air inside them and to protect astronauts from extreme temperatures.
We gain most of our information about the items in space from different kinds of telescopes. Some of them are space telescopes, put in outer space for a better view. Space probes also explore planets, comets and other space objects that are not too far.
Natural satellite
A natural satellite in astronomy is a smaller body which moves around a larger body. The smaller body is held in orbit by gravitation. The term is used for moons which go around planets, and it is also used for small galaxies which orbit larger galaxies.
Bodies which orbit planets are called moons. They vary in size. The Earth has only one moon. Some other planets have many moons, and some have none. When people write just ""the" moon", they are usually talking about the moon of the Earth. Earth's moon is written with a capital letter, "Moon". The Latin word for the moon is "luna", which is why the adjective used to talk about the moon is "lunar". For example, lunar eclipse.
Anything that goes around a planet is called a satellite. Moons are natural satellites. People also use rockets to send machines into orbit around the Earth. These machines are called artificial (man-made) satellites.
Moons do not make their own light. We can see the Earth's moon because it acts like a mirror, and reflects the light of the Sun. The same half of the moon faces toward Earth at all times, no matter where it moves. But different parts of the moon are lit up by the Sun, so it looks different at different times of the month. This change as seen from Earth is called the phases of the moon, or lunar phases.
A moon's cycle is the time the moon takes to change from looking very bright and round to looking very small and thin, and then back to bright and round again. In the case of the Earth's moon, this is about four weeks. It does this about 13 times in one year. The moon's cycle is about 28 days, a bit shorter than a calendar month.
The Apollo 11 mission helped Neil Armstrong and Buzz Aldrin become the first people to walk on the Moon. They did this on July 20, 1969.
The orbit of a moon or other satellite is affected by two forces: gravity, and the centripetal force.
For example, the Earth's moon is kept in orbit by the gravitational pull from the Earth. This is also the way the Earth is attracted to the Sun, and stays in its orbit. The orbit of the Earth's moon actually causes the tides and waves on Earth.
No moons that belong to moons have been found. In most cases, the tidal effects of the main body would make such as unstable.
However, math completed after the recent finding of a possible ring system around Saturn's moon Rhea show that Rhean orbits would be stable. Also, the rings are thought to be narrow, something that is known with shepherd moons.
The finding of 243 Ida's moon Dactyl in the early 1990s was the proof that some asteroids have moons; indeed, 87 Sylvia has two. Some, such as 90 Antiope, are double asteroids with two same-sized parts.
The biggest moons in the Solar System (those bigger than about 3000 km across) are Earth's moon, Jupiter's Galilean moons (Io, Europa, Ganymede, and Callisto), Saturn's moon Titan, and Neptune's captured moon Triton.
The following is a table grouping the moons of the solar system by diameter. The column on the right has some notable planets, dwarf planets, asteroids, and Trans-Neptunian Objects for comparing. It is normal for moons to be named after people from mythology.
Planets in our Solar System that have moon(s):
Planets in our Solar System that do not have moons:
Galaxies are found in groups called galaxy clusters which are also held together by gravitation. Our own Milky Way is the second largest galaxy in our Local Group (the largest is Andromeda). Many smaller galaxies and star clusters are also held in the Local Group, outside the two main galaxies. They are all in orbits round one of the centres of gravity. That means most of them move round either Andromeda or the Milky Way. so it seems natural for astronomers to use the term 'satellite' for these as well.
Our Local Group is itself part of an even larger group, the Virgo Supercluster. There are other, even larger, groups of galaxies: see the Great Wall for an example.
Gravity
Gravity, or gravitation is one of the fundamental forces of the universe. In this article, we discuss it in three parts:
Some physicists think gravity is caused by gravitons, but they are still unsure.