The term "evolution" usually refers to the biological evolution of living things.
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But the processes by which planets, stars, galaxies, and the universe form and change over time are also types of "evolution. In the late s the American astronomer Edwin Hubble made a very interesting and important discovery.
Hubble made observations that he interpreted as showing that distant stars and galaxies are receding from Earth in every direction. Moreover, the velocities of recession increase in proportion with distance, a discovery that has been confirmed by numerous and repeated measurements since Hubble's time. The implication of these findings is that the universe is expanding. Hubble's hypothesis of an expanding universe leads to certain deductions.
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One is that the universe was more condensed at a previous time. From this deduction came the suggestion that all the currently observed matter and energy in the universe were initially condensed in a very small and infinitely hot mass. A huge explosion, known as the Big Bang, then sent matter and energy expanding in all directions.
This Big Bang hypothesis led to more testable deductions. One such deduction was that the temperature in deep space today should be several degrees above absolute zero. Observations showed this deduction to be correct. In fact, the Cosmic Microwave Background Explorer COBE satellite launched in confirmed that the background radiation field has exactly the spectrum predicted by a Big Bang origin for the universe.
As the universe expanded, according to current scientific understanding, matter collected into clouds that began to condense and rotate, forming the forerunners of galaxies. Within galaxies, including our own Milky Way galaxy, changes in pressure caused gas and dust to form distinct clouds. In some of these clouds, where there was sufficient mass and the right forces, gravitational attraction caused the cloud to collapse.
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If the mass of material in the cloud was sufficiently compressed, nuclear reactions began and a star was born. Some proportion of stars, including our sun, formed in the middle of a flattened spinning disk of material. In the case of our sun, the gas and dust within this disk collided and aggregated into small grains, and the grains formed into larger bodies called planetesimals "very small planets" , some of which reached diameters of several hundred kilometers.
In successive stages these planetesimals coalesced into the nine planets and their numerous satellites. The rocky planets, including Earth, were near the sun, and the gaseous planets were in more distant orbits. The ages of the universe, our galaxy, the solar system, and Earth can be estimated using modem scientific methods.
The age of the universe can be derived from the observed relationship between the velocities of and the distances separating the galaxies. The velocities of distant galaxies can be measured very accurately, but the measurement of distances is more uncertain. Over the past few decades, measurements of the Hubble expansion have led to estimated ages for the universe of between 7 billion and 20 billion years, with the most recent and best measurements within the range of 10 billion to 15 billion years.
A disk of dust and gas, appearing as a dark band in this Hubble Space Telescope photograph, bisects a glowing nebula around a very young star in the constellation Taurus. Similar disks can be seen around other nearby stars and are thought to provide the raw material for planets. The age of the Milky Way galaxy has been calculated in two ways.
One involves studying the observed stages of evolution of different-sized stars in globular clusters. Globular clusters occur in a faint halo surrounding the center of the Galaxy, with each cluster containing from a hundred thousand to a million stars. The very low amounts of elements heavier than hydrogen and helium in these stars indicate that they must have formed early in the history of the Galaxy, before large amounts of heavy elements were created inside the initial generations of stars and later distributed into the interstellar medium through supernova explosions the Big Bang itself created primarily hydrogen and helium atoms.
Estimates of the ages of the stars in globular clusters fall within the range of 11 billion to 16 billion years. A second method for estimating the age of our galaxy is based on the present abundances of several long-lived radioactive elements in the solar system. Their abundances are set by their rates of production and distribution through exploding.
According to these calculations, the age of our galaxy is between 9 billion and 16 billion years. Thus, both ways of estimating the age of the Milky Way galaxy agree with each other, and they also are consistent with the independently derived estimate for the age of the universe. Radioactive elements occurring naturally in rocks and minerals also provide a means of estimating the age of the solar system and Earth. Several of these elements decay with half lives between million and more than billion years the half life of an element is the time it takes for half of the element to decay radioactively into another element.
Using these time-keepers, it is calculated that meteorites, which are fragments of asteroids, formed between 4. The same radioactive time-keepers applied to the three oldest lunar samples returned to Earth by the Apollo astronauts yield ages between 4. The oldest known rocks on Earth occur in northwestern Canada 3.
In Western Australia, zircon crystals encased within younger rocks have ages as old as 4. The best estimates of Earth's age are obtained by calculating the time required for development of the observed lead isotopes in Earth's oldest lead ores. These estimates yield 4.
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The origins of life cannot be dated as precisely, but there is evidence that bacteria-like organisms lived on Earth 3. These early organisms must have been simpler than the organisms living today. Furthermore, before the earliest organisms there must have been structures that one would not call "alive" but that are now components of living things. Each of these molecules is in turn composed of four kinds of subunits known as nucleotides.
The sequences of nucleotides in particular lengths of DNA or RNA, known as genes, direct the construction of molecules known as proteins, which in turn catalyze biochemical reactions, provide structural components for organisms, and perform many of the other functions on which life depends. Proteins consist of chains of subunits known as amino acids. The sequence of nucleotides in DNA and RNA therefore determines the sequence of amino acids in proteins; this is a central mechanism in all of biology. Experiments conducted under conditions intended to resemble those present on primitive Earth have resulted in the production of some of the chemical components of proteins, DNA, and RNA.
Some of these molecules also have been detected in meteorites from outer space and in interstellar space by astronomers using radio-telescopes. Scientists have concluded that the "building blocks of life" could have been available early in Earth's history. An important new research avenue has opened with the discovery that certain molecules made of RNA, called ribozymes, can act as catalysts in modem cells.
It previously had been thought that only proteins could serve as the catalysts required to carry out specific biochemical functions. Thus, in the early prebiotic world, RNA molecules could have been "autocatalytic"—that is, they could have replicated themselves well before there were any protein catalysts called enzymes.
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Laboratory experiments demonstrate that replicating autocatalytic RNA molecules undergo spontaneous changes and that the variants of RNA molecules with the greatest autocatalytic activity come to prevail in their environments. Some scientists favor the hypothesis that there was an early "RNA world," and they are testing models that lead from RNA to the synthesis of simple DNA and protein molecules. These assemblages of molecules eventually could have become packaged within membranes, thus making up "protocells"—early versions of very simple cells.
For those who are studying the origin of life, the question is no longer whether life could have originated by chemical processes involving nonbiological components. The question instead has become which of many pathways might have been followed to produce the first cells. Will we ever be able to identify the path of chemical evolution that succeeded in initiating life on Earth? They published their results January 14 in the journal Communications Physics. Eric Blackman, a co-author on the paper, said the inspiration for their experiment was based on how most textbooks teach the phenomenon of MRI.
People have assumed for a long time that MRI will make disks of material spread out, pushing close material closer to the center, where it can fall into a central star or black hole, and outer material farther away. Researchers can see the results of material piling into the center of a system — a star is born, or a black hole shoots out active jets. But measuring the flow of material accurately enough to test MRI is beyond our current abilities.
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So Blackman and his colleagues took the simplest approach, with springs instead of magnetic fields and weights instead of clouds of charged materials. They filled concentric rotating cylinders with water, and attached a weighted ball with a spring to the center. By spinning the cylinders, they could reproduce the effects of MRI. And it turns out that MRI works just as researchers have long predicted, pushing close materials in and farther materials out.
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Astronomy Day. When a disk of material spins in the cosmos, how do the objects involved react? Researchers recently used a giant swirling tank of water to find out. Stars, planets, and black holes all grow by consuming material from the center of a spinning disk.