PHYSICAL UNIVERSE
PHYSICAL UNIVERSE
GLOSSARY
baryons The term used by cosmologists to mean any of the normal matter in the universe made of protons and neutrons. In particle physics it means only those particles that are made of three quarks.
conservation of angular momentum Angular momentum is a property of how much an object (or set of objects is spinning). It is larger when the object spins faster, but also when the size of the circles it spins around are larger. It is always conserved, so the reason ice skaters spin faster as they draw in their arms is to conserve angular momentum; the same process means that stars collapse into rapidly rotating neutron stars, and explains why solar systems and most galaxies are spinning discs, and why accretion discs are found on so many scales in the universe.
dark energy A mysterious energy source in the universe, with negative pressure, that can speed up cosmic expansion. No one knows what it is but all constraints suggest it is constant with time and space (the ‘cosmological constant’).
dark matter Material we see only by its gravitational impact but that creates no light. We know that galaxies and clusters of galaxies both contain large amounts of dark matter, and we know from observations of the early universe that most of it cannot be baryons (normal matter). Dark matter should not be confused with dark energy, which has a similar name, but is a very different thing.
dwarf planet A planet, like Pluto, which is both orbiting the Sun, and has formed into a roughly spherical shape, but is not sufficiently large to clear its orbit of other similar size objects as the classical or major planets all have.
exoplanet Short for ‘extrasolar planet’, which is simply a planet orbiting a star other than the Sun. Observations in recent years suggest that almost all stars have planets.
gravitational lenses In Einstein’s theory of gravity even light is bent around massive objects. A gravitational lens is an object so massive that it noticeably bends light from objects behind it, distorting their images into arcs, and sometimes complete rings.
inverse-square law Any law in physics in which something decreases in size or intensity with the square of the distance from its source. Both the force of gravity, and the intensity of light are examples of inverse-square laws.
metals Astronomers do know the chemical definition of a metal – we use the term ‘metals’ as a shorthand to indicate elements other than hydrogen and helium, not as a redefinition of the term.
protostar A dense hot clump of interstellar material that is almost hot enough to become a star, protostars are embedded in protoplanetary discs, which as well as being the place where planets will eventually form, also help feed material onto the protostar as it grows into a fully fledged star.
quasar or quasi-stellar object A point of light created by the accretion disc around a supermassive black hole in a very distant galaxy. As material spirals into a black hole it forms a doughnut-shaped accretion disc that gets very hot due to friction between particles and makes a lot of light. Because the galaxy is so distant you see only a point, and often cannot see the host galaxy as it is too dim.
stellar core The very centre of a star, where the extreme temperatures and pressures allow nuclear fusion to occur, generating the energy that powers the star, and creating heavy elements that might one day go on to form a planet.
X-ray binaries A binary star system in which one star is a relatively normal star that is feeding material onto its companion neutron star or black hole. Because of the conservation of angular momentum this creates an accretion disc around the compact object, which gets extremely hot and creates X-ray emission.
KEPLER’S LAWS OF ORBITAL MOTION
the 30-second blast
In 1609 the German astronomer and mathematician Johannes Kepler published Astronomia Nova, in which he described his discoveries about the motion of celestial bodies. Analysing many years’ worth of carefully obtained data of the planets, he noted two significant findings: first, that planets revolve around the Sun in an elliptical path with the Sun located at one focus of the ellipse rather than at the ellipse’s centre; and, second, that planets move faster when they are closer to the Sun in such a way that their paths sweep out equal areas within that ellipse over equal periods of time. Ten years later, in his work Harmonices Mundi, Kepler published a third key mathematical relationship about planetary motion, that the square of each planet’s orbital period is proportional to the cube of the planet’s distance from the Sun. These three mathematical laws, known as ‘Kepler’s laws of orbital motion’, reflect the fundamental basis of all the motion in the universe caused by gravity and rotation. His first law, for example, is true because the acceleration of gravity follows an inverse-square law; the second law, on the other hand, is the result of a phenomenon known as the ‘conservation of angular momentum’, something that occurs in every freely spinning system on Earth and throughout the cosmos.
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Johannes Kepler discovered three laws of motion that describe the motion of objects in the universe – the foundation of all orbital dynamics and space travel today.
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Kepler’s third law – often called the ‘Harmonic Law’ – is a mathematical relationship that arises from the interplay between the first two laws. It is the basic mathematical tool that astronomers use today to measure the mass of any celestial object or system – asteroids, planets, stars, galaxies and more – ranging in size from a few metres across to millions of light years in diameter.
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3-SECOND BIOGRAPHIES
TYCHO BRAHE
1546–1601
Danish astronomer who became the imperial mathematician of Bohemia; he assembled the world’s most detailed measurements of planetary motions in his time
JOHANNES KEPLER
1571–1630
German astronomer, and successor to his mentor Brahe, who built on the latter’s work; he combined astronomical observations with mathematical calculations to figure out how the planets orbit the Sun
The mathematical laws that bear Kepler’s name govern the orbital motions of stars, satellites and space stations.
NEWTON’S LAWS OF MOTION
the 30-second blast
Newton’s laws were astounding intellectual leaps in their time, connecting as they did the motion of everyday objects (such as an apple falling off a tree) with the (at the time mysterious and ‘cosmic’) orbits of the planets. Newton realized that a simple law of gravity, describing a force that increased in proportion with mass and decreased with the square of the distance from that mass, could explain all of Kepler’s empirical laws of orbital motion as well as why everything on Earth appears to be pulled towards the ground. Along with his description of gravity, Newton published three rules for how objects react when a force is applied. The first law states that objects only change their speed or direction of motion in response to a force. The second states that acceleration is proportional to the force divided by the mass. The third states that forces between two objects will be balanced: an action will have an equal and opposite reaction. These deceptively simple laws allow for the physical understanding of a huge range of objects, from bridges to levers to pulleys – and to the motion of the planets. Generations of physics students have now learned Newton’s laws of motion, and his universal law of gravity continues to be an excellent description of gravity, provided the mass isn’t too large or the distance too small.
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Newton’s laws of motion and gravity provide a basic physical foundation to explain the motion of the planets in our solar system, stars in our galaxy and galaxies in our universe.
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Newton’s laws can be used to give a surprisingly accurate estimate of the amount of mass needed in the universe to stop its expansion. We can also use Newton’s law of gravity to find that there is dark matter in galaxies in the universe. Einstein’s model for gravity is only needed very close to large masses (such as the Sun) or for the most compact objects or furthest distances.
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KEPLER’S LAWS OF ORBITAL MOTION
3-SECOND BIOGRAPHY
SIR ISAAC NEWTON
1642–1727
English physicist and mathematician, who made an astonishingly broad set of contributions to foundational matters in both subjects; most famous for his laws of motion and gravity, he was also for a time active in politics and economics
Connecting the motion of falling apples to that of the planets in space, Newton’s laws of motion form the basis of our understanding of how forces make objects move.
BLACK HOLES
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The extreme gravity of black holes powers exotic and fascinating phenomena throughout the universe, including such astronomical wonders as X-ray binaries, gravitational lenses and quasars. What black holes don’t do – despite a popular misconception – is to suck matter into themselves like cosmic vacuum cleaners. From a distance, a black hole won’t attract an object any more forcefully than anything else of the same mass; they are simply objects with gravity so strong that the escape velocity at their surface exceeds the speed of light. However, if something actually does fall in, there will be no way out, except possibly as tiny bits of subatomic matter a long, long time in the future. Black holes can be created when nuclear fusion in giant stars many times more massive than our Sun fails to prevent their stellar cores from collapsing catastrophically under their own weight. Once formed, black holes grow as matter falls into them, their diameters increasing in direct proportion to their total mass. Astronomers know there are many billions of black holes in the observable universe, and supermassive black holes, the largest of all, which accrete matter for millions of years to reach their huge masses, lurk at the centres of large galaxies, including our own Milky Way.
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A black hole is an object in space where gravity is so strong that even light cannot get out, thus making them invisible.
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The nature of a ‘dark star’, the gravity of which could prevent light from leaving its surface, was first calculated and proposed by the British scientist John Michell in 1783. The idea lay fallow for more than a century until Albert Einstein’s general theory of relativity was confirmed in 1919. That allowed for the possibility that a large mass concentrated in a small volume could curve space-time so sharply that there would be no way for any object to escape.
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JOHN MICHELL
1724–93
British astronomer and seismologist; other than his idea that ‘dark stars’ might exist he is also known for suggesting that earthquakes are caused by the movement of rocks below the Earth’s surface
JOHN ARCHIBALD WHEELER
1911–2008
American physicist who popularized the term ‘black hole’; his research ranged from atomic and nuclear physics to relativity and gravitation
While black holes themselves are dark, their surroundings often include discs, jets and streams of brightly lit matter.
GALAXIES
the 30-second blast
Almost all the stars in the universe form, live and die within a galaxy. Galaxies come in two basic types: spirals, which are disc shaped and can have beautiful arms, rings and/or bars; and ellipticals, which are largely featureless blobs of stars. Most star formation takes place in spiral galaxies, while elliptical galaxies tend to be collections of mostly old stars. Until the 1920s the words ‘galaxy’ and ‘universe’ meant essentially the same thing – in fact, one of the early names for a galaxy was ‘island universe’, which reflected an understanding of the vast distances between galaxies. Despite these distances, however, galaxies are not as isolated or as static as they might seem, and over cosmic timescales many galaxies actually do interact and collide, collecting together to form giant groups, clusters and superclusters, this interaction causing them to change shape over time. It is still an open research question what exactly causes some galaxies to stop forming stars and others to continue, or how spirals turn into ellipticals, but it seems to be a mixture of nature (the number of stars and amount of gas and how it all moves around) and nurture (the effect of the local galactic environment and what collisions have occurred over time).
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A galaxy is a massive collection of stars, gas, dust and dark matter, each one like an island floating in the universe.
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Our own galaxy, the Milky Way – sometimes called simply ‘the Galaxy’ – is of the spiral type with a stellar bar. It is a relatively massive spiral with relatively low star-formation rates. Our nearest large neighbour, the Andromeda galaxy, is also a large, older spiral and is moving towards us. It is likely the Milky Way and Andromeda will merge in about four billion years.
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3-SECOND BIOGRAPHIES
HEBER CURTIS
1872–1942
HARLOW SHAPLEY
1885–1972
American astronomers famous for holding a public debate in 1920 on the nature of the spiral nebula: Curtis believed they were extragalactic, while Shapley thought the Milky Way was much bigger and the nebulae were part of it. The positions they held are now known to have both been partly right: Shapley on size and Curtis on locations
Galaxies come in an astounding array of shapes and sizes, from barred spirals to ones that are shaped like a sombrero hat.
STARS
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Stars are immense spheres of almost unimaginably hot gas. They start their lives as low-density cold gas clouds in interstellar space that collapse under gravity, causing their atoms to heat up. Once a protostar’s core gets hot enough, nuclear fusion will begin, and a star is born. This process releases energy as hydrogen is turned into helium, and this heats up the star so that it glows red, blue or even white hot. Once a star runs out of hydrogen in its core its so-called ‘main-sequence’ life is over, and after possibly fusing a series of heavier elements (depending on its mass), it will either fade into a white dwarf or explode in a supernova to leave behind a neutron star or a black hole. A few billion years from now our Sun will swell into a red giant, swallow the Earth and other inner planets and then lose its outer layers and leave behind a white dwarf. More massive stars, however, will end in violent supernovae; these explosions spread heavy elements around the universe allowing systems of rocky planets to form around other stars and may even be the triggers that cause new generations of gas clouds to collapse into stars.
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Stars are cosmic generators – 200 billion of them in a typical galaxy like the Milky Way make the light and most of the heavy elements that fill our universe.
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A star’s mass determines almost everything about it. Our Sun is a relatively modest-mass star so it glows yellow hot, has enough hydrogen to last 10 billion years and will end as a white dwarf. A star a tenth of the mass of the Sun will be dim and red and live for hundreds of billions of years, while one ten times more massive will glow white hot and burn hydrogen so rapidly that it will last less than 100 million years and then explode as a violent supernova.
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ANNIE JUMP CANNON
1863–1941
American astronomer, most famous for developing the stellar classification sequence we still use; it is said that during her lifetime she classified more than 300,000 stars
CECILIA PAYNE-GAPOSCHKIN
1900–1979
British-American astronomer, said to have written (in 1925) the most impactful PhD thesis ever, when she proposed correctly that stars were made mainly of hydrogen and helium. Initially her realization was rejected as too extraordinary (stars differ so much from Earth’s make-up), but it is now known to be true
A star’s colour reveals its temperature, with blue stars being hotter than red. Our Sun is a yellow star, intermediate in temperature.
BETWEEN THE STARS
the 30-second blast
When astronomers describe galaxies as giant collections of stars, gas, dust and dark matter in the universe, what they mean by gas and dust is the interstellar medium, the material from which all new stars and planets are born and to which old ones eventually return after they die. Mostly this is made up of loosely connected and widely spread-out atoms (less dense than any vacuum found on Earth) of hydrogen and helium collected in giant gas clouds known as nebulae, with just trace amounts of ‘heavy elements’–anything that is heavier than hydrogen or helium. Gravity slowly, so slowly, brings the material together, and once it starts to become a little denser in one location the process begins to gather speed. Eventually, the core of a protostar forms and heats up, becoming increasingly hot until sometimes it gets hot enough for nuclear fusion to take place. The dust in the cloud collects and starts to make clumps rotating around the outskirts of the protostar – and these are the seeds from which all planets are made. Once a star dies it returns much of its material to the interstellar medium, material from which the next generation of stars and planets will be made.
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The name given to the stuff found in the space between star systems is the ‘interstellar medium’.
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The interstellar medium provides us with some of the most beautiful sights in astronomy, with colourful nebulae revealing the many different types of elements that can be found in cosmic gas clouds. The variegated hues shine like fireworks in space – in fact, the physics that makes the light is genuinely similar to that which makes the different colours in fireworks. Atoms in the clouds are heated up and glow with their characteristic colours – for example, hydrogen looks red, oxygen blue and sodium yellow.
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3-SECOND BIOGRAPHY
ZOLT LEVAY
1952–
Hungarian-American astronomer responsible for putting together most of the best-known images of the interstellar medium. He cleaned up raw images from the Hubble Space Telescope, merging images from different bands and choosing the colour balance to showcase the science of the objects
Space is the original recycler – most of the material that makes stars is eventually returned back into space at the end of a star’s life, sometimes in spectacularly beautiful ways.
PLANETS
the 30-second blast
For much of human history, the ‘planets’ – the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn – were understood as those celestial objects that wandered the heavens from one night to the next and were distinct from the stars, which maintained apparently fixed positions in the night sky. During recent centuries, however, the scientific definition of a planet has changed many times to reflect our increased understanding of the universe. Today, the International Astronomical Union (IAU) defines a planet in our solar system as an object that has the following three properties: it primarily orbits the Sun; it is massive enough that its internal gravity has made it a spherical shape; and it is by far the largest object in the vicinity of its orbital path. By these criteria, the planets that orbit our Sun are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Across the further-flung universe, however, the IAU has not yet proclaimed any planetary definitions, and since the 1990s astronomers have discovered thousands of planets orbiting stars other than our own. These ‘exoplanets’ defy easy classification because of their wide variety of individual and orbital characteristics. As the number of known exoplanets grows, astronomers may soon have to reconsider – again – what is and what is not a planet.
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Whereas there are only eight planets inside our solar system, thousands of planets have been discovered orbiting other stars, and many more are waiting to be found.
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After being classed as a planet for more than 70 years, Pluto was ignominiously relegated to the status of ‘dwarf planet’ in 2006 by a vote of the IAU. That reclassification, however, did not diminish Pluto’s scientific value or beauty in any way, as images and data from the New Horizons space probe clearly showed in 2016. Other dwarf planets in our solar system include Ceres (the largest asteroid), Eris, Makemake and Haumea.
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WILLIAM HERSCHEL
1738–1822
German-born British astronomer who discovered Uranus in 1781 as well as infrared radiation
URBAIN LE VERRIER
1811–77
French mathematician who predicted Neptune’s existence in 1846; this was subsequently confirmed through observations based on Le Verrier’s calculations by German astronomers Johann Gottfried Galle and Heinrich d’Arrest
CLYDE TOMBAUGH
1906–97
American astronomer who discovered Pluto in 1930
Every planet in our solar system – and, indeed, every other solar system – is a unique world of wonder and mystery.
LIFE AS WE KNOW IT
the 30-second blast
What is life as we know it? It is perhaps the greatest irony of our existence that we know we are alive, yet we cannot properly define what it means to be alive. The best that scientists come up with is to describe what life does and what life needs. Living things on Earth all seem to become alive from being previously non-living, age while they are alive, reproduce following instructions somehow encoded in their bodies and, finally, at some point, stop being alive. Living things on Earth all appear to need liquid water, a small set of other critical substances such as carbon and nitrogen and a reasonably steady source of heat or other energy. Astronomers looking for life elsewhere have been focusing their efforts on finding places in the universe with conditions similar enough to those of Earth with life’s requirements available in a single environment. Until less than three decades ago scientists could not say for sure whether any planets existed outside our own solar system, but today thousands of such planets have been scientifically confirmed. With the technology currently available, however, we cannot yet say if any of these planets harbour life; perhaps in another three decades examples of extraterrestrial life will be as plentiful as known exoplanets are today.
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So far, Earth is the only place in the universe where we have found what we recognize as living organisms.
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The most promising places in our solar system to look for extraterrestrial life seem to be our planetary neighbour Mars, Jupiter’s moons Europa and Ganymede along with Saturn’s moons Titan and Enceladus. In other planetary systems, astronomers are searching for life primarily on planets orbiting in habitable zones around their host stars, where temperatures will be just warm enough for water to stay liquid. A dozen or so such planets have been detected, but so far life on them has not.
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JILL TARTER
1944–
American astronomer and pioneer in the scientific search for extraterrestrial life, especially of life forms that might be able to think or communicate in ways intelligible to humans
CARL SAGAN
1934–96
American astronomer who popularized and championed the scientific search for extraterrestrial life
If humans were to encounter truly extraterrestrial life, would we even recognize it as being alive?
