Strand K Astrophysics Unit 2 Galaxies, Stars and the Solar System Contents Page The Early Universe 2 The Life Cycle of Stars 4 Features of the Solar System 7
K21 The Early Universe Running the current model of the expanding universe backwards some 13 billion years places the entire mass, and therefore the entire energy of the universe in a very small region of space (we call this region the singularity) This region would have been so hot and energetic that our current understanding of physics breaks down, because particle energies at this time were so vast Within 01s of the big bang however, the universe had expanded and cooled to 10 12 K, and at this temperature subatomic particles such as electrons and quarks (quarks are the sub atomic particles from which protons and neutrons are made) could exist The universe continued to expand and cool, and 100s after the big bang the temperature had reduced (10 8 K), and the subatomic particles had reduced in energy such that quarks could come together, forming protons and neutrons For the next 100,000 years the universe remained in this state, continuing to expand and cool, allowing the formation of ionised hydrogen and helium gas Separation distances between particles became larger due to electromagnetic interactions and the universe became transparent to radiation It was at this point that the cosmic microwave background was emitted Stable, neutral atoms of hydrogen and helium then formed The universe remained dark for the next 100 million years, but continued to expand and cool The patchy gas of hydrogen and helium reduced in energy sufficiently for gravity to take affect, condensing the gas and increasing its temperature When these condensed regions of gas reached sufficient temperatures, fusion reactions could take place and the first few stars (emitting the first light in the universe) formed This period is known as the cosmic dawn The stars that we see in the night sky today are all part of our own Milky Way galaxy, a spiral arm galaxy with a diameter of approximately 150 million light years The sun is just one relatively ordinary star of a system of between 100 and 400 billion stars that form the galaxy, bound together by gravitation It is estimated that our own galaxy contains some 100 billion planets, and evidence suggests that the galaxy is rotating about a supermassive black hole (Sagittarius A*) at its center We now know that there are at least hundreds and possibly thousands of billions of galaxies in the observable universe The separation between galaxies is so vast that light from the furthest visible galaxies takes billions of years to reach us 2
Exercise K21 1 In terms of the big bang model, what is meant by the singularity? 2 The laws of physics as we understand them break down at the singularity (at the moment the big bang occurred) Why is this? 3 Protons and neutrons could not be formed until at least 100s after the big bang It took a further 100,000 years before neutral hydrogen atoms could form Why did it take so long? 4 How long would it take light from a star on one side of the milky way galaxy to reach an observer on the other side of the milky way galaxy? Challenge Question 5 Explain what is meant by the cosmic dawn 3
K22 The Lifecycle of Stars Stars are formed within giant hydrogen gas and dust clouds called nebulae Within the nebulae, clumps of gas and dust contract under gravitational forces to form a protostar As the protostar continues to contract under gravity it heats up until there is enough energy to force protons close enough together to fuse (15 million C), initiating a chain reaction and igniting the star The lifecycle of the star is predetermined by the mass of the star, and is shown diagrammatically in Figure K221 Nebula Average Star Massive Star Red Giant Red Super Giant White Dwarf Black Dwarf Supernovae Figure K221 Neutron Star Black Hole For a star of 15 solar masses (15 times the size of our own sun or less); The hydrogen within the star is fused together to form helium, releasing energy During this time the star expands due to the heat energy released in the fusion reaction, until the thermal expansion balances out the gravitational contraction The star is now in its stable burning period, which we call the main sequence The star remains in this phase for around 10 billion years, continually turning hydrogen into helium, which collects within the stars core (center) The hydrogen helium burn occurs only in the stars core during main sequence because here the thermal energy is greatest 4
As the hydrogen in the core begins to run out, the fusion reaction slows and the thermal energy reduces The thermal expansion no longer overcomes the force of gravity and the core contracts The fusion reaction shifts toward the outer shell of the star where hydrogen still remains, and the outer shell of the star expands, becoming larger and cooler The star has now reached the red giant phase Within the core of the red giant, energies become sufficient under the contraction to initiate the fusion of helium to form carbon Eventually the helium in the core runs out, and the outer shell of the red giant drifts away from the core to form a planetary nebulae The core of the star shrinks under gravity forming a white dwarf, a small hot remnant of a star Eventually the white dwarf cools such that it stops shining At this point the remnant of the star has become a black dwarf If the star is above 15 solar masses (some stars are in excess of 50 times the size of our sun), it starts off its lifecycle in just the same way as a small star, burning hydrogen to form helium during its main sequence burn However, the more massive the star the quicker it burns its supply of hydrogen Whereas an average sized star remains main sequence for 10 billion years, a large star can use up all the hydrogen in the core within 1 million years The core then shrinks and the outer layer expands, and forms a red super giant Within the red super giant the core of the star and hence the gravitational forces acting are so large that the helium laid down in the main sequence burn is fused to form carbon Then the carbon is fused to form oxygen and so on, the series of fusion reactions forming layers of heavier and heavier elements down on the core much like the layers of an onion, with the heaviest elements at the center as shown by Figure K222 Core Hydrogen Helium Carbon Oxygen Silicon Iron Figure K222 When the star reaches the point at which energies arising from gravitational forces are insufficient to initiate a fusion reaction of the next heaviest element, or if the star is sufficiently massive and the iron fusion stage is reached (iron can not be burnt in a fusion reaction) the stars core goes out, and collapses in on itself In less than a second the star explodes in a process known as supernovae, for a short time burning brighter than all the stars in an entire galaxy combined, blowing off the outer shell of the star, and forming all the elements in the periodic table in the explosion that were not formed during the core burn A stars lifecycle is the only process in which elements heavier than helium are made naturally, and as such the elements from which the computer you are using or the page that you are reading (or the carbon and oxygen that make up the largest percentage of your body) were made within a long dead star, and distributed throughout the galaxy by a supernovae 5
Sometimes part of the stars core survives the supernovae as a core remnant If the core remnant is less than 15 solar masses, the core collapses under gravity until the coulombic repulsion between electrons and protons balances the force of gravity The core becomes stable forming a black dwarf If the remnant is between 15 and 3 solar masses, the coulombic repulsion is insufficient to balance the gravitational force squeezing the core The core continues to shrink, causing the protons within the core to spontaneously change into neutrons The star continues to shrink, until the strong nuclear force turns repulsive and balances the gravitational forces At this point a super dense neutron star is formed This star so dense that a single cubic meter of material from a neutron star would weigh the same as the entire Atlantic Ocean Neutron stars are typically around 8 miles in diameter, and yet can be 05 million times heavier than the Earth, and spin at 700 revolutions per second! If the core remnant is greater than 3 solar masses, not even the strong nuclear force is enough to stop the core collapsing in on itself under gravitation and forming a black hole The gravitational pull in a black hole is so great that nothing can escape from it, not even light, and the density of matter in a black hole cannot be measured Black holes distort the space around them, and can often suck neighboring matter into them, including stars! Exercise K22 1 Explain the following terms; A Nebulae B Protostar C Main Sequence Star D Core remnant 2 Why does hydrogen fuse to form helium only in the core of a main sequence star? 3 When does a star become a red giant? 4 Consider the following table Which of these stars will form red super giants and then explode in a supernovae? Star Solar Mass 1 Cygnus OB2-1 44 2 Sirius 2 3 Sun 1 Challenge Question 5 Describe the lifecycle of our nearest star (the Sun) from birth to death 6
Mercury K23 Features of the Solar System The Solar System is a system of objects that orbit the Sun in either circular or elliptical motion, bound by gravity These objects include our own planet, plus seven other planets, dwarf planets, moons, comets, asteroids and dust clouds Scientists believe that the solar system was formed 5 billion years ago when a giant interstellar gas cloud collapsed to form the Sun, with the left overs coalescing to form the planets 9985% of the mass in the solar system is contained within the Sun It is a yellow dwarf star, with a surface temperature of approximately 6000 C and a diameter approximately 109 times the diameter of the Earth By mass the sun is made up of 71% Hydrogen, 28% Helium and the remaining 1% comprising heavier atoms such as Carbon, Nitrogen, Oxygen, Silicon and Iron It has no fixed surface and the temperature is too high for matter to exist as a solid or a liquid Due to this different parts of the sun rotate at different rates The parts of the surface near the equator complete a rotation in 25 Earth days, whereas the parts near the pole take 36 days Of the remaining 015%, the majority makes up the planets sown by Figure K231 Here the diameters of each planet are shown to scale In addition the separation between planets is also to scale, but the diameter of the planets and the separation are not to scale relative to each other If they were, and Jupiter were to remain the same diameter as shown, the distance between the Sun and Pluto would be greater than 100 page widths A tediously accurate representation of the diameter and separation of the planets in the solar system may be found here Earth Sun Mars Venus Jupiter Saturn Uranus Neptune Pluto 10AU 20AU 30AU 1AU = 1 Earth-Sun distance = 149,598,000 kilometers Figure K231 The first four planets, Mercury, Venus, Earth and Mars are known as the Inner Planets, since they are the closest to the Sun, or as the Terrestrial Planets because they all comprise of rock and metal, just like our own planet Venus, Earth and Mars have substantial atmospheres whist Mercury has virtually no atmosphere The Earth has one natural satellite, the moon, whereas Mars has two moons, Deimos and Phobos Mercury is the smallest of the eight planets in the solar system and has the closest orbit to the sun It has no known moons Mercury has a diameter of approximately 3000 miles It is made up of a high percentage of metal, making it the densest planet in the solar system It takes Mercury 88 Earth days to orbit 7
the sun The fast orbit however is offset by the planets slow spin 1 day on Mercury is 59 Earth days long Venus is the second planet from the sun Its orbit lies between the orbits of Mercury and Earth and has no known moons It is the closest planet to the Earth and after the moon is the most brilliant natural object in the night sky Venus has a diameter of approximately 12,100 km making it slightly smaller than Earth (Earth s diameter is approximately 12,750 km) Although further away from the sun than Mercury, Venus is the hottest planet in the solar system This is because it has a very large atmosphere made up mostly of Carbon Dioxide This thick, dense atmosphere traps the heat radiated from the planet s surface and the sun (greenhouse effect) making the average temperature on Venus about 460 C Venus spins about its axis very slowly, taking 243 Earth days to complete one rotation, which is the length of a day on Venus It completes one orbital revolution of the sun in 225 Earth days making Venus the only planet where a day is longer than a year Venus rotates about its axis in a retrograde motion ie in a direction opposite to the other planets Venus and Neptune are the only planets which rotate counter clockwise while the other 6 planets rotate clockwise Earth is the third planet from the sun It is the only planet that hosts all known life Its orbit lies between Venus and Mars and has one moon The Earth has a diameter at the equator of 12,756km It spins about its axis once every 24 hours (1 Earth day) and takes 365256 days to orbit the sun The Earth s atmosphere is made up of 78% Nitrogen, 21% Oxygen and the remaining 1% consists of other gases such as Argon, Carbon Dioxide, Methane and Hydrogen Mars is the fourth planet from the sun The orbit of Mars lies between Earth s orbit and Jupiter s orbit Between Mars and Jupiter lies the main asteroid belt Mars has two small moons, Phobos and Deimos, and is the second smallest planet in the solar system Its diameter at the equator is approximately 6,800km It takes Mars 687 days to complete one revolution of the sun Thus, one year on Mars is equivalent to almost 2 Earth years Mars is called the red planet due to the reddish brown Iron Oxide on its surface Its atmosphere is made up mostly of Carbon Dioxide, however the atmosphere is very thin making the average temperature on the surface average about -70 C Between Mars and Jupiter lies the Asteroid Belt Asteroids are leftovers that never quite made it to be a planet It is estimated that there are more than 750,000 of them with diameters larger than 1 km and millions of smaller asteroids A little known dwarf planet called Ceres, about 950 km in diameter, resides here The next four Outer Planets are giant planets, known as the Jovian Planets (Jovian means Jupiter like ) Jupiter and Saturn are large Gas Giants, and are made mostly of hydrogen and helium 8
Jupiter is by far the largest planet in the solar system with a mass 25 times the mass of the rest of the planets combined Whilst it may have a solid core it does not have a solid surface Jupiter has a volume 1,321 times the volume of Earth, and yet is only 318 times as massive It also has 69 known natural satellites The four largest moons, Io, Europa, Ganymede, and Callisto are some of the largest in the solar system, visible from Earth with binoculars, and are collectively known as the Galilean moons Jupiter completes one rotation in 9 hours 55 minutes, which is the length of one Jupiter day, and it takes about 119 earth years to orbit the sun Saturn is 95 times the size of the Earth and probably has a rocky core After Jupiter it is the second largest planet in the solar system Saturn takes 30 Earth years to complete one revolution around the sun, and one day on Saturn is 108 hours Saturn is surrounded by a ring system composed of ice particles, rocky debris and dust 62 moons are known to orbit Saturn, of which fifty-three are officially named! Titan is Saturn's largest moon, and the second largest in the Solar System Titan is larger than the planet Mercury, although less massive, and is the only moon in the Solar System to have a substantial atmosphere The next two Jovian planets are Uranus and Neptune Both are Ice Giants because they primarily consist of ice, and are far less massive than Jupiter or Saturn Both Neptune and Uranus are less than 20 times the mass of the Earth Uranus and Neptune have similar liquid interiors due to very high temperatures and pressures inside the planets They are made up of the melted ices of water, methane and ammonia, along with molten rock and metals and small amounts of Hydrogen and Helium They each have massive atmospheres made up of approximately 75% Hydrogen and 25% Helium with small amounts of methane, water and ammonia Finally comes, Pluto 39 AU from the Sun Once considered the ninth planet of the solar system, Pluto was downgraded to a dwarf planet in 2006 due to the discovery of multiple objects in orbit around the Sun that were larger (the dwarf planet Eris for example) Charon, the largest of Pluto's moons, is sometimes described as part of a binary system with Pluto, as the two bodies orbit a center of gravity above their surfaces (ie they appear to orbit each other) An easy way to remember the order of the planets (including poor Pluto) is the mnemonic My Very Easy Method Just Speeds Up Naming Planets, to give Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto (not a planet ) 9
The planets and asteroid belt are locked into circular or elliptical orbits around the Sun by the Sun s immense gravitational pull, which stops the planets flying off into space The planets orbit in a similar, singular plane with the exception of Pluto, which orbits on a tilt as shown by Figure K232 Halley s Comet Notice the arrow passing through each planet signifying the axis of rotation of the planet All planets including the Earth have a tilted Figure K232 rotational axis It is this tilt, combined with the orbit of the Earth about the Sun that gives rise to the seasons In the first half of the Earths orbit the northern hemisphere gets direct rays from the Sun and in the second half of the orbit the southern hemisphere gets direct rays from the Sun (its not about a difference in distance, that is a misconception) A planet such as Uranus rotates on a tilt in excess of 90, and so must experience seasons of extremes! Comets, such as Halley s comet, also orbit the Sun Comets have extreme elliptical orbits, Halley s comet orbits in an ellipse which extends nearly out to the orbit of Pluto, completing an orbit once every 79 years (Halley was last visible from Earth in 1986 and wont appear again until 2061) A comet is essentially a dirty ice ball, comprising of rock, dust, and frozen gases When heated by the Sun, the gases sublimate and produce an atmosphere surrounding the comet known as the coma The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which always points away from the Sun Exercise K23 1 List the planets in order of distance from the sun, starting with the nearest 2 Identify which planets are terrestrial, and which are Jovian 3 Which two planets are classed as ice giants? 4 List 3 naturally occurring features of the solar system that are NOT planets Challenge Question 5 Explain how the tilt of the Earth s rotational axis gives rise to summer and winter You may need to research your answer 10