HUBBLE SPACE TELESCOPE

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1 ASTRO 202 Age of the Universe Tuesday February 19, 2008 STARS: How and where do they form? From clouds of dust and gas primarily hydrogen -in our galaxy and other galaxies Part of the Orion nebula (Hubble Space Telescope image) Prof. Don Campbell HUBBLE SPACE TELESCOPE D = 2.4 m => resolution = 0.04 arcsec at λ = 0.5 μm (Could see a coffee cup on Earth from its orbit) Eagle Nebula 1

2 How do stars work - i.e. where does all the light come from? How do stars work - i.e. where does all the light come from? From burning (fusing) hydrogen to form helium and then heavier elements Helium nucleus 2 protons, 2 neutrons Energy released given by E = mc 2 (Einstein) Hydrogen atom nucleus 1 proton Luminosity of Sun = 4 x watts (about 400,000,000,000,000,000 power stations a power station is ~1,000 mega watts) (From N. Strobel notes) (m = mass difference, c = velocity of light) How long will the Sun live roughly in its current form? We know how much energy the Sun emits per second 4 x Joules We know that when 4 hydrogen nuclei burn to form a helium nucleus that the helium nucleus weighs ~0.7% less than the 4 hydrogen nuclei. This mass difference is converted into energy via E = mc 2 Therefore: 1 kilogram of hydrogen gives x 1 x c 2 joules of energy = 6 x Joules (1 watt = 1 Joule/sec) How many kilograms of mass is burnt per second? = 4 x divided by 6 x = 6.6 x kilograms/sec ~ 660 million tons/sec What is the mass of the Sun? From the distance of the Earth around the Sun and its orbital period 1 year about the Sun we know that: The Sun s mass = 2 x kilograms (i.e. 2 x tons) How much of the Sun s mass (mainly hydrogen) is in the center at high enough temperature and pressure to fuse into helium: Mass available to burn = ~10% of mass of Sun = 2 x tons How long before all this mass is burnt : Lifetime as a normal star = 2 x / 6.6 x 10 8 seconds = 3 x seconds 1 year ~ 3 x 10 7 seconds ~ 10 billion years 2

3 A diversion: If the Sun radiates 4 x watts Power per square meter at Earth = 1.4 kilowatts Average house uses ~1.5 kilowatts 24-hours a day Solar cells ~10% efficient, only work in daytime Therefore need ~30 m 2 of solar cells to power a house Problems: day/night storage rainy or snowy days (winter), etc Not all stars have the same mass Range is about 0.1 to 50 times mass of the Sun Luminosity / luminosity of the Sun Modern power station ~ 1,000 mega watts ~ 1 km x 1 km of sunlit surface Not a lot but difficult to collect and store energy efficiently The star s size and surface temperature increase with mass so the luminosity (energy radiated) increases very rapidly with mass Massive stars have short, brilliant lives millions of years Small stars have long, dull lives billions of years Hertzsprung Russell diagram Gets to about half the distance to Earth 3

4 Planetary nebula outer envelope of the star being blown off while the center part contracts to a white dwarf about the size of Earth Circled stars are white dwarfs Crab Nebula supernova remnant the original star supernovaed in 1054 AD (actually 6,000 years before that since it is that far away). Stars more than 5 times the mass of the Sun end their lives in a massive explosion called a supernova that blows off the outer part of the star and the inner part shrinks down to a neutron star about 20 kilometers in diameter or, if the star is at least 25 times more massive than the Sun, to a black hole. Oldest globular clusters contain stars that are no bigger than ~0.7 x mass of the Sun. Knowing how stars evolve this tells us that the age of the globular cluster is 10 to 18 billion years so Universe must be at least this old. 4

5 THE AGE OF THE UNIVERSE An Earth centered universe until the 16 th century To determine this one has to have an understanding of what the universe is? Ptolemaic (Greek) view of the universe - Sun and planets form the first 7 crystal spheres and the stars the 8 th. Outside that is the sphere of the heavens. The Revolution in Astronomy Copernicus and Galileo Galileo Galilei (1564 (?) 1642) Nicholas Copernicus ( ) On the Revolution of the Heavenly Spheres (1543) Jupiter and its moons (1609) Galileo s telescope about 2 cm effective aperture the first astronomical telescope 5

6 This sorted out the solar system as far as it was known then but: What About the Stars? The Universe? The stars are suns like our own and there are countless suns freely suspended in a limitless space, all of them surrounded by planets like our earth, peopled with living things The sun has no central position in the boundless universe. Giordano Bruno (~1580) Until the late 19th and early 20th century it was assumed that the Universe and the Galaxy were one. Some people thought that the nebulae observed amongst the stars were other island universes but most assumed that they were part of our galaxy universe. Issue finally resolved by Edwin Hubble in s Charles Messier compiled list of 100 fuzzy or diffuse non-stellar objects (nebulae) in the sky Messier objects 1864 General Catalogue of Nebulae by John Herschel 1888 New General Catalogue (NGC numbers) by J. Dreyer Some of these nebulae showed spiral structure debate raged until 1920 as to whether these nebulae were in our galaxy or external to it. Issue resolved by Edwin Hubble in 1923 using new 100 inch Mt Wilson telescope in California using Cepheid variables as standard candles. Need to measure distance and velocities of nebulae. How do we measure velocities? Use the Doppler technique: For galaxies moving away from us light is REDSHIFTED For galaxies moving towards us light is BLUESHIFTED MEASURING DISTANCES: Need a standard candle That is, a natural source of bright light that always has the same luminosity so that if we measure its intensity at Earth we can tell how far away it is. The observed intensity decreases as the square of the distance. Wavelength shift proportional to velocity Z = wavelength shift divided by the original wavelength Easy to measure Stars - Vary in their luminosity although if we can measure their temperature we can estimate their size and luminosity not very exact. Galaxies - Vary a lot in size and luminosity brightest ones in a group often have similar luminosities Supernova Brightness varies depending on size of exploding star but can be seen to great distances Need to calibrate all the relative distances from these methods Henrietta Leavitt made the big early breakthrough 6

7 Henrietta Leavitt ( ) What type of star are Cepheid variables? massive stars near the end of their life. Cepheid variable star light curve Factor of 2 brightness variation Cepheids Hertzsprung-Russell diagram - massive stars are hotter and more luminous Leavitt used Cepheid variables in the Magellanic clouds because they are all about the same distance from Earth (now known to be about 150,000 light years). She found that the brightness of the star correlated with the period of the brightness variation. But the distance to the Magellanic clouds was not known so she could not determine luminosity vs period the absolute scale. Small and large Magellanic clouds looking south from lake Titicaca, Bolivia Distance ~150,000 light years Harlow Shapley used Cepheid variables in our galaxy to calibrate Leavitt s relationship giving luminosity vs brightness. Cepheids were then a STANDARD CANDLE 7

8 Calibrating Cepheids - Distances to nearby stars from stellar parallax Harlow Shapley measured size of our galaxy (300,000 light years, 10 times bigger than thought at the time) about 1918 using Cepheid variables but insisted that that was it the nebulosities were part of the galaxy. (His value for the size was later determined to be too high by about a factor of 3) In 1923 Edwin Hubble measured cepheid variables in the Andromeda nebulae and showed that it was a million light years away most nebulae were in fact distant galaxies and the universe was much larger than we thought. It is less than one hundred years since we gained a reasonably accurate concept of the universe. Edwin Hubble at work M31 Andromeda galaxy HUBBLE S LAW - What did Hubble do? Hubble s law relates distance and velocity of galaxies in the universe. Velocities of many nebulae (galaxies) were known Hubble measured the distances to galaxies using Cepheid variables detected by the then new 100 inch Mt Wilson telescope biggest in the world. Found correlation between recessional velocity and distance THE FURTHER AWAY A GALAXY IS THE FASTER IT IS RECEDING FROM OUR GALAXY (1929). If you time reverse this then at some time in the past all the matter in the galaxies was in the same place this time is the age of the universe Mt Wilson 100 inch telescope completed 1919 used by Hubble to measure distances and velocities of galaxies. 8

9 200 inch PALOMAR TELESCOPE Figure 1: Radial velocity as a function of distance x 106 Distance Velocity law Velocity = H 0 x distance (or distance/velocity = 1/H 0 ) H 0 = the Hubble constant 1/H 0 is the Hubble time ~ the age of the universe Hubble measured 1.8 billion years much too low Galaxy types: Spiral galaxy Galaxy cluster Abell 2218 at a distance of ~2 x 10 9 light years Hubble Space Telescope photo Elliptical galaxy Irregular galaxy 9

10 Cepheid variables too dim to be seen in distant galaxies so need secondary standard candles that are calibrated using Cepheid variables. Tully-Fisher method: Found that the brightness of galaxies in the infra-red is related to their size. Surface brightness of galaxies: Similar for certain types of galaxies. Type II Supernova: Explosion of giant stars but energy release varies. Big breakthrough: Type 1a supernova (only about 10 years ago) Type Ia Supernova Accreting white dwarf Type I supernova about 5 billion times brighter than the Sun, 50 million times brighter than the brightest Cepheid variable so can detect about 7,000 times further away. A Type 1a supernova explosion at the distance of the nearest star, ~3 light years, would be almost as bright as the Sun. Hubble Constant (H o ) Expansion Rate of the Universe Major discovery from measuring the velocity and distance of galaxies with Type 1a supernova: H o in km/sec/mpc given by slope of the best fit line. Current best estimate is approx. 72 km/sec/mpc Age of Universe ~ H -1 o (about 14 billion yrs) THE EXPANSION OF THE UNIVERSE IS ACCELERATING Expect that it would be slowing due to gravity so a big surprise! Acceleration must be due to DARK ENERGY that is, we don t understand it Using Einstein s equation E = MC 2 can show that the equivalent mass of this dark energy comprises 70% of the mass of the Universe. One parsec (pc) ~ 3 light years ~ km 10

11 DARK MATTER Composition of the Cosmos There must be much more matter in the Universe than what we add up from stars and gas that we can observe. Many clues that this matter must exist. Clusters of galaxies: Velocities of individual galaxies so high and census of matter so small that the cluster should fly apart because not enough gravitational force to keep them together. Not normal luminous matter protons, electrons, etc or we would detect it don t have a clue as to what it is. Coma cluster of galaxies ~300 million light years away, ~1,500 galaxies 11

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