Introduction to Astrophysics

Transcription

1 Introduction to Astrophysics Colin SKYPONDERER

2 OUR MILKY WAY GALAXY

3 As usual notes are at: Password is t2=mr3&

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6 NAMES From the Greek word galaxias meaning milky As it appears in the sky: a white dusty highway across the heavens

7 Size 100,000 LIGHT YEARS 6,323,900,000 AU ONLY 1,000 LIGHT YEARS THICK Is it bigger than this?

8 Contents and Age billion stars Age unknown. However, oldest stars in galaxy is around 13.2 billion years old

9 Parts

10 Sun

11 Sun s Orbit Earth takes year to orbit Sun = 1 Earth year Sun takes 1 cosmic year to orbit galaxy 1 cosmic year = 220 million years A man travels the world in search of what he needs and returns home to find it. George Moore

12 Sun on the move Sun does pass through spiral arms We pass through a major spiral arm every 100 million years or so taking around 10 million years to pass through each one

13 Up and down Sun yo-yos up and down as it orbits the galaxy because of gravity When on north side of galaxy more exposed to extra-galactic cosmic rays Hits top of its climb every 70 million years or so

14 Mass Extinctions?

15 GALACTIC CENTRE

16 What are we orbiting?

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19 Distance vs Time T 2 = M R 3 IF: T IN EARTH YEARS M IN SOLAR MASSES R IN AU

20 Mass? 4 million solar masses

21 Size? S2 has a pericenter of just 17 light hours (120 AU) Size of orbited object must be significantly smaller than this otherwise star would be ripped apart (Roche limit) What can be 4 million solar masses in a space the size smaller than solar system?

22 Black holes

23 Black holes

24 What's at the bottom?

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26 LIGO

27 Accretion disc In-falling material gets sped up and ripped apart by immense gravitational forces of black hole. This forms a flattened disk called an accretion disk. As a result of this process high energy radiation is created mostly in the form of X-rays.

28 Sgr A* Black hole's food is gas from winds blown off by massive, young hot stars near centre. But these are relatively far away from centre - previously estimated to consume only 1% of the gas.

29 But... X-ray observations by Chandra space telescope suggest it is munching on only 1% of that 1% Some black holes are a thousand million times more active than this.

30 Past activity Observations of a cloud called Sagittarius B2 show variations due to interactions with X-rays and gamma ray. The cloud is 300 light years from galactic centre. Lots of radiation must have been produced 300 years ago at a level a million times more active than today.

31 G2 Cloud Close Approach Cloud 3 times the mass of the Earth has more than doubled its speed in last eleven years Was due to be swallowed in early 2014 but managed to escape. It is thought the cloud contains a star whose gravity held the cloud together. It is thought this single star formed when the black hole's gravity pulled two stars together.

32 Blowing Bubbles Well defined edges of bubbles hint at sizeable and rapid release of energy

33 MILKY WAY BAR... Observations in 2005 by the Spitzer Space Telescope helped create the most detailed survey of the inner galaxy to date. The results heavily suggest the presence of a bar at the centre of the Milky Way just like that seen in NGC 1073 NGC 1073

34 A DRINK AT THE BAR... Radio astronomers were looking at Sagittarius B2 searching for amino acids the building blocks of life But they also found the cloud contained ethyl formate a chemical that smells of rum.

35 GALAXY CLASSIFICATION

36 SPIRAL ARMS

37 WINDING PROBLEM

38 DENSITY WAVES

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42 HALO Roughly spherical cloud of old stars and globular clusters 90% of visible material in the halo lies within 100,000 light years. ~ 150 globular clusters orbit on edge of Milky Way. 40% do so retrograde

43 GLOBULAR CLUSTERS Over 100,000 stars within 100 or so light years There are only around 15,000 stars within 100 light years of Earth Some of the oldest stars known with ages around 13 billion years Formation mechanism unclear

44 Rotation problem

45 Rotation problem

46 Oort and Zwicky

47 Vera Rubin ( )

48 Invisible mass?

49 Dark Matter Nothing visible in this part of the galaxy But something must be there because its gravity is affecting the rotation of the Milky Way Must behave in same way as normal matter but it doesn't interact with light - dark matter Now thought that dark matter makes up 95% mass of Milky Way

50 Gravitational Lensing

51 Dark Matter Beach Ball Orbits of dwarf galaxies around the Milky Way allow the distribution of dark matter to be inferred More dark matter seems to lie above the galaxy than next to it

52 COFFEE BREAK

53 STANDARD MODEL OF PARTICLE PHYSICS WHAT CAN DARK MATTER BE? More on quarks in Week 6 Neutrinos Electron

54 LUX, HOMESTAKE MINE, US LARGE UNDERGROUND XENON EXPERIMENT

55 370 KILOGRAMS OF -100C XENON SHIELDED BY 70,000 GALLONS OF WATER

56 PHOTOMULTIPLIER TUBES PICK UP FLASHES CAUSED BY WIMPS HITTING XENON

57 ICECUBE, ANTARCTICA

58 8 6 H O L E S E A C H C O N TA I N I N G 60 NEUTRINO DETECTORS

59 SOLAR NEUTRINOS

60 GALACTIC CENTRE

61 ALPHA MAGNETIC SPECTROMETER [AMS-02]

62 CATCH UP FROM LAST WEEK

63 Atmospheres Hot Jupiter G type star (like Sun) 1.01 Solar Masses Planet orbits 10 times closer than Mercury

64 Atmospheres

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66 Atmospheres Super Earth

67 Atmospheres Super Earth

68 PLANETARIUM

69 EXTRA NOTES ON THE DRAKE EQUATION

70 POPULATION?

71 Drake Equation What factors do we need to consider when trying to estimate the number of intelligent civilisations in the Milky Way? Discuss

72 Drake's important factors 1). The number of new stars forming in the galaxy per year 2.) Fraction of those stars that have planets 3.) Number of planets per solar system with right environment for life 4.) Fraction of those planets on which life actually appears 5.) Fraction of those living planets that go on to develop intelligent life 6.) Fraction of those intelligent civilisations who go on to develop to release detectable signals into space 7.) The length of time they release these signals for

73 The number of new stars forming in MW per year Measurements suggest that the Milky Way converts between solar masses' worth of gas into new stars every year This should deplete the galaxy's gas reserves But star formation rate seems relatively constant 7 stars per year

74 Feeding from neighbourhood Ionized High Velocity Clouds (ihvcs) from beyond halo are falling onto galaxy and replenishing star forming material

75 Keeping track of Drake's factors 1). The number of new stars forming in the galaxy per year 7 2.) Fraction of those stars that have planets 3.) Number of planets per solar system with right environment for life 4.) Fraction of those planets on which life actually appears 5.) Fraction of those living planets that go on to develop intelligence life 6.) Fraction of those intelligent civilisations who go on to develop to release detectable signals into space 7.) The length of time they release these signals for

76 How many stars have From last week it is thought every star has an average of 1.6 planets orbiting around it - i.e all stars have planets

77 Keeping track of Drake's factors 1). The number of new stars forming in the galaxy per year 7 2.) Fraction of those stars that have planets 1 3.) Number of planets per solar system with right environment for life 4.) Fraction of those planets on which life actually appears 5.) Fraction of those living planets that go on to develop intelligent life 6.) Fraction of those intelligent civilisations who go on to develop to release detectable signals into space 7.) The length of time they release these signals for

78 Number of planets per solar system with right environment for life 70% of stars in the galaxy are thought to be in binary systems Gravitational interplay unable to be stable enough for life

79 Habitability and Stability Earth is 4,600 million years old Life on Earth started around 3,000 million years ago. Intelligent creatures have been around for a blink of an eye In fact if Earth's history were condensed to a single Earth year then oldest fossilised creatures appear late March Recorded human history begins at 23:59:30 on December 31st

80 Long lived stars If you need a similar time scale (4,000 million years) then O, B, A and F stars can be discounted as they burn out too quickly.

81 Kick out K and M too Two ends combined immediately rules out 40% of the stars in the galaxy (60% are G stars)

82 Habitable zone Habitable zone for Sun is around AU

83 Skittling Jupiters Many Jupiter-sized planets are thought to migrate inwards towards their star. This is why we find lots of hot Jupiters in exoplanet searches. Jupiter like planets are thought to migrate inwards in 94% of systems

84 Fraction of stars with planets that have right conditions for life This adds up to 20 million habitable planets in Milky Way Other estimates place it as high as 60 billion (if you include tidally locked planets) So let's say it is somewhere in between - 30 billion There are ~ 200 billion stars in the Milky Way Means a ratio of stars to habitable planets of 1:0.15

85 Keeping track of Drake's factors 1). The number of new stars forming in the galaxy per year 7 2.) Fraction of those stars that have planets 1 3.) Number of planets per solar system with right environment for life ) Fraction of those planets on which life actually appears 5.) Fraction of those living planets that go on to develop intelligent life 6.) Fraction of those intelligent civilisations who go on to develop to release detectable signals into space 7.) The length of time they release these signals for

86 Fraction of those planets on which life actually appears Now we're moving from astrophysics into biology and things get even more speculative. We don't even know for sure how life began on Earth, how can we say how often that process happened elsewhere? What we do know is that life did start pretty much as soon as it could conceivably do

87 Keeping track of Drake's factors 1). The number of new stars forming in the galaxy per year 7 2.) Fraction of those stars that have planets 1 3.) Number of planets per solar system with right environment for life ) Fraction of those planets on which life actually appears ) Fraction of those living planets that go on to develop intelligent life 6.) Fraction of those intelligent civilisations who go on to develop to release detectable signals into space 7.) The length of time they release these signals for

88 Fraction of those living planets that go on to develop intelligent life If we know little about the origins of life, we know even less about the development of intelligence. Certainly it has taken many fortunate steps and evolutionary twists and turns to get here. Intelligence seems considerably less likely than 'simple' life

89 Keeping track of Drake's factors 1). The number of new stars forming in the galaxy per year 7 2.) Fraction of those stars that have planets 1 3.) Number of planets per solar system with right environment for life ) Fraction of those planets on which life actually appears ) Fraction of those living planets that go on to develop intelligence life ) Fraction of those intelligent civilisations who go on to develop to release detectable signals into space 1 7.) The length of time they release these signals for

90 How long can we stick around? Lowest estimate I can find is 420 years. This is based on analysis of duration of 60 post-roman civilisations. A little pessimistic. Highest estimate would be 1,000 million years due to expansion of the Sun and the boiling of Earth's oceans. What could kill us off in the meantime?

91 One up on the dinosaurs

92 Asteroids and Comets Mass extinctions every million years. Last one 65 million years ago thought to be caused by an asteroid hitting Mexico.

93 Solar Flares

94 Supernovae and Gamma-Ray Bursts Both give out highly energetic gammarays Earth's atmosphere protects us from direct irradiation However, gamma-rays would split nitrogen in atmosphere React with oxygen to create nitric oxide This then can then react with ozone destroying 95% of ozone layer We'd be left without a shield for several years and mostly likely get fried

95 But... A supernova would have to be within light years to cause the damage. No stars that could go supernova currently within that distance. One calculation suggests that supernovae have a 17% chance of affecting the Earth over its entire lifetime. GRBs are thought to have an extra-galactic origin and so the alignment would have to be outrageously perfect to affect Earth.

96 Keeping track of Drake's factors 1). The number of new stars forming in the galaxy per year 7 2.) Fraction of those stars that have planets 1 3.) Number of planets per solar system with right environment for life ) Fraction of those planets on which life actually appears ) Fraction of those living planets that go on to develop intelligence life ) Fraction of those intelligent civilisations who go on to develop to release detectable signals into space 1 Drake multiplied the first six numbers together... 7 x 1 x 0.15 x 0.1 x x 1 =

97 Drake Equation...then last step is to multiply by last factor (the length of time) L (years) N , , , ,000, ,000, ,000, ,000,000,000 10,000

98 Where is everybody? Fermi Paradox If the Universe Is Teeming with Aliens - Where Is Everybody?: Fifty Solutions to Fermi's Paradox and the Problem of Extraterrestrial Life

99 NEXT WEEK: Our Local Group + Solar System, Galaxy, Universe