Old and New Perspectives on the Milky Way

Transcription

1 Jacopo Tintoretto, The Origin of the Milky Way, c Aquarius halo A, Cooper et al Old and New Perspectives on the Milky Way Eline Tolstoy & Amina Helmi, Kapteyn Astronomical Institute

2 The Milky Way in the Night Sky Easily seen by the naked eye, the Milky Way has always been a well known feature in the sky, but our understanding that it is a galaxy did not occur until relatively recently Fred Espenak

3 The Discovery of the Milky Way Aristotle ( BC) in Meteorologica, mentions earlier work by Democritus ( BC) and Anaxagoras ( BC) suggesting that the Milky Way is composed of stars far distant from Earth. He argued strongly against this theory, and believed that the Milky Way was in the Earth s atmosphere. Olympiodorus the Younger ( AD, Alexandria) disagreed with Aristotle as did Alhazen ( AD, Cairo), who made the first recorded attempt to measure the Milky Way s parallax. Nasir al-din al-tusi ( AD, Persian) wrote in his Tadhkira, The Milky Way is made up of a very large number of small, tightly-clustered stars, which, on account of their concentration, appear as cloudy patches. It took a while to realise that the Milky Way is extremely distant, and the conjecture that it was made up of individual stars could not be proven.

4 The Discovery of the Milky Way Galileo Galilei ( AD) first used a telescope, in 1609, to view the Milky Way and was able to prove that it was made up of a vast number of stars congeries stellarum.. Thomas Wright ( AD, English), in 1750, suggested the Milky Way is an optical effect due to our immersion in what locally approximates to a flat layer of stars. Immanuel Kant ( AD, Königsberg/Kaliningrad) expanded on Wright s ideas in 1755, and suggested the Milky Way was a disc of stars, which formed from a spinning cloud of gas. He also first suggested that other nebulae could also be rotating discs of stars, opening the new idea of astronomy beyond our solar system, and even our Milky Way and the beautify concept of Island Universes. the modern vision of the Milky Way has arrived.

5 An Island Universe the first survey of the Milky Way In 1785, William & Caroline Herschel first mapped the Milky Way, and although they didn t realise that much of it was obscured, it was a reasonably accurate diagram. They counted stars along 683 lines of sight using 48-inch telescope. assumed all stars have same luminosity, and so the relative brightness gives the relative distance. assumed they could see unobscured all the way to the edge of the system.

6 Stellar Parallax Determining Distances In1838 using a heliometer Bessel measured the first parallax of a star, 61 Cygni, of 0.3 arcseconds, indicating that the star is 10.3 ly away (current measurement 11.4 ly). Friedrich Wilhelm Bessel ( ) There are ~25 stars within 3.5pc of us.

7 Photographic Plates allow accurate and calibrate-able consistent measurements to be uniformly recorded. major revolution in astronomy The first daguerrotype of the moon was made by J.W. Draper in The first star was not recorded until 1850, when a daguerrotype of Vega was made.

8 magnitude The Hertzsprung-Russell Diagram Ejnar Herzsprung ( ) = colour-magnitude diagram HOT COOL BRIGHT Henry Norris Russell ( ) FAINT BLUE colour RED Hertzsprung 1911 Russell 1914

9 The Kapteyn Universe used photographic star counts estimated distances statistically based on parallaxes & proper motions of nearby stars. neglected interstellar absorption of starlight (he assumed that stars were faint only because they are far away, not because interstellar absorption blocks some of the light). dust free! Kapteyn 1922 roughly solar-centric Universe

10 Distribution of Globular Clusters dust free! 100=10kpc clearly not solar-centric Universe Henrietta Leavitt 1912 Shapley 1918

11 The Great Debate On april 26th 1920 at the American National Academy of Sciences there was a famous public debate between Harlow Shapley and Herber Curtis about the nature of the spiral nebulae, and our place in the Milky Way What is the size of our own Galaxy? Are spiral nebulae members of our Galaxy? Or are they separate `island universes well beyond? Kapteyn s universe Shapley s model of Milky Way Shapley Curtis

12 DUST Visible La polvere

13 DUST Infra-red Visible La polvere

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15 Following in Kapteyn s Footsteps: The Stellar Milky Way The Sun orbits the Galactic Centre at approximately 220 kilometres per second The Sun is a very ordinary star in a very ordinary galaxy

16 Galactic Archaeology Position and velocities information: trajectories, origin and gravitational field (today and in the past) from parallax (distance), proper motions, radial velocities (Doppler shift in spectra)! Chemical abundances: conditions of the medium (gas clouds) at time of formation from spectra (chemical elements present as absorption lines)! Ages: from spectra of decaying elements; from temperature and luminosity

17 Stellar Distances Nearly ~ stars in Hipparcos catalogue within ~100pc of the Sun ESA Hipparcos satellite, launched B-V Perryman et al. 1995, A&A, 304, 69

18 The Solar Neighbourhood 100pc diameter B1+B2, 10Msun ~10-20Myr K-giant, 1.1Msun ~7Gyr G2, 1.1Msun ~6Gyr A1, 2Msun ~250Myr K5, 2Msun ~1 Gyr In 2 million years Pioneer 10 will pass through the Aldebaran system Sun is a 4.6Gyr old G2 star

19 The Solar Cylinder in Motion 130pc diameter

20 The Milky Way in motion

21 Motions in the Solar Neighbourhood Geneva-Copenhagen survey movie: 250 million years F & G dwarfs with Hipparcos parallaxes, Tycho-2 proper motions Nordström et al. 2004

22 Chemical Evolution of the Milky Way Disk GAIA-ESO survey Recio-Blanco et al A&A, 567, A5

23 The Milky Way Halo: Field of Streams Credit: S. Koposov and the SDSS-III collaboration

24 inflows from Alejandra Recio-Blanco

25 The Future: GAIA Launched: 20th December 2013 First data release: Summer 2016!!! Measure positions and motions of stars for 10 9 stars (10,000 x larger than predecessor); over a volume 1,000,000 larger; 100 more precisely revolution is expected Picture: S. Brunier/ESO; GRAPHIC SOURCE: ESA Nature, 3rd Oct 2013

26 The Future: ESO E-ELT

27 Fin