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A primer on Distances in the Universe Image: Splung.com physics 5/19/11 Reinhold Dorn ESI 2011 2 5/19/11 Reinhold Dorn ESI 2011 3 Stellar magnitude – a measure of the brightness of stars Astronomers talk about two different kinds of magnitudes: apparent and absolute. The apparent magnitude, m, of a star expresses how bright it appears, as seen from the earth, ranked on the magnitude scale. Two factors affect the apparent magnitude: 1. How luminous the star is 2. How far away the star is from the earth. Absolute magnitude, M, expresses the brightness of a star as it would be ranked on the magnitude scale if it was placed 10 pc (32.6 ly) from the earth. Since all stars would be placed at the same distance, absolute magnitudes show differences in actual luminosities. Some astronomical objects and their apparent magnitudes from Earth 5/19/11 Reinhold Dorn ESI 2011 4 The Hertzsprung-Russell diagram Hertzsprung-Russell diagram by Richard Powell Image: Richard Powell The H-R Diagram is an extremely useful. It shows the changes that take place as a star evolves. Most stars are on the Main Sequence because that is where stars spend most of their lives, burning hydrogen to helium. As stars live out their lives, changes in the structure of the star are reflected in changes in stars temperatures, sizes and luminosities, which cause them to move in tracks on the H-R Diagram. 5/19/11 Reinhold Dorn ESI 2011 5 It is important to understand a basic fact how planets and stars orbit: The Barycenter - the common center of mass Two bodies with an extreme Two bodies with similar mass orbiting difference in mass orbiting around a around a common barycenter with common barycenter (i.e. Sun-Earth elliptic orbits (i.e. binary stars) system) 5/19/11 Reinhold Dorn ESI 2011 6 Another important fact: REDSHIFT > The Doppler effect for electromagnetic waves (light) Red shift (and blue shift) is the relative difference between the observed and emitted wavelengths. Absorption lines in the optical spectrum of a supercluster of distant galaxies (right) compared to absorption lines in the optical spectrum of the Sun (left) 5/19/11 Reinhold Dorn ESI 2011 7 Where are we in the Universe? – our solar neighborhood (12.5 ly) Number of stars within 12.5 light years = 33 Most stars are red dwarfs (1/10 of the Sun's mass and less than 1/100 the luminosity. About 8% of all the stars in the universe are red dwarfs. The nearest star - Proxima - is a typical example. 5/19/11 Reinhold Dorn ESI 2011 Reference: www.atlasoftheuniverse.com 8 Where are we in the Universe? – our solar neighborhood (250 ly) Shown are the 1500 most luminous stars within the 250 ly. All of these stars are much more luminous than the Sun and most of them can be seen with the naked eye. About 1/3 of the stars visible with the naked eye lie within 250 light years 5/19/11 Reinhold Dorn ESI 2011 Reference: www.atlasoftheuniverse.com 9 Where are we in the Universe? – The Milky Way (50k ly) Spiral galaxy of at least two hundred billion stars. Our Sun is within the Orion Arm about 26 000 light years from the centre. Towards the centre of the Galaxy the stars are packed together much closer than they are where we live. Also shown a nearby dwarf galaxy - the Sagittarius dwarf - which is slowly being swallowed up by our own galaxy. 5/19/11 Reinhold Dorn ESI 2011 Reference: www.atlasoftheuniverse.com 10 Where are we in the Universe? – The Satellite Galaxies (500k ly) 5/19/11 Reinhold Dorn ESI 2011 Reference: www.atlasoftheuniverse.com 11 Attempt to show the entire visible The galaxies in the Universe. universe tend to collect into vast sheets and superclusters of galaxies surrounding large voids giving the universe a cellular appearance. Because light in the universe only travels at a fixed speed, we see objects at the edge of the universe when it was very young up to 14 billion years ago. 5/19/11 Reinhold Dorn ESI 2011 Reference: www.atlasoftheuniverse.com 12 This photo was taken on the 11th of December 2005 by Hännes Heyer (ESO). It shows some of the docking stations for the Auxiliary Telescopes of the Very Large Telescope Interferometer in Chile. Alpha Centauri is the star almost at the centre of the image, the Southern cross lowest of the two "Pointers" to the Southern Cross visible just above the "Coal Sack", in the middle of the image. Coal sack The Milky Way is clearly visible in all its majesty, as well as on the right, the Magellanic Clouds. Proxima Centauri The photo is facing the East. (Canon EOS 5D, 40 and 20 sec exposures). Alpha Centauri 5/19/11 Reinhold Dorn ESI 2011 13 What is a extrasolar planet ? 5/19/11 Reinhold Dorn ESI 2011 14 Stars are a billion times brighter …than the planet hidden in the glare The diameter of the Sun is 100 times that of the Earth and our earth fits ~ 1 million times into our sun. Like this firefly Lighthouse: http://planetquest.jpl.nasa.gov 5/19/11 Reinhold Dorn ESI 2011 15 Habitable zone, possible life ? 5/19/11 Reinhold Dorn ESI 2011 16 The planets discovered so far are closer in mass to Jupiter. This is what has been found This is what we are looking for Image Credit: http://planetquest.jpl.nasa.gov Jupiter’s diameter is 11x the diameter of the Earth, and Jupiter has over 300 times the mass. 5/19/11 Reinhold Dorn ESI 2011 17 There are two ways to look for exoplanets. Direct methods: Indirect methods: Measure photons coming Measure photons from the from the planets itself. host star which is influenced by the presence of a planet Problem is the big contrast or planetary system. difference between the star and the planet and the small angular separation. Can measure the effect of the planet in terms of the light coming from the star or the effect on the stars motion 5/19/11 Reinhold Dorn ESI 2011 18 As of today, six main methods are used in ground based Astronomy to detect extrasolar planets. • Indirect detection Pulsar Timing (Timing of the regular signals emitted by the star itself ) Radial velocity (Cyclic Shift of Stellar Abs. Line) Transits (Cyclic Dip in Stellar Light Curve) Gravitational Microlensing (Anomaly in Lensed Light Curve) Astrometry (Cyclic Shift of Stellar Light Centroid) • Direct detection Imaging (take a picture of the exoplanet) 5/19/11 Reinhold Dorn ESI 2011 19 Pulsar Timing 5/19/11 Reinhold Dorn ESI 2011 20 The Radial Velocity Method (also called Doppler spectroscopy) If the star is moving towards us If the star is moving away from us > Color is blueshifted. > Color is redshifted. 5/19/11 Reinhold Dorn ESI 2011 21 The Radial Velocity Method Information about a distant star can be obtained by recording its spectrum. Changes in the radial velocity of the star cause the lines in the star's spectrum to shift, also known as the Doppler effect. Radial velocity can only measure one of the three space parameters of the stars motion! Periodic changes in the star’s radial velocity depend on the planet’s mass and the inclination of its orbit to our line of sight. Inclination of the planetary orbit is not known > only minimum value for the mass of the planet possible to determine. The radial velocity method has proven to Earth effect on the be the most successful in finding new planets. Sun = 9 cm/s ! 5/19/11 Reinhold Dorn ESI 2011 22 HARPS at the ESO 3.6 m Telescope in La Silla / Chile A Trio of Super- Earths discovered with HARPS in 2008 ΔRV =1 m/s 2-fiber fed Δλ=0.00001 A ΔRV =1 m/s 15 nm ΔT =0.01 K Δp=0.01 mBar 1/10000 pixel Harps cryostat with CCD 5/19/11 Reinhold Dorn ESI 2011 23 Planet Delectability with radial velocities With Kepler’s laws it can be shown that the semi-amplitude of the RV variation, K1 is given by: mp is the mass of the planet, m∗ is the mass of the star, i is the inclination angle of the orbital plane against the line of sight, p is the orbital period, g is the gravitational constant, and e is the orbital eccentricity. Jupiter @ 1 AU : 28.4 m s-1 Jupiter @ 5 AU : 12.7 m s-1 Neptune @ 0.1 AU : 4.8 m s-1 Neptune @ 1 AU : 1.5 m s-1 -1 Super-Earth (5 M⊕) @ 0.1 AU : 1.4 m s -1 Super-Earth (5 M⊕) @ 1 AU : 0.45 m s Earth @ 1 AU : 9 cm s-1 5/19/11 Reinhold Dorn ESI 2011 24 Higher RV precision? Yes, the Laser comb Remember: Earth effect on the Sun = 9 cm/s + Development ThAr - lamp A frequency comb consists of thousands of equally spaced frequencies over a bandwidth of several THz. Laser comb Steinmetz et al. 2008 5/19/11 Reinhold Dorn ESI 2011 25 Laser comb implementation Astro-Comb Th-Ar 5/19/11 Reinhold Dorn ESI 2011 26 ……and with future Instrumentation projects for the VLT and E-ELT CODEX 2 cm/s @ 42 m E-ELT HARPS 1m/s at 3.6 m ESPRESSO 10 cm/s @ 8.2mShift VLTon detector: 3.2 Å Shift on detector: 16 Å Shift on detector: 17 nm Silicon lattice constant: 5.4 Å HZ Note: Unkown factor of sin i, unless orbital inclination can be measured by other method July 20 2009 27 Transits (Photometry) If a planet passes in front of its parent star's disk > the visual brightness of the star drops Main disadvantage: Planetary transits are only observable for planets whose orbits are in the line of sight of the observer.
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