M31: The Andromeda Galaxy 2 million light years from us http://antwrp.gsfc.nasa.gov/apod/
Milky Way Galaxy 25,000 light years, Or ~ 8 kpc 200 billion stars Galactic year = 225 million yr Our sun is 4.6 billion yr old 1 pc = 3.26 ly
“Milky Way” – a milky patch of stars that rings the Earth Galactos = milk in Greek
The Structure of the Milky Way (1) Disk Nuclear Bulge Sun Halo Globular Clusters
The Structure of the Milky Way (2) Galactic Plane Galactic Center The structure is hard to determine because: 1) We are inside 2) Distance measurements are difficult 3) Our view towards the center is obscured by gas and dust
William and Caroline Herschel, 1785 Herschel could not see very far because of the interstellar gas and dust. He concluded that we live near the center of a relatively small disk of stars.
Strategies to Explore the Structure of Our Milky Way I. Select bright objects that you can see throughout the Milky Way and trace their directions and distances II. Observe objects at wavelengths other than visible (to circumvent the problem of optical obscuration), and catalogue their directions and distances III. Trace the orbital velocities of objects in different directions relative to our position
R d The key to determining the size and the shape of the Galaxy: measure the distances to the most distant stars and star clusters Parallax works only for the closest stars within 500 pc To probe the distance at larger scales, we must find standard candles – very bright objects of known luminosity (or absolute magnitude) Then we can measure their intensities or apparent magnitudes and find the distance using the inverse square law:
The key to the distance scale in the Universe: variable stars cepheids Henrietta Leavitt (1868-1921) Discovered and catalogued over 2000 variable stars in Small Magellanic Cloud
Delta Cephei Discovered in 1784 by John Goodricke (deaf-mute)
Also explained a puzzle of Algol! John Goodricke 1764-1786
Leavitt noticed that the brightest variable stars had longest periods
-7 -6 -5 Average magnitude M<V> -4 -3 -2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Period (log P) Period-luminosity relation Since all cepheids in SMC are at the same distance from us, the same relationship should be between their periods and absolute magnitudes! Leavitt did not know the distance to SMC and could not calibrate this relation. It was done by Shapley
Proposed to use globular clusters as tracers of the mass distribution in our Galaxy Harlow Shapley (1885-1972)
Exploring the Galaxy Using Clusters of Stars Two types of star clusters: Open clusters h and c Persei 1) Open clusters: young clusters of recently formed stars; within the disk of the Galaxy 2) Globular clusters: old, centrally concentrated clusters of stars; mostly in a halo around the Galaxy Globular Cluster M 19
Globular Clusters • Dense clusters of 50,000 – 1 million stars • Old (~ 11 billion years), lower-main-sequence stars • Approx. 200 globular clusters in our Milky Way Globular Cluster M80
Locating the Center of the Milky Way Distribution of globular clusters is not centered on the sun… …but on a location which is heavily obscured from direct (visual) observation
Infrared View of the Milky Way Near infrared image Interstellar dust (absorbing optical light) emits mostly infrared Galactic Plane Nuclear bulge Infrared emission is not strongly absorbed and provides a clear view throughout the Milky Way
Cepheid Distance Measurements Comparing absolute and apparent magnitudes of Cepheids, we can measure their distances (using the 1/d2 law)! The Cepheid distance measurements were the first distance determinations that worked out to distances beyond our Milky Way! Cepheids are up to ~ 40,000 times more luminous than our sun => can be identified in other galaxies.
Cepheid Variables: The Period-Luminosity Relation The variability period of a Cepheid variable is correlated with its luminosity. The more luminous it is, the more slowly it pulsates. => Measuring a Cepheid’s period, we can determine its absolute magnitude!
Cepheids: what happens when the balance between the thermal pressure and gravity becomes unstable
Pulsating Variables: The Instability Strip For specific combinations of radius and temperature, stars can maintain periodic oscillations. Those combinations correspond to locations in the Instability Strip Cepheids pulsate with radius changes of ~ 5 – 10 %.
6 Instability Strip Classical Cepheids Cepheids 4 Mira LPVs PNNVs VW Virginis Irregular LPVs 2 log (L/L) RR Lyrae Scutis DDVs Solar-type stars 0 DBVs Main Sequence ZZ Ceti (DAVs) -2 5.0 4.5 4.0 3.5 log Teff There are several different types of variable stars
Mechanism of pulsations: the battle between opacity, the temperature and gravity Explained by Sergei Zhevakin in 1950s
A simple pulsation cycle • At one point in the pulsation cycle, a layer of stellar material loses support against the star’s gravity and falls inwards. • This inward motion tends to compress the layer, which heats up and becomes more opaque to radiation. • Since radiation diffuses more slowly through the layer (as a consequence of its increased opacity), heat builds up beneath it. N.B. These diagrams are definitely not to scale!!
A simple pulsation cycle 2 • The pressure rises below the layer, pushing it outwards. • As it moves outwards, the layer expands, cools, and becomes more transparent to radiation. • Energy can now escape from below the layer, and pressure beneath the layer drops. • The layer falls inwards and the cycle repeats.
This animation illustrates two stellar pulsation cycles. General idea is OK, but it does not work – pulsations will be damped
In most stars there are two main partial ionisation zones. Partial ionisation zones • The hydrogen partial ionisationzone is a broad region with a characteristic temperature of 1 to 1.5 104 K, in which the following cyclical ionisations occur: • The helium II partial ionisationzone is a region deeper in the stellar interior with a characteristic temperature of 4 104 K, where further ionisation of helium takes place:
Pulsating Variables: The Valve Mechanism Partial He ionization zone is opaque and absorbs more energy than necessary to balance the weight from higher layers. => Expansion Upon expansion, partial He ionization zone becomes more transparent, absorbs less energy => weight from higher layers pushes it back inward. => Contraction. Upon compression, partial He ionization zone becomes more opaque again, absorbs more energy than needed for equilibrium => Expansion
The pulsation properties of a star depend primarily on where its partial ionisation zones are found within the stellar interior. -10 -9 H Surface -8 -7 He log (1-Mr/Mstar) -6 -5 -4 Centre -3 Teff ~ 7500K • The location of the partial ionisation zones is determined by the star’s temperature. • For stars hotter than Teff ~ 7500K, the partial ionisation zones are located too close to the star’s surface, where there is not enough mass to drive the oscillations effectively.
-10 -9 Surface -8 -7 log (1-Mr/Mstar) H -6 -5 -4 Centre He -3 Teff ~ 5500K • For stars cooler than Teff ~ 5500K, on the other hand, the partial ionisation zones are deep in the stellar interior. • However at low temperatures, energy transport via convection becomes quite efficient in the stellar interior, preventing the build-up of heat and pressure beneath the driving pulsation layer.
Computer modelling of stellar pulsation suggests that it is primarily the helium II ionisation zone which is responsible for the observed oscillations of stars on the instability strip. V Teff R/Rmin r Time (days) Modelling pulsations • The hydrogen ionisation zone, however, still plays an important role, producing an observable phase lag between the star’s maximum brightness and its minimum radius. Observed properties of a classical Cepheid Note the phase lag between the star’s maximum brightness and its minimum radius.
The structure of the Galaxy • Two components: • Disk • Spherical (halo and bulge)
Methods to map out the spiral arms: • galactic (or open) clusters • (found in spiral arms) • O & B stars: bright, short lived • HI 21 cm line • (HI is the astronomer's name for hydrogen atoms) • when the electron "flips its spin", it emits a 21 cm radio wave • (radio frequency passes through dust) • HII regions • HII ionized Hydrogen (found preferentially in spiral arms) • Molecular clouds ( CO)