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13. the Realm of the Nebulae

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13. the Realm of the Nebulae Astronomy 110: SURVEY OF ASTRONOMY 13. THE REALM OF THE NEBULAE 1. Distances and Types of Galaxies 2. Hubble’s Law and Galaxy Evolution 3. Peculiar and Active Galaxies Galaxy History Revealed in This Colorful Hubble View 1. DISTANCES AND TYPES OF GALAXIES a. An Extragalactic Distance Scale b. Galaxy Morphology c. Groups and Clusters Parallax Distances Nearby stars appear to shift back and forth as we orbit the Sun. 2p Dec June The parallax angle p is inversely proportional to D the distance D: p 1 360° D ≃ AU 2π p AU Using parallax, we can measure stellar distances out to a few hundred light-years. Luminosity Distances The same luminosity L must pass through each sphere. A sphere of radius D has area A = 4πD2 So brightness is inversely proportional to (distance)2: L L L B = = D = A 4πD2 √ 4πB An star of known luminosity L is a standard candle. 106 Main Sequence Fitting All stars in a cluster have the same distance, so plot HR diagrams for clusters using apparent brightness. MS in Hyades appears 7.5 times brighter than MS in Pleiades; why? Pleiades are √7.5 ≃ 2.7 times further away than Hyades! Distance Scale: Summary 1. Parallax measurements within the solar system gave an accurate value for the astronomical unit: 1 AU = 1.496×108 km 2. Using the Earth’s orbit as a baseline, stellar parallax provides a distance to the Hyades cluster: 6 DHyades = 9.56×10 AU = 151 ly 3. Main sequence fitting yields the distance to other clusters in the galaxy in terms of DHyades. At each step, known distances are used to find unknown distances. Cepheid Variable Stars 3.5 4.0 Brightness (mag) 4.5 Period 2 4 6 8 10 12 Time (days) delta Cephei Massive stars become Cepheid Variables at one phase of their lives after leaving the main sequence. During this phase, they vary in size, temperature, and brightness in regular ways with well-defined periods. Period-Luminosity Relationship. I Cepheids in the Large Magellanic Cloud were found to obey a relationship between period and apparent brightness. Large Magellanic Cloud 100 These stars are all at the 10 ightness same distance, so their 1.0 apparent brightnesses are absolute luminosity e apparent br 0.1 proportional to their relativ absolute luminosities. 0.01 So Cepheids must obey a period-luminosity relationship! Period-Luminosity Relationship. II To be useful for distance measurements, the P-L relationship must be calibrated in units of L⊙ by measuring absolute luminosities of some Cepheids. Cepheids in star clusters are handy for this, since distances are available via main-sequence fitting. Once this is done, a Cepheid’s luminosity can be found from its period. The Distance to Andromeda (M31) Is M31 another galaxy, or part of the Milky Way? The luminosities of several Cepheids in M31 were determined from their periods via the P-L relationship. Andromeda Nebula: Var! Given their luminosities and brightnesses, distances to 6 these Cepheids could be computed: DM31 ≃ 2.4×10 ly. M31 is far beyond the Milky Way! M31: The Andromeda Galaxy White Dwarf Supernovae: Standard Bombs These supernovae have a very narrow range of peak luminosities since they always occur in the same way. To calibrate this peak, we must observe supernovae in galaxies with distances known from Cepheid variables. An Extragalactic Distance Scale 10-3 ly 1 ly 103 ly 106 ly 109 ly Interlocking methods allo be measur ed fairl w distances up to ~10 y reliabl y. 10 ly to Spiral Galaxy, Inclined M63: The Sunflower Galaxy Spiral Galaxy, Edge-On NGC 4565: Needle Galaxy Barred Spiral Galaxy NGC 1365: A Nearby Barred Spiral Galaxy ‘Grand Design’ Spiral Galaxy M51 Hubble Remix Disk Galaxy With Large Bulge The Sombrero Galaxy from VLT Lenticular Galaxy With Dust NGC 2787: A Barred Lenticular Galaxy Giant Elliptical Galaxy With Companions Galaxies Away Dwarf Elliptical Galaxy Companions to M31 M32: Blue Stars in an Elliptical Galaxy M31: The Andromeda Galaxy Irregular Galaxy (Large Magellanic Cloud) The Large Cloud of Magellan Peculiar Galaxy The Colliding Galaxies of NGC 520 Hubble’s Galaxy Classification Irregular and peculiar galaxies not included. The Hubble Tuning Fork — Classification of Galaxies The Local Group: Over 30 Galaxies two large spirals with satellites one smaller spiral many dwarf elliptical and irregular galaxies Local Group The Virgo Cluster: Over 1000 Galaxies! Distance: ~6 × 107 ly three massive elliptical galaxies many MW-sized galaxies M86 in the Virgo Cluster A Rich Regular Galaxy Cluster Distance: ~2.5 × 108 ly mostly elliptical galaxies Galaxies of the Perseus Cluster A Rich Irregular Galaxy Cluster Distance: ~5 × 108 ly many disk galaxies some are colliding The Hercules Cluster of Galaxies A Compact Group Distance: ~6 × 107 ly one elliptical galaxy three spiral galaxies Galaxy Group Hickson 44 2. HUBBLE’S LAW AND GALAXY EVOLUTION a. The Expanding Universe b. Looking Back in Time c. Class Survey The Doppler Shift Doppler Effect Doppler Effect A stationary source sends If the source is moving, out waves of the same the waves bunch up wavelength in all ahead of its motion, and directions. spread out behind. The Doppler Shift: Light We get a similar effect with light. The change in wavelength λ depends on the source’s velocity v red-shift blue-shift toward or away from us: λ - λ v shift rest = Note: valid for v ≪ c λrest c where λshift is the observed (shifted) wavelength, λrest is the wavelength with the source at rest, and c is the speed of light. The Redshift Text Most galaxies have spectra systematically shifted toward the red, implying that they’re moving away from us. The Redshift: An Example Define the redshift: λ - λ shift rest = z λrest line λrest λshift z v = c z (nm) (nm) (km/s) Hβ 486.1 500.9 0.0304 9120 Hγ 434.1 447.3 0.0304 9120 Hδ 410.2 422.7 0.0304 9120 Arp 188 and the Tadpole's Tidal Tail 1931ApJ....74...43H The Expansion of the Universe Plotting galaxy velocities, v, against their distances, d, revealed a relationship: v ≃ H0 d, where H0 is Hubble’s “constant”: The Expansion of the Universe Plotting galaxy velocities, v, against their distances, d, revealed a relationship: v ≃ H0 d, where H0 is Hubble’s “constant”: H0 ≃ 22 km/s/Mly. Two consequences: (1) galaxy redshifts can be used to estimate distances; (2) the universe is expanding. The Cartoon History of the Universe The Universe Has No Center! Observed from Galaxy A Observed from MW Observed from Galaxy B MW MW MW B B A B A A All observers see other galaxies moving away from their position with speeds proportional to distances. The expansion does not define a center! The Universe Has No Center! The universe shows no sign of edges — it seems to be infinite in all directions. Cosmological Principle: The universe looks roughly the same everywhere. • Matter is evenly distributed on very large scales. • There is no center and no edges. • Not proved but consistent with observations. The Universe Has An Age! Assume that galaxies move apart at constant speeds; how long ago were they all ‘on top of each other’? A galaxy at distance d = 1000 Mly moves away at speed v = H0 d = (22 km/s/Mly) × 1000 Mly = 22000 km/s The time required to travel this distance is d 1000 Mly 9.5×1021 km = = = 4.32×1017 s = 13.7 Gyr v 22000 km/s 22000 km/s (Note: d cancels out; you get the same time for any d!) 13.7 Gyr is a good estimate for the Universe’s age! High Redshift Redshifts z bigger than one can’t be interpreted dtoday in terms of galaxy velocity: v = c z then d A correct interpretation: then dtoday 1 + z = dthen Example: from redshift galaxy z = 2 to today, galaxy location distances have tripled. then Looking Back in Time Light travels at finite speed, so when we look out into space we are also looking back in time! Many of these galaxies are billions of lightyears away, so we’re seeing them as they were billions of years ago. Galaxy History Revealed in This Colorful Hubble View Lookback Time Lookback time is related to redshift: longer times correspond to higher redshifts. z tback (Gyr) then 1 7.73 2 10.3 3 11.5 13.7 ∞ galaxy location then z ≈ 2 High-Redshift Galaxies 200-million-year-old baby galaxies 200-million-year-old baby galaxies These galaxies have redshifts z ≈ 7 to 7.5, implying lookback times of ~ 13 Gyr; back then, the age of the universe was only 700 Myr. High-Redshift Galaxies 200-million-year-old baby galaxies 200-million-year-old baby galaxies • Irregular shapes; no apparent symmetry • Very high rates of star formation • Powerful outflows of gas 1. What do we need to know about a star before we can use it as a standard candle? A. Mass B. Diameter C. Age D. Luminosity E. Temperature 2. We compute the peak luminosity of a white-dwarf supernovae in another galaxy by determining its distance using __________ and measuring its __________. A. parallax; temperature from spectra B. Cepheids in the same galaxy; apparent brightness C. main-sequence fitting; apparent brightness D. Cepheids in the same galaxy; makeup from spectra E. parallax; mass using orbital motion 3. What kind of galaxy is this? A. Irregular B. Elliptical C. Barred spiral D. Regular spiral E. High-Redshift NGC 1365 4. Where do we see evidence of recent star formation? D A B C E NGC 1365 5.
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