Star Party Presentations for 41N Latitude - Halo Globular Clusters August September

Older Low-Metallicity Globular Clusters in the Milky Way Halo - August September - Star Party Presentations for 41N Latitude - Demonstration center RA19h 2°Dec - Southern Cygnus - 25° radius

K. Fisher fisherka@csolutions.net Rev. 9/15/2007C; Orig. 9/14/2007

Purpose

Using a GOTO mount in a series of quick slews around Cygnus, show three of five older low-metallicity globular clusters and one younger high-metallicity globular cluster by increasing distance and compare their relative visual angular and physical sizes. Younger high-metallicity globulars surrounding the Milky Way's core will be the subject of another star party presentation sheet.

Low-metallicity globular clusters contain large numbers of short-period low-metallicity Population II RR Lyrae "cluster" variables. The RR Lyr variable stars have periods of less than one day. The April 15, 2007 Astronomy Picture of the Day (APOD) features a single night of time-lapse exposures of globular cluster M3. Speeded up by time-lapse photography, the numerous RR Lyrae cluster variables in M3 can be seen blinking on and off. Amazingly, these magnitude changes translate into the surface of these distance stars pulsating in and out at a rate between 10 and 170 kilometers/second.

Overview finder Fig. 1

Observing List

Table 1 - August Halo Globular Cluster Tour

ID ID_alt Dist kpc Rad_vel kms^{-1 [1]} Coordinates Con Sync star Slew_degs approx² v_mag Mpsas Dia. arcmin Size kpc³ Relative physical size Spec Fe/H_log M_sol Notes_____________

M71 NGC6838 3.8 -23 J195345.60+184612.0
G056.74-04.57 Sge Altair 0° 6.1 21.5 6 0.007 100% G1 -0.73 3.67x10⁴ Seen through the galactic disk. Exhibits extinction.

M13 NGC6205 7.0 -247 J164140.80+362736.0
G059.01+40.91 Her Vega 45.5° 5.8 21.1 23 0.047 671% F6 -1.54 6.27x10⁵ None

M92 NGC6341 8.1 -120 J171707.20+430748.0
G068.33+34.86 Her Rastaban (bet Dra) 5.5° 6.3 19.6 8 0.019 271% F2 -2.29 3.64x10⁵ None

M56 NGC6779 9.9 -136 J191633.60+301048.0
G062.65+08.34 Lyr Alberio 19° 8.9 20.1 5 0.014 200% F5 -1.94 1.89x10⁵ Also seen through galactic disk.

C47 NGC6934 15.2 -411 J203409.60+072414.4
G052.10-18.89 Del Altair; Enif 25° 9.8 19.1 2 0.009 129% F7 -1.54 2.51x10⁵ C42 and C47 are south of the galactic disk.

C42 NGC7006 40.7 -378 J210128.80+161048.0
G063.76-19.41 Del Altair; Enif 8° 11.4 20.1 1 0.011 157% F1 -1.68 2.52x10⁵ Dim and small. Needs larger aperture and non-urban skies.

RR Lyr HD182989 0.23 -72.4 J192528.80+424648.0
G074.95+12.30 Lyr Vega; Alberio 33.5° 7.1-8.1 var N/a N/a N/a N/a F5 N/a 0.64 Archetype for RR Lyr short-period globular cluster variables (0.56 days). Use eyepiece diffraction grating to view low-res spectrum,

ID	ID_alt	Dist kpc	Rad_vel kms^{-1 [1]}	Coordinates	Con	Sync star	Slew_degs approx²	v_mag	Mpsas	Dia. arcmin	Size kpc³	Relative physical size	Spec	Fe/H_log	M_sol	Notes_____________
M71	NGC6838	3.8	-23	J195345.60+184612.0 G056.74-04.57	Sge	Altair	0°	6.1	21.5	6	0.007	100%	G1	-0.73	3.67x10⁴	Seen through the galactic disk. Exhibits extinction.
M13	NGC6205	7.0	-247	J164140.80+362736.0 G059.01+40.91	Her	Vega	45.5°	5.8	21.1	23	0.047	671%	F6	-1.54	6.27x10⁵	None
M92	NGC6341	8.1	-120	J171707.20+430748.0 G068.33+34.86	Her	Rastaban (bet Dra)	5.5°	6.3	19.6	8	0.019	271%	F2	-2.29	3.64x10⁵	None
M56	NGC6779	9.9	-136	J191633.60+301048.0 G062.65+08.34	Lyr	Alberio	19°	8.9	20.1	5	0.014	200%	F5	-1.94	1.89x10⁵	Also seen through galactic disk.
C47	NGC6934	15.2	-411	J203409.60+072414.4 G052.10-18.89	Del	Altair; Enif	25°	9.8	19.1	2	0.009	129%	F7	-1.54	2.51x10⁵	C42 and C47 are south of the galactic disk.
C42	NGC7006	40.7	-378	J210128.80+161048.0 G063.76-19.41	Del	Altair; Enif	8°	11.4	20.1	1	0.011	157%	F1	-1.68	2.52x10⁵	Dim and small. Needs larger aperture and non-urban skies.
RR Lyr	HD182989	0.23	-72.4	J192528.80+424648.0 G074.95+12.30	Lyr	Vega; Alberio	33.5°	7.1-8.1 var	N/a	N/a	N/a	N/a	F5	N/a	0.64	Archetype for RR Lyr short-period globular cluster variables (0.56 days). Use eyepiece diffraction grating to view low-res spectrum,

1 - Radial heliocentric velocity. Apparent speed (kilometers per second) and direction (negative is moving toward) relative to Sol.
2 - Approximate slew degrees from last object to current object
3 - Computed using half-angle formula, distance and angular size.

Visualizations and finder charts

Table 2 - Visualizations - Globular Clusters

ID Relative angular dia. 1°-1.2° e.p. Relative physical dia. parsecs⁴ Finder

M71
Fig. 2

Fig. 3

Fig. 4

M13
Fig. 5

Fig. 6

Fig. 7

M92
Fig. 8

Fig. 9

Fig. 10

M56
Fig. 11

Fig. 12

Fig. 13

C47
Fig. 14

Fig. 15

Fig. 16

C42
Fig. 17

Fig. 18

Fig. 19

4 - Scaled to relative physical size (M71=100%) based on Table 1, above

ID	Relative angular dia. 1°-1.2° e.p.	Relative physical dia. parsecs⁴	Finder
M71	Fig. 2	Fig. 3	Fig. 4
M13	Fig. 5	Fig. 6	Fig. 7
M92	Fig. 8	Fig. 9	Fig. 10
M56	Fig. 11	Fig. 12	Fig. 13
C47	Fig. 14	Fig. 15	Fig. 16
C42	Fig. 17	Fig. 18	Fig. 19

Table 3 Visualizations - Other

Item Graphic Item Graphic

Ortho map x-y 16kpc
Fig. 20
Ortho map z-x 16kpc
Fig. 21

Ortho map x-y 100kpc
Fig. 22
Ortho map z-x 100kpc
Fig. 23

Globular Metallicity - Pop. Table
Fig. 24
Glob Metallicity - 3D distrib.
Light blue = younger metal-rich
Dark blue = older metal-poor

Fig. 25

RR Lyr variables in C47

External content:
Time lapse of RR Lyraes in M3 (APOD)
Fig. 26
Pulsating variable types - HR diagram
Fig. 27

RR Lyr Light curve
Fig. 28
RR Lyr finder
Fig. 29

Earth orbit schematic
Fig. 30

Item	Graphic	Item	Graphic
Ortho map x-y 16kpc	Fig. 20	Ortho map z-x 16kpc	Fig. 21
Ortho map x-y 100kpc	Fig. 22	Ortho map z-x 100kpc	Fig. 23
Globular Metallicity - Pop. Table	Fig. 24	Glob Metallicity - 3D distrib.	Light blue = younger metal-rich Dark blue = older metal-poor Fig. 25
RR Lyr variables in C47 External content: Time lapse of RR Lyraes in M3 (APOD)	Fig. 26	Pulsating variable types - HR diagram	Fig. 27
RR Lyr Light curve	Fig. 28	RR Lyr finder	Fig. 29
Earth orbit schematic	Fig. 30

Use

The observing list and discussion topics are intended for use by a single-scope participant-presenter at a local club star party. Multiple star-party telescope owners can divide the observing list objects (above) and discussion topics (below) and present a row of two or three scopes that show a non-duplicative set of discussion topics and of globulars by distance.

The observing list in Table 1 has been used at suburban transition zone star parties and, in part, at urban light-polluted star parties.

Discussion topics

Star party presenters can re-enforce observations at the eyepiece with the visualizations in Table 2 to discuss the following principles:

Magnitude and distance are related.
The apparent and physical size of extended objects is not the same. Sometimes an object is smaller because it is smaller; other times it is just further away. Figs. 2 through 18.
Integrated magnitude is not always an accurate visual indicator of the relative brightness of extended objects because the magnitude scale not compensate for the dispersion of integrated brightness caused by the different apparent diameters of objects. Figs. 2 through 18. Magnitudes per square arcsec is an alternative surface brightness scale that also captures the effect of an extended object's diameter on apparent brightness. E.g. Fig. 6.
The estimated solar mass of each globular cluster is listed in Table 1. This is not the same of same as the number of stars in the cluster, but solar mass gives an impression of a cluster's relative star count. For example, M13 contains about 627,000 solar masses of matter.
What if M13 were right next door to Sol? What would it look like. The Upper Scorpius OB Association, which includes most of the bright stars of Scorpius, has a center about 145 parsecs from Sol and is visible to the south or southwest during August and September. Antares is about 114 parsecs distant. M13 has a diameter of about 47 parsecs and contains a mass of 627,000 Sols. If M13 was placed at the same distance as Antares, it would subtend 21° or about the one-third more than the size of an outstretched hand with fingers spread out. Image a night sky with the southern horizon filled with a constellation-sized object emitting the light of 600,000 Suns!
The galactic coordinate system is tilted at about 60 ° relative to the local horizon system. Fig. 1; Fig. 30. Two the observing list targets are in the south galactic hemisphere; the remainder are in the north galactic hemisphere. Figs. 1, 21 and 23. A consequence of the tilt of Earth's orbit with respect to the galactic plane is at various times of the year, portions of the galactic disk appear as trending North-South across the local horizon (at N41° latitude), East-West trending across the local horizon, or approximately coincident with the horizon. In August, the Earth is between its summer solstice and autumnal equinox positions. Fig. 30; Fig. 1. In this orbital position, the galactic disk is rising above the eastern horizon at 9:00pm and at midnight is oriented approximately North-South with respect to the local horizon. Fig. 1.
The Milky Way rotates toward (the spinward side) Cygnus and Altair, around the galactic core in Sagittarius. Fig. 20; Fig. 1. All clusters in the observing list are in the first galactic quadrant - a spinward quadrant. Fig. 20 and 22. All these globulars having radial velocities showing apparent movement toward the Earth. Table 1. Using such globulars in the 1920's, Shapley initially confirmed the rotation direction of the Milky Way and estimated the physical size of the Milky Way. Compare Figure 1 in Shapley's 1919 "Twelfth Paper: Remarks on the Arrangement of the Sidereal Universe." Mt. Wilson Obs. Contrib. No. 157:209-234; compare Figures 3 and 4 in Shapley's Fourteenth Paper (external content); compare Zinn, R. 1985. Figure 3 - [Metal Rich and Metal Poor Globular Clusters]. In "The globular cluster system of the galaxy. IV - The halo and disk subsystems." 1985ApJ...293..424Z (external content).
There are two populations of globular clusters - an older metal-poor population and a younger metal-rich population. (Figs. 24 and 25; Zinn (1985)) The older population of metal-poor globular clusters ( log[Fe/H] < -0.8 ) are distributed spherically in the halo and around the Milky Way's disk. Fig. 24; Fig. 21; Fig. 22. The observing list globulars are in the galactic halo above the northern and southern sides of the galactic disk. Figs. 21 and 23; Table 1 (galactic coords). The one metal rich globular cluster M71 ( log[Fe/H] > -0.8 ) is part of a younger population of clusters that are near or in the thin disk or that are near the galactic core. Fig. 24; Fig. 22; Fig. 25.
In this context, "metallicity" refers to the proportion of elements in a star that are heavier than the most basic element - hydrogen. Fig. 24. It does not refer to the traditional metals found in the periodic table of elements.
The metallicity of a star depends on its source material. Younger clusters are made from the metal-rich remnants of older first generation stars that made the first heavier metals. That older clusters are relatively metal poor is an indication that they formed with or just after the first generation stars - and before or just after the creation of heavier metals by dying first generation stars. The age and integrated metallicity of the observing list globulars are see in their spectral class "turn-off" points.
That there are two populations of globular clusters containing differing levels of metals (Figs. 24 and 25; Zinn (1985)) raises unresolved questions about how our Milky Way evolved. Did the globulars all form in place? Do the two metallicity populations of globulars contradict this theory of how the Milky Way formed? Or has our Milky Galaxy absorbed other galaxies and the younger higher-metallicity globulars represent the globulars that were torn from an ancient galaxy that later was cannibalized by the Milky Way. Fig. 25; Pulliam (Mar. 2006, S&T); Brook (2003) (hierarchical consolidation); Martin (2004) (Canis Major Dwarf galaxy accretion); Forbes (2004) (not Canis Major Dwarf galaxy accretion)).
That there is some metallicity in older metal-poor globular clusters (and in Population II stars in the galactic halo) implies the existence of the first-generation of metal-less Population III stars. Population III stars are a theoretical stellar population that has not been well-defined by empirical evidence. Population I stars (higher-metallicity younger stars in the galactic disk that travel in ordered orbits parallel to the disk) and Population II stars (older lower-metallicity stars that travel in oblique orbits crossing the disk and extending into the galactic halo) are well-defined from empirical evidence.
Globular clusters in the observing lists are packed with RR Lyrae short-period variables. Fig. 26. Because of this, RR Lyrae variables are sometimes referred to as "globular cluster variables."
Relatively miniscule proportions of "metals" (Fig. 24) in the upper layers of a star effects its evolutionary path, including the life-cycle of RR Lyrae variables found in globular clusters. Heavy than hydrogen elements in the outer atmosphere of the star change its opacity and alters its evolutionary path. The initial mass of a star, along with its initial metallicity, are the primary determinates of its evolutionary path. Fig. 27. For short-period variables - delta Sct, RR Lyra, Cephieds and Population II Cephieds, the metallicity of the star:
- Effects the star's position in instability strip. Fig. 27 - delta Sct; RR Lyra; Pop. II Cephieds; Cephieds.
- Effects the length of the star's period when in the instability strip. Fig. 27; Fig. 28.
RR Lyrae variables have a small, low-mass (1/2 Sol) hot, helium-burning core. The core is surrounded by an expansive outer shell about 5 times the radius of the Sun. These physical parameters are a recipe for a variable star - a low-mass hot core inflates an oversized outer shell. Due to its large radius, the outer shell is unstable and collapses. Some physical parameters of RR Lyra - the prototype of the RR Lyrae class of variables - are:

Mass: 0.64 Sols
Radius (variable): approx. min: 5.0 Sol-radii max: 6.2 Sol-radii (Fokin (1997) (model))
Period: ~13 hours
Magnitude change: v7.1-8.1
Radial velocity of surface: in class, ranges from 10 to 60 kilometers/sec (Smith (2004); Fokin (1997)).
B-V and spectral type: 0.16; F5
RR Lyr class subtype: ab
Radial velocity relative to Sol: -72 k/sec

Amazingly, the surface of RR Lyra travels out about 1.2 million kilometers - about the radius of the Sun - and then falls back to minimum. This total travel distance of 2.4 million kilometers is covered in about 13 hours at an average rate of about 50 kilometers/second.
Differences in the shape of the variable's light expansion curve define three subclasses to the RR Lyrae variable type: Classes ab, c and d - also written as "RRab" "RRc", and "RRd".

RR Lyrae subclass ab has a rapid rise to peak and a rapid decline with one-half the period spent near minimum. Periods are generally 12 to 20 hours. About 90% of all known RR Lyrae variables fall in this subclass.
RR Lyrae subclass c has a more sinusoidal shaped light-curve and periods between eight to twelve hours.
RR Lyrae subclass d is similar to class a, but the period contains irregular overtones.

In subclass RRab, the rise to peak and fall to minimum is rapid with a half-period of quiescence. The average surface speed may not be representative for the RRab subclass. Fokin's 1997 RR Lyrae stellar models suggests that that shock waves may at some times travel at 170 kilometers/s. The rate of change in the surface can be inferred from change in the star's overall brightness and from Doppler shifts in absorption lines seen in high-dispersion spectra, e.g. BII and Ba II. (Fokin (1997)).

Fig. 31 - RR Lyrae subclass a,b and c light curves from Plate VII, Bailey (1902)

References and sources

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Acknowledgements

Centre de Donn�es astronomiques de Strasbourg - Simbad: This project has made use of the SIMBAD database, operated at CDS, Strasbourg, France.

Centre de Donn�es astronomiques de Strasbourg - Catalogue Service: This project has made use of numerous catalogues redistributed through the CDS Astronomer's Catalogue Service, operated at CDS, Strasbourg, France. The use of those sources by this reference is acknowledged and appreciated.

NASA Astrophysics Data System/Computation Facility at the Harvard-Smithsonian Center for Astrophysics - NASA ADS Abstract Services: This project has made use of NASA's Astrophysics Data System.

Chevalley, Patrick.: Cartes du Ciel Planetarium Software. This project makes use of celestial charts prepared using Cartes du Ciel v. 2.76. The use of this source by this reference is acknowledged and appreciated.

Chevalley, Patrick.: Variable Star Observer's Software (VAROBS). This project makes use of light curves prepared using Varobs v.2.3. The use of this source by this reference is acknowledged and appreciated.

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