Andre James Clayden
03-12-2001
HET603 Swinburne Astronomy Online

Introduction

Star clusters are groups of stars, which are close together in space, rather than just accidentally lined up one behind the other.

There are two different kinds star clusters, open clusters and globular clusters.

They populate the halo or bulge2 with a significant concentration towards the Galactic centre. The image blow shows the galactic centre

This essay will examine the characteristics of the cluster types, including distribution, stellar content and origin of different kinds of clusters.

GLOBULAR CLUSTER

Spectacular ball of many thousands of stars, called Globular clusters. These are gravity bound concentrations of approximately “ ten thousand to one million stars within sometimes slightly flattened spheroid volumes6 of typical radii 10-25pc.” (AE) This was also noted by DSP.  The stars are strongly concentrated towards the center of the cluster with a steep density gradient outward.

Globular clusters may be classified after their apparent degree of concentration.                
“Spectral classification, F-type clusters are very poor in heavy elements (metals) and are found throughout the halo, whereas G-type clusters are only moderately metal poor and lie closer to the nuclear bulge of the galaxy” (DSP) agrees with this distinction AE. These are also generally smaller and less concentrated. 

Spectroscopic study of globular clusters show that they are much lower in heavy elements” (AA).  Most clusters stars are composed primarily of hydrogen and helium whereas more recently formed stars, like our sun contain metals”. “ The metallic elements only became apparent at a later date, after being manufactured in supernova explosion. This was support by AE, DSP, and W5

 Spectral types OBAFGKM represent a temperature sequence, defining the star’s “color” and surface brightness from hotter (O stars) to cooler (M stars), and from hotter (0) to cooler

 “Globular cluster distances are most reliably obtained through studies of variable stars primarily of the RR Lryrae type, which have reasonably well determined mean absolute magnitudes.” (W6) For clusters lacking in known variable stars the apparent magnitude of the main sequence and horizontal branch stars are used.

Globular clusters are believed to be very old, and formed an earlier generation of star. “The most recent estimates yield ages of 12 to 20 billion years”, (AE) this was supported by W6 but the best observation is perhaps 14 to 16 billion years. (AE)

“ The average distance between stars is about one light-year. It seems also, both from observation and from gravitation theory,” (BA) that the stars in a cluster are not evenly distributed.  The outer ones are probably a couple of light-years or so apart, whereas those near the centre are separated by only a fraction of a light – year.

A good place to look for star clusters is in the Milky Way.

THE BRIGHTEST GLOBULAR CLUSTERS

Our galaxy has “ 200 Globular clusters most in highly eccentric orbit that take them far outside the Milky Way”(DSP). There are four that are comparatively bright and can even be see with the unaided eye. Two of these lie in the northern hemisphere, M5 in Serpens and M13 in Hercules with  “ apparent of magnitudes 5.7.” (CD1)

Two more in the southern hemisphere, “Omega Centauri (NGC5139) with apparent magnitude of 3.6 and 47 Tucanae (NGC104) with the apparent magnitude of 4.0”. (CD1)

OMEGA CENTAURI

This awe inspiring object Omega Centauri, it is the “largest globular cluster in our Milky Way galaxy, within its bulk, astronomy have estimated up to 5 million solar masses,”(W1) what was stated by DSP is “ ten times as massive as other big globular clusters which includes M13 in Hercules and M15 in Pegasus”(BA). Omega Centauri is by far the most luminous and brightest globular cluster visible to the naked eye with a apparent “magnitude of 4.5”(CD1) Though a small 60mm telescope or binoculars this bright “fuzz-ball” you can even make out its shape.

“Omega Centauri might be the remnant of a nucleus of a small galaxy, which has merged with our Milky Way”(W8) a statement with which was in agreerence by W1, DSP.

Studies indicate that the stars of this cluster did not form at once, but over a 2 billion year period of time.  This indicates starburst peak and multiple populations were found in the globular cluster.1 These bodies agreed with this statement BA, DSP, and W1

“Omega Centauri contains a large number of RR lyrae5 variable stars, which enable its distance to be determined as 17000 light years and age of 16 billion years, absolute diameter of about 150 million light years.”(DSP) AA, AE, and W8 reached similar conclusion

Omega Centauri is one of the oldest objects associated with the Milky Way; indeed its age is comparable to that of the universe itself.

HISTORY OF GLOBULAR CLUSTERS

Edmund Halley (1656-1710) published a list of six luminous spot or patches in the “Philosophical transactions of the royal society of 1715”, including his own discoveries of globular clusters.  Omega Centauri was mentioned, (on a journey 1677 St Helena).

1914 –1918 Shapley (USA) found that globular star clusters formed a cloud round the center of the galaxy – conception of the structure of the galaxy made more precise”(AA) and also noted in BA

 OPEN OR GALACTIC CLUSTERS

Open or galactic clusters are physically related groups of stars held together by mutual gravitational attraction. They are believed to originate from the large cosmic gas/dust clouds in the Milky Way, and continue to orbit the galaxy, in many clouds visible as diffuse nebulae.

Most open clusters have only a short life as a stellar swarm.  As they drift along their orbit, some of their members escape the cluster, due to velocity changes in mutual closer encounters, tidal forces in the galactic gravitational field and encounter with field stars and interstellar clouds crossing their way.

The escaped individual stars continue to orbit the Galaxy on their own, as field stars.  All field stars in our, and the external galaxies are thought to have their origin in clusters.

“All the stars in a cluster may begin to form almost simultaneously but they do not all become main-sequence3 stars at the same time.”(AA)For this reason star clusters are valuable laboratories for comparing the evolution of different stars.

“High-mass stars evolve more rapidly than low-mass protostars while high mass protostars become hot ultra luminous stars of spectral types O and B.Several are 100 million years old, only few of them have age counted by billions of years.”(AE) These statement where supported by DSP, W4

There are “ 27 open or galactic clusters in the Messier Catalog.”(4CD1) In the plane of the spiral arms some of the stars are gathered into loose associations or groupings, Some open clusters like the most famous in the northern hemisphere Pleiades – “M45 with apparent magnitude of 1.5 and NGC188 with the apparent magnitude of 8.1”(CD1)”. One of the oldest known open clusters with a approximate age of over 5 billion years, the Praesepe M44”(W11) group of stars in Cancer with a apparent” magnitude of 3.7”(CD1) and in the southern hemisphere Jewel Box (NGC4755) with a apparent” magnitude 4.2 surrounding Kappa Crucis. Other fine example of Open clusters are M6    Butterfly cluster with a apparent magnitude of 5.3, M7 Ptolemy cluster with a apparent magnitude of 4.1 in Scorpius, and NGC3532 in Carina”(CD1) the largest swarm of all Open clusters.

PLEIADES

The Pleiades in the constellation of Taurus a well-known open cluster visible to the unaided eye.

This star cluster actually contains some “500 stars,”(AA) which were formed as a group about “100 million years ago, making these stars 1/50^th “(W3) the age of our sun.  As a group, they travel through space in the same direction (a so – called moving star cluster). “This stellar system is some 30 light years in diameter and 380 light years distant.”(W3)

“The brightest stars in the Pleiades are Alocyone with a apparent magnitude of 6.49; Pleione with the apparent magnitude of 6.34 is 1000 times more luminous than stars like our sun”(W3) This was also noted in, AA These white giants of spectral class B rotate rapidly round their axes, up to a hundred times faster than the sun. The cluster contains clouds of dust that appear as so-called reflection nebulae near the brightest stars.

“The faintest stars are still 40 times brighter than our own sun would appear at a similar distance.”(W3)

HISTORY OF STAR CLUSTERS

138AD Ptolemy had mentioned M7 & Coma star cluster

First thought to be Nebulae it was Galileo who in 1609 discovered that what are composed of stars when observing M44

1610 Galileo counted 40 stars in the Pleiades with his telescope

CONCLUSION

In this essay we noted a conclusion that star cluster are defined into two groups open and globular clusters.

Open clusters are physically related groups of star held together by mutual gravitational attraction originating in the cosmic clouds the stars in open clusters may begin to form almost simultaneously but may not become main sequence stars at the same time.

Having a age of between 100 million and few billion making them young and a spectral type of O, B. Making them a good example of stellar evolution.

As a stellar swarm it life is short, the member star escape the cluster due to velocity and tidal forces in the galactic gravitational field.

A good example of open cluster visible to the naked eye being Pleiades M44 a well-known naked eye open cluster containing some 500 stars which as groups are traveling through space in the same direction

This open cluster is about 100 million years old it is 1/50 the age of sun and has diameter of 30 light years and 380 light years distant.

The brightest stars in the cluster are 1000 times more luminous then our sun.

These white giants have spectral type of B.

The cluster contains reflection nebulae near the brightest stars.

Globular cluster are stark difference, the stars gravity bound concentrations of tens of thousands to one million stars slightly flattened spheroid volume with of 12 to 20 billion years with a spectral type of F, G being pour in heavy elements

There are about 200 globular cluster in our galaxy most in highly eccentric orbit that takes them far outside the galaxy.

Some globular contain RR Lryrae variables which offers the most reliable way to determine the distance

A good example of Globular cluster is Omega Centauri a naked eye object with distance of 17000 light years away and diameter of 150million light years Omega Centauri has a age of 16 billion years which making it one of the oldest object associated with our galaxy.

Omega Centauri has estimated 5million solar masses. Studies indicate that it did not form at once but over a 2 million year period. This indicates star burst peak and multiple populations.  

REFERENCES:

AE -The Astronomy Encyclopaedia by Patrick Moore

Pp. 416 to 418

DSP-Deep space by Colin A Ronan

Page 56 to 59

BA- Book of Astronomy by Dr. H.C.King

Page 115 to 119

UFE-Universe fifth edition

Kaufman Freedman

Chapter 18

AA- The Amateur Astronomer

Consultant Editor John Gribbin

Page 124 to 126

URLS- REFERENCES

W1 Astronomy Picture Of The Day October 4,1996 Omega Centauri

Robert Nemiroff (MTU) & Jerry Bonnell (USRA)

http://ww.phy.ncku.edu.tw/~astrolab/mirrors/apod/ap961004.thml

W2 History Of The Discovery of The Deep sky Objects http://seds.lpl.arizona.edu/messier/xtra/history/deepskyd.html

W3 The Pleiades

 http://www.ras.ucalgary.ca/~gibson/pleiades/

W4 Star Clusters

Hartmut Frommert

http://www.seds.org/messier/cluster.html

W5 Cepheid Variable Stars a distance Indicator.

University of Oregon, Davison E Soper 

http://zebu.uoregon.edu/~soper/MilkyWay/cepheid.html

W6 Variable Stars

Astronomy 162 professor Barbara Ryden

http://www-astronomy.mps.ohio-state.edu/~ryden/ast162__4/notes18.html

W7 Variable Stars In Globular

1997, Astrophyics Kaluzny j. Kubiak M, Szymanski.

http://bulge.princeceton.edu/~ogle/PAPERS/kal197b.html

W8 NGC5139 Omega Centauri

Hartmut Frommert

http://sed.lpl.arizon.edu/messier/xtra/ngc/5139.html

W9 Celestial Planisphere Manual open clusters http://www.aurlia.com/mapas/page/o24.htm

W10  http://imgine.gsfc.nasa/doc/science/know-12/star.html

CD1-cd rom Sky 5.0