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Patrick Moore’s Practical Astronomy Series

Other Titles in this Series

Navigating the Night Sky How to Identify the Stars and Constellations Guilherme de Almeida

Observing and Measuring Visual Double Stars Bob Argyle (Ed.)

Observing Meteors, Comets, Supernovae and other transient Phenomena Neil Bone

Human Vision and The Night Sky How to Improve Your Observing Skills Michael P. Borgia

How to Photograph the Moon and Planets with Your Digital Camera Tony Buick

Practical Astrophotography Jeffrey R. Charles

Pattern Asterisms A New Way to Chart the Stars John Chiravalle

Deep Sky Observing The Astronomical Tourist Steve R. Coe

Visual Astronomy in the Suburbs A Guide to Spectacular Viewing Antony Cooke

Visual Astronomy Under Dark Skies A New Approach to Observing Deep Space Antony Cooke

Real Astronomy with Small Telescopes Step-by-Step Activities for Discovery Michael K. Gainer

The Practical Astronomer’s Deep-sky Companion Jess K. Gilmour

Observing Variable Stars Gerry A. Good

Observer’s Guide to Stellar Evolution The Birth, Life and Death of Stars Mike Inglis

Field Guide to the Deep Sky Objects Mike Inglis

Astronomy of the Milky Way The Observer’s Guide to the Southern/Northern Sky Parts 1 and 2 hardcover set Mike Inglis Astronomy of the Milky Way Part 1: Observer’s Guide to the Northern Sky Mike Inglis Astronomy of the Milky Way Part 2: Observer’s Guide to the Southern Sky Mike Inglis Observing Comets Nick James and Gerald North Telescopes and Techniques An Introduction to Practical Astronomy Chris Kitchin Seeing Stars The Night Sky Through Small Telescopes Chris Kitchin and Robert W. Forrest Photo-guide to the Constellations A Self-Teaching Guide to Finding Your Way Around the Heavens Chris Kitchin Solar Observing Techniques Chris Kitchin How to Observe the Sun Safely Lee Macdonald The Sun in Eclipse Sir Patrick Moore and Michael Maunder Transit When Planets Cross the Sun Sir Patrick Moore and Michael Maunder Light Pollution Responses and Remedies Bob Mizon Astronomical Equipment for Amateurs Martin Mobberley The New Amateur Astronomer Martin Mobberley Lunar and Planetary Webcam User’s Guide Martin Mobberley (Continued after Index)

Dr Mike Inglis FRAS SUNY [email protected]

Library of Congress Control Number: 2007925262

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency. Enquiries concerning reproduction outside those terms should be sent to the publishers.

Patrick Moore’s Practical Astronomy Series ISSN 1617- ISBN-13: 978-1-85233-890-9 e-ISBN-13: 978-1-84628-736- Springer Science+Business Media Springeronline.com

©Springer-Verlag London Limited 2007

The use of registered names, trademarks, etc. in the publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant laws and regulations and therefore free for general use.

The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made. Observing the Sun, along with a few other aspects of astronomy, can be dangerous. Neither the publisher nor the author accepts any legal responsibility or liability for personal loss or injury caused, or alleged to have been caused, by any information or recommendation contained in this book.

For Dad and Alan, who are already amongst the stars

viii Contents

• 2.3 Emission Nebulae
• 2.4 Dark Nebulae
• 2.5 Reflection Nebulae
• 2.6 Molecular Clouds
• 2.7 Protostars
• 2.8 The Jeans Criterion
• Chapter 3 Stars
  • 3.1 The Birth of a Star
    • of Mass 3.2 Pre-Main-Sequence Evolution and the Effect
  • 3.3 Mass Loss and Gain
  • 3.4 Clusters and Groups of Stars
  • 3.5 Star Formation Triggers
  • 3.6 The Sun—The Nearest Star
  • 3.7 Binary Stars and Stellar Mass
  • 3.8 Lifetimes of Main-Sequence Stars
  • 3.9 Red Giant Stars
  • 3.10 Helium-Burning and the Helium Flash
  • 3.11 Star Clusters, Red Giants, and the H-R Diagram
    • Clusters 3.12 Post-Main-Sequence Star Clusters: The Globular
  • 3.13 Pulsating Stars
  • 3.14 The Death of Stars
  • 3.15 The Asymptotic Giant Branch
  • 3.16 Dredge-Ups
  • 3.17 Mass Loss and Stellar Winds
  • 3.18 Infrared Stars
  • 3.19 The End of an AGB Star’s Life
  • 3.20 Planetary Nebulae
  • 3.21 White Dwarf Stars
  • 3.22 High-Mass Stars and Nuclear Burning
    • of the Elements 3.23 Iron, Supernovae, and the Formation
    • Pulsars, Neutron Stars, and Black Holes 3.24 The End Result of High-Mass Stars’ Evolution:
• Chapter 4 Galaxies
  • 4.1 Introduction
  • 4.2 Galaxy Types
  • 4.3 Galaxy Structure
  • 4.4 Stellar Populations
  • 4.5 Hubble Classification of Galaxies
  • 4.6 Observing Galaxies
  • 4.7 Active Galaxies and AGNs
  • 4.8 Gravitational Lensing
    • and the Hubble Law 4.9 Redshift, Distance,
  • 4.10 Clusters of Galaxies Contents ix
  • 4.11 Endnote
• Appendix 1 Degeneracy
• Appendix 2 Books, Magazines, and Astronomical Organizations
  • Books, Magazines, and Organizations
  • Star Atlases and Observing Guides
  • Astronomy and Astrophysics Books
  • Magazines
  • Organizations
• Topic Index
• Object Index

xii Preface and Thanks

During the time spent writing this book, usually alone, usually at night, usually tired, I had the company of some wonderful musicians whose music is truly sublime. They are Steve Roach, David Sylvian, John Martyn, and the Blue Nile. Many friends have helped raise my spirits during those times when not all was going right according to the Inglis Master Plan. They listened to me complain, laughed at my jokes, and helped me remain sane—for the most part. So I want to say thank you to my British friends—Pete, Bill, Andy and Stuart—and my new friends here in the USA—Sean and Matt. It is nice to know that beer is the universal lubricant of friendship, whether it is McMullens or Blue Point. Astronomy is a very important part of my life, but not as important as my family; my brother, Bob, is a great friend and a strong source of support, especially during my formative years as an astronomer. My mother, Myra, is amazing, full of energy, spirit, and laughter, and has been supportive of my dream to be an astronomer since I was knee-high to a tripod. She is truly an example to us all. And of course Karen, my partner. I am not exaggerating when I say this book would not have seen the light of day without her help. “Diolch Cariad.” For making my life so much fun, cheers!

Dr. Mike Inglis Long Island, USA, 2006

Acknowledgements

I would like to thank the following people and organisations for their help and permission to quote their work and for the use of the data they provided:

The European Space Organization, for permission to use the Hipparcos and Tycho catalogues.

My colleagues at Suffolk County Community College, USA, for their support and encouragement.

The astronomers at Princeton University, USA, for many helpful discussions on the whole process of star formation.

The astronomers at the University of Hertfordshire, UK, for inspirational lectures and discussions.

Gary Walker, of the American Association of Variable Star Observers, for infor- mation on the many types of variable stars.

Cheryl Gundy, of the Space Telescope Science Institute, USA, for supplying astro- physical data on many of the objects discussed.

Dr. Stuart Young, of the University of Hertfordshire, UK, for discussions and information relating to star formation and the Hertzsprung-Russell diagram, and impromptu tutorials on many aspects of astronomy.

Dr. Chris Packham, of the University of Florida, USA, for his help on pointing out several mistakes I have made over the years and for his input regarding AGNs.

Karen Milstein, for the superb and professional work that she did reading through the initial proofs of the book, when there seemed to be more errors than facts!

xiii

Overview

To most normal people, astrophysics—the science of stars, galaxies, and the universe we live in—would seem to be a topic suited to a university-level textbook, and so the idea of a guide to astrophysics for the amateur astronomer may not, on first appearance, make any sense. However, let me assure you that anyone can understand how a star is born, lives its life, and dies, how galaxies are thought to evolve and what their shape can tell us about their origins and age, and even how the universe began and how it may end. In fact, very little mathematics is needed, and when it is used, it is only a matter of multiplication, division, subtraction, and addition! 1 What is more, there are many wonderful objects that can be observed in the night sky that will illustrate even the most obtuse astrophysics concepts. All one needs is a willingness to learn and a dark night sky. Learning about, say, the processes that give rise to star formation, or what happens to a very large star as it dies, or even why some galaxies are spiral in shape whereas others are elliptical can add another level of enjoyment and wonder to an observing session. For instance, many amateur astronomers are familiar with the star Rigel, in the constellation Orion, but how many of you know that it is a giant star, with a mass more than 40 times that of the Sun, and it is nearly half a million times more luminous than the Sun? How many know that the closest large galaxy, M31 in Andromeda, has a supermassive black hole lurking at its center with a mass more than 50 million times that of the Sun? Or that the Orion Nebula, regarded by many as the premier nebula in the sky, is in fact an enormous stellar nursery where stars are actually being born as you read this book? Knowing details such as these can add another level of enjoyment to your observing sessions.

xv

xvi Astrophysics is Easy

Each section of this book addresses a specific aspect of astrophysics. The first part focuses on the concepts needed for a complete understanding of the remainder of the book, and as such will be divided into specific topics, such as the brightness, mass, and distance of stars, and so on. Then we will look at the tools of an astronomer, namely spectroscopy. It is true to say that nearly all of what we know about stars and galaxies was and is determined by this important technique. We shall spend a fair amount of time looking at something called the Hertzsprung-Russell diagram; if ever a single concept or diagram could epitomize a star’s life (and even a star cluster’s life), the H-R diagram, as it is known, is the one to do it. It is perhaps the most important and useful concept in all of stellar evolution, and it is fair to say that once you understand the H-R diagram, you understand how a star evolves. Moving on to the objects themselves, we start with the formation of stars from dust and gas clouds, and conclude with the final aspect of a star’s life, which can end in a spectacular event known as a supernova, resulting in the formation of a neutron star and perhaps a black hole! On a grander scale, we delve into galaxies, their shapes (or morphology, as it is called), distribution in space, and origins. The topics covered are chosen specifically so that examples of objects under discussion can be observed; thus, at every point in our journey, an observing section will describe the objects that best demonstrate the topics discussed. Many of the objects, whether they be stars, nebulae, or galaxies, will be visible with modest optical instruments, and many with the naked eye. In a few exceptional cases, a medium-aperture telescope may be needed. Of course, not all observable objects will be presented, but just a representative few (usually the brightest examples). These examples will allow you to learn about stars, nebulae, and galaxies at your own pace, and they will provide a detailed panorama of the amazing objects that most of us observe on a clear night. For those of you who have a mathematical mind, some mathematics will be provided in the specially labelled areas. But, take heart and fear not—you do not have to understand any mathematics to be able to read and understand the book; it is only to highlight and further describe the mechanisms and principles of astrophysics. However, if you are comfortable with maths, then I recommend that you read these sections, as they will further your understanding of the various concepts and equip you to determine such parameters as a star’s age and lifetime, distance, mass, and brightness. All of the maths presented will be simple, of a level comparable to that of a 4th-year school student, or an 8th-grader. In fact, to make the mathematics simpler, we will use rough (but perfectly acceptable) approximations and perform back-of-the-envelope calculations, which, surpris- ingly, produce rather accurate answers! An astute reader will notice immediately that there are no star maps in the book. The reason for this is simple: in previous books that I have written, star maps were included, but their size generated some criticism. Some readers believed the maps were too small, and I tend to agree. To be able to offer large and detailed star maps of every object mentioned in this book would entail a doubling of its size, and probably a tripling of cost. With the plethora of star-map software that is available these days, it is far easier for readers to make their own maps than to present any here.

CHAPTER ONE

Tools of the Trade

1.1 Distance

To determine many of the basic parameters of any object in the sky, it is first necessary to determine its proximity to us. We shall see later how this is vitally important because a star’s bright appearance in the night sky could signify that it is close to us or that it is an inherently bright star. Conversely, some stars may appear faint because they are at immense distance from us or because they are very faint stars in their own right. We need to be able to determine which is the correct explanation. Determining distance in astronomy has always been, and continues to be, fraught with difficulty and error. There is still no consensus as to which is the best method, at least for distances to other galaxies and to the farthest edges of our own galaxy—the Milky Way. The oldest method, still used today, is probably the most accurate, especially for determining the distances to stars. This simple technique is called Stellar Parallax. It is basically the angular measurement when the star is observed from two different locations on the Earth’s orbit. These two positions are generally six months apart, and so the star will appear to shift its position with respect to the more distant background stars. The parallax (p) of the star observed is equal to half the angle through which its apparent position appears to shift. The larger the parallax (p), the smaller the distance (d) to the star. Figure 1.1 illustrates this concept. If a star has a measured parallax of 1 arcsecond (1/3600th of a degree) and the baseline is 1 astronomical unit (AU), which is the average distance

1

2 Astrophysics is Easy

Sun

p

Earth [January]

d

Earth [July]

Earth [January]

Nearby star

Sun

1 AU

p d

Earth [July]

Nearby star

1 AU

1 2

Figure 1.1. Stellar Parallax. (1) The Earth orbits the Sun, and a nearby star shifts its position with respect to the background stars. The parallax (p) of the star is the angular measurement of the Earth’s orbit as seen from the star. (2) The closer the star, the greater the parallax angle (PA).

from the Earth to the Sun, then the star’s distance is 1 parsec (pc)—“the distance of an object that has a parallax of one second of arc.” This is the origin of the term parsec, which is the unit of distance used most frequently in astronomy. 1 The distance (d) of a star in parsecs is given by the reciprocal of its parallax (p), and is usually expressed as thus:

d =

p

Thus, using the above equation, a star with a measured parallax of 0.1 arcseconds is at a distance of 10 pc, and another with a parallax of 0.05 arcseconds is 20 pc distant.

Box 1.1: Relationship between Parallax

and Distance

d =

1 p

d = the distance to a star measured in pc p = the parallax angle of the measured star in arcseconds

This simple relationship is a significant reason that most astronomical distances are expressed in parsecs, rather than light years (l.y.). The brightest star in the night sky is

4 Astrophysics is Easy

There are other distance determination methods used for the objects farthest from us—galaxies. These methods include the Tully Fisher method and the very famous Hubble Law. All of these methods—Cepheid variable, Tully Fisher, and the Hubble Law— will be addressed in greater detail later in the book. A final note on distance determination is in order. Do not be fooled into thinking that these methods produce exact measurements. They do not. A small amount of error is inevitable. This error is usually about 10 or 25%, and even an error of 50% is not unheard of. Remember that a 25% error for a star estimated to be at a distance of 4000 l.y. means it could be anywhere from 3000 to 5000 l.y. away. Table 1.1 lists the 20 nearest stars. Let us now discuss some of the nearest stars in the night sky from an obser- vational point of view. The list (Table 1.1) is by no means complete but includes those stars that are easily seen. Many of the nearest stars are very faint, and thus present an observing challenge; they are not included here. Throughout the book, I have used the following nomenclature with regard to stars: the first item will be its common name, followed by its scientific designation. The next item will be its position in right ascension and declination. The final item will identify the months when the star is best positioned for observation (the month in bold type is the most favorable time of observation). The next line will present standard data and information pertinent to the star under discussion: its apparent magnitude, followed by its absolute magnitude, other specific data relating to the star, and, finally, the constellation in which the star resides.

Table 1.1. The 20 nearest stars in the sky

Star Distance, l.y. Constellation

1 Sun ---- ---- 2 Proxima Centauri 4.22 Centaurus 3 Alpha Centauri A^7 4.39 Centaurus 4 Barnard’s Star 5.94 Ophiuchus 5 Wolf 359 7.8 Leo 6 Lalande 21185 8.31 Ursa Major 7 Sirius A^7 8.60 Canis Major 8 UV Ceti A^7 8.7 Cetus 9 Ross 154 9.69 Sagittarius 10 Ross 248 10.3 Andromeda 11 Epsilon Eridani 10.49 Eradinus 12 HD 217987 10.73 Piscis Austrinus 13 Ross 128 10.89 Virgo 14 L 789–6 A 7 11.2 Aquarius 15 61 Cygni A 11.35 Cygnus 16 Procyon A^7 11.42 Canis Minoris 17 61 Cygni B 11.43 Cygnus 18 HD173740 11.47 Draco 19 HD 173739 11.64 Draco 20 GX Andromadae 7 11.64 Andromeda

Tools of the Trade 5

1.1.1 The Nearest Stars to Us^9

Proxima Centauri V645 Cen 14 h^ 29.7 m^ − 62  41 ′^ Mar-Apr-May 11.01v m 8 15.45M 4.22 l.y. 0.772 ′′^ Centaurus

This is the second-closest star to the Earth and the closest star to the Solar System, and thus it is included albeit faint. It is a red dwarf star and also a flare star with frequent bursts, having maximum amplitude of around one magnitude. Recent data indicate that it is not, as previously thought, physically associated with  Centauri, but is, in fact, on a hyperbolic orbit around the star and just passing through the system.

Sirius A Canis Majoris 06 h^45  1 m^ − 16  43 ′^ Dec-Jan-Feb −1.44 m 1.45M 8.6 l.y. 0.379′′^ Canis Major

Sirius, also known as the Dog Star, is a lovely star to observe. It is the sixth-closest and brightest star in the sky. It is famous among amateur astronomers for the exotic range of colors it exhibits due to the effects of the atmosphere. It also has a dwarf star companion—the first to be discovered. A dazzling sight in any optical device.

Procyon  Canis Minoris 07 h^ 39.3 m^ + 56  13 ′^ Dec-Jan-Feb 0.40 m 2.68M 11.41 l.y. 0.283′′^ Canis Minor

Procyon is the fifteenth-nearest star and the eighth brightest. Like its neighbor Sirius, Procyon has a white dwarf companion star, but it is not visible through amateur telescopes.

Barnard′s StarHD21185 17 h^ 57.8 m^ + 4  38 ′^ Apr-May-Jun 9.54 m 13.24M 5.94 l.y. 0.549′′^ Ophiuchus

The third-closest star is a red dwarf. What makes this star so famous is that it has the largest proper motion of any star 10 —0.4 arcseconds per year. Barnard’s Star, also known as Barnard’s Runaway Star, has a velocity of 140 km per second; at this rate, it would take 150 years for the star to move the distance equivalent to the Moon’s diameter across the sky. It has also been thought that the star belonged to the Galaxy’s Halo Population.

61 Cygni A V1803 Cyg 21 h^ 06.9 m^ + 38  45 ′^ Jul-Aug-Sep 5.20 (^) v m 7.49M 11.35 l.y. 0.287′′^ Cygnus

This is a very nice double star with separation 30.3 arcseconds and a PA of 150 (see section 3.7). Both stars are dwarfs and have a nice orange color. It is famous for being the first star to have its distance measured successfully, by F. W. Bessel in 1838, using stellar parallax.

GX And Grb34 00 h^ 18.2 m^ + 44  01 ′^ Aug-Sep-Oct 8.09 (^) v m 10.33M 11.65 l.y. 0.280′′^ Andromeda