When you look up at a clear night sky, what do you see? Probably your first answer would
be "stars". That would be my first answer as well. But what we actually
see are points of light. How have we come to conclude that those points of light are
like our Sun, that they are very far away,
and that they are arranged spatially in the system we now recognize as our galaxy?
For thousands of years humanity came up with many notions about those points of light. Often
those notions had much to do with supporting the psychological and practical needs of the society.
Even Tycho Brahe was motivated to make the most precise and systematic measurements
of his time, in part, because he wanted to make astrology more precise. It was only during the
Scientific Revolution that we could conclude that stars are similar to our Sun.
Yet even then, in the 19th century, there
were scientists who felt we would never learn the true nature of stars, because the stars are
so far away.
* * *
The physics discoveries of the 19th and 20th centuries, together with more careful
and sophisticated astronomical work changed all that. The links on this page essentually
describe what we understand based on that work.
Some important highlights:
- Successful parallax measurements, beginning in 1838, and then also proper motion studies
of nearby stars gave us our first firm indications of how far away stars are.
- It was realized that spectral measurements reveal to us the composition and physical
conditions at the surfaces of stars.
- The notion of Conservation of Energy logically led to the notion that stars must
have finite lives. (There is only so much energy available in them.)
Therefore, they must also have births and deaths. Therefore:
- How do they do begin and end their lives?
Could we observe their births and deaths?
- If stars are born and die, do they show indications of age? Do they evolve?
- Observational evidence during the 19th century showed that there are different
kinds of stars with different physical characteristics. Attempts were made to fit observations
with physics. The required detective work took decades to unfold,
with twists that sometimes led to
mistakes. That our spectral classification system goes "O, B, A, F, ..." and
not "A, B, C, D, ..." is a vestige of one of those twists. The depth and detail
in the links below show how far we have already come in this detective work. For some aspects,
this is still an active research field.
- We weren't able to understand how stars shine until the discovery of radioactivity and
nuclear physics.
- It was eventually realized that the nuclear reactions inside stars are the primary
source of the elements heavier than hydrogen and helium. It was also realized that there
is a cycling of material in the galaxy between the stars and the interstellar medium.
Each generation of new stars gets a larger mix of heavier elements. That in turn helped
explain differences between the galaxy's oldest stars (Population II) in the globular
clusters and the newer stars (Population I) in the galactic plane.
- Paralax and proper motion can only be measured for stars within a few hundred
light years of us. More clever techniques had to develop in order to determine
distances much further away. Several classes of variable stars were noticed to have
specific brightnesses. Detecting those stars in clusters and in nearby galaxies became
a key to determining distances on the galactic scale.
- Dust drastically reduces the distance we can see in the plane of the Milky Way at optical
wavelengths. Our understanding of the rest of the galaxy is aided by studies at non-optical
wavelengths (particularly hydrogen in the radio spectrum). We also study the dynamics of moving
bodies on large scales; we use as evidence the doppler shifts and the distribution
of those bodies. We can also compare our galaxy to other galaxies.
- The development of astronomy observation at non-optical wavelengths during the 1950's and
1960's revolutionized astrophysics.
Grade school level:
- Windows to the Universe
-
Stars @ Zoom Astronomy / Enchanted Learning.
Highlights some facts. Not my first choice to look at.
Intermediate/Undergraduate:
- Sloan Digital Sky Survey Education Projects
Basic and more advanced (intermediate) levels.
- Review of the Universe
(Intermediate to Advanced)
-
Astronomy 108 The Astounding Cosmos
Survey of modern astronomy course for non-science majors.
Clear introductions to the relevant subjects. Dr. Joe Howard.
Salisbury State University, Maryland.
-
Prof. Gene Smith's Astronomy Tutorial
Based on an introductory college course for liberal arts majors. Includes some detailed
physics (astrophysics).
University of California, San Diego Center for Astrophysics & Space Sciences
-
Nick Strobel's Astronomy Lecture Notes
Comprehensive college level course notes.
-
Curious About Astronomy? Stars
University introduction to the life cycle of stars. Plus many
basic and intermediate level Questions & Answers. Cornell University.
-
Introductory Level Astronomy Supplemental Pages
by Kristen and Scott Miller, University of Marland.
- Dr. Siobahn Morgan.
(Thank you for writing me.)
Univ. of Northern Iowa.
-
Astronomy 162: Stars, Galaxies, and Cosmology
Introductory college course notes. Univ. of Tennessee, Knoxville.
-
Astronomy 162 -
Intorduction to Stellar, Galactic and Exra-Galactic Astronomy
Course notes by Professor Barbara Ryden, Ohio State Univ.
-
Astro 124 Introduction to Stars, Galaxies and the Universe
John F. Hawley. University of Virginia. See the lecture notes at the bottom.
-
Astronomy 103
J.C. Evans. George Mason University.
The
astronomy supplement has extensive discussions.
-
Astronomy 122 - Stars
Davison E. Soper, Univ. of Oregon. Basic level. Part of a sequence of astronomy courses.
-
ASTRO 201 Cornell University:
- Imagine the Universe
A website about astrophysics for teen level and above. High energy astrophysics is particularly
emphasized. However other aspects are covered as well.
Note there are several levels of presentations. There is more material here than is
obvious; therefore I suggest using the sitemap. Includes links to more resources at universities
and book suggestions for high school or university level.
High Energy Astrophysics Science Archive Research Center @ NASA GSFC.
- Sol Station
Informative and cute. Many pages to look at. Includes some data tables and maps.
-
HyperPhysics - Astrophysics Concepts
Georgia State Univ.
The Sun has its own page here.
Intermediate:
- Stars
Imagine the Universe, NASA GSFC.
-
Portraits of Stars and their Constellations
By Jim Kaler,
Prof. Emeritus of Astronomy, University of Illinois.
Lots of information about Stars, including their physical nature and
classifications used in observing them. Some information about several variable star
types.
Features photographs of the constellations with detailed physical
descriptions of the major naked-eye stars within each constellation. Also some
sky lore.
- The Internet Stellar Database
By Roger M. Wilcox. You can look up data of stars within 75 light years of us, plus the
brighter more well known stars that are further. Also some pages about the background
astrophysics.
-
Nick Strobel's Astronomy Lecture Notes
Comprehensive college level course notes. Chapters on Determining Star Properties,
the Interiors of Stars, and Stellar Evolution.
- Dr. Siobahn Morgan.
(Thank you for writing me.)
Univ. of Northern Iowa.
Introductory college course website. Supplemental material includes a constellation section.
- Review of the Universe
- Stars
(Intermediate to advanced).
- Stars
By John Baez. Some slightly random comments by a mathematical physicist
about classification and physics.
- There is much more in-depth information about stars in the subsections below.
Links about noticing the appearance of stars as constellations are on the
Observing the Sky page. There is a section for
Constellation photography on the Photo page.
Links about using the stars for time
telling and navigation can be found on the Time & Navigation page.
- Hertzprung-Russell (H-R) versus Color Magnitude (C-M) diagrams
You may see references to H-R diagrams and C-M diagrams. They seem to describe the
same characteristics of stars. But there is a slight difference in their meaning.
And so what is that difference?
The initial discovery was that there is a definite correlation between Absolute Magnitude
and the Spectral Type of a star. If you plot on a chart the spectral class versus
magnitude of each star, you get what is called the Hertzprung-Russell (H-R) diagram.
The H-R diagram is derived by measuring stellar spectra.
It
was found that on such charts, the plotted position of each star depends on the star's
mass and on how the star evolves with time.
It was then realized that the spectral type also corresponds with the temperature of the
star's outer atmosphere. The temperature directly determines the color we
observe. And so if we measure star colors and plot that measurement with magnitude, we
should see a similar pattern. A Color Magnitude (C-M) diagram is derived from the measuring
the star colors.
In practice, we determine color by measuring the brightness of
the star through two different colored filters. The brightness is measured with photometers.
The photometry measurement is always more precise than determining spectral classes. And so
in professional practice, the C-M diagram is preferred.
-
The Stellar Spectral Types OBAFGKM
Astro Picture of the Day.
-
Astronomical Concept Pocket Reference
A brief explanation of the H-R diagram (Star Types article) and apparent Magnitudes in
terms of the brighter naked eye stars. Fremont, MI High School.
-
Star Colors and Spectroscopy
A paper for amateur astronomers about how star colors were correlated with spectral
types and Absolute Magnitudes. We see this correlation on Hertzprung-Russell (H-R)and
Color Magnitude (C-M) diagrams. The story is told with some history.
By John A. Blackwell. Regulus Astronomy Newsletter.
-
Spectroscopy - Information Concealed in Starlight
An essay about the historical development of astronomical spectroscopy. In support of
virtual 3D space atlas projects.
By Brian von Konsky. Curtin University of Technology, Australia.
- WPO Amateur Spectroscopy
Includes a beginner's guide. Maurice Gavin.
- Sloan Digital Sky Survey Education Projects
Basic and more advanced (intermediate) levels.
Star colors, spectral types, Hertzprung Russel Diagrams, Parallax.
- Royal Observatory Greenwich
Look for Spectroscopy and Photometry in "Astronomy Fact Files".
-
Introductory Astronomy Supplemental Pages by Kristen and Scott Miller.
Univ. of Maryland
-
What's in a Star Chem Connections
-
The Classification of Stellar Spectra Jesse Allen. GSFC. [Link not current]
-
Nick Strobel's Astronomy Lecture Notes
One of the chapters is about Determining Star Properties. Includes discussion of
distances, magnitude and luminosity, color index, composition and physical properties.
The physical properties discussion includes how studying binaries can reveal mass and size.
Ends with discussion of H-R (color magnitude) diagrams.
-
PHY103 Stars and Galaxies
Includes some worked problems about magnitude, luminosity, color and the H-R diagram.
Contrasts the H-R diagrams of an open cluster and a globular cluster
(47 Tuc). There is also a glossary.
-
Astronomy 108 The Astounding Cosmos
Survey of modern astronomy course for non-science majors. Dr. Joe Howard.
Salisbury State University, Maryland.
-
Astronomy 122 - Stars Davison E. Soper, Univ. of Oregon.
-
Astronomy 162: Stars, Galaxies, and Cosmology @ Univ. of Tenn.
-
Introductory Level Astronomy Supplemental Pages
by Kristen and Scott Miller, University of Marland.
- Rundetaarn (The Round Tower)
in Denmark.
The astronomy section has some history including a page about
Ejnar Hertzsprung and
this
Interactive Hertzprung-Russel Diagram
- See
100 Billion Suns--The Birth, Life, and Death of the Stars
by Rudolph Kippenhahn. Basic Books.
- Another good intermediate level book for introducing you to the details of measuring stellar
properties is:
The Milky Way By Bart & Priscilla Bok.
1984. Harvard University Press.
- Also see the Astronomy by Wavelength page,
especially
Spectroscopy -- Spectral Measurement.
- Also see Observing:
Stars and
How Bright are the Stars? -- Magnitude
- Also see
Measurement topics - On what is measured
A star is said to begin its late stages of life once it leaves the main sequence.
That is, once a star can no longer fuse hydrogen at its core, a new sequence of events start
which eventually lead to the star's end. Just what exactly happens depends mostly on how
massive the star is. If a star is not massive enough, it will not have a heavy enough atmosphere
above its core to sustain the fusion of elements beyond a certain stage.
Such a star will then eventually
blow off its outer atmospheres to form a planetary nebula. And the star itself will
evolve into a white dwarf.
If the star is more massive, it will go through stages
of fusing heavier elements until it fuses iron. Then it will explode as a
supernova. The resulting nebula is called a supernova remnant. The star itself
becomes a very dense degenerate star called a neutron star.
If the spin axis is aligned in a
favorable direction, the magnetic pole may regularly point towards us as the star spins.
Then we may detect such a star as a pulsar.
In both cases, the star immediately after leaving the main sequence evolves into a Red Giant star.
The outer atmosphere
of the star expands and so becomes cooler and redder. However the star also becomes brighter.
Seen on a Hertzprung-Russel (H-R) diagram (or color magnitude diagram) the star would move towards
the upper right.
In general, the more massive the star is, the more complex will be its development in later stages.
The complexity can be seen from the track on the H-R diagram. Of course, humans have not lived
long enough to watch any star evolve. Our knowledge comes from computer modeling and checking
the models with observational data, especially from star clusters.
Open star clusters are particularly helpful because one can assume the stars of the cluster
were born at approximately the same time. Therefore, the distribution of a cluster's stars
on the H-R diagram indicates the cluster's age. The more massive stars use up their hydrogen and
so leave the main sequence first. Less massives stars burn their hydrogen longer. The most
massive main sequence star you can find in a cluster therefore indicates the age of the
cluster.
The more massive stars will tend to be found somewhere in the H-R diagram's upper left as Red Giants.
Or they may have already evolved past that. Some have become hotter and appear bluer (towards the
right on the H-R diagram) on their way to becoming white dwarfs. Since their track runs across
the H-R diagram, they are called Horizontal Branch stars. Stars evolved past that are
found in the lower right as white dwarfs.
By studying the H-R diagrams of clusters and the tracks of stars developing in the computer
models, some interesting characteristics have become known. One is that certain classes of
variable stars, such as Delta Cepheids, are stars passing through particular regions of the
H-R diagram. On the computer models these stars show an instability as they try to expand and so
fall back in; but then they again have the conditions that made them expand. And so they oscillate.
Another interesting feature in some cluster diagrams is the appearance of "Blue
Stragglers", stars that appear as blue main sequence stars where one would expect just
red giants. These stars are now thought to be the result of collisions between stars.
A complication arises when stars are binary or in multiple star groups. If one of the stars
is more massive and so evolves into a large red giant, and if the stars are close enough,
the outer material of the Red Giant will flow onto the neighboring star. This leads to
material accreting around and on the neighboring star, which at some point may start to
ignite. Those stars are "Cataclysmic Variables". Novae are among the
Cataclysmic Variables.
There is some thought that the most massive stars end as Black Holes.
Simon Jeffery's article below is a very good starting point for learning about the late
stages of stellar evolution. Further below are links to a number of groups that
study Cataclysmic Variables.
Geneal discussions:
Studying particular kinds of objects:
- For Planetary Nebulae and Supernova Remnants
see Nebulas below.
- For Cataclysmic Variables see Variable Stars
below.
- Also see links about Black Holes.
-
Violence in the Cosmos
-
Death of Stars
Short discussions of Novae, X-Ray Bursters, and Supernovae.
- Royal Observatory Greenwich
Look for Pulsars and White Dwarfs in "Astronomy Fact Files".
-
Chandra (AXAF) Science Center Public Information Server
- Constellation-X
GSFC
BBC Horizon - The Death Star October 18, 2001.
Gamma-ray bursts and Hypernovae. Includes program transript and links.-
Blue Stragglers in Globular Clusters
Sol Station.
Red Giant Phase
White Dwarfs
- These are the stars remaining after Type II
Supernovas (see below).
-
Introduction to neutron stars C. Miller (U. Maryland).
-
Pulsars @ Imagine the Universe, NASA.
-
What are Pulsars - a tutorial in stellar astronomy
NASA Science.
- Pulsars
Royal Observatory Greenwich.
-
ATNF Astrophysics Pulsar-Supernova Remnant Project CISRO (Australia)
-
A Tutorial on Radio Pulsars
-
Radio Pulsar Glitch Studies
An article at African Skies. Particular attention on the Crab and Vela pulsars.
- BATSE Pulsar Studies
-
Virtual Trips to Black Holes and Neutron Stars Page
- Jocelyn Bell-Burnell and the Discovery of Pulsars
Jocelyn Bell was a graduate student at Cambridge studying the scintillation of Quasars with a
fixed array of radio antennas. She was the one who first noticed the Pulsars.
In the beginning no one knew what the pulsars were. There was even some concern that
the pulsar signals might be from aliens and the results were kept quiet for a short while.
Eventually she published her results for her Ph.D. thesis. That spurred other observers to
look for more pulsars. And it encouraged theorists to explain them. Eventually her thesis
advisor was awarded a Nobel Prize, but she was not. Her story is interesting for the
science, for the insight into graduate work, and for the human interest.
-
Little Green Men, White Dwarfs or Pulsars?
by Jocelyn Bell-Burnell, about her discovery of Pulsars in 1967,
Cosmic Search Vol. 1, No. 1 - January 1979. Originally published in
Annals of the New York Academy of Science, vol. 302, pages 685-689, Dec., 1977.
- See the online talk about pulsars by Jocelyn Bell-Burnell at
The Vega Trust.
- A Science Odyssey PBS mini-series:
- Moments of Discovery - Pulsars
The discovery of optical pulsars as told by the discoverers John Cocke and Michael
Disney, with background by Phillip Morrison. 10 pages. You can also listen with Real Audio.
- Also see Supernova Remnants links in Nebulas. Especially see
the section about The Crab Nebula (M1).
- Also see links about Black Holes.
Our primary method of verifying star properties, especially mass, is
by measuring the orbits of nearby binary star systems; binaries in nearby
open clusters, such as the Hyades, are especially useful.
- Binary Stars
By Don Ware, Peoria Astronomical Society. A nice introduction to the topic.
-
Astronomy 162: Stars, Galaxies, and Cosmology @ Univ. of Tenn.
- Double Star Library
Bill Hartkopf, U.S. Naval Observatory.
- Double Stars
Royal Observatory of Belgium (Brussels). Includes short web pages about why double stars
are studied and about the types of double stars found.
- Some brief notes on
Double Stars
I wrote, based on Paolo Maffei's 1978 book Beyond the Moon.
-
Interacting Binar Stars
a user friendly tutorial, by John Blondin. NCSU.
-
Binary Star Simulator
- Binary Stars Software
for Macintosh, by Claud Lacy, Univ. of Arkansas at Fayetteville
-
Eclipsing Binary Stars Dan Bruton.
-
ASTRO 201 General Topics Cornell University course highlights:
-
X-Ray Binaries Imagine the Universe, NASA GSFC.
Includes additional discussion of Mass Exchange in Binary Systems, Binaries that Pulse and Flash,
and Determining Orbits and Masses (brief). Also, links to more resources at universities
and book suggestions.
-
Double Stars
A page about amateur astronomy observing. Hanwell Community Observatory.
- Also see the links about Cataclysmic Variable Stars in the
Variable Star section.
- On a larger scale, stars which form together and
remain gravitationally bound are seen as
Star Clusters, Associations (see the section below).
Nebulae are generally associated with two stages in stellar evolution: 1) the coalescing
material where new stars form, and 2) the late stages of a star's life when the star throws off
material. Within the second category there are several kinds of nebulae, depending on the kind
of star involved. The supernova remnants come from very massive stars. Planetary nebulae
(not an accurate name for them) are thrown off atmospheres of less massive stars.
-
Violence in the Cosmos
The section on nebulas describes the basic types.
Mike Guidry, University of Tennessee, Knoxville. Based on a popular level talk given
in 1995.
- SEDS Messier Database
In the mid 1700's the hot subject of astronomy was finding comets. The professional telescopes
then were the size of amateur telescopes now. When a comet is near the sun, one can see its
bright tail. But when it is farther out in the solar system, it just appears as a faint cloudy
patch. If that patch moves from week to week, one then knows it is a comet. Charles Messier
compiled a list of about 100 fuzzy objects that he could see in his telescope which didn't move.
The list served to help other astronomers, so they wouldn't waste their time trying to track those
objects. Later, astronomers realized that those objects are either nebulae
in our galaxy
or other nearby galaxies. Nowadays the Messier list is popular with amateur
astronomers because it gives you a nice list of relatively bright deep space objects one can see
with small telescopes.
- The Web Nebulae SEDS
-
Nebulae @ Plasmas - the 4th State of Matter
-
Planetary Nebula Sampler
-
Image Gallery of Planetary Nebulae
Institut für Astronomie und Astrophysik Tübingen (Germany)
- Encyclopedia of Astronomy and Astrophysics
Look up articles under Interstellar Medium Objects.
-
Astronomy Picture of the Day Archive
There are several sections about nebulae. Usually each page gives a taste of the
research and gives some links.
-
Infrared Astronomy at the University of Calgary
Research on proto-planetary nebulae and planetary nebulae in the Infrared.
- Bruce Balick University of Washington.
Research about planetary nebulae. Specifically "the late phases of stellar evolution and
the hydrodynamical evolution of the gas that is ejected in these phases".
-
Doug Johnstone's Astrophysics Home Page. See his research pages, including
-
Supernova Remnants Imagine the Universe, NASA GSFC.
-
Constellation X Space Telescope - Science - Life Cylces of Matter
Includes high energy astrophysics work on supernova remnants.
-
X-Ray Sources - Supernovae and Supernova Remnants Chandra X-Ray Telescope
notes
-
List of Supernova and Supernova Remnant Pages on the WWW
by Marcos J. Montes.
-
Catalogue (or Catalog) of Galactic Supernova Remnants (SNRs)
-
HubbleSite: News about Nebulas
- Crab Nebula (M-1):
As Phllip Morrison said during the early 1970's, the Crab Nebula served as a
"Rosetta Stone" of astrophysics for that time.
The Crab Nebula had been suspected as the remnant of a
supernova recorded by the Chinese in 1054. And during a century of professional telescopic
observation, the nebula was seen to expand. That was about the state of understanding until
the 1960's. Then many new puzzle pieces
emerged from many different branches of astronomy. Early telescopes sent up in rockets,
satellites and balloons observed the Crab at wavelengths that could not be seen from the
ground. They all saw the Crab as one of the brightest objects in the sky. That implied the
crab had a tremendous power source. New radio techniques began to detect pulsating sources,
"Pulsars". The Crab was eventually found to have a very fast pulsar, flashing about
30 times per second. Optical techniques soon caught up and identified which visible star was
the pulsar. Meanwhile, neutron star theory was revived and was eventually able to successfully
describe the pulsar and the supernova expolosion that must have created it. Because the Crab
Nebula explosion was so recent, and its remnants so close and relatively easy to study, it was
used as a benchmark to study more distant nebulae and pulsars. As supernova explosions became
better understood, an important part of the stellar evolution cycle also came to be understood.
And so the Crab opened the door to lots of new and interesting astrophysics.
-
History of Crab Nebula Observations
-
Crab Nebula @ JAXA Space Notes.
-
The Crab Nebula Pulsar Royal Observatory Greenwich.
-
The Crab Nebula @ Chandra AXAF
-
The Crab Nebula @ Cornell Astro 201
-
Crab Space Movie NASA News September 19, 2002.
Movie of the Crab neutron star rotating and spewing antimatter jets into space.
From observations by HST and Chandra X-Ray observatories.
- Crab Nebula
Astronomy Picture of the Day December 2, 2005.
-
The Supernova that Made the Crab Nebula July 2006
Max Planck Institute for Astrophysics.
-
Crab Nebula and Pulsar
Sol Station.
- Also see Late Stages of Stellar Evolution above,
especially Neutron Stars / Pulsars.
-
Rosette Nebula @ Cornell Astro 201
- Also see Interstellar Medium below.
- Also see Late Stages of a Star & Stellar Death
above.
- Also see
Before the Main Sequence and Stellar Birth above.
Interstellar Medium Books
Interstellar Medium Articles
- Also see meteorite links and
articles for evidence
of interstellar grains found in meteorites that seem to predate the birth of our
Solar System.
College course notes:
-
Leicester Univ. Guide to Space:
The Milky Way - our Galaxy in the Galaxies Section.
-
Our Galaxy The Milky Way, La Via Lattera Padua Observatory, Italy.
No longer updated, but still online.
-
Astronomy 162: Stars, Galaxies, and Cosmology University of Tennessee, Knoxville.
-
The Milky Way Galaxy
Astronomy 162 course notes by
Professor Barbara S. Ryden, Ohio State University.
- Dr. Siobahn Morgan.
(Thank you for writing me.)
Univ. of Northern Iowa.
-
Introductory Level Astronomy Supplemental Pages
by Kristen and Scott Miller, University of Marland.
- Astronomy 103:
astronomy supplement
J.C. Evans. George Mason University.
-
The Milky Way @
ASTRO 201 General Topics Cornell University.
Some highlights of concepts.
-
Astronomy 123 - Galaxies Davison E. Soper, Univ. of Oregon
- Also see General and Introductory Links above, including
Intermediate / Undergraduate level links.
Stars & the Milky Way
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