Physics 3250: An Introduction to Astrophysics....Spring 2020
(Left) NGC 6302, the "Butterfly" Nebula, a bi-polar expanding death
shroud of a dead star (click to enlarge - white dwarf is the very dim
dot at the geometric center), our Sun's fate 7 billion years hence.
(Right) A large star cluster containing recently-formed very massive
stars (or rather the massive stars are most luminous and so stand out),
20,000 ly away in the constellation of Carina. Both images were
obtained by the Hubble Space Telescope.
Our Sun formed within a cluster, albeit likely much smaller, 4.57
billion years ago.
Instructor: Kirk Korista
Office: 2226 Everett Tower
Office phone: (269) 387-4971
email: kirk 'dot' korista 'at' wmich 'dot' edu
1120 Everett Tower
Physics Department phone: (269) 387-4940
Western Michigan University home page is here
The course textbook is An Introduction to Modern Astrophysics,
2nd Edition (2007) by Bradley Carroll & Dale Ostlie. If you have purchased
the Pearson publisher version (all printings prior to October 2017), a list of corrections of typos in this version can be found
Important! As of October 2017, the 2nd Edition
of this this book is published by Cambridge University Press.
Its webpage is found
here (click on 'Resources' for some neat stuff, but none of it required in this course), and *most* of the errata (typos) in the Pearson version
have been corrected in the Cambridge University Press version. You are responsible for those corrections relevant to the
material covered in this course, including use of the updated physical
constants. The pagination for both publisher versions of the 2nd Edition are identical - so no problems there.
If you have any questions about possible errors or for any reason need help, please see me!
The course syllabus is
The University statements on
rights and responsibilities. The Office
of Student Conduct.
listing of the Chapters to be covered (pay attention to possible
Measuring Stars: Read the Preface;
(read the very brief Section 1.4; other sections are optional
background material, and not
tested), Chapter 2 (all; in section 2.4, I am mainly interested that
you understand the results
of the virial theorem, e.g., Equations 2.46 and 2.47 and their applications), Chapter 3 (to
p.75, then skim 3.6 - understand
for reading context what
is meant by a star's apparent magnitude and absolute magnitude (pp. 60-62), and
a by star's "color" based on comparing the light that passes through two filters, but don't sweat
the details), together
Chapter 5 (all), Chapter 6 (skim for background information - no
homework and not
tested); Chapter 7 (Section 7.1 -
an introduction, plus Sections 7.2 and 7.3 through p.189. HW#3
has a sample problem on Visual
Binary systems discussed in Section 7.2,
extra credit problem on Spectroscopic
Binary systems discussed in
not be examined on the details pertaining to Spectroscopic Binary
systems, but you ought to be familiar with the basic idea of
Doppler effect to acquire information about the orbit speeds, their
ratio, and what that ratio tells us about the two stars' masses.).
a) A few animations illustrating the motions of two bodies under a
is an animation showing how the Sun and Jupiter (1047x less massive;
not to scale)
orbit their center of mass
11.86 years. This "pivot point" always lies nearer to the more massive
object. In reality, the Sun's motion is complicated by pull of the
other planets, but most of the Sun's motion is due to the pull by
Jupiter -- Jupiter's is both very massive (318x the mass of Earth) and
relatively near (5.2 AU) to the Sun. (If interested in the details, see
illustrative tutorial; you won't be tested on these details.) The
following are additional
animations illustrating orbits of two masses around the
equal masses (circular orbit),
equal masses (elliptical orbit; note the conservation of angular
momentum in action - a bit subtle in this example),
but comparable masses, different
and more disparate masses, and
different masses (the last 3 of these all have circular orbits).
Note that both masses in each 2-body orbit example have the
same orbital period. Which
is more massive? Which has a greater orbital acceleration? greater
orbit speed? The conservation of linear momentum, m1v1
= m2v2, tells us that the ratio of masses is
related to the ratio of the orbit speeds. The ratio of the two masses
also inversely related
to the ratio of their distances from the center of mass. The textbook
and the lectures will cover what you need to know about the 2-body
problem for this class, but further details can be found at
link and additional links therefrom.
b) You might find
simulation of an eclipsing binary system (binary stars that eclipse
one another from our point of view) to be interesting. I won't be
examining you on the particulars of eclipsing binary systems (discussed
in Section 7.3).
However, the methods by which we can measure various physical
properties of stars (in particular, each of the two star's masses,
radii, and the ratio of their surface temperatures) found in these
special binary systems are so clever that I figured you might find them
to be of interest.
The Physics of Stellar Spectra:
(most; in Section 9.5, you will not be examined on any of the details
pertaining to the
profiles (shapes) of spectral absorption lines or on the details pertaining to the primary measure
of their strengths
called "Equivalent Width" (e.g., the curve of growth concept). However, the twin concepts of the effects of
the quantum mechanical probability
for a specific interaction between matter and a photon and the factors
(mainly T) that determine the
number density or column density of absorbers
present should be understood. I will
not examine you on concepts related to
radiation emerging from the star's surface except along the local normal
(theta = 0) - so we are *probably* skipping the concept of "Limb Darkening". Do read
the final section called Computer Modeling of Stellar Atmospheres). Read section
11.2 to the top of p.370 of Chapter 11 (especially those subsections that are
obvious applications of those sections of Chapter 9 we discussed in class or
covered in HW) alongside Chapter 9. Ask yourself this question: why does a
star's spectrum appear the way it does? The 3 key major concepts are
temperature, optical depth (and its relation to
photon mean free path), and the quantum interaction between light and
matter. If you're comfortable discussing these
and how they determine a star's spectrum, for a given set of elemental
abundances, you should do well on the exam. And don't forget the most
important diagram in all of astrophysics -- the HR diagram, and its backwards horizontal axis
is the stellar effective surface temperature. I guess
temperature is a key concept in this unit.
Stars -- how they're built and how they work:
Chapter 10 (all, but just skim sections "The Mixing Length Theory..." and
"Polytropic Models..." - you won't be held accountable for the details of these
two topics). Pay closest attention to the readings associated with what we
covered in class and in your HW. Read section 11.1 of Chapter 11 along with
Chapter 10 (especially those subsections that are obvious applications of
Chapter 10). You will want to review the
discussion of stellar continuous opacity on pp.244-251 in Chapter 9.
I would love to get to
stellar evolution (birth, life, deaths of stars), but other than introductions
these topics will need to
wait for a future course. If you're interested
in amazing new field of helioseismology and how
we use it to plumb the physical conditions of the interior of our Sun
(and now in nearby stars!), this is introduced in section 5 of Chapter
14. However, you
won't be examined on helioseismology.
Some quotable quotes about
Science is built up with facts, as a house is with stones. But a
of facts is no more a science than a heap of stones is a house.
...science is not a database of unconnected factoids but a set of
designed to describe and interpret phenomena, past or present, aimed at
a testable body of knowledge open to rejection or confirmation.
Shermer (Scientific American,
Like all sciences, astronomy advances most rapidly when confronted
exceptions to its theories... -from An Introduction to Modern
Carroll & Dale Ostlie)
Science is a way of trying not
yourself. The first principle is that you must not fool yourself, and
you are the easiest person to fool. -Richard P. Feynman (Physics
What binds us to space-time is our rest mass, which prevents us from
at the speed of light, when time stops and space loses meaning. In a
of light there are neither points nor moments of time; beings woven
light would live "nowhere" and "nowhen"; only poetry and mathematics
capable of speaking meaningfully about such things. -Yuri
professor of Mathematics at Northwestern University
Recommended Reading for Fun
If you really want to blow out your mind, read this book,
in the Void...Why nothing is important, by astronomer
Odenwald. You might also be interested in
reading some of his essays.
I also give my highest recommendation to this book by physicist Brian
Fabric of the Cosmos: Space, Time, and the Texture of Reality. And
for an excellently written, if a bit dated, grand overview of what we
know about the
cosmos, I recommend Timothy Ferris'
The Whole Shebang - A State of the
article from the popular science magazine,
Scientific American, discussing
several common misconceptions of the Big Bang Theory.
video by the music group "Muse"
(yeah, dating myself a little) has something to say about the true nature of gravity (if you ignore the
flapping hair and shirt collar). I'm not joking; and given the titles of several of their hits, such as "Starlight",
"Supermassive Black Hole", and "Neutron Star Collision", among others, one of the guys must have taken a course in
What do the apparent "arrow of time" and entropy have to do with
the origin of the universe? Sean Carroll's
to Here - The Origin of the Universe and the Arrow of Time
addresses this at an understandable level. And this...
The Big Picture - On the Origins of Life, Meaning,
and the Universe about covers it, eh?
This should be required reading for all budding scientists:
Importance of Stupidity in Scientific Research .
Finally - you're in for a treat if you read
Richard P. Feynman's
QED: The Strange Theory of
Light and Matter.
(Pay attention for updates!)
- First exam covering Chapters
2,3,5 (and Chapter 7: mainly the the big ideas concerning how we
star masses in binary systems; see reading assignments above) and HW
#1,2,3 has been scheduled for: Thursday, February 20.
You need a few pencils, an eraser,
- The first meeting of
Pizza will be on TBD @ Bilbo's on Stadium. We'll meet over
there at about 4 pm - if you need a ride, please email me!
- Friday, 28 February
- Sunday, 8 March: mid-semester break.
- Second exam covering Chapters 8,9,11.2
(see reading assignments, above, for relevant sections) is scheduled
for TBD. You need a few pencils, an eraser, and a calculator.
- Final Exam:
scheduled for Thursday, April 23, 10:15-12:15 .
It will be comprehensive, with emphasis placed on material
covered in the last unit. You need a few pencils, an eraser, and a calculator.
We will meet in 2211 Rood Hall (usual
(With the exception of the MS-Excel spreadsheet
noted in bold immediately below, the following is optional. However, you are welcome to
use the FORTRAN coding to create your own codes in C/C++ or MS Excel spreadsheets.)
computes the emergent specific intensity from a stellar photosphere
under simple assumptions.
This MS-Excel spreadsheet
(orbital_vr-vtheta.xlsx) computes relative values of system orbit speed v,
v(radial), and v(theta) as functions of theta for a user choice in orbit
eccentricity e. A plot accompanies the tabulated results, and students can use
this as a template for producing scientifically useful plots.
MS-Excel spreadsheet (voigt_profile.xlsx)
computes an absorption line profile (Voigt function) and absorption
line curve of growth. Both sets of concepts are described in Chapter 9 of the textbook.
Note -- this material is no longer covered in any detail in this couse.
for executing the following Fortran programs; these programs might be useful on
some of the homework assignments.
Compute your own simple elliptical orbit: fortran code
Compute your own radial velocity plot vs. time for a binary system:
Compute your own blackbody flux spectrum, in cgs units/Angstrom:
Evaluate the blackbody flux at a particular wavelength, in cgs
units/Angstrom: fortran code
Compute Boltzmann relative level populations N2/N1, N3/N1, N3/N2,
well as N1/N(HI), N2/N(HI), and N3/N(HI) for H-like species: fortran
Compute ionization distributions from the Saha equation: fortran
Compute the n = 2 population of H and the n = 3 population in He+,
to the total H and Helium species: fortran code
Compute the emergent intensity (in cgs units/Angstrom) as a
depth from a toy model photosphere of a star: fortran code
Compute two very simple stellar structure models: fortran code
Compute your own zero age main sequence star - a realistic, yet
using all of the physics of stars introduced in this course: fortran
Text (ascii) files containing the 2000
Standard Solar Models: mass, density, temperature, pressure, etc as functions of
the radial coordinate within the Sun. These were recent state of the art stellar interior models for our Sun.
A tabulation of Fundamental
for executing these Fortran programs, in case you missed the link above
Other, completely optional, computer stuff of potential interest....
A few cosmic web
One of the absolute
coolest astronomical images, ever (IMHO).
My personal page on the issue of light
The Kalamazoo Astronomical Society (local amateur astronomy club)
of the Day; highly recommended - a new picture every day (since
What's the latest cosmic discovery by the super fantastic
Hubble Space Telescope?
See the Earth having a bad day.
The new sciences of astrobiology
(here is a
graduate programs, and here is a blog "Life Unbounded") and astrochemistry
(see also here; and here is a
life's building blocks in the cosmos -- and maybe life itself, one day.
from the Kalamazoo
Gazette March 12, 1998, illustrating a common misunderstanding
of how science
works (and a poor understanding of cosmology, which they criticize).
Here is my
response to that editorial,Viewpoint March 25, 1998.
An example of how ignorance and lack of critical thinking skills
can kill...you have to read it
to believe it.
to discussions of science, pseudoscience, and issues regarding science
You may have wondered about...the
meaning of color in astronomical images; and links to discussions
of light and color.
description of the physics of the La
points of orbits, including a more detailed derivation - here.
This is discussed in Chapter 18 of your textbook, and while we will not
cover this specific material you might find it an interesting aside
concerning binary star systems (which we will cover).
Your Chance to view through
Free Public Telescopic Observing Sessions of the night sky
the Kalamazoo Nature Center
beginning after twilight, if skies are clear; here is a map),
by the Kalamazoo Astronomical
Society (KAS),whose meetings (indoors,
with cool presentations) are open to the public. Yours truely is a
member and has given many presentations. Here
the scheduled list of telescopic observing session events (check the
calendar year) and other
For all observing sessions: dress appropriately; events are outdoors,
there are no restroom facilities. The KAS will supply the telescopes;
you need is a pair of eyes. However, if you have binoculars or your own
feel free to bring it out. KAS members will also be happy to help you
your telescope. (Note: on occasion, the KNC staffs the gate and
collects a fee per auto. If they aren't there to collect, then the
event is free as the KAS intends.)
Image Caption: The great
Nebula in Orion. UV light of newly formed massive stars heats the skin
of a giant molecular cloud to ~8000K - emission lines of hydrogen and
other elements color the hot skin. You are peering into a cavern at the
edge of the great cloud (some 1500 light years distant), carved out by
strong UV radiation and gaseous winds of the newly formed stars
(heavily over-exposed near the center of the image). Still-forming
stars populate the cavern within gaseous "cocoons", while still others
remained buried within the molecular cloud. The "smokey" looking gas is
to visible light absorption and scattering by embedded dust grains. (credit: European
Southern Observatory and Igor Chekalin)
Here below, I've collected some of the latest images from our
advanced telescopes (ground and space-based), plus a few animations and
some really cool computer simulations to illustrate cosmic phenomena.
These are for your casual use and perusal to help your appreciation of
the cosmos - nothing below is required knowledge for this course.
You'll note that I also include stuff about galaxies and cosmology.
Some day, I hope to offer a second course in astrophysics covering
those topics. Any how...
``Listening'' to the story of the cosmos
- Overhead view of Yerkes
Observatory in Williams Bay, WI, and the 40-inch refractor here
- The Cerro Tololo Interamerican Observatory (Chile) 4-meter
dome and beautiful night sky
- The Very
Large Telescope Array (Chile) and one of its 8.2-meter
- A cartoon of the twin 10-meter (segmented mirror) Keck
I & II Observatories, atop Mauna Kea, Hawaii, and a photograph
of these two observatories plus the 8-meter Subaru - note the ocean of
- The new
MMT telescope (6.5-meter) in southern Arizona
- The Very Large Array Radio
Observatory in New Mexico
- Three photos of the Hubble Space Telescope: in
the Shuttle bay for repairs, in
action, and why astronauts cannot suffer from vertigo,
the HST within the Shuttle bay for repairs, 600 km above the western
- A movie of speckle
images of the bright star Betelgeuse, 30 msec per frame,
- A movie from the Keck Telescope showing the
Galactic Center before and after the adaptive optics is turned on,
the effects of atmospheric "twinkling". Here
interferometery techniques allow astronomers to measure accurate
of hundreds of stars. Here observations of some of the smallest known
is compared to theoretical predictions of radius vs. mass for stellar
400 million years (red dashed) and 5 billion years (black solid).
- perhaps akin to a failed star (brown dwarf ) - is shown for
(solid triangle). The brown dwarf stars occupy the region of the
the curves are roughly horizontal. Here
is the press release with some futher details. Speaking of brown dwarf
is an illustration comparing our Sun, an M dwarf star 75x mass of
Jupiter, L and T class brown dwarf stars 65x and 35x the mass of
Jupiter, and Jupiter itself. The same sequence is shown
here, as they would appear (roughly) to eyes sensitive to IR
Our star: the Sun on the outside
Star birth, life, & death
- as it appears at visual wavelengths: the photosphere,
- an extreme
close-up of the Sun's photosphere: sunspot
- a gif movie of a
- sunspots in motion,
showing the Sun's rotation; here
is another movie
is a plot showing the sunspot cycle: monthly average number vs.
time from 1750 to the present.
- the Sun's upper
atmosphere observable during eclipse
- observing the chromosphere in the light of hydrogen-alpha
hops from the third to the second energy level of hydrogen), 656.3 nm here
spicules, in the light of hydrogen-alpha
- the chromosphere in the light
of ionized calcium, 393.4 nm
gif movie showing a series of narrow band filter images from the Dutch Open
Telescope that show how
a region near a sunspot changes with height, from the photosphere to
over 1000 km above into the chromosphere. Explanation: The light
of different absorption lines in the solar spectrum escapes at
different heights in the solar atmosphere. This property enables us to
observe solar phenomena simultaneously in multiple layers. The three
images that constitute this morph were taken in the blue G band
(wavelength 430.5 nm, due to CH molecules), the violet Ca II H line
(396.8 nm, once-ionized calcium atoms), and the red Balmer Halpha line
(656.3 nm, neutral hydrogen atoms). They show three slices of solar
active region AR10675 on September 29, 2004. The G band shows the
photospheric solar surface covered by convective granules and tiny
bright magnetic elements between these, as well as a sunspot about as
large as the Earth (inset). The strong Ca II H line samples the low
chromosphere (a few hundred kilometers higher up) in which the
granulation appears reversed intensity and magnetic elements appear
considerably brighter. The Halpha line shows fibrils in the high
chromosphere (at a few thousand km in height) that are obviously
controlled by magnetic fields. Many field lines originate in the spot
and the magnetic elements and connect elsewhere.
- the transition zone, in the light of ionized
helium, 30.4 nm
- the outer corona,
in visible light (during eclipse)
- the corona in the light
of three emission lines: 8x,9x ionized iron (17.1 nm), 11x ionized iron
nm), plus 14x ionized iron (28.4 nm)
loops in extreme UV light, also here
- the corona in continuous
- an mpeg movie showing images
of our Sun across the electromagnetic spectrum, from the
into the corona. These images were taken at approximately the same
the brightest active regions in the Sun's hot upper atmosphere
to the darker/cooler sunspots that lie beneath in its photosphere.
- a cartoon showing the major outer
- large solar prominence in the light of ionized helium, 30.4 nm here
solar flare event in the light of ionized helium, 30.4 nm
- an mpeg movie
of our Sun's active corona (November 2000); here
October-November 2004 in gif format in the light of 17.1 nm 8-9x
ionized iron; and this
is another (very large mpeg movie) in the light of 11x ionized iron
(19.5 nm) during October-November 2003 in which historically enormous
coronal mass ejection and X-ray flare events took place.
magnetic carpet: what powers the Sun's chromosphere and corona
- the changing Sun: X-ray
images of the Sun's corona over the 11 year magnetic activity cycle
- a cartoon of the solar
wind interacting with Earth's magnetic field
- some pictures of aurorae: 1,
and here in SW
Michigan (October 28, 2000)
- a photo of an aurora
taken by astronauts on-board the International Space Station; note the
blue veil that is the Earth's lower atmosphere (troposphere and
and that aurorae occur Earth's tenuous upper atmosphere. The white
feature on Earth's surface is a snow-filled 212 million year old impact
crater in northern Canada.
Our star: the Sun on the inside
The spectra of stars
to pages on aurorae (and predictions thereof) and other Sun-Earth
to pages containing information, high resolution pictures and
movies of our Sun
InterStellar Medium (ISM) and the birth of
- A temperature sequence of stellar spectra
- The spectrum of Arcturus, a K1
- Our Sun's spectrum, a G2
sequence star, and an exploded
representation of our Sun's spectrum. The vertical axis measures
the observed specific flux
above Earth's atmosphere,
the horizontal scale measures wavelength in nanometers (nm). The
ultraviolet (UV), visible and infrared (IR) portions of the Sun's
spectrum are marked. Note the many
(narrow downward 'spikes') formed in its photosphere and lower
where atoms and ions absorb light at particular wavelengths (photons of
energies). Note also the
overall shape of
the Sun's spectrum is a good match
the spectrum of a simple thermal (blackbody) radiator of temperature T
= 5777 K
green). The equivalent in total energy of this thermal (blackbody)
spectrum is emitted within
- The spectrum of Procyon, a F5
- Our Galaxy, the Milky
dark cloud toward the center of our Galaxy
- Another dark,
cold (10 K), molecular cloud, some 520 light years away and about
light year across, is partially transparent at infrared wavelengths.
This is demonstrated in a dramatic way in
this image, which shows the same image through each of the 6
broad-band filters separately - from 440 nm (blue visible light), to
550 nm (green visible light), and then 900 nm, 1250 nm, 1650 nm and
2160 nm in the infrared (the filter central wavelengths are given in
units of microns = 1000 nm in the figure). This
is a map measuring the extinction of light through the cloud. The
first and outermost contour represents a loss of visible light at 550
nm by a factor of 40. Each contour inside is a step up in visible light
extinction by a factor of 6.2. The innermost contour represents a
factor of 1014 loss of visible light!
- a tiny grain
of cosmic dust, just 10 microns across (size of a human white blood
- lots of molecules
are found in the cosmos, and many of these are the organic building
- The constellation
- The Orion Nebula, nearest stellar nursery, at visual
wavelengths, and also here.
The Orion star forming complex is just ~1 million years old, and 1500
light years away.
- Hubble Space Telescope visible and near-IR image of the Orion
Nebula. This view is 13 light years across.
- The central Trapezium region of the Orion Nebula at infrared
wavelengths, and a broader view at infrared
wavelengths; here's another
infrared image of the Orion starforming complex.
- The Orion Nebula's Trapezium, visual
vs. infrared, as observed by the Hubble Space Telescope. Note the
numbers of stars that come into view at infrared wavelengths - infrared
is not as easily blocked by dusty gas as is visible light. The dimmest
of light in the infrared image are "Brown Dwarfs", not massive enough
- Two image galleries (1,
of solar systems forming around new stars in the Orion Nebula, and a cartoon.
The dark, dusty disks might be forming planets.
- The Horsehead
dark nebula in Orion; the bright star on the left is the leftmost
belt - seen here in a spectacular style. The three bright stars of
the "belt of Orion" are Alnitak, Alnilam, Mintaka.
- Short wavelength (blue) light emitted by stars scatters easily
dust grains within surrounding gas clouds - these dusty clouds reflect
especially blue light, and are known as reflection
We may get as far as the bizarre
deaths of stars in this course - but feel free to sample more of our
- The Eagle
Nebula, as imaged at visible wavelengths by the Hubble Space
and a close-up of one of the "pillars"
showing the photo-evaporating gaseous globules, and a broad
view of the surrounding region, including the main star cluster, at
wavelengths - note the near transparency of the Eagle Nebula at these
Here is an unbelievably
beautiful visible light, 20 light year wide, view of the molecular
complex surrounding the Eagle Nebula. Narrow band green, blue and red
were used to sort out emission from hydrogen atoms, twice-ionized
and singly-ionized sulphur atoms, respectively. This star forming
is 6500 light years away.
- Here is the Cone
Nebula star forming complex, 2500 light years away. Note the newly
stars emerging from the cone-shaped molecular gas near the bottom of
diagram. A close-up from the Hubble Space Telescope is here;
the "cone" is about 1 light year across.
- Just 2 light years across and perhaps just a few hundred thousand
years old, a
brand new stellar nursery lying in the constellation Cygnus, as
in near-infrared light
- 50 light years across and extremely young, a newly
cluster of very
young, massive stars in the nearby dwarf galaxy, the Large
Cloud. And here
is NGC 346, 200 light years across and 210,000 light years distant
in the Small Magellanic Cloud, 3-5 million years old..
- A gargantuan
star formation region & star cluster (some 1500 light years
in a nearby spiral galaxy called M33. An even better view is here.
- Perhaps the largest star forming complex in the local universe,
in the nearby dwarf galaxy the Large Magellanic Cloud 160,000 light
years away, the
Tarantula Nebula Star forming complex. Shown is just the
central region; images of the whole complex lie in the "Star Clusters"
section, below. If it were at the distance of the Orion Nebula (1500
light years), it
would be visible in the day time and span about 1/4 of the sky!
Trifid Nebula, an emission (red part) and reflection (blue part)
- The Rosette
Nebula; here's another view
- The Keyhole
Nebula in Carina
- A young stellar cluster in a star forming region (NGC 3603): VLT
- Young stars with dusty gas disks around them: 1,2,3,4,5.
When the disks are observed from the side, they are dark when viewed at
visible wavelengths and bright when viewed
infrared wavelengths. Why might that be?
- Near the end of a medium mass star's life, unstable hydrogen and
helium fusion occur within two concentric shells surrounding a
shrinking inert core. The result is that the star gradually
ejects its outer envelope in multiple ejection episodes (note the
concentric expanding spherical
shells), leaving behind a dense remnant known as a white dwarf (the
point of light at the center).
- Hubble Space Telescope image of newly
emerging white dwarf star and planetary nebula
- 20 years before this
image was taken by the Hubble Space Telescope in 1996, no planetary
was present in Henize 1357 (now dubbed the Sting Ray Nebula). 18,000
the nebula spanned 130 solar system diameters in 1996 and continues to
- Along with collaborators, George Jacoby and Gary Ferland, I did research on
planetary nebula, lying about 7000 light years away. A hot (150,000 K)
dwarf lies at the center of this star's former envelope - now an
spherical shell 5.5 light years across (a corresponding angular
160 arc seconds).....a death shroud. This is our Sun's fate, 7 billion
hence. Spiral galaxies at enormous distances may be seen lying in the
of this image.
- Hubble Space Telescope image of planetary nebulae: the Ring
Nebula and NGC
deep exposure image
of the Ring Nebula, taken by a ground-based telescope, shows the
multiple ejection episodes that the central star underwent to become
the white dwarf (at the center of the ring). Note the distant
in the background.
- Here are some more pictures of the death shrouds of
stars: the Dumbbell Nebula from the VLT
Nebula, and the hour-glass shaped Ant
Nebula. The cores of the dead stars (aka white dwarfs) lie at or
- Hubble Space Telescope image of the red
- Hubble Space Telescope image of massive
star blowing a hot, ionized wind
- Hubble Space Telescope image of the dying supermassive star, Eta
and its surroundings at visible
wavelengths. An explosion in the 1846 produced the twin lobes of
gas, and temporarily made it one of the brightest stars in the sky.
- Here is a computer
simulation of an exploding star or "supernova"
- VLT image of the supernova remnant, the Crab
Nebula in the constellation of Taurus. The expanding glowing gas is
former envelope of a star that exploded in the year 1054 (as observed
Earth), and lies 6300 light years away. The star's remnant core is the
of the pair of stars near the center of the nebula, and lies at the
of this extreme close-up shown in this
composite image from the Hubble Space Telescope (visible light:
and Chandra X-ray Observatory (X-rays: blue). It is a rapidly
(30 times per second!) neutron star, called a pulsar. Here
spectacular image from the Hubble Space Telescope, and a composite
image showing X-ray (Chandra X-ray Observatory: blue), visible
Space Telescope: green), and infrared emission (Spitzer Space
star, called Cassiopeia A, exploded in 1680 (as observed on Earth);
is shown here in three X-ray "color" (energy) bands, and it lies
light years away. A several million degree neutron
star lies at the
of the explosion. A second observation, with an alternative color
scheme, is shown
- Pages containing images of Supernova
the most recent nearby example of an exploding star.
- the Veil
Nebula: interstellar gas shocked (compressed/heated) by the blast
of a star that exploded 5000 years ago, 1440 light years away in the
- Also in Cygnus, but 6000 light years away, seen in 25 and 60
(mid-infrared) and 21 and 74 cm (radio) - the
cosmic dance of star birth and death and the recycling of the
medium. The larger hollow shells of gas are supernovae ejecta
exploded star envelopes), including the brownish one in the lower left.
bright white knots are stellar cocoons of star formation. The red dots
some immensely distant, yet enormously luminous, quasars (we're not
these objects in this course).
- A drawing of an accretion disk surrounding a black
hole, and another one here.
Star Clusters: an astronomer's laboratory
- WWW links to sites
in relativity, neutron stars, black holes, gravitational lenses, etc
Some day, we may have a second
course in Astrophysics that covers
and cosmology - but feel free to continue touring the universe...I'll
continue to dream....
- The ages
of several open (or galactic) star clusters
in the Milky Way galaxy determined by two independent means: main
sequence "turn-off" and the time it takes white dwarf stars to cool to
a particular temperature. The straight line indicates an equality
between the methods. It is important to note that the types of stars
used in and the physics involved in the two methods to determine the
star cluster age are completely different.
- At the center of the enormous Tarantula
Nebula star forming complex, 30
Doradus: a dense cluster of newly born massive stars (just 2-3
years old) in the Large Magellanic Cloud (a dwarf irregular galaxy
the Milky Way). Another spectacular
view...This is one of the largest (more than 1000 light years
and most active stellar nurseries known in the local universe.
- Lying near to 30 Doradus in the Large Magellanic Cloud, Hodge
301: a young, 20 million year old, open star cluster
- Visible to the naked-eye (just 430 light years away), a 100
year old open star cluster: the Pleiades
contains well over a thousand stars, and another
view (courtesy of Matthew
T. Russell). The "spikes" of light surrounding the bright stars are
due to the diffraction of light at the secondary mirror supports, and
their appearance has been enhanced in these two images for artistic
- Here is another open star cluster
and 250 million years old: M11
- Ancient globular clusters M80,
and giant 47
Tucanae, with 100,000 to a few million stars! Typically, globular
lie 15-50,000 light years away in the "halo" of the Milky Way. Here
M3, 34,000 light years from Earth and 180 light years across, it
a half million stars.
- The core of the most massive globular cluster belonging to our
Omega Centauri. This
view spans just 13 light years and contains over 50,000 stars. The full
cluster contains 10 million stars, is 150 light years across, and lies
light years away. It may actually be a captured dwarf elliptical galaxy.
- The "nearby" Andromeda galaxy has open
star clusters, too. Why do you think the globular clusters of the
galaxy have angular diameters that are on average 100 times smaller
the globular clusters belonging to our Galaxy?
The Milky Way
Galaxies and Cosmosolgy
Other Galaxies in the "local" universe
- The Milky Way as it appears from the northern
1288: a good match to the shape and size of the Milky Way
- spiral galaxy NGC
4565, observed edge-on
- very dusty, edge-on spiral NGC
all-sky image of the Milky Way. See any similarities to the above
- An artistic
yet scientific rendition of our Galaxy, also showing the relative
location of the Sun (illustration courtesy: NASA/JPL-Caltech/R. Hurt
- An infrared view toward the Galactic
Center, and 1
million stars in the direction of the Galactic Center
- A view of the nucleus
of our Galaxy, at 90 cm radio wavelengths
- Two young
star clusters with VERY massive stars near the Galactic center, at
wavelengths. The Pistol Star (brightest orange one in right
is at least 100 solar masses with a luminosity of 10 million Suns! It
have lost nearly half its mass through an enormous wind.
- Comparisons of a piece
of our Sun's spectrum (at very high spectral resolution) with those
two ancient stars of the Milky Way with extremely low heavy element
- a gif animation demonstrating the observational properties of Cepheid
variable stars; here is a movie
of RR-Lyrae variable stars changing their intrinsic brightnesses as
outer layers pulsate, all lying within the globular cluster known as
All RR-Lyrae stars have the same average luminosity of about 45x the
of the Sun, while the more luminous Cepheid variable stars follow a
Period - Avg. Luminosity
relationship. Both are used to determine distance within the Milky Way
- A graph illustrating the
effects of a "dark matter halo" within which the Milky Way galaxy
and most galaxies are embedded.
Interacting or Colliding Galaxies:
- more than 2 million light years distant the nearest spiral
(M31) and the smaller M
The central bulge of M31 is just visible to the unaided eye in good
conditions, and the whole of the galaxy's visible disk spans
190 arcminutes (11,400 arcseconds, about 3 degrees or 6
widths of the Moon) in the sky. This
is what the Andromeda galaxy looks like in the light of dust grains
with gas clouds heated by the absorption of visible and UV light of
stars - that is,
thermal radiation at a wavelength of 24 microns (24,000 nm). And
finally, the same galaxy looks
in ultraviolet wavelengths. This light is dominated by hot,
luminous, massive main sequence stars in regions of active star
- a nearly "perfect," grand-design spiral, M74,
at a distance of 30 million light years.
- some more beautiful spiral galaxies: M 51 or the "whirlpool
- the large red areas are star-forming regions with hot main sequence
emitting lots of energetic UV light that excites/ionizes nearby gas to
emission lines - in this case the red line of the hydrogen Balmer
series, and here
is a side-by-side comparison of M51 at visible vs.
infrared wavelengths: "blue" = starlight, "red"=mainly hot dust within
gas clouds), NGC
2997 from the VLT, M100,
ground and from the
Telescope 25 million light years distant, NGC
- a massive spiral galaxy 30 million
away - from the VLT and another
view from the Hubble Space Telescope and yet another
in infrared light from the Spitzer Space Telescope (blue = stars,
red = star-heated warm dust inside gas clouds), a barred spiral M83,
from the VLT, and distinctly "barred" spiral galaxies NGC
7424 and NGC
1300. The last of these lies 70 million light years away.
- A member of the Local Group, dwarf
elliptical Leo 1
- The giant elliptical galaxy: M87
and accompanying globular clusters in the Virgo Cluster.
- The Milky Way's two neighbors, dwarf irregulars: Large
Magellanic Cloud at 169,000 ly distant and Small
Magellanic Cloud at 200,000 ly distant
- another dwarf irregular NGC
6822 in the Local Group, 1.5 million ly distant
- another irregular in the nearby M81 Group: M82;
large numbers of newly formed stars in the galaxy's center blow a
(shown in red, as glowing hydrogen gas in the previous image),
a lot of dust
- two members of a nearby small group of galaxies: the
M81 group, 12 million light years away (M82 is the one on the right)
- About 50 million light years away, the most nearby cluster of
central regions of the Virgo
cluster, and the view of a much more distant cluster called Coma,
320 million light years away.
Distant Galaxies and Cosmology
- 2 colliding
spiral galaxies, 65 million light years away, also as observed by the Hubble
Space Telescope, and also here
at infrared wavelengths which penetrate the dusty star forming regions.
Note the brand new super star clusters,
formed within colliding gas clouds. Here is a an MPEG movie
of a computer simulation of their collision (credit: John Dubinski,
Note that this is NOT a cartoon. It is an actual simulation of what
when two spiral galaxies collide; all of the masses follow the laws of
- more galaxian collisions: the Tadpole
(420 million light years away; tail of stars is 280,000 ly long) and Mice
galaxies (300 million light years away).There are over 4000 background
in the Tadpole image that spans an area in the sky equal to the angular
spanned by a dime at a distance of about 53 feet. The two colliding
in the `Mice' pair were nearly identical spiral galaxies that began
dance roughly 160 million years ago. Here
is a computer simulation spanning several hundred million years showing
it all happened and the fate of these two clashing titans (courtesy Josh
& John Hibbard). If you've a high speed internet connection, you
might give this
monster (23 Mbyte) simulation of the same collision a try. Note the
many stellar arcs that form around the central, mostly
spherical, star concentration. This is reminiscent of the bizarre giant
elliptical galaxy, Centaurus A
just 13 million light years away; see the star rings here.
- the result of two colliding galaxies: the
polar ring galaxy
- two more galactic wrecks, 500 and 600 million light years away:
galaxy (and a
multiwavelength view: purple X-rays, blue ultraviolet, green
visible and red mid-infrared) and Hoag's
- 2 MPEG movies
representing computer simulations of a possible collision between the
Way and Andromeda Galaxies, 3 billion years hence (credit: John
CITA). A higher resolution (10.7Mbyte) version is here.
- And if you just can't get enough of watching 2 spiral gladiators
themselves (and become an elliptical galaxy), here
is another MPEG movie, courtesy of the Max Planck Institute for
- These simulations
are set to music!
- A spiral galaxy NGC
3370 in the constellation of Leo, lying 98 million light years
Its angular diameter is approximately 200 arc seconds, whereas the
Way's neighboring spiral galaxy M31 (linked
above) spans 11,400 arc
at a distance of 2.5 million light years. Note also the many galaxies
much smaller angular diameters - more distant still. NGC
1309 is another beautiful spiral galaxy, 100 million light years
away, with very distant galaxies visible in the background.
- giant elliptical NGC
4881 and a spiral galaxy in the Coma Cluster, 320 million light
- Two dense clusters of galaxies Abell
2218 and Abell
1689. Both are about 2 billion lyrs distant and demonstrate the
of space due to the masses of these galaxy clusters - gravitational
lensing, as predicted by Einstein's theory of General Relativity. Here
is another demonstration of this strange phenomenon.
- The distant dense galaxy cluster, Cl0024+1654
(4 billion light years away) is shown in red. Astronomers used the
images of roughly 7000 background (more distant) galaxies to construct
map of the dark matter distribution, shaded in
The dark matter in this cluster represents 80-85% of the total matter!
distortions are caused by the warping of spacetime (gravitational
due to the presence of the huge amount of mass in this galaxy cluster.
shows that the dark matter is (1) highly concentrated toward the center
this cluster, and (2) that it follows closely the visible mass
of the galaxies themselves. The area colored light blue near the
center is the hot X-ray emitting gas as observed by the Chandra X-ray
Observatory (the nucleus of this enormous galaxy cluster in visible
light is shown here;
note the blue arcs of light: gravitationally distorted images of more
galaxies). The full image represents an area of the sky equal to that
the full Moon.
- A very distant (7 billion lyrs) galaxy
cluster, showing many galaxies are interacting. Galaxy interactions
this scale are rare in the local universe (here and now).
- The 2dF galaxy survey: a census of 1/4
million galaxies within two thin wedges in the sky (each dot = 1
but on the scale of this map galaxies are physically smaller than the
We lie at the intersection of the two wedges.
- The Sloan Digital Sky Survey: the
positions of roughly 200,000 galaxies within
thin wedges in the sky plotted with distance in megaparsecs (Mpc) (each
dot = 1 galaxy, as above), within 2.8 billion
years of Earth located at the intersection of the two wedges. Here
but showing the scale in units of lookback time (how long ago light we
observe now left the source) in years. The final survey will include
galaxies and their redshifts, resulting in a 3-dimensional census of
covering approximately one-quarter of the sky out to several billion
- The 2MASS infrared all-sky
survey of 1.6 million galaxies. This snapshot of the sky was taken
3 infrared filters: blue, green and red have been coded to represent
at wavelengths of 1.2, 1.6, and 2.2 microns. This image shows the same
of structure of walls and voids, as the 2dF and Sloan galaxy surveys
above, except that the galaxies appear flat against the sky (distance
has been suppressed). The "blue" band running through the middle is
light from stars within the disk of our Galaxy.
- A map
of our "local" universe, showing galaxy superclusters within 1
light years of the Milky Way Galaxy whose home supercluster is called
located at the center of this map.
Deep Field North: This image taken by the Hubble Space telescope in
of 1995 is one of the most important ever taken of the cosmos. It looks
through 1-12 billion years in cosmic history - more than 2000 galaxies
within this "blank"
area of the sky equal to the angular area spanned by a dime at a
of about 74 feet! A similar image acquired from the southern hemisphere
in 1998 appears (in part) here.
- Look at this
image taken by a ground-based telescope of our nearest large
the spiral Andromeda galaxy (aka M 31; note also the two dwarf
2.5 million light years away. Note the tiny green box with a
of stars visible inside of it. It spans an area of the sky equivalent
that spanned by a dime at a distance of about 53 feet. Here
is what the ACS camera on-board the Hubble Space Telescope recorded
collecting light over an 84 hour period within that tiny green box.
are hundreds of thousands of stars (most of which lie in the halo of M
a few of the brightest stars belong to our Milky Way Galaxy). There are
also thousands of very distant galaxies, many of them
just like M31 and the Milky Way. Snapshot
closeups of 6 representative areas of the image are found here. A
globular cluster belonging to M 31 appears in the lower right panel.
is an mpeg pan across the image.
- The Hubble
Ultra Deep Field: From September 2003 to January 2004, the Hubble
Telescope collected light through several broad-band filters across the
and near infrared (centered on 4350, 6060, 7750, 8500 Angstroms, ACS
into the infrared (1.1 and 1.6 microns, NICMOS camera). Here
is the image from its ACS camera - with 10,000 galaxies in a view
back between 1 billion and 13+ billion years of time. This view spans
in the sky equivalent to that spanned by a dime at 53 feet. Higher
versions of the image and further explanations may be found at links
This mpeg movie shows a zoom-in
to the empty spot in the sky in the constellation of Fornax, and
mpeg movie pans
across the image. Ned Wright's web site allows you to flick the ACS and
images back and forth.
- Images of representative,
high redshift (z = 3, lookback time 12 billion years) galaxies, and
close-up of another one, observed at infrared wavelengths. Here are
separate galaxy building blocks, each a super star cluster spanning
lyrs across, and lying at the same high redshift (z = 2.4, looking back
billion years in time) within 2 million lyrs of each other within the Hubble
Deep Field (North) image. Most of these likely later merged to form
single galaxy, just as we see these protogalaxies
doing here. How do these galaxies compare to the ones found in the
universe (two sections above)? This
series of images shows direct comparisons between the way
galaxies appear at different stages in the history of the universe
and now on the left; high redshift long ago on the right).
- The "spiderweb
at a redshift of 2, as it was assembling itself some 10.6 billion years
ago. The width of the expanded view is approximately 300,000 light
as they were in a universe approximately 900 million years old (z ~ 6),
from the Hubble Ultra Deep Field; each box spans about 30,000 light
years on a side.
- a gravitationally
lensed protogalaxy, as it appeared 13 billion years ago:
500 lyrs across, containing just a few million solar masses of stars.
- A sketch showing two
possible histories of star and galaxy formation in the universe
that where it says "Milky Way galaxy forms" refers to when the present
began organizing - the spheroidal component is older still), and an
rendition of the
birth of stars and protogalaxies beginning perhaps 200 million
after the Big Bang (corresponding to a redshift z = 20). Astronomers
are just now observing faint,
shaped smudges of light at redshifts of 5-7. Future technology will
more detailed studies of the first
gargantuan star clusters that served as the building blocks for
protogalaxies and galaxies. Maybe they'll look like this
nearby starbursting irregular galaxy, 13 million lyrs away. This dwarf
blue compact galaxy is just 59 million light years away, yet a
large fraction of its
stars are less than 1 billion years old (it also contains an old
population of stars some 10 billion years in age).
- The evolution of large scale structure in the universe as
governed by the force of gravity - all of
simulations presented below start at very large redshifts (z = 20-50)
fluctuations in the matter density. All are displayed in co-moving
coordinates, i.e., those that expand with the universe and so this
expansion is not displayed in the simulations.
- An MPEG movie showing the simulation of the gravitational
clumping of matter (dominated by dark matter), within a co-moving
volume of space spanning 140 million light years on a side. Note the
filamentary structures that develope forming the walls of bubbles. It
is along the interesections of the dark matter filaments that large
galaxy clusters form.
- An MPEG movie of a simulation of the formation
of a galaxy cluster within a co-moving volume of space spanning 14
million light years on a side. Note the merging of small structures to
form larger structures.
- An MPEG movie of a simulation of the formation of a very large
galaxy supercluster (the box spans about 100 million lyrs and is
million lys deep). Here
is a series of stills on the same size scale depicting the
of the formation of this galaxy supercluster (left panels), for
1, and 0 (top to bottom). Red represents high density, blue very low
with orange, yellow, and green in between. The right panel shows the
for a different set of assumptions about the matter/energy content in
- Two more MPEG movies simulating the formation of galaxy
notice especially in the second how small structures collapse/form, and
with other small structures to form larger structures, and so on.
- here is an imaginary
fly-through of the dark matter "foam" at the present epoch, around
galaxies and galaxy clusters and superclusters formed and are now
- The development of large scale structure depends upon the
actual mixture of matter and energy in the universe, as these
comparisons at redshifts of 3,
(now) show. Four simulations of structure at each of the redshifts
shows the density of matter (brighter = denser) for a slice of space
with dimensions of 1 billion light years on a side. This serves as a
consistency check on our observations that measure the matter/energy
content as well as on ourmodels that predict how structure developes as
a function of redshift. The model appearing in the lower left
represents today's best case.
- the expanding
raisin bread analogy to the expanding universe, and here is an "expanding
neighborhood" analogy - everybody moves away from everybody else.
is a more detailed animation demonstrating the concept of how we
observe galaxies in an expanding universe. The following describes the
problem of defining a distance in an expanding universe. Two galaxies
are 4 billion light years apart when the universe is only 1 billion
old (at the start of the animation). The first galaxy (on the left)
emits a pulse of light. For example, this might be the first huge burst
of star formation. Because space is expanding, the second galaxy (on
the right) does not
receive that pulse until the universe is 14 billion years old (the
By this time the two galaxies are separated by about 28 billion light
pulse of light has been travelling for 13 billion years, so the view
the astronomers residing in the second galaxy receive is an image of
galaxy going through its initial big burst of star formation when the
universe was just 1 billion years old and the two galaxies were
separated by 4 billion light years. Astronomers residing presently in
the galaxy on the right would observe the galaxy on the left to have a
redshift of about z = 5.9 (courtesy: Richard Powell).
- The predicted products
of nucleosynthesis during the first few 100 seconds or so after the
expansion of the universe. The relative abundances of these light
depend on the density of ordinary matter (protons, neutrons,
at that time and thus on the value of the mean matter density today
horizontal axis). These two plots (1,
show how protons and neutrons were converted into the
elements as a function of time during which the universe expands and
temperature drops. This
one shows the same as the first with horizontal bars indicating
of these light elements. The vertical grey bar shows a common solution
for the observed abundances of all of the light elements, indicating
expected relative mean matter density in the universe.
- The cosmic
background radiation (CBR) blackbody spectrum, from COBE (1989-1990)
- WMAP's all-sky
view of the 1 part in 10,000 and smaller fluctuations
in temperature of the CBR, representing very small inhomogenieties
the matter around which structure will form from gravitational
- as shown here.
back over the early history of the universe
- Simplified 2-D
representations of possible 3-D geometries of the universe,
upon the matter-energy density, and predicted
vs. observed fluctuations in the CBR depending upon the geometry of
- A "standard candle": Type
Ia supernova - 10 billion solar luminosities at maximum light -
off just 50 million lyrs away in the central regions of a spiral
the Virgo Cluster (most of the galaxy is not shown here).
- WWW links to sites
in cosmology and structure evolution.
Kirk T. Korista
Professor of Astronomy
updated last: 7 January 2020
Department of Physics
Western Michigan University
Kalamazoo, MI 49008-5252