Unit 3
Chapter 15
The Sun
Properties
 Radius: 700,000 km
 Mass: 2.0 × 1030 kg
 Density: 1400 kg/m3 or 1.4 xH2O
 Rotation: Differential
 Period: about a month
 Surface temperature: 5800 K
 The apparent surface of the Sun is the Photosphere
Luminosity
 total energy radiated by the Sun – can be calculated from the fraction of that energy that
reaches Earth.
 This diagram illustrates how one can extrapolate from the radiation hitting Earth to the entire
output of the Sun.
Mathematical models,
 consistent with observation and physical principles, provide information about the Sun’s
interior.
 In equilibrium, inward gravitational force must be balanced by outward pressure
 Although the average density of the Sun is only slightly more dense than water the density at
the core is extremely high. The temperature is relatively low at the surface but is near 15 million
K in the core.
The Solar interior
 has been understood theoretically for years.
 In recent years we have been able to investigate it by observation.
 Doppler shifts of solar spectral lines indicate a complex pattern of vibrations that can be used
much like seismic waves on the Earth to determine the interior construction of the Sun.
Energy transport
 in gases is limited to convection and radiation.
 In the radiation zone the particles are so tightly packed that the energy can only pass by
radiation. The radiation zone is relatively transparent; the cooler convection zone is opaque.
The Heart of the Sun
 Before we can discuss how the heat of the Sun is generated we have to consider the following:
Four Forces of Nature
Force
Strength
Strong Nuclear (pull)
1040
Electro-Magnetic
1028
Weak Nuclear (push)
1026
Gravitational
1

Einstein’s famous equation for the equivalence of matter and energy
Proton –Proton Chain
 This is the first step in the three-step fusion process that powers most stars.
 The ultimate result of the process is


The helium stays in the core;
The energy is in the form of gamma rays, which gradually share their energy with the body of
the Sun as they travel out from the core, emerging in all of the wavelengths of the
electromagnetic spectrum;
 The neutrinos escape without interacting
The Solar Atmosphere
 The cooler chromosphere, is the pink layer above the photosphere. It is hard to see directly as
the sun is too bright, unless Moon completely covers the photosphere.
 The much less dense corona can also be seen during a total eclipse. Temperatures here range
from 1 to 4 million K.
 Solar corona changes along with sunspot cycle; it is much larger and more irregular at sunspot
peak
Features of the the Sun’s Surface
 The visible top layer of the Sun, the Photo-sphere, is the top convection zone and is granulated.
The areas of upwelling hot gas are bright, surrounded by areas of sinking cooler gas.
 Sunspots are the most fascinating of all the solar features. Galileo saw them in 1610 and they
have been studied actively ever since. They appear dark because they are slightly cooler than
the surroundings. The dark central part is the umbra and lighter surrounding part is the
penumbra
 The Sun reverses magnetic polarity every 11 years. Sun spots cycle through a maximum every
polarity cycle . It takes two 11 year half-cycles to make one full 22 year cycle
 Schematic Formation of Sunspots : Notice the interaction between magnetic field lines and the
differential rotation. Magnetic lines trapped in the plasma stretch and break to form Sun Spots
They are like the ends of bar magnets with lines of force looping between them.
 At the beginning of a half cycle most of the Sun spots are near 30 degrees north and south. By
the end most are nearer the equator
 The number of Sunspots go through a maximum each half cycle. Some half cycles show more
activity than others.
 Over the years efforts have been made, with few successes, to correlate sun spot activity with
events on the Earth. The coincidence of the Maunder minimum with the mini-ice-age in Europe
is often designated as a cause-and-effect.
 A Solar flare is a large explosion on Sun’s surface, driving matter into space in seconds or
minutes. This matter adds to the Solar Wind.
 Hot matter escapes Sun often through coronal holes in Coronal Mass Ejections, which can be
seen in X-ray images as bright white spots
Neutrinos
 are emitted directly from the core of the Sun, and escape, interacting with virtually nothing.
 Being able to observe these neutrinos would give us a direct picture of what is happening in the
core. Unfortunately, they are no more likely to interact with Earth-based detectors than they are


with the Sun; the only way to spot them is to have a huge detector volume and to be able to
observe single interaction events.
This is a large solar neutrino detector. Interactions (dim flashes) take place in the liquid, which
reaches the top of the dome when full, Detection is by the glass photomultiplier tubes.
Detection of solar neutrinos has been going on for more than 30 years now; despite very
different detection methods and energy sensitivities, all experiments agree that they are seeing
about 30–50% of the expected number of neutrinos. Could be:
•
Problem with solar model
•
Problem with our understanding and detection of neutrinos
• The second option seems more likely today.
Chapter 16
The Nature of Stars
Stellar Distance Scales
• Light Year = the distance that light travels in one year
• Parsec = the distance to a point where 1 AU subtends one second of arc
• Remember that nearby stellar distances can be measured using parallax
Nearest star to the Sun is
• Proxima Centauri which is a member of a 3-star system: Alpha Centauri complex
• Model of distances:
o Sun is a marble, Earth is a grain of sand orbiting 1 m away
o Nearest star is another marble 270 km away
o Solar system extends about 50 m from Sun; rest of distance to nearest star is basically
empty
Brightest stars
• were known to, and named by, the ancients (Procyon)
• In 1604, stars within a constellation were ranked in order of brightness, and labeled with
Greek letters (Alpha Centauri)
• In the early 18th century, stars were numbered from west to east in a constellation (61
Cygni)
• As more and more stars were discovered, different naming schemes were developed (G5115, Lacaille 8760, S 2398)
• Now, new objects are simply labeled by their celestial coordinates
Brightness Scales
 Apparent magnitude
o Hippachus
o 1st to 6th
o Spica (1st Mag), Vega (0 Mag)
 Brightness (Luminocity)
o measured by light meter
Absolute Magnitude
 An Apparent Magnitude difference of 5 represents a Brightness ratio of 100/1
 The Absolute Magnitude (M) of stars is defined as the apparent magnitude that the star
would have if were at 10 parsecs distance. Then the following ratio holds:
M
m
 2
2
10
d
Note: This is not exact – there are constants left out
Luminosity, or absolute brightness,
o is a measure of the total power radiated by a star.
 Apparent brightness is how bright a star appears when viewed from Earth; it depends on
the absolute brightness but also on the distance of the star Two stars that appear equally
bright might be a dimmer, nearer star and a brighter, farther star
Temperature
 The color of a star is indicative of its temperature. Red stars are relatively cool, while blue ones
are hotter.
 Spectral classes make up a Temperature Sequence
O, B, A, F, G, K, M
o Hottest
o M coolest
o Oh Be A Fine Girl Kiss Me
Size
 For the vast majority of stars that cannot be imaged directly, size must be calculated knowing
the luminosity and temperature:


 Supergiant stars are more than 100 solar radii
 Giant stars are between 10 and 100 solar radii
 Upper main sequence stars are 8 to 100 solar radii
 Average stars are .5 to 8 solar radii
 Dwarf stars are ,1 to .5 solar radii
Ejnar Hertzsprung (8 Oct, 1873 - 21 Oct, 1967) was a Danish chemist and astronomer. Henry Norris
Russell (Oct 25, 1877 – Feb 18, 1957) was an American Astronomer.
 Together they invented one of most useful graphs in Astronomy
 The Hertzsprung– Russell Diagram
o The H–R diagram plots stellar luminosity against surface temperature. This is an H–R
diagram of a few prominent stars
o Once many stars are plotted on an H–R diagram, a pattern begins to form. These are the
80 closest stars to us; note the dashed lines of constant radius.The darkened band is
called the main sequence, as this is where most stars are.
o An H–R diagram of the 100 brightest stars looks quite different, These stars are all more
luminous than the Sun. Two new categories appear here – the red giants and the blue
giants.
o Clearly, the brightest stars in the sky appear bright because of their enormous
luminosities, not their proximity.
 Major Sections of the H-R Diagram.
o They start with the Main Sequence
o then the two Giant Stages
o then finally the White Dwarf Stage
 About 90% of stars lie on the main sequence; 9% are giants and 1% are white dwarfs.
Spectroscopic parallax:
 has nothing to do with parallax,
 but does use spectroscopy to extend our ability to determine the distance to a star
1. Measure the star’s apparent magnitude (brightness) and spectral class (temperature)
2. Use temperature to estimate luminosity
3. Apply inverse-square law to find distance
 Spectroscopic parallax can extend the cosmic distance scale to several thousand parsecs.
 The spectroscopic parallax calculation can be misleading if the star is not on the main sequence.
The width of spectral lines can be used to define luminosity classes
Determination of Stellar Masses
 Many stars are in binary pairs; measurement of their orbital motion allows determination of the
masses of the stars.
 Visual binaries can be measured directly; this is Kruger 60:
 Study of spectral lines reveals the motion of spectroscopic binaries and hence their spacing.
From that the masses are calculated.
 The mass of a star is also correlated with its radius, and very strongly correlated with its
luminosity.
 Mass is also related to stellar lifetime

Using the mass–luminosity relationship

The most massive stars have the shortest lifetimes – they have a lot of fuel but burn it at a very
rapid pace.
On the other hand, small red dwarfs burn their fuel extremely slowly, and can have lifetimes of a
trillion years or more.

Chapter 17
Formation of Stars
Star formation
 is an ongoing process in the Universe. Star-forming regions are seen in our galaxy as well as
others
 Star formation happens when part of a dust cloud begins to contract under its own gravitational
force; as it collapses, the center becomes hotter and hotter until nuclear fusion begins in the
core.
 When looking at just a few atoms, the gravitational force is nowhere near strong enough to
overcome the random thermal motion. Even a massive cloud of gas and dust will remain just a
cloud until some shock wave or pressure wave arrives to initiate its gravitational collapse
 The collapse process from nebula to star is similar for all stars and can be followed by observing
the temperature produced by the compression
Stages of Star Formation
 Stage 1: An interstellar cloud starts to contract, probably triggered by a shock or pressure wave.
As it contracts, the cloud fragments into smaller, irregular size pieces.
 Stage 2:
o





Individual cloud fragments begin to collapse. Once the density is high enough, there is
no further fragmentation.
Stage 3:
o The interior of the fragment has begun heating, and is about 10,000 K.
Stage 4:
o The core of the cloud is now a protostar, and the surface temperature is high enough to
make its first appearance on the H–R diagram
Stage 5
o Planetary formation around the star has likely begun, but the protostar itself is still not
in equilibrium – all heating that effects the system comes from the gravitational
collapse.
The last stages can be followed on the H–R diagram.
The protostar’s luminosity decreases even as its temperature rises because it is becoming
more compact.
Stage 6
o the core reaches 10 million K, and nuclear fusion begins.
o The protostar has become a star but it not yet on the main sequence.
o This stage is often called the T Tauri stage and it is a period of adjustment. T Tauri stars
are mostly between 105 and 108 years in age, are of low mass (0.5 to 3.0 M¤),
surrounded by hot, dense envelopes; and are losing mass via stellar winds with typical
v¥= ~100 km/s.
Stage 7
o
o
The star continues to contract and increase in temperature, until it is in equilibrium.
The star has reached the main sequence and will remain there as long as it has hydrogen
to fuse.
The Main Sequence
 The main sequence is a band, rather than a line, because stars of the same mass can have
different compositions.
 Most important: Stars do not move along the main sequence! Once they reach it, they are in
equilibrium, and do not move until their fuel begins to run out.
Chapter 18
Evolution from the Main Sequence
During its stay on the main sequence, any fluctuations in a star’s condition are quickly restored; the star
is in equilibrium
Again to follow the post-main-sequence evolution of a star we will resort to the stage method. Not
every star adheres to this sequence but it serves to describe the steps that many stars take’
Even while on the main sequence, Stage 7, the composition of a star’s core is changing.
 Eventually, as hydrogen in the core is consumed, the Star leaves the main sequence,
 Stage 8.
o Its evolution from then on depends very much on the mass of the star:
o


Low-mass stars go quietly. Medium-mass stars struggle. High-mass stars go out with a
bang!
o When the fuel in the core is used up the fusion ceases. The result is a contraction of the
Star and the formation of a new fusion furnace in a shell around the helium core.
Stage 9: The Red-Giant Branch.
o The now much larger surface of the furnace causes outer layers of the star to expand
and cool. It is now a red giant, extending out beyond the orbit of Mercury. Despite its
cooler surface temperature, its luminosity increases enormously due to its large size.
Stage 10: Helium fusion.
o Once the core temperature has risen to 100,000,000 K, the helium in the core can fuse,
through a three-alpha process:
o
The 8Be nucleus is highly unstable, and will decay in about 10–12 sec unless an alpha
particle fuses with it first. This is why high temperatures and densities are necessary.
o The helium flash: The pressure within the helium core is almost totally due to “electron
degeneracy” – two electrons cannot be in the same quantum state, so the core cannot
contract beyond a certain point. This pressure is almost independent of temperature so
when the helium starts fusing, the pressure cannot adjust and the core explodes
completely disrupting the surrounding shell furnace.
o Helium begins to fuse extremely rapidly; within hours to days the enormous energy
output is over, but the star is now on its way to White Dwarf
Stage 13
o Disruption of the hydrogen furnace throws the star out of equilibrium and it starts to
shrink, but it has much heat to dissipate from the Helium Flash. The result is the surface
gets smaller as the surface temperature gets higher, causing movement across the
graph toward the blue while maintaining nearly the same brightness.
Stages 11 and 12 depend very much on the mass of the star.
o From .5 to 1.4 solar masses the transition from the horizontal branch White Dwarf
goes smoothly.
o From 1.4 to about 5.5 solar masses they must shed the extras mass to get down to the
Chandrasakar limit of 1.4 solar masses, then they can transition to White Dwarf.
The Instability Strip, still Stage 10
 As the dying star moves along the horizontal branch it encounters a region, discovered by
Hertsprung, called the Instability Strip. The star becomes a variable star changing brightness
slightly in a very few days.
 Henrietta Levitt discovered a direct relationship of Period to Luminosity of the Cephied
Variables and the RR Lyra Variables
Some stars with more than about 5.5 solar masses have a different problem.
 The Helium flash becomes a permanent nuclear furnace.
 The Helium core fuses helium to carbon and the shield furnace continues to fuse Hydrogen to
Helium and the star is now in a some what stable state. Many stars go into a new Red Giant
condition for a period. This is the Asymptotic Giant Branch, Stage 11
Chapter 19
Death of Stars
Low Mass Stars on the lower Main Sequence of the H_R Diagram have extremely long lifetimes. Their
entire original mass of Hydrogen is available as fuel. When all the hydrogen of the star is used (fused to
Helium) it collapses quietly to a Helium White Dwarf
Stars with masses of about ½ M to about 8 M follow roughly the evolution of a 1 M star.
Mass Loss Among Red Giants
 Stars just larger than 1.4 M lose their extra mass through accelerated stellar wind
 Stars with masses up to 8 or 9 M often have their outer layers go unstable and explode. The
result is a Nova
o A Nova is seen from Earth as a sudden brightening of an existing star. The explosion is
very bright for a few days to weeks as the gas expands. Then it fades as the gas expands
and cools.
o This process can be repeated every 3 or 4 hundred years until the star reaches the mass
limit then it can go White Dwarf.
More than half the stars in the sky are double stars and are close enough to share matter when one
goes Giant. The larger star goes giant 1st and dumps its extra mass to the smaller Main Sequence star
until it is under the mass limit then it quietly goes to White Dwarf. When the now bloated 2nd star goes
giant it feeds back to the White Dwarf where it is ejected by explosion and we see it as a Nova.




A large star has a great number of shell fusion furnaces.
The ‘ashes’ from one furnace serves as fuel for the next.
The inner most ash layer is Iron.
A high-mass star can continue to fuse elements in its core right up to iron (after which the fusion
reaction is energetically un-favored).
As heavier elements are fused, the reactions go faster and the stage is over more quickly.
 A 20-solar-mass star will fuse carbon for about 10,000 years, but its iron core lasts less than a
year.
 On the left, nuclei gain mass through fusion; on the right they loose it through fission.
 Iron is the crossing point; when the core has fused to iron, no more fusion can take place.
 Many elements are formed during normal stellar fusion.
 Some are made during the supernova explosion.
A supernova is a one-time event – once it happens, there is little or nothing left of the progenitor star.
 There are two different types of supernovae, both equally common:
o Supernova I, which is a supernova explosion around a core which implodes
o Supernova II, which is an explosion of the core resulting in the complete destruction of
the star
A super nova has not occurred in our part of the Milky Way since the invention of the telescope so we
have not had the opportunity to study one up close. We have seen many in other galaxies as well as
remnants in our galaxy.
Supernovae I arise in two ways.
 The first kind, a single star SNI, comes from an explosion in the Silicon layer around the Iron
core of a large star.
 The second kind, comes from interaction of large binary stars.
 The iron is very reluctant to fuse. Sometimes the Oxygen and Silicon layers around the core
become unstable and explode, imploding the Iron to a Neutron Star
 Normally a large star would die as a Supernova. In a binary situation, however, it dumps its
excess mass over to its smaller companion and becomes a White Dwarf.
 The now very large companion finishes its life and goes giant dumping its excess matter on
the white dwarf.
 The multi-layered star around the white dwarf is very unstable and explodes in a Supernova
imploding the White Dwarf to a Neutron Star.
 The classic results of a Supernova I are the expanding debris of the explosion, a neutron star
and a Pulsar
The Crab Nebula
 is the result of a Supernova in 1054. It was observed and location recorded by the Chinese. We
see the expanding debris of the explosion today at that location.
Often as a large star ages much of the fuel is used up and deposited as ‘ash’ in the iron core. The inward
pressure on the iron core is enormous, due to the high mass of the star. As the core continues to
become more and more dense, the protons react with one another to become neutrons + a flood of
neutrinos + much energy.
These local hot spots initiate fusion of the iron which triggers formation of all of the elements more
massive than iron + more neutrinos and much more energy. The energy builds up in a cascade effect
producing a gigantic explosion and the complete destruction of the star, known as a Supernova II
 The classic results of a Supernova II are:
o Collapse of the iron core
o Flood of neutrinos
o Super explosion debris cloud
o Complete disassembly of the star
While doing a theoretical study of Supernovae Zwysic and Baade in the 1930’s predicted the existence of
Neutron Stars but they had never been seen even with the 200 inch Hale telescope on Mount Palomar.
The first one found, much later, was associated with a Pulsar in the Crab nebula.
Neutron Stars
 1–3 solar masses, are so dense that they are very small.
 about 10 km in diameter, compared to Manhattan.
 As the parent star collapses, the neutron core spins very rapidly, conserving angular
momentum. Typical periods are fractions of a second.
 Again as a result of the collapse, the neutron star’s magnetic field becomes enormously strong.
 In 1967 Jocelyn Bell led a group of graduate students at the University of Cambridge in England
in a search for Radio Sources in the sky. They discovered a source that emitted extraordinarily
regular pulses. After some initial confusion, it was realized that this was rardiation from a
neutron star, spinning very rapidly.
Why do neutron Stars pulse?
 Strong jets of matter and beams of light are emitted at the magnetic poles, as that is where they
can escape. If the rotation axis is not the same as the magnetic axis,
 the two beams will sweep out circular paths. If the Earth lies in one of those paths, we will see
the star blinking on and off.
 The velocities of the material in the Crab nebula can be extrapolated back, using Doppler shifts,
to the original explosion point.
Review of Death of Stars
Chapter 21
Our Milky Way
Inside Our Galaxy
 In as much as we are on the inside of the Galaxy , our view is incomplete.
 From Earth, see relatively few stars when looking out of galaxy (red arrows), many when looking
into the disk (blue arrows) of the galaxy. When looking toward the bulge at the center, most our
view is blocked by dust and dense gas.
 One of the first attempts to measure the size and shape of the Milky Way was done by Herschel
using visible stars. Unfortunately, he was not aware that most of the Galaxy, particularly the
center, is blocked from view by vast clouds of gas and dust.
Distance Meqasurement
 Giant stars, after the Helium Flash, pass into the Instability Strip.
 The variability of these stars comes from a dynamic imbalance between gravity and pressure –
they have oscillations around stability. The upper plot is an RR Lyrae star. All such stars have
about the same luminosity curve, with periods from 0.5 to 1 day. The lower plot is a Cepheid
variable; Cepheid periods range from about 1 to 100 days.
 The usefulness of these stars comes from their period–luminosity relation
The Period-Luminosity Relation allows us to measure the distances to these bright giant stars.
 RR Lyrae stars all have about the same luminosity; knowing their apparent magnitude and using
the inverse square law allows us to calculate the distance.
 Cepheids have a luminosity that is strongly correlated with the period of their oscillations; once
the period is measured, the luminosity is known and we can proceed as above.
 We have now expanded our cosmic distance ladder one more step .. this time a giant step.
Many RR Lyrae stars are found in globular clusters. These clusters are not all in the plane of the Galaxy,
so they are obscured by dust and can be measured.
 This yields a much more accurate picture of the extent of our Galaxy and our place within it.
 Once we could measure distances in the Milky Way, we could determine our position in it. This
artist’s conception shows the various parts of our Galaxy,and location of our Sun.
 Harlow Shapely and his students were instrumental in determining the size and shape of the
Galaxy.
 The Galactic Halo is made of globular clusters and formed very early. The halo is essentially
spherical. All the stars in the halo are very old, and there is no gas and dust in the clusters.
 The Galactic disk is where the youngest stars are, as well as star forming regions (emission
nebulae), and large clouds of gas and dust.
 Surrounding the Galactic center is the Galactic bulge, which contains a mix of older and younger
stars.
 Stellar orbits in the disk are in a plane and in the same direction; orbits in the halo and bulge
are much more random.
 This infrared view of our Galaxy shows much more detail of the Galactic center than the visiblelight view does, as infrared seems to penetrate the gas and dust.
Measurement of the position and motion of gas clouds shows that the Milky Way has a spiral form
One method that astronomers have used to calculate the Mass of the Milky Way galaxy is to use
Kepler's 3rd law. We take the radius of our orbit and the period. Plugging those numbers into Kepler's
3rd law we can estimate the mass inside our orbit. This gives just shy of 100 billion solar masses. Other
mass estimates of the Milky Way are up to 1 trillion solar masses.
Once all the Galaxy is within an orbit, the Orbital Velocity should diminish with distance, as the dashed
curve shows. It doesn’t; more than twice the mass of the Galaxy would have to be outside the visible
part to reproduce the observed curve. This mass is called Dark Matter
Dark Matter is space matter we cannot see because, unlike stars and galaxies, it does not give off light.
There is much more dark matter in the Universe than bright. Some scientists think 90 percent of matter
is dark. Astronomers know about dark matter because its gravity pulls on stars and galaxies, changing
their orbits and the way they rotate.
Galactic Center,
 it is entirely obscured by dust.
 Because of interstellar dust along the line of sight, the available information about the Galactic
Center comes from observations at gamma ray, hard X-ray, infrared and radio wavelengths.
 Coordinates of the Galactic Center were first found by Harlow Shapely in his 1918 study of the
distribution of the globular clusters.
 Direct study of a black hole such as the one widely suspected to exist at the center of our galaxy
is tricky, as black holes swallow up nearby light, rendering themselves virtually invisible. But
researchers can infer properties of a black hole from its hearty gravitational influence on nearby
stars.
 A large star S2 in that region was found to orbit a dark concentration of mass, estimated at 3.7
million times the mass of our Sun The laws of physics have ruled out any explanation but one-that this is indeed an enormous black hole.
Chapter 21
Galaxies
Spiral Galaxies
 The components of spiral galaxies are the same as in our own Galaxy: disk, core, halo, bulge,
spiral arms.
 Type Sa tends to have the most tightly bound spiral arms, with Types Sb and Sc progressively
less tight, although the correlation is not perfect.
 Similar to the spiral galaxies are the barred spirals. Historically they were recognized later but
were found to be equally numerous. Like the spirals they are designated by how tightly wound
they are.
Elliptical galaxies
 have no spiral arms and no disk.
 Two main sizes,
o giant ellipticals of trillions of stars,
o dwarf ellipticals of less than a million stars.
 Ellipticals also contain very little, if any, cool gas and dust, and show no evidence of ongoing star
formation.
 Many do, however, have large clouds of high temperature gas, extending far beyond the visible
boundaries of the galaxy.
 Ellipticals are classified according to their shape from E0 (almost spherical) to E7 (the most
elongated).
S0 (Lenticular) and SB0 galaxies
 have a flattened disk and bulge, but no spiral arms and, like the Ellipticals, have little
interstellar gas and dust
The irregular galaxies
 have a wide variety of shapes, but no fixed shapes.
 The Small and Large Magellanic Clouds are close neighbors to our own Milky Way
When Edwin Hubble announced that many of the strange nebulae in the sky were really Galaxies, he
was tasked with sorting and classifying them. Hubble’s “tuning fork” is a convenient way to remember
the galaxy classifications, although it has no deeper meaning
Cepheid variables allow measurement of distance to galaxies to about 25 Mpc away. However, some
galaxies have no Cepheids, and most are farther away than 25 Mpc. New distance measures are needed.
•
Tully–Fisher relation correlates a galaxy’s rotation speed (which can be measured using the
Doppler effect) to its luminosity.
•
Type I supernovae all have about the same luminosity, since the process by which they happen
doesn’t allow for much variation, so the inverse square can be applied
With these additions then, the cosmic distance ladder has been extended to about 1 Gpc
The Local Group
Here is the distribution of galaxies within about 1 Mpc of the Milky Way.
There are three spirals in this group – the Milky Way, Andromeda, and M33. These and their satellites –
about 45 galaxies in all – form the Local Group.
Such a group of galaxies, held together by its own gravity, is called a galaxy cluster
A nearby galaxy cluster is the Virgo cluster; it is much larger than the Local Group, containing about
3500 galaxies.
Red Shift
 All galaxies (with a few nearby exceptions) show a Doppler Red Shift that suggest they are
moving away from us.
 This chart shows red shift correlated with their recession velocity and distance:
 These plots show the relation between distance and recession velocity for the five galaxies in
the previous figure, and then for a larger sample
The Doppler Effect relates the change in wavelength to the recession velocity
Δλ / λ0 = Vr / C
The Hubble Law relates recession velocity with distance
V = Hd
This puts the final step on our distance ladder. However the Hubble Law does need some final tweaking
when the recession velocities approach relativistic values
Active Galaxies
 About 20–25% of galaxies don’t fit well into the Hubble scheme – they are far too luminous.
Such galaxies are called Active Galaxies. They differ from normal Galaxies in both the luminosity
and type of radiation they emit
 The radiation from active galaxies is called non-stellar radiation.
 Many luminous galaxies are experiencing an outburst of star formation, probably due to
interactions with a neighbor. These galaxies are called starburst galaxies
 The galaxies we will discuss now are those whose activity is due to events occurring in and
around the galactic center.
 This active galaxy has star-formation rings surrounding a very luminous core called an Active
Galactic Core
 Active galaxies are classified into three types: Seyfert galaxies, radio galaxies, and quasars.
Seyfert galaxies resemble normal spiral galaxies, but their cores are thousands of times more
luminous.
 The rapid variations in the luminosity of Seyfert galaxies indicate that the core must be
extremely compact
 Radio galaxies emit very strongly in the radio and X-ray portions of the spectrum. Many have
enormous lobes, invisible to optical telescopes, perpendicular to the plane of the galaxy.
 Core-dominated and radio-lobe galaxies are probably the same phenomenon viewed from
different angles.
 Quasars – quasi-stellar objects – are star-like in appearance, but have very unusual spectral
lines. Eventually it was realized that quasar spectra were normal, but enormously redshifted:
 Solving the spectral problem introduces a new problem – quasars must be among the most
luminous objects in the universe, to be visible over such enormous distances.
 Active galactic nuclei have some or all of the following properties:
•
high luminosity
•
non-stellar energy emission
•
variable energy output, indicating small nucleus
•
jets and other signs of explosive activity
•
broad emission lines, indicating rapid rotation
• This is the leading theory for the energy source in an active galactic nucleus: a black hole,
surrounded by an accretion disk. The strong magnetic field lines around the black hole
channel particles into jets perpendicular to the magnetic axis.
• One might expect the radiation from such a powerful source to be mostly X rays and gamma
rays and we often see quasars that way, but apparently it is often “reprocessed” in the
dense clouds around the black hole and we see it reemitted at longer wavelengths
• Particles in the jet will emit synchrotron radiation as they spiral along the magnetic field
lines; this radiation is decidedly non-stellar.
• In an active galaxy, the central black hole may be billions of solar masses.
• The accretion disk is whole clouds of interstellar gas and dust; they may radiate away as
much as 10–20% of their mass before disappearing.