Unit 5
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.
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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.
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Tully–Fisher relation correlates a galaxy’s rotation speed (which can be measured using the
Doppler effect) to its luminosity.
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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:
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high luminosity
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non-stellar energy emission
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variable energy output, indicating small nucleus
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jets and other signs of explosive activity
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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.
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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.