Download here - SDSU Astronomy Department and Mount Laguna Observatory

ASTRO 101
Principles of Astronomy
Instructor: Jerome A. Orosz
(rhymes with
“boris”)
Contact:
• Telephone: 594-7118
• E-mail: [email protected]
• WWW:
http://mintaka.sdsu.edu/faculty/orosz/web/
• Office: Physics 241, hours T TH 3:30-5:00
Homework/Announcements
• For Chapter 11, skip sections 11.9, 11.11,
11.14, 11.16, 11.17, 11.18, 11.19
• For Chapter 12, sections 12.1-12.7
• Tuesday, May 7: wrap-up and review
• Tuesday May 14, Final
Homework/Announcements
• Chapter 12 homework Due May 7: Question 5
(What observations led Harlow Shapley to
conclude we are not at the center of the
Galaxy?)
NEXT:
Our Galaxy
and
Other Galaxies
A Sense of Scale
• So far, we have discussed things that are
relatively nearby:
• The Sun, Moon, and solar system planets. The size
is several “light hours”.
• Stars: many stars that you can see without a
telescope are within a few hundred light years.
A Sense of Scale
• The Sun (and its planets) and these nearby stars
are part of a vast collection of stars bound by
gravity:
 Such a collection of stars is called a “galaxy”.
 This structure contains roughly 1011 stars and is
100,000 light years across.
• There are about 50 billion galaxies similar to our
own in the observable universe!
And now for something completely
different:
The Meaning of Life
Defining the Milky Way
• The Milky Way is only one of many galaxies…
Island Universes
• We know today that galaxies are the
fundamental building blocks of the Universe.
• Billions of them are known, and we often talk of
them as if they are single objects (rather than a
collection of objects).
Island Universes
• You see galaxies almost everywhere you look,
provided you expose for a long enough time.
Island Universes: The History
• Astronomers in the 1700s were very interested
in comets. These objects appear as fuzzy blobs
when seen through a telescope.
Island Universes: The History
• Comets look obviously
non-stellar, especially in
modern photographs.
• It is much harder to see
find detail visually (i.e.
without a photograph).
Island Universes: The History
• Astronomers in the 1700s were very interested
in comets. These objects appear as fuzzy blobs
when seen through a telescope.
• Many workers undertook surveys for comets.
Many fuzzy objects were found that were not
comets. Charles Messier made a catalog of
them, for which he is famous. His comet
discoveries have been forgotten.
Island Universes: The History
• Many workers undertook surveys for comets.
Many fuzzy objects were found that were not
comets. Charles Messier made a catalog of
them, for which he is famous. His comet
discoveries have been forgotten.
• Some of these “fuzzy blobs” turned out to be
clouds of gas relatively nearby the Sun. Other
blobs showed “pinwheel” structure.
Island Universes: The History
• This object from
Messier’s catalog (the
Orion Nebula) turned
out to be a cloud of gas
and dust.
Island Universes: The History
• This object from Messier’s catalog (the
Andromeda Galaxy), turned out to be
something different…
Island Universes: The History
• Many workers undertook surveys for comets.
Many fuzzy objects were found that were not
comets. Charles Messier made a catalog of
them, for which he is famous. His comet
discoveries have been forgotten.
• Some of these “fuzzy blobs” turned out to be
clouds of gas relatively nearby the Sun. Other
blobs showed “pinwheel” structure.
• The nature of the “spiral nebulae” was not
solved until the 1920s.
Island Universes: The History
• This figure illustrates how objects of very
different sizes can appear to have the same
angular sizes.
Island Universes: The History
• The nature of the “spiral nebulae” was not
solved until the 1920s.
• Why was it so hard? Early astronomers had no
way to judge the distances to these objects. As
a result, it was not obvious if these things were
very large and very far away, or relatively small
objects seen close by.
• Edwin Hubble solved the problem in the 1920s
using a certain class of variable stars.
Island Universes: The History
• Edwin Hubble used the 100
inch telescope at Mount Wilson
(just outside L.A.), which was
the largest telescope available
in the 1920s. He is shown here
at the 60 inch telescope at
Palomar Observatory, just north
of here.
• He could see individual stars in
some of the spiral nebulae!
Inverse Square Law
Luminosity
F = L/(4d2)
Brightness or flux
Distance
L=4d2F
One usually measures F, d (parallax) to find L
Inverse Square Law
Luminosity
F = L/(4d2)
Brightness or flux
Distance
d2=L/(4F)
Sometimes one can measure F and L to find d…
Distance Indicators
• Some stars vary in brightness on a regular basis.
In many cases, this is because they are pulsating.
The radius and temperature changes over the
cycle, giving rise to brightness variations.
Distance Indicators
• Some stars vary in brightness on a regular basis.
In many cases, this is because they are pulsating.
The radius and temperature changes over the
cycle, giving rise to brightness variations.
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
Distance Indicators
• Some stars vary in brightness on a regular basis. In many cases, this is
because they are pulsating. The radius and temperature changes over the
cycle, giving rise to brightness variations.
Distance Indicators
• For a certain class of
pulsators, the luminosity
is proportional to the
period of pulsation.
• If you observe the star
over a long enough time,
you can measure the
period and then compute
the luminosity.
Distance Indicators
• For a certain class of
pulsators, the luminosity
is proportional to the
period of pulsation.
• If you observe the star
over a long enough time,
you can measure the
period and then compute
the luminosity.
Distance Indicators
• For a certain class of
pulsators, the luminosity
is proportional to the
period of pulsation.
• If you observe the star
over a long enough time,
you can measure the
period and then compute
the luminosity. So what?
Distance Indicators
• If you observe the star
over a long enough time,
you can measure the
period and then compute
the luminosity. So what?
• If you know how bright a
star appears, and also its
luminosity, then you can
compute the distance!
Distance Indicators
• If you observe the star
over a long enough time,
you can measure the
period and then compute
the luminosity. So what?
• If you know how bright a
star appears, and also its
luminosity, then you can
compute the distance!
Island Universes
• Hubble found that the great nebula in Andromeda was
well over 1,000,000 light years away (Modern
measurements give 2,400,000 light years).
• This is much larger than the size of the Milky Way.
The spiral nebulae are outside our galaxy!
Island Universes
• Hubble found that the great nebula in Andromeda was
well over 1,000,000 light years away (Modern
measurements give 2,400,000 light years).
• Also, given the distance and the angular size on the
sky, one can compute the physical size.
Island Universes
• Hubble found that the great nebula in Andromeda was
well over 1,000,000 light years away (Modern
measurements give 2,400,000 light years).
• Also, given the distance and the angular size on the sky,
one can compute the physical size. >100,000 light years
The Structure of Our Galaxy
• How do we measure the structure of the Milky,
given that we are inside it?
• To appreciate why this is so hard, let us briefly
revisit stellar evolution…
Stellar Evolution
• The basic steps are:




Gas cloud
Main sequence
Red giant
Rapid mass loss (planetary nebula or supernova
explosion)
 Remnant
• The stars form from clouds of gas and dust, and
much of this material is put back into space
before the star “dies.” So what?
Interstellar Dust
• The space between the stars often contains
lots of gas (e.g. hydrogen, oxygen, etc) and
dust particles. This interstellar dust can
inhibit our ability to observe at certain
wavelengths. This point was not fully
understood or appreciated until the mid
1930s.
Interstellar Dust
• The dust is composed
of tiny slivers of
graphite and silicates,
possibly coated with
water ice.
• Note that the scale on
this diagram is 10-7
meters!
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
Interstellar Dust
• Light passing through an
interstellar dust cloud will
be dimmed.
• However, the amount of
dimming depends on the
wavelength of the light:
blue light is scattered
more easily than red light.
The object appears
redder.
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
Why is the Sky Blue?
• Blue light travels a relatively short distance before
it is scattered by molecules in the air. Red light
goes much further before being scattered.
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
Interstellar Dust
• Interstellar dust makes
a star appear dimmer
and redder if that star
is behind a cloud of
dust.
Intersteller Dust
• Interstellar dust makes
a star appear dimmer
and redder if that star
is behind a cloud of
dust.
• This will complicate
matters when we try to
judge the structure of
our galaxy…
Finding the Structure
• We are inside a large collection of stars. How
do we make sense of the structure of this system
from the inside? It is not easy…
Finding the Structure
• A band of diffuse
light going around the
sky can be seen from
a dark location. This
band was known to
the ancient Greeks
and Romans and was
called the “Milky
Way”.
Finding the Structure
• Using a telescope
Galileo discovered
that the Milky Way is
actually made up of
millions of stars, each
individually too faint
to see without a
telescope.
• Away from this band,
fewer stars are seen.
Finding the Structure
• Astronomers in the 1800s did large “star gauging”
surveys all over the sky. They essentially counted
stars using visual observations through a
telescope.
Finding the Structure
• Relatively large numbers of stars are seen in a band
cutting across the sky. This band was known to the
ancient Greeks and Romans and was called the “Milky
Way”.
• Away from this band, fewer stars are seen.
Finding the Structure
• From the star counts, it was concluded that the
“Universe” is a flattened structure (aspect ratio of 5:1)
with a diameter of about 8000 light years. The Sun was
essentially at the center.
Finding the Structure
• From the star counts, it was concluded that the
“Universe” is a flattened structure (aspect ratio of 5:1)
with a diameter of about 8000 light years. The Sun was
essentially at the center.
Finding the Structure
• From the star counts, it was concluded that the
“Universe” is a flattened structure (aspect ratio of 5:1)
with a diameter of about 8000 light years. The Sun was
essentially at the center.
• Either we are at a very special place, or something is
wrong.
Finding the Structure
• From the star counts, it was concluded that the
“Universe” is a flattened structure (aspect ratio of 5:1)
with a diameter of about 8000 light years. The Sun was
essentially at the center.
• Either we are at a very special place, or something is
wrong. Interstellar dust was not accounted for.
Finding the Structure
• From the star counting observations and other
observations, we know the galaxy is a flattened
structure.
Finding the Structure
• From the star counting observations and other
observations, we know the galaxy is a flattened
structure.
• However, interstellar dust obscures the distant
parts of it, leading to severe observational biases.
Finding the Structure
• From the star counting observations and other
observations, we know the galaxy is a flattened
structure.
• However, interstellar dust obscures the distant
parts of it, leading to severe observational biases.
• Is there another way to probe the structure?
Finding the Structure
• From the star counting observations and other
observations, we know the galaxy is a flattened
structure.
• However, interstellar dust obscures the distant
parts of it, leading to severe observational biases.
• Is there another way to probe the structure?
There are several: the earliest alternative was a
study of globular clusters…
Finding the Structure
• In 1918 Harlow Shapley estimated the distances to 93
globular clusters. Shapley found a strong concentration
of globular clusters in the direction of Sagittarius.
Finding the Structure
• In 1918 Harlow Shapley estimated the distances to 93
globular clusters. Shapley found a strong concentration
of globular clusters in the direction of Sagittarius.
• If the GC’s are uniformly distributed through the
galaxy, then the Sun is well off the center!
Finding the Structure
• In 1918 Harlow Shapley
estimated the distances to
93 globular clusters.
Shapley found a strong
concentration of globular
clusters in the direction of
Sagittarius.
• If the GC’s are uniformly
distributed through the
galaxy, then the Sun is
well off the center!
Finding the Structure
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
• In 1918 Harlow Shapley estimated the distances to 93
globular clusters. Shapley found a strong concentration
of globular clusters in the direction of Sagittarius.
• If the GC’s are uniformly distributed through the
galaxy, then the Sun is well off the center!
Finding the Structure
• Counting individual stars in the optical
leads to biases if interstellar dust is not
accounted for. One has the illusion that we
are at the center.
• The globular clusters give a much more
unbiased view. The center of the galaxy is
roughly 25,000 light years away in the
direction of Sagittarius.
Finding the Structure
• In 1918 Harlow Shapley estimated the distances to 93
globular clusters. Shapley found a strong concentration
of globular clusters in the direction of Sagittarius.
• If the GC’s are uniformly distributed through the
galaxy, then the Sun is well off the center!
Finding the Structure
• There are three main parts:
– The bulge/nucleus at the center (roughly spherical with a
radius of about 3000 light years).
Finding the Structure
• There are three main parts:
– The disk which is about 100,000 light years across and about
2000 light years thick. It contains the young stars, the gas,
and the dust. The Sun is in the disk about 2/3 the way out.
Finding the Structure
• There are three main parts:
– The halo, which is a roughly spherical and relatively diffuse
region a few hundred thousand light years across. The halo
contains very old stars and the globular clusters.
Finding the Structure
Finding the Structure
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
The Structure of the Milky Way
• The bulge contains relatively old stars.
• The disk contains gas and dust, young stars,
and some older stars. The gas and dust are
confined to a relatively thin region.
• The halo contains very old stars, and
essentially no gas and dust.
Next
• Structure in the Disk
• The Galactic Center
Finding the Structure of the Disk
• In the 1920s and 1930s, Jan Oort studied the
motions of thousands of stars. He concluded
that most stars are on roughly circular orbits
about the center of the galaxy (as one would
expect from Newton’s laws).
Finding the Structure of the Disk
• In the 1920s and 1930s, Jan Oort studied the
motions of thousands of stars. He concluded
that most stars are on roughly circular orbits
about the center of the galaxy (as one would
expect from Newton’s laws).
• In addition, he showed that stars closer to the
center have higher angular speeds.
Finding the Structure of the Disk
• In addition, he showed that stars closer to the
center have higher angular speeds.
• If one can estimate the orbital speeds and the
distance to the center, then one can find the mass
of the galaxy (or at least its central parts).
Finding the Structure of the Disk
• In addition, he showed that stars closer to the
center have higher angular speeds.
• If one can estimate the orbital speeds and the
distance to the center, then one can find the mass
of the galaxy (or at least its central parts).
Modern measurements yield a mass of about
1011 solar masses for the Milky Way galaxy.
The Structure of the Disk
• Is the disk of the Milky Way galaxy
axisymmetric, or are there patterns within
it?
Finding the Structure of the Disk
• It is very hard to study the structure of the disk of the
galaxy because we are inside it. We cannot travel
above it as in the above example. Is there another
way?
Finding the Structure of the Disk
• It is very hard to study the structure of the disk of the
galaxy because we are inside it. We cannot travel
above it as in the above example. Is there another
way? Yes. Observe at radio wavelengths.
The 21 cm line of Hydrogen
• For the study of Galactic structure, radio
observations have some advantages:
The 21 cm line of Hydrogen
• For the study of Galactic structure, radio
observations have some advantages:
 Radio photons can easily penetrate the
interstellar dust. You can essentially see every
part of the disk.
The 21 cm line of Hydrogen
• For the study of Galactic structure, radio
observations have some advantages:
 Radio photons can easily penetrate the
interstellar dust. You can essentially see every
part of the disk.
 Neutral hydrogen, which is the most abundant
element in the universe, has an emission line at
21.1 cm, so you can measure radial velocities
from Doppler shifts.
The 21 cm line of Hydrogen
• In quantum mechanics, electrons and
protons have a property called “spin”,
although it is really not the same as a
spinning ice skater.
The 21 cm line of Hydrogen
• In quantum mechanics, electrons and
protons have a property called “spin”,
although it is really not the same as a
spinning ice skater.
• This “spin” has only two values: “up” or
“down”.
The 21 cm line of Hydrogen
• In quantum mechanics, electrons and
protons have a property called “spin”,
although it is really not the same as a
spinning ice skater.
• This “spin” has only two values: “up” or
“down”. Why is this important?
The 21 cm line of Hydrogen
• A hydrogen atom where the proton and the electron
have the same “spin” has a slightly higher energy than
an atom where the “spins” are opposite.
The 21 cm line of Hydrogen
• A hydrogen atom where the proton and the electron
have the same “spin” has a slightly higher energy than
an atom where the “spins” are opposite.
• An isolated hydrogen atom can spontaneously go from
one state to the other, giving off a photon with a
wavelength of 21.1 cm in the process.
The 21 cm line of Hydrogen
• Hydrogen is very abundant, so 21 cm line
emission comes from all over the Milky Way
galaxy’s disk.
• Just build a large antenna with a good radio
receiver, and make a spectrum near a
wavelength of 21 cm in many lines-of-sight
throughout the disk…
The 21 cm line of Hydrogen
• The spectrum gives you the intensity as a function of
wavelength (or radial velocity).
• Using some assumptions about how the clouds orbit,
one can compute the distances to the clouds in that
line-of-sight.
Image from Nick Strobel’s Astronomy Notes
The Structure of the Disk
• Detailed radio observations show that the hydrogen
gas is not uniformly distributed in the disk.
• There are distinct “spiral arms”, but with branches
and spurs, similar to what is seen in other galaxies.
The Structure of the Disk
• This map is made from
infrared observations of
dust.
• There are distinct “spiral
arms”, and possibly a
“bar” in the center.
Spiral Arms
• Spiral structure is
seen in many
galaxies outside the
Milky Way galaxy.
• What causes this
structure?
Differential Rotation
• Stars near the center take less time to orbit the center
than stars further out.
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
Differential Rotation
• Stars near the center take less time to orbit the center
than stars further out.
• A linear structure can quickly turn into a spiral pattern.
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
Differential Rotation
• Stars near the center take less time to orbit the center
than stars further out.
• A linear structure can quickly turn into a spiral pattern.
However, it is a bit too quick…
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
Differential Rotation
• Stars near the center take less time to orbit the center
than stars further out.
• A linear structure can quickly turn into a spiral pattern.
However, it is a bit too quick…
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
Differential Rotation
• Stars near the center take less time to orbit the center
than stars further out.
• A linear structure can quickly turn into a spiral pattern.
However, it is a bit too quick… ???
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
Density Waves
• Spiral arms are not
“material” waves, but
rather “density” waves.
In other words, the same
stars do not stay in the
arm.
• The “spiral density
wave” is a compression
wave.
Density Waves
• Spiral arms are not
“material” waves, but
rather “density” waves.
In other words, the same
stars do not stay in the
arm.
• The “spiral density
wave” is a compression
wave.
Density Waves
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
Next:
• The Galactic Center
• Dark Matter