ASTRO 101

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ASTRO 101

• Principles of Astronomy

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Instructor Jerome A. Orosz
(rhymes with boris)Contact

• Telephone [phone]
• E-mail orosz_at_sciences.sdsu.edu
• WWW http//mintaka.sdsu.edu/faculty/orosz/web/
• Office Physics 241, hours T TH 330-500

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Text Perspectives on Astronomy First
Editionby Michael A. Seeds Dana Milbank.
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Astronomy Help Room Hours

• Monday 1200-1300, 1700-1800
• Tuesday 1700-1800
• Wednesday 1200-1400, 1700-1800
• Thursday 1400-1800, 1700-1800
• Friday 900-1000, 1200-1400
• Help room is located in PA 215

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Coming Up

• Chapter 9 (The Milky Way Galaxy)
• Chapter 10 (Other Galaxies)
• Chapter 11 (Cosmology in the 21st Century)
• Homework due December 3 Question 1, Chapter 11
  (How does the darkness of the night sky tell you
  something about the Universe?)

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Questions from Before

• How do you tell that the Universe is expanding?
  By observing the speed of a galaxy vs. its
  distance.
• If the Universe is expanding, what is it
  expanding into? It can be expanding into a
  dimension that is not accessible to us.
• What is a quasar? A quasar is a supermassive
  black hole that resides at the core of a galaxy.
  The black hole is accreting matter, which causes
  a great release of energy.

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Questions for Today

• Can the Universe be stationary?
• What is the Big Bang?
• What is the cosmic microwave background?

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NextActive GalaxiesActive Galactic
NucleiQuasars
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Energetic Objects

• Historically, there were three types of unusually
  energetic objects studied
• Active Galaxies
• Active Galactic Nuclei
• Quasars
• They turn out to be similar things

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Active Galactic Nuclei (AGN)

• Observationally, AGN are powerful sources of
  energy, usually associated with the center of a
  distant galaxy.
• These objects are by far the most powerful
  objects in the universe (the energy output easily
  exceeds the total output of all of the stars in
  the Milky way).
• Multi-wavelength observations are essential for
  understand what is happening (X-ray and radio
  observations are especially important).

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AGN

• Images of two galaxies from the Hubble Space
  Telescope. The one at the left is normal,
  whereas the one on the right has a very bright
  nucleus.

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AGN

• Early in the 20th century Carl Seyfert and others
  cataloged galaxies with optically bright nuclei.
  Galaxies of this class are called Seyfert
  Galaxies.
• From optical spectroscopy it was known that these
  objects had large amounts rapidly moving
  outflowing gas (velocities up to 10,000 km/sec),
  much more than normal galaxies.

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Seyfert Galaxies

• Seyfert galaxies are usually spirals with small,
  highly luminous nuclei.

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AGN

• The broad emission lines indicate the presence of
  an energetic process not associated with normal
  stars.
• The nature of the engine was not known.

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AGN

• After WW II, radio astronomy developed. People
  pointed radio receivers at the sky and to their
  surprise discovered many strong radio sources.
• At first, the positional accuracy was not good,
  so associating a radio source with an object on
  an optical photograph was not easy.
• Eventually, it was found that many of the strong
  radio sources were associated with Seyfert
  galaxies.

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AGN

• A radio map of Cygnus A, with optical images to
  scale. The radio emission extends well beyond
  the optical edge.

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AGN

• So far, we have
• Galaxies with optically bright nuclei.
• Many of these galaxies are very strong radio
  and/or X-ray sources.
• jets can be seen in the radio or X-rays these
  are focused outflows of material and radiation.
  It is believed that the material (electrons and
  protons mainly) is moving close to the speed of
  light.
• What could the energy source be???

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Quasars

• Some of the radio sources were not associated
  with bright galaxies at all.
• When the positional accuracy was improved, it was
  found that many of these radio sources were
  associated with point-like sources on optical
  photographs. On the images, they look like
  stars.
• The term quasar means quasi-stellar object.

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Quasars

• Quasars look point-like, just like the stars do.
  Note how the galaxies are resolved and show
  structure.

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Quasars

• Here are optical images of four of the first
  known quasars. They look point-like, just like
  the stars do.
• However, optical spectroscopy shows that these
  are NOT stars!

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Quasars

• However, optical spectroscopy shows that these
  are NOT stars!
• Emission lines at high redshifts are seen.

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Quasars

• However, optical spectroscopy shows that these
  are NOT stars!
• Emission lines at high redshifts are seen.

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Quasars

• Here are the spectra of various quasars.
• Emission lines at high redshifts are seen.

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The Expansion of the Universe

• Recall Hubbles observation the further a galaxy
  is away, the faster it is receding from us. That
  is, vH0d, so the recession velocity can be used
  to get the distance for an extragalactic source.
  This does not work for stars within our galaxy
  since the Milky Way is bound by its own gravity.

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The Expansion of the Universe

• Also note that distance can be thought of as time
  in the past. For example, if a galaxy is 1
  billion light years away, we are presently seeing
  the quasar as it was 1 billion years ago.
• Velocity distance lookback time

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Quasars

• These objects are very compact on optical
  photographs. They are also strong radio sources.
• Emission lines are seen, indicating a non-stellar
  energy source.
• The large Doppler shifts seen imply large radial
  velocities, which in turn implies a large
  distance using Hubbles law.

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Quasars

• The large Doppler shifts seen imply large radial
  velocities, which in turn implies a large
  distance using Hubbles law.
• The velocity can be a substantial fraction of the
  speed of light (16 in the previous example).
• The implied distance is billions of light years
  (2 billion light years in the previous example).
• These objects appear relatively bright, and the
  great distances imply exceedingly large
  luminosities.

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Quasars

• These objects appear relatively bright, and the
  great distances imply exceedingly large
  luminosities.
• It is also known that they can vary in brightness
  over timescales of days to weeks. This implies
  that the physical size is relatively small.

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Quasars

• The timescale of the variability sets a limit on
  the physical size of the object.
• Most quasars are smaller than 1 light-month.

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AGN and Quasars

• We have two types of unusual sources.
• In one case, the nucleus of a relatively nearby
  galaxy is producing large amounts of energy.
• In the other case, an unresolved source is doing
  the same.
• Supermassive black holes provide a framework to
  explain these objects

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AGN and Quasars

• First, consider possible energy sources

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AGN and Quasars

• First, consider possible energy sources
• Nuclear reactions (4H --gt 1He energy).
• Efficiency 0.07 mc2.
• Needs very high temperatures (e.g. stellar
  cores).
• Can be ruled out based on the small sizes
  inferred for quasars.

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AGN and Quasars

• First, consider possible energy sources
• Matter-antimatter annihilation.
• 100 efficient, so small volumes needed is not a
  problem.
• Antimatter appears to be rare, and
  matter-antimatter interactions give a very
  characteristic energy spectrum, which is not
  seen.

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AGN and Quasars

• First, consider possible energy sources
• Gravitational energy.
• Can be efficient (0.1 mc2).
• For the highest efficiency you need a very
  compact gravitating body. This naturally gives
  a high energy production in a small volume of
  space.

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AGN and Quasars

• First, consider possible energy sources
• Gravitational energy.
• Can be efficient (0.1 mc2).
• For the highest efficiency you need a very
  compact gravitating body. This naturally gives
  a high energy production in a small volume of
  space.
• A supermassive black hole can be the energy
  source for AGN and quasars.

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Active Galactic Nuclei

• Active galactic nuclei are believed to be powered
  by the accretion of matter onto a supermassive
  black hole.
• The material is heated as it falls, releasing
  large amounts of energy in a small volume of
  space.

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Active Galactic Nuclei

• Active galactic nuclei are believed to be powered
  by the accretion of matter onto a supermassive
  black hole.
• A spinning black hole can in principle explain
  the emission of jets.

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Active Galactic Nuclei

• Active galactic nuclei are believed to be powered
  by the accretion of matter onto a supermassive
  black hole.
• Direct evidence for supermassive black holes is a
  bit hard to come by
• In some cases, X-ray spectroscopy reveals
  emission lines with distortions caused by intense
  gravitational fields.
• In some cases, the motions of stars or gas
  (inferred from Doppler shifts) imply a high
  concentration of mass within a small space
  (recall the center of the Milky Way).

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Active Galactic Nuclei

• On the left, we see an image of M87, showing the
  jet and bright nucleus.
• On the right, spectroscopy of the accretion disk
  reveals gas with a very high orbital velocity.

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Active Galactic Nuclei

• On the right, spectroscopy of the accretion disk
  reveals gas with a very high orbital velocity.
• The orbital velocities imply a central mass of
  around 3 billion solar masses.

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The Nature of Quasars

• These are very luminous and very compact sources.
• Accretion onto a black hole can provide the
  energy source.

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The Nature of Quasars

• These are very luminous and very compact sources.
• Accretion onto a black hole can provide the
  energy source. However, how do you feed the
  black hole?

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The Nature of Quasars

• Using the Hubble Space Telescope, it has been
  possible to image faint galaxies surrounding a
  few relatively nearby quasars.

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The Nature of Quasars

• Using the Hubble Space Telescope, it has been
  possible to image faint galaxies surrounding a
  few relatively nearby quasars.
• The implication is quasars are powerful versions
  of active galactic nuclei.

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The Nature of Quasars

• Quasars are common at large distances and rare at
  relatively close distances.

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The Nature of Quasars

• Quasars are common at large distances and rare at
  relatively close distances.
• Thus, quasars were common when the universe was
  young, and disappeared as the universe aged.

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The Nature of Quasars

• Thus, quasars were common when the universe was
  young, and disappeared as the universe aged.

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The Nature of Quasars

• Thus, quasars were common when the universe was
  young, and disappeared as the universe aged.
• Probably the black holes ate up all of their
  food. No infalling gas, no energy source to
  shine.

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Quasars as Probes of the Universe

• Quasars can be seen across the universe. They
  can help us study the chemical composition of gas
  over time. Gas clouds between us and the quasar
  absorb light at different wavelengths depending
  on the distance.

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Energetic Objects

• Historically, there were three types of unusually
  energetic objects studied
• Active Galaxies
• Active Galactic Nuclei
• Quasars
• They turn out to be similar things supermassive
  black holes in the centers of galaxies.

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NextThe Origin of the Universe
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The Origin of the Universe

• But first, a few questions to ponder

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• What if the universe were infinitely large and
  infinitely old?

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• What if the universe were infinitely large and
  infinitely old?
• Everything that is possible to happen, has
  already happened.

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• What if the universe were infinitely large and
  infinitely old?
• Everything that is possible to happen, has
  already happened. In fact, it has happened an
  infinite number of times.

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• What if the universe were infinitely large and
  infinitely old?
• Everything that is possible to happen, has
  already happened. In fact, it has happened an
  infinite number of times.
• Somewhere out there, there are an infinite number
  of exact copies of you, me, this room, the Earth,
  etc.

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• What if the universe were infinitely large and
  infinitely old?
• Everything that is possible to happen, has
  already happened. In fact, it has happened an
  infinite number of times.
• Somewhere out there, there are an infinite number
  of exact copies of you, me, this room, the Earth,
  etc. On average, you have to go out unimaginely
  far to find these copies, however.

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• Why is the night sky dark? Shouldnt every line
  of sight lead to a star?

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• Why is the night sky dark? Shouldnt every line
  of sight lead to a star?

Image from Nick Strobel (http//www.astronomynotes
.com)
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• Why is the night sky dark? Shouldnt every line
  of sight lead to a star?
• The dark night sky implies the universe has a
  finite age (e.g. it is not infinitely old).

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The Beginning

• The darkness of the night sky implies that the
  universe had a beginning.
• The expansion of the universe observed today
  implies the universe began from a very compact
  state sometime in the past.

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The Expansion of the Universe

• In addition, it was found that the further a
  galaxy was away, the faster it was receding from
  us. That is, vH0d.

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The Expansion of the Universe
Image from Nick Strobel (http//www.astronomynotes
.com)
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The Expansion of the Universe

• It is important to note that we are not at the
  center of expansion!

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The Origin of the Universe

• The Big Bang.
• Odds and Ends
• Expansion of the Universe.
• Oblerss Paradox and the dark night sky.
• The age of the universe.
• The geometry of the universe.
• The cosmic microwave background
• Ripples in the background.
• The early universe and inflation.

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The Big Bang

• The Big Bang theory is an attempt to describe the
  conditions of the Universe as it evolves with
  time.

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The Big Bang

• The Big Bang theory is an attempt to describe the
  conditions of the Universe as it evolves with
  time.
• But first, lets perform a thought experiment
  using water

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• Start with a really cold state with high order,
  and add energy

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• Start with a really cold state with high order,
  and add energy
• Ice

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• Start with a really cold state with high order,
  and add energy
• Ice add energy and it melts to liquid.

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• Start with a really cold state with high order,
  and add energy
• Ice add energy and it melts to liquid.
• Water

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• Start with a really cold state with high order,
  and add energy
• Ice add energy and it melts to liquid.
• Water add energy and it turns to steam (e.g. a
  gas).

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• Start with a really cold state with high order,
  and add energy
• Ice add energy and it melts to liquid.
• Water add energy and it turns to steam (e.g. a
  gas).
• Steam

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• Start with a really cold state with high order,
  and add energy
• Ice add energy and it melts to liquid.
• Water add energy and it turns to steam (e.g. a
  gas).
• Steam Keep adding energy and at some point the
  average particle energy exceeds the chemical
  binding energy, giving H and O atoms.

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• Start with a really cold state with high order,
  and add energy
• Ice add energy and it melts to liquid.
• Water add energy and it turns to steam (e.g. a
  gas).
• Steam Keep adding energy and at some point the
  average particle energy exceeds the chemical
  binding energy, giving H and O atoms.
• Atoms

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• Start with a really cold state with high order,
  and add energy
• Ice add energy and it melts to liquid.
• Water add energy and it turns to steam (e.g. a
  gas).
• Steam Keep adding energy and at some point the
  average particle energy exceeds the chemical
  binding energy, giving H and O atoms.
• Atoms Keep adding energy and at some point the
  electrons leave the atom, leaving the nucleus
  behind.

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• Start with a really cold state with high order,
  and add energy
• Atoms Keep adding energy and at some point the
  electrons leave the atom, leaving the nucleus
  behind.
• Nuclei

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• Start with a really cold state with high order,
  and add energy
• Atoms Keep adding energy and at some point the
  electrons leave the atom, leaving the nucleus
  behind.
• Nuclei Keep adding energy and at some point the
  protons and neutrons become unbound.

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• Start with a really cold state with high order,
  and add energy
• Atoms Keep adding energy and at some point the
  electrons leave the atom, leaving the nucleus
  behind.
• Nuclei Keep adding energy and at some point the
  protons and neutrons become unbound.
• Baryons

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• Start with a really cold state with high order,
  and add energy
• Atoms Keep adding energy and at some point the
  electrons leave the atom, leaving the nucleus
  behind.
• Nuclei Keep adding energy and at some point the
  protons and neutrons become unbound.
• Baryons Keep adding energy and the protons and
  neutrons break apart to make quarks, which are
  the fundamental unit.

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• Now start with a really hot state and let it
  cool

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• Now start with a really hot state and let it
  cool
• Quarks form baryons

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• Now start with a really hot state and let it
  cool
• Quarks form baryons
• Baryons form nuclei

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• Now start with a really hot state and let it
  cool
• Quarks form baryons
• Baryons form nuclei
• Nuclei and electrons form atoms

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• Now start with a really hot state and let it
  cool
• Quarks form baryons
• Baryons form nuclei
• Nuclei and electrons form atoms
• Atoms form molecules.

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The Big Bang

• The universe began in a single point, called a
  singularity.
• The universe expanded, and the physical
  conditions change with time the density gets
  lower and the temperature drops.
• At the earliest times, there was a mixture of
  photons and elementary particles (quarks,
  electrons). As the temperature dropped,
  elementary nuclei (H and He) formed.

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The Big Bang

• The universe expanded, and the physical
  conditions change with time the density gets
  lower and the temperature drops.
• At the earliest times, there was a mixture of
  photons and elementary particles (quarks,
  electrons). As the temperature dropped,
  elementary nuclei (H and He) formed. Electrons
  were separated from the nuclei, and photons could
  not travel far owing to the free electrons.

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The Big Bang

• At the earliest times, there was a mixture of
  photons and elementary particles (quarks,
  electrons). As the temperature dropped,
  elementary nuclei (H and He) formed. Electrons
  were separated from the nuclei, and photons could
  not travel far owing to the free electrons.
• Around 400,000 years after the big bang, the
  electrons combined with the nuclei to form
  neutral atoms, and the photons traveled freely.

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The Big Bang

• Around 400,000 years after the big bang, the
  electrons combined with the nuclei to form
  neutral atoms, and the photons traveled freely.
• Eventually large scale structures formed
  (galaxies and clusters of galaxies), leading to
  what we see today.
• Is there any evidence at all for this? Yes
• The expansion.
• Cosmic Microwave Background.
• Helium abundance.

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The Microwave Background

• The universe started off very hot, and radiation
  quickly interacted with matter. This radiation
  would have a black body spectrum.
• After a time of roughly 400,000 years, the
  radiation and matter no longer interacted.
  However, the radiation should be present today.
• The spectrum was a black body spectrum with a
  temperature of about 3000 K.

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The Microwave Background

• In the dense ionized gas, photons interact with
  the charged particles.
• In the neutral gas, the photons do not interact.

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The Microwave Background

• The universe started off very hot, and radiation
  quickly interacted with matter. This radiation
  would have a black body spectrum.
• The spectrum was a black body spectrum with a
  temperature of about 3000 K. However, the
  universe has expanded, and all photons have had
  their wavelengths stretched. The black body
  spectrum should look much cooler, roughly 3 K
  today.

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The Microwave Background

• The spectrum was a black body spectrum with a
  temperature of about 3000 K. However, the
  universe has expanded, and all photons have had
  their wavelengths stretched. The black body
  spectrum should look much cooler, roughly 3 K
  today.
• A 3 K black body has a spectrum that peaks in the
  microwave region, and the expectation is that the
  universe is filled with microwave photons.

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The Microwave Background

• In the 1960s, Penzias Wilson detected faint
  microwave radiation from all over the sky. At
  first they did not understand what they found.
• Recently NASA has used satellites to measure this
  radiation more accurately.

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The Microwave Background

• The spectrum is precisely described by a black
  body with a temperature of 2.728 K.

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The Microwave Background

• The temperature was about 3000 K after
  recombination, but has cooled to just under 3 K
  as the universe has expanded.

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Big Bang Nucleosynthesis

• The universe started off with elementary
  particles (quarks, electrons, etc.).
• As the universe cooled a bit, the quarks formed
  protons and neutrons.
• For a brief time, it was hot and dense enough for
  nuclear fusion to take place

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Big Bang Nucleosynthesis

• Isotopes of hydrogen and helium are formed.
• Because the universe was cooling as it expanded,
  only a limited amount of helium was made (about
  25 of the total).

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Big Bang Nucleosynthesis

• Isotopes of hydrogen and helium are formed.
• Because the universe was cooling as it expanded,
  only a limited amount of helium was made (about
  25 of the total).
• Observations of the oldest stars show the
  predicted He abundance.

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Big Bang Nucleosynthesis

• Only H and He were made in the early times. All
  other elements were subsequently made in stars
  and dispersed back into space at much later times.

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The Big Bang

• The big bang theory accounts for three basic
  observations
• The expansion of the universe.
• The microwave background
• The abundance of helium in the oldest stars.

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The Origin of the Universe

• The Big Bang.
• Odds and Ends
• Expansion of the Universe.
• Oblerss Paradox and the dark night sky.
• The age of the universe.
• The geometry of the universe.
• The cosmic microwave background
• Ripples in the background.
• The early universe and inflation.