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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• Office hours cancelled for October 8 and October
  13

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Exam 1

• N48
• Average 57.2
• Maximum 91
• Minimum 24
• std. dev. 15.6
• The average corresponds to roughly a C- using the
  guidelines in the syllabus.

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Coming Up

• Chapter 5 The Sun
• Chapter 6 The family of stars
• Homework due October 15 Question 7, Chapter 6
  (Why does the luminosity of a star depend on both
  its radius and temperature?)

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Questions for Today

• How hot is the Sun?
• How do you take the temperature of a star?

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The Sun and the Stars

• In ancient times, there were certain categories
  of objects in the sky
• The Earth.
• The Moon.
• The five planets.
• The Sun.
• The fixed stars.

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The Sun and the Stars

• In ancient times, there were certain categories
  of objects in the sky
• The Earth.
• The Moon.
• The five planets.
• The Sun.
• The fixed stars.

Since the time of Copernicus, we have known these
are the same kinds of objects.
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The Sun and the Stars

• In ancient times, there were certain categories
  of objects in the sky
• The Earth.
• The Moon.
• The five planets.
• The Sun.
• The fixed stars.

Since the time of Copernicus, we have known these
are the same kinds of objects.
Since the late 1800s, spectroscopy has shown that
the Sun is simply the nearest star.
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The Sun and the Stars

• The Sun is the nearest example of a star.
• Because it is so near, it is the only star whose
  surface we can study in any detail. The other
  stars are so far away that their apparent
  angular diameter is much too tiny to resolve
  with even the largest telescopes.

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The Angular Size of a Star

• Because of the huge distances, the angular sizes
  of most stars are about 1000 times or more
  smaller than what you can resolve with even the
  biggest telescopes. (In some cases, the angular
  diameters can be measured using a technique
  called interferometry, but this method does not
  yield an image.)

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The Sun and the Stars

• The Sun is the nearest example of a star.
• Basic questions to ask

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The Sun and the Stars

• The Sun is the nearest example of a star.
• Basic questions to ask
• What do stars look like on their surfaces? Look
  at the Sun since it is so close.
• How do stars work on their insides? Look at both
  the Sun and the stars to get many examples.
• What will happen to the Sun? Look at other stars
  that are in other stages of development.

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The Sun

• There are two broad areas of solar research
• The study of the overall structure of the Sun.
• The study of its detailed surface features.
• Think of the distinction of climate and
  weather on Earth
• Climate refers to global trends.
• Weather refers to local conditions.

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The Surface of the Sun

• The surface of the Sun can be complex.
• Surprisingly, observing the Sun can be quite
  difficult, owing to the immense heat.
• The study of the solar surface is usually done
  using many different wavelengths, from the X-rays
  to radio. Different features show up well in
  certain wavelengths.

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The Solar Surface

• The Sun has no solid surface. The part we see is
  called the photosphere.
• A visual light image captures different features
  than an ultraviolet light image.

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The Solar Surface

• The Sun has no solid surface. The part we see is
  called the photosphere.
• High resolution images of the photosphere show
  granulation.

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Granulation

• From the measurement of Doppler shifts, we know
  that the granules are blobs of gas that are
  rising and falling.
• The granules are similar to what one sees in
  boiling water on Earth.
• Energy from the interior is being transported
  outwards by motions in the gas. This type of
  energy transport is called convection.

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Solar Oscillations

• By detailed analysis of the Doppler shifts of
  different parts of the photosphere, we know that
  the photosphere oscillates (i.e. it vibrates much
  like a bell).

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Solar Oscillations

• By detailed analysis of the Doppler shifts of
  different parts of the photosphere, we know that
  the photosphere oscillates (i.e. it vibrates much
  like a bell).
• These vibrations are somewhat similar to sound
  waves in the air on Earth.

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Solar Oscillations

• By detailed analysis of the Doppler shifts of
  different parts of the photosphere, we know that
  the photosphere oscillates (i.e. it vibrates much
  like a bell).
• These vibrations are somewhat similar to sound
  waves in the air on Earth.
• Since the speed of sound in a gas depends on the
  temperature and density of the gas, the study of
  solar oscillations can reveal details about the
  solar interior.

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Sunspots

• Sunspots are darker regions on the Suns surface.
• They can be observed in the optical, and were
  first discovered by Galileo in 1610.

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Sunspots

• Note the complex structure in the spot and its
  surroundings.

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The Solar Cycle

• In the mid 1800s, a Swiss astronomer made
  detailed observations of sunspots in order to
  search for transits of a possible planet interior
  to Mercury.

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The Solar Cycle

• No planets were found, but it was discovered that
  the number of sunspots varies with an 11 year
  cycle.
• This is not fully understood.

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Sunspots

• Galileo used sunspots to track the rotation of
  the Suns surface

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Sunspots

• Galileo was the first to sunspots to track the
  rotation of the Suns surface.

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Sunspots

• Galileo was the first to sunspots to track the
  rotation of the Suns surface.
• The Sun does not rotate as a solid body. The
  equator rotates once every 25 days. At 45o
  latitude, it takes 27.8 days.

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The Sun and Space Weather

• Violent activity can occur in regions near
  sunspots.
• A solar flare is a giant eruption of particles
  and radiation.
• The radiation and particles can interact with the
  Earths upper atmosphere, disrupting satellite
  communications and power grids.

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The Sun and Space Weather

• Violent activity can occur in regions near
  sunspots.
• A solar flare is a giant eruption of particles
  and radiation.
• The cause of these giant flares is not
  understood, although magnetic fields are thought
  to play a role.

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Next

• Other Stars

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Stellar Properties

• The Sun and the stars are similar objects.
• In order to understand them, we want to try and
  measure as many properties about them as we can
• Temperature at the surface
• Power output (luminosity)
• Radius
• Mass
• Chemical composition

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Observing Other Stars

• Recall there is basically no hope of spatially
  resolving the disk of any star (apart from the
  Sun). The stars are very far away, so their
  angular size as seen from Earth is extremely
  small.

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Observing Other Stars

• Recall there is basically no hope of spatially
  resolving the disk of any star (apart from the
  Sun). The stars are very far away, so their
  angular size as seen from Earth is extremely
  small.
• Recently using interferometry, it has been
  possible to measure the angular diameters of the
  nearest stars. This is not really the same as
  imaging their surfaces.

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Observing Other Stars

• Recall there is basically no hope of spatially
  resolving the disk of any star (apart from the
  Sun). The stars are very far away, so their
  angular size as seen from Earth is extremely
  small.
• The light we receive from a star comes from the
  entire hemisphere that is facing us.

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Observing Other Stars

• Recall there is basically no hope of spatially
  resolving the disk of any star (apart from the
  Sun). The stars are very far away, so their
  angular size as seen from Earth is extremely
  small.
• The light we receive from a star comes from the
  entire hemisphere that is facing us. That is, we
  see the disk-integrated light.

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Measuring Photons

• There are 4 fundamental properties one can
  measure for a photon
• Its energy/wavelength/frequency.
• Its direction.
• Its time of arrival.
• Its polarization, which a measure of the
  direction of the electric and magnetic fields.
• The first three properties are the most commonly
  measured ones in astronomy.

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Measuring Photons

• Recall the 4 fundamental properties one can
  measure for a photon
• Its energy/wavelength/frequency.
• Depending on the detection system, there is a
  limit to the spectral resolution, which is the
  ability to tell one wavelength from another. At
  some point, the energy difference between two
  photons becomes too small to measure with your
  device.

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Observing Other Stars

• To get an understanding of how a star works, the
  most useful thing to do is to measure the
  spectral energy distribution, which is a plot of
  the intensity of the photons vs. their
  wavelengths (or frequencies or energies).

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Observing Other Stars

• To get an understanding of how a star works, the
  most useful thing to do is to measure the
  spectral energy distribution, which is a plot of
  the intensity of the photons vs. their
  wavelengths (or frequencies or energies).
• There are two ways to do this
• Broad band, by taking images with a camera and
  a colored filter.

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Observing Other Stars

• To get an understanding of how a star works, the
  most useful thing to do is to measure the
  spectral energy distribution, which is a plot of
  the intensity of the photons vs. their
  wavelengths (or frequencies or energies).
• There are two ways to do this
• Broad band, by taking images with a camera and
  a colored filter.
• High resolution, by using special optics to
  disperse the light and record it.

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Broad Band Photometry

• There are several standard filters in use in
  astronomy.
• The filter lets only light within a certain
  wavelength region through (that is why they have
  those particular colors).

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Color Photography

• The separate images are digitally processed to
  obtain the final color image.

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Color Photography
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Color Photography
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Broad Band Photometry

• Broad band photometry has the advantage in that
  it is easy (just need a camera and some filters
  on the back of your telescope), and it is
  efficient (relatively few photons are lost in the
  optics).

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Broad Band Photometry

• Broad band photometry has the advantage in that
  it is easy (just need a camera and some filters
  on the back of your telescope), and it is
  efficient (relatively few photons are lost in the
  optics).
• The disadvantage is that the spectral resolution
  is poor, so subtle differences in photon energies
  are impossible to detect.

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Broad Band Photometry

• Despite the disadvantages, broad band photometry
  is useful.
• For example, it is immediately evident that
  different stars have different colors (the
  image on the left is a composite of three images
  taken in different filters.

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Magnitudes

• Historically (e.g. Hipparcos in the First
  Century), the brightness of stars as seen by the
  eye have been measured on a magnitude scale
• The brightest stars were first magnitude.
• The faintest stars were sixth magnitude.
• Brighter objects have smaller magnitudes.

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Magnitudes

• In modern times, it was discovered that the human
  eye has a nonlinear response to light if one
  source of light has twice the photons as a second
  source, then the first source would not appear by
  eye to be twice as bright.
• The response of the eye is logarithmic, so that
  differences of magnitudes correspond to ratios of
  flux.

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The Magnitude Scale

• The modern of the magnitude scale is set up so
  that a difference of 5 magnitudes corresponds to
  a ratio of brightnesses of 100.
• Bright objects can have negative apparent
  magnitudes.

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High Resolution Spectroscopy

• To obtain a high resolution spectrum, light from
  a star is passed through a prism (or reflected
  off a grating), and focused and detected using
  some complicated optics.

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High Resolution Spectroscopy

• Using a good high resolution spectrum, you can
  get a much better measurement of the spectral
  energy distribution.

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High Resolution Spectroscopy

• Using a good high resolution spectrum, you can
  get a much better measurement of the spectral
  energy distribution.
• The disadvantage is that the efficiency is lower
  (more photons are lost in the complex optics).
  Also, it is difficult to measure more than one
  star at a time (in contrast to the direct imaging
  where several stars can be on the same image).

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Stellar Properties

• The Sun and the stars are similar objects.
• In order to understand them, we want to try and
  measure as many properties about them as we can
• Temperature at the surface
• Power output (luminosity)
• Radius
• Mass
• Chemical composition

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Spectral Classification

• In the early 1800s, Joseph Fraunhofer observed
  the solar spectrum. He saw dark regions, known
  as spectral lines (these tell us what elements
  are there).

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Spectral Classification

• In the early 1800s, Joseph Fraunhofer observed
  the solar spectrum. He saw dark regions, known
  as spectral lines (these tell us what elements
  are there).
• Starting in the late 1800s, it became possible to
  take the spectra of stars with similar detail.

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Spectral Classification

• By the early 1900s, astronomers at Harvard
  College Observatory had collected the spectra of
  hundreds of thousands of stars (one at a time!).

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Spectral Classification

• By the early 1900s, astronomers at Harvard
  College Observatory had collected the spectra of
  hundreds of thousands of stars (one at a time!).
• What does one do with them?

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Spectral Classification

• By the early 1900s, astronomers at Harvard
  College Observatory had collected the spectra of
  hundreds of thousands of stars (one at a time!).
• What does one do with them? You classify them,
  based on certain characteristics, and hope you
  can make sense out of all of them.

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Spectral Classification

• By the early 1900s, astronomers at Harvard
  College Observatory had collected the spectra of
  hundreds of thousands of stars (one at a time!).
• What does one do with them? You classify them,
  based on certain characteristics, and hope you
  can make sense out of all of them.
• Most of the early classification work was done by
  women (since they were paid less than men there
  could be more of them on the staff).

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Spectral Classification

• At first, there was no physical understanding.
• The earliest classification scheme was based on
  the strength of the hydrogen lines, with classes
  of A, B, C, D, E, F, G, H, I, J, K, L, M, N, O.
• Class A had the strongest hydrogen lines, class O
  the weakest.

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Spectral Classification

• At first, there was no physical understanding.
• The earliest classification scheme was based on
  the strength of the hydrogen lines, with classes
  of A, B, C, D, E, F, G, H, I, J, K, L, M, N, O.
• Class A had the strongest hydrogen lines, class O
  the weakest.
• Later on, only a few of these classes were kept.

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Spectral Classification

• At first, there was no physical understanding.
• The earliest classification scheme was based on
  the strength of the hydrogen lines, with classes
  of A, B, C, D, E, F, G, H, I, J, K, L, M, N, O.
• Class A had the strongest hydrogen lines, class O
  the weakest.
• Later on, only a few of these classes were kept.
  Then, subclasses were added (e.g. G2), based on
  other elements.

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Spectral Classification

• At first, there was no physical understanding.
• The earliest classification scheme was based on
  the strength of the hydrogen lines, with classes
  of A, B, F, G, K, M, O.
• Eventually, physical understanding came. It was
  discovered that the spectral type was a
  temperature indicator. As a result, a more
  natural ordering of the spectral types became O,
  B, A, F, G, K, M (the old classes were retained).

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Spectral Classification

• Here are digital plots of representative stars in
  the spectral sequence.
• Note the variation in the strength of the
  hydrogen lines.

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Spectral Classification

• This is a computer simulation of the different
  types.

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Spectral Classification

• Why do the spectral classes look different from
  one another?
• The temperature. The electrons in the atoms are
  responsible for the spectral lines, and the
  energies of the electrons are change with
  changing temperature. Example an O-star is so
  hot that the hydrogen atoms have lost their
  electrons, so no lines of hydrogen are seen.

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Spectral Classification

• http//www.astronomynotes.com

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Spectral Classification

• This is a computer simulation of the different
  types.

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Spectral Classification

• A measurement of the spectral type gives the
  surface temperature of the star.

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Spectral Classification

• A measurement of the spectral type gives the
  surface temperature of the star.
• O-stars are the hottest, with surface
  temperatures of up to 60,000 K.
• M-stars are the coolest, with temperatures of
  only 3000 K.
• The temperature of the Sun (a G2 star) is 5770 K.

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Stellar Properties

• The Sun and the stars are similar objects.
• In order to understand them, we want to try and
  measure as many properties about them as we can
• Temperature at the surface Use the spectral
  type
• Power output (luminosity)
• Radius
• Mass
• Chemical composition

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