Integrative Studies 410 Our Place in the Universe

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INST 410 Our Place in the Universe
The Stars
gburk_at_otterbein.edu

• Office Science 238
• Secretary Sandra Salee
• FAX [phone]
• OfficeHours by appointment.

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The Stars Part I

• What properties can we measure?
• distance
• velocity
• temperature
• size
• luminosity
• chemical composition
• mass

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How Stars Got Their Names

• Some have names that go back to ancient times
  (e.g. Castor and Pollux, Greek mythology)
• Some were named by Arab astronomers (e.g.
  Aldebaran, Algol, etc.)
• Since the 17th century we use a scheme that lists
  stars by constellation
• in order of their apparent brightness
• labeled alphabetically in Greek alphabet
• Alpha Centauri is the brightest star in
  constellation Centaurus
• Some dim stars have names according to their
  place in a catalogue (e.g. Ross 154)

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Distances to the Stars

• Parallax can be used out to about 100 light years
• The parsec
• Distance in parsecs 1/parallax (in arc seconds)
• Thus a star with a measured parallax of 1 is 1
  parsec away
• 1 pc is about 3.3 light years
• The nearest star (Proxima Centauri) is about 1.3
  pc or 4.3 lyr away
• Solar system is less than 1/1000 lyr

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Our Stellar Neighborhood
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Scale Model

• If the Sun a golf ball, then
• Earth a grain of sand
• The Earth orbits the Sun at a distance of one
  meter
• Proxima Centauri lies 270 kilometers (170 miles)
  away
• Barnards Star lies 370 kilometers (230 miles)
  away
• Less than 100 stars lie within 1000 kilometers
  (600 miles)
• The Universe is almost empty!
• Hipparcos satellite measured distances to nearly
  1 million stars in the range of 100 pc
• almost all of the stars in our Galaxy are more
  distant

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Measuring Velocities

• Can use the Doppler shift to determine the radial
  velocity of distant objects
• The transverse velocity can be measured from the
  motion of stars with respect to their background
  over a period of years

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Barnards Star

• Velocity is 88 km/s (55 miles/sec)
• Moved 227 in 22 years relative to background

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Three Things Light Tells Us

• Temperature
• from black body spectrum
• Chemical composition
• from spectral lines
• Radial velocity
• from Doppler shift

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Review The Electromagnetic Spectrum
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Black Body Spectrum (gives away the temperature)
Peak frequency

• All objects - even you - emit radiation of all
  frequencies, but with different intensities

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Measuring Temperatures

• Find maximal intensity
• ? Temperature (Wiens law)

Identify spectral lines of ionized elements ?
Temperature
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Wiens Law

• The peak of the intensity curve will move with
  temperature, this is Wiens law
• ? T const. 0.0029 m K
• So the higher the temperature T, the smaller
  the wavelength ?, i.e. the higher the energy of
  the electromagnetic wave

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Spectral Lines Fingerprints of the Elements

• Can use spectra to identify elements on distant
  objects!
• Different elements yield different emission
  spectra

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• The energy of the electron depends on orbit
• When an electron jumps from one orbital to
  another, it emits (emission line) or absorbs
  (absorption line) a photon of a certain energy
• The frequency of emitted or absorbed photon is
  related to its energy
• E h f
• (h is called Plancks constant, f is
  frequency)

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Luminosity and Brightness

• Luminosity L is the total power (energy per unit
  time) radiated by the star
• Apparent brightness B is how bright it appears
  from Earth
• Determined by the amount of light per unit area
  reaching Earth
• B ? L / d2
• Just by looking, we cannot tell if a star is
  close and dim or far away and bright

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The Magnitude Scale

• A measure of the apparent brightness
• Logarithmic scale
• Notation 1m.4 (smaller ?brighter)
• Originally six groupings
• 1st magnitude the brightest
• 6th magnitude the dimmest
• The modern scale is more complex
• The absolute magnitude is the apparent magnitude
  a star would have at a distance of 10 pc 2M.8

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Which of the following magnitudes is the
brightest?

• -1.4 m
• 0.0m
• 1.3m
• 14m

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Measuring the Sizes of Stars

• Direct measurement is possible for a few dozen
  relatively close, large stars
• Angular size of the disk and known distance can
  be used to deduce diameter

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Indirect Measurement of Sizes

• Distance and brightness can be used to find the
  luminosity
• L ? d2 B (1)
• The laws of black body radiation also tell us
  that amount of energy given off depends on star
  size and temperature
• L ? R2 ? T4 (2)
• We can compare two values of absolute luminosity
  L to get size

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Sizes of Stars

• Dwarfs
• Comparable in size, or smaller than, the Sun
• Giants
• Up to 100 times the size of the Sun
• Supergiants
• Up to 1000 times the size of the Sun
• Note Temperature changes!

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Two stars have the same chemical composition,
spectral type, and luminosity class, but one is
10 light years from the Earth and the other is
1000 light years from the Earth. The farther
star appears to be

• a) 100 times fainter.
• b) 10,000 times fainter.
• c) 100,000,000 times fainter.
• d) the same brightness since the stars are
  identical.

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Classification of the Stars Temperature

• Class Temperature Color Examples
• O 30,000 K blue
• B 20,000 K bluish Rigel
• A 10,000 K white Vega, Sirius
• F 8,000 K white Canopus
• G 6,000 K yellow Sun, ? Centauri
• K 4,000 K orange Arcturus
• M 3,000 K red Betelgeuse

Mnemotechnique Oh, Be A Fine Girl/Guy, Kiss Me
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Hertzsprung-Russell-Diagram

• Hertzsprung-Russell diagram is luminosity vs.
  spectral type (or temperature)
• To obtain a HR diagram
• get the luminosity. This is your y-coordinate.
• Then take the spectral type as your x-coordinate.
  This may look strange, e.g. K5III for Aldebaran.
  Ignore the roman numbers ( III means a giant
  star, V means dwarf star, etc). First letter is
  the spectral type K (one of OBAFGKM), the arab
  number (5) is like a second digit to the spectral
  type, so K0 is very close to G, K9 is very close
  to M.

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Constructing a HR-Diagram

• Example Aldebaran, spectral type K5III,
  luminosity 160 times that of the Sun

L
1000
Aldebaran
160
100
10
1
Sun (G2V)
O B A F G K M
Type
0123456789 0123456789 012345
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The Hertzprung-Russell Diagram

• A plot of absolute luminosity (vertical scale)
  against spectral type or temperature (horizontal
  scale)
• Most stars (90) lie in a band known as the Main
  Sequence

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A very hot, but dim star shows up where in a
Hertzsprung-Russel diagram?

• Left upper corner
• Middle
• Lower left corner
• Upper right corner

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Hertzsprung-Russell diagrams

• of the closest stars of the brightest stars

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Mass and the Main Sequence

• The position of a star in the main sequence is
  determined by its mass
• ?All we need to know to predict luminosity and
  temperature!
• Both radius and luminosity increase with mass

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Stellar Lifetimes

• From the luminosity, we can determine the rate of
  energy release, and thus rate of fuel consumption
• Given the mass (amount of fuel to burn) we can
  obtain the lifetime
• Large hot blue stars 20 million years
• The Sun 10 billion years
• Small cool red dwarfs trillions of years
• ?The hotter, the shorter the life!

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• Most complete Hertzsprung-Russel diagram to date
  over 20,000 stars shown

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Binary Stars

• Some stars form binary systems stars that orbit
  one another
• visual binaries
• spectroscopic binaries
• eclipsing binaries
• Beware of optical doubles
• stars that happen to lie along the same line of
  sight from Earth
• We cant determine the mass of an isolated star,
  but of a binary star

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Visual Binaries

• Members are well separated, distinguishable

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Spectroscopic Binaries

• Too distant to resolve the individual stars
• Can be viewed indirectly by observing the
  back-and-forth Doppler shifts of their spectral
  lines

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Eclipsing Binaries (Rare!)

• The orbital plane of the pair almost edge-on to
  our line of sight
• We observe periodic changes in the starlight as
  one member of the binary passes in front of the
  other

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Spectroscopic Parallax

• Assuming distant stars are like those nearby,
• from the spectrum of a main sequence star we can
  determine its absolute luminosity
• Then, from the apparent brightness compared to
  absolute luminosity, we can determine the
  distance (B ? L / d2 again!)
• Good out to 1000 pc or so accuracy of 25

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Solar Activity and Temperature
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A Stellar Zoo of odd species
Wolf-Rayet Stars
Wolf-Rayet stars are hot (25-50,000 degrees K),
massive stars (20 solar mass) with a high rate
of mass loss. These stars are fusing carbon and
nitrogen and generate intense winds of material
being blown off
Wolf-Rayet 124 by HST
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A Stellar Zoo of odd species
Quasars Quasi-stellar Radio Sources
Enormously energetic, huge red-shifts, most
luninous objects in the universe, probably
powered by rotating massive black holes
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A Stellar Zoo of odd species
Supernova When a massive star runs out of fuel
and its core collapses. Source of the metals
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A Stellar Zoo of odd species
Carbon Stars
Carbon stars loose a significant fraction of
their total mass in the form of a stellar wind
which ultimately enriches the interstellar gas
ashes of nuclear helium fusion and the source of
material for future generations of stars. TT Cyg
is about 1,500 light-years away in Cygnus
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A Stellar Zoo of odd species
Magnetars
On 27 December 2004, the radiation from an
extremely powerful explosion on the surface of
SGR 1806-20 (the numbers indicate its position in
the sky) reached Earth and lasted more than 6
minutes. During the first 200 ms, the amount of
energy released was equivalent to what our Sun
radiates in 250 000 years. It is the brightest
event known to have impacted the Earth from an
origin outside our solar system. SGR 1806-20
is located at around 50 000 light-years from
Earth on the far side of our Milky Way galaxy, in
the direction of the Sagittarius constellation. A
similar blast within 10 light years would have
destroyed the ozone layer and be similar to a
major nuclear blast. Fortunately, the closest
known magnetar is 13 000 lightyears distant
from ESA - 31 Oct 2007
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A Stellar Zoo of odd species
Pulsars and Magnetars
The Death Stars of science fiction, these
spinning neutron stars emit intense radiation
beams
Artistic impression of the two pulsars orbiting
around the common centre of mass in 2.4 hours.
The faster rotating pulsar spins 45 times per
second or almost 3000 times per minute. In the
same time, the slower rotating pulsar spins only
22 times or every 2.8 seconds. Credit Michael
Kramer (Jodrell Bank Observatory, University of
Manchester)