Telescopes (continued).

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Lecture 36

• Telescopes (continued).
• Basic Properties of Stars.

Chapter 17.8 ? 17.16

• Observatories and Spacecrafts
• Stellar Brightness, Distances, Luminosities

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Refractors
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Refractors
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Reflectors
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Reflectors
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Types of Telescopes
Optical and Infrared telescopes Radio telescopes
(use metal mirrors) Interferometeres (link
several separate telescopes together to improve
angular resolution)
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Observatories
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Radiotelescopes
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Satellites

• The first satellite ? 1957 Soviet Sputnik
• First astronomical satellites ? late 1960s
• The Hubble Space Telescope (HST) ? 1990
• The X-ray Chandra Observatory ? 1999
• The Spitzer Space (IR) Observatory ? 2003

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Satellites
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Important Stellar Parameters
Stars have similar internal structures and energy
sources.
The most important parameter, which causes
differences in a stars appearance, is its mass.
The mass determines the stars lifetime, surface
temperature, radius, and luminosity at any moment.
Astronomers classify stars according their
luminosities and surface temperatures.
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Stellar Luminosity
Luminosity is the total amount of power the star
radiates into space. It is measured in power
units (Watts).
Brightness of a star in the sky depends on the
distance towards a star and its luminosity.
The apparent brightness is the amount of light
reaching us per unit area.
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Apparent Brightness
Apparent brightness obeys an inverse square law
with distance.
At the distance of Jupiter is 5 A.U., the Sun is
25 times dimmer than on Earth.
Alpha Centauri radiates almost the same amount of
light as the Sun, but it is located 27,000 times
further away from Earth than the Sun. Thus, its
apparent brightness is 70 billion times less than
that of the Sun.
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The Inverse Square Law for Light
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Luminosity Distance Relation

Luminosity Apparent brightness
-------------------
4 p (distance)2
The units of apparent brightness are Watts per
square meter.
Luminosity is also measured in the units of solar
luminosity (LSun 3.8 1026 Watts).
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Measuring the Apparent Brightness
Stars emit radiation of all wavelengths. No
detector is sensitive to the entire spectrum.
Usually we measure apparent brightness in a small
range of the complete spectrum.
Eyes are sensitive to visible light. When we
measure the apparent brightness in the visible
region, we can calculate only the visible?light
luminosity.
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Stellar Parallax
Parallax is the annual shift in a stars apparent
position in the sky due to the Earths orbital
motion.
The parallax angle is half the annual shift.
The parallax angle of the nearest star, Proxima
Centauri, is 0.77 arcseconds.
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Parsec
An object with a parallax of 1 arcsecond is
located at the distance of 1 parsec.
1 pc 3.26 light-years 3.09 1013 km
1 d (in
parsecs) --------------------------
p (in arcseconds)
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Stellar Magnitudes
Historically stellar brightness is described in
magnitudes suggested by Hipparchus.
The brightest stars received the designation
first magnitude, the next brightest second
magnitude, etc.
The faintest stars visible by the naked eye are
sixth magnitude.
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Stellar Magnitudes
The modern magnitudes system is more precisely
defined. Since a star may have any brightness,
fractional apparent magnitudes are possible. For
example, a star of magnitude 1.00 is 2.5 times
brighter than a star of magnitude 2.00.
The brightest star in the sky is Sirius with an
apparent brightness of 1.46. The faintest stars
observed with HST are of 30th magnitudes.
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Surface Temperature
Surface temperature determines a star color.
The coolest stars are red, the hottest ones are
blue.
Only the brightest star colors can be recognized
by the naked eye. The color can be determined
better by comparing a stars brightness in
different filters.
Betelgeuse has a temperature of 3,400 K, Sirius
9,400 K, the hottest stars up to 100,000 K.
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Spectral Type
The surface temperature also determines the line
spectrum of a star.
Hot stars display lines of highly ionized
elements, while cool stars show molecular lines.
Stars are classified by assigning a spectral
type. The hottest stars are called spectral type
O, followed by B, A, F, G, K, M as the surface
temperature declines.
Oh Be A Fine Girl, Kiss Me
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Stellar Masses
It is harder to measure stellar masses. The best
method is to apply Keplers third law in
combination with Newtons law of gravity.
This procedure can only be applied to orbiting
objects Visual binary a resolved pair of stars
(Mizar) Eclipsing binary a pair orbiting in the
plane of our line of sight Spectroscopic binary
an object with regularly moving spectral lines or
with 2 line systems.
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The Hertzsprung-Russell Diagram
Invented by Ejnar Hertzsprung (Denmark) and Henry
Norris Russell (USA) in 1912.
The diagram is a plot of stellar luminosities
against their surface temperatures.
Temperature increases leftward. Luminosity
increases upward.
H-R diagram
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Patterns in the H-R diagram
Main sequence location of the most stars (from
upper left to lower right corner) Luminosity
class V
Supergiant branch along the top (class I) Giant
branch just below the supergiants (class III)
White dwarfs near left corner (small size, high
temperature)
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Star Clusters
Open clusters and globular clusters.
Open clusters contain a few thousands stars and
span 30 light-years (10 pc). Pleiades Globular
clusters can contain more than a million stars
and span 60-150 light-years.
Stars in clusters are at the same distance from
the Sun and are formed at about the same time. It
is easy to determine clusters ages.
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Star Clusters
Age of cluster lifetime of stars at
main-sequence turnoff point.
Most open clusters are relatively young (lt5
billion years). Globular clusters are typically
old objects (12-16 billion years), the oldest
objects in the galaxy. They place a limit on the
possible age of the Universe.
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Summary
The differences between stars are due to their
initial mass and current age. The HR diagram is
one of the most powerful tools of
astronomers. Stars spend most of their lives at
the main sequence. The most massive stars live a
few million years, while the least massive stars
live more than the current Universe age.