Lecture 36
Telescopes (continued).
Basic Properties of Stars.
Chapter 17.8  17.16
• Observatories and Spacecrafts
• Stellar Brightness, Distances, Luminosities
Refractors
Refractors
Reflectors
Reflectors
Types of Telescopes
Optical and Infrared telescopes
Radio telescopes (use metal “mirrors”)
Interferometeres (link several separate telescopes
together to improve angular resolution)
Observatories
Radiotelescopes
Satellites
•
•
•
•
•
The first satellite  1957 Soviet Sputnik
First astronomical satellites  late 1960’s
The Hubble Space Telescope (HST)  1990
The X-ray Chandra Observatory  1999
The Spitzer Space (IR) Observatory  2003
Satellites
Important Stellar Parameters
Stars have similar internal structures and energy
sources.
The most important parameter, which causes
differences in a star’s appearance, is its mass.
The mass determines the star’s lifetime, surface
temperature, radius, and luminosity at any moment.
Astronomers classify stars according their
luminosities and surface temperatures.
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.
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.
The Inverse Square Law for Light
Luminosity – Distance Relation
Luminosity
Apparent brightness = ------------------4 π (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).
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.
Stellar Parallax
Parallax is the annual shift in a star’s apparent
position in the sky due to the Earth’s orbital
motion.
The parallax angle is half the annual shift.
The parallax angle of the nearest star, Proxima
Centauri, is 0.77 arcseconds.
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)
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’’.
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.
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
star’s brightness in different filters.
Betelgeuse has a temperature of ~3,400 K,
Sirius ~9,400 K, the hottest stars – up to 100,000 K.
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
Stellar Masses
It is harder to measure stellar masses.
The best method is to apply Kepler’s third law in
combination with Newton’s 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.
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
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)
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.
Star Clusters
Age of cluster = lifetime of stars at main-sequence
turnoff point.
Most open clusters are relatively young (<5 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.
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.