Stellar Astrophysics

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Stellar Astrophysics
An understanding of the nature of stars, their
structure, source of energy and evolution, is
arguably the greatest astronomical discovery
of the 20th century.
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What are Stars ??
Do all stars have the same properties ? What are
the important properties of stars ? Where are
the stars ? What powers the stars ? How long do
stars exist ? What are the life-cycles of stars ?
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• To explore the stars, astronomers must obtain
• data on a variety of stellar properties.
• To understand and relate these data, keep in
• mind the following queries
• ? How is a particular property measured ?
• ? What is its significance ?
• ? In which units is it expressed ?

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Stellar Parameters

• Intrinsic
• Luminosity
• Size
• Surface Temperature
• Composition
• Mass
• Spectrum I? (?)
• Color(s)
• Rotation rate
• Magnetic field strength
• Activity Cycle
• Stellar wind strength
• Age

• Extrinsic
• Position
• Distance
• Radial velocity
• Proper motion

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The Distance Scale
Determination of intrinsic stellar parameters
requires a measure of their
distances Multiple methods are employed to
estimate distances to celestial objects,
collectively termed the distance scale The
most accurate, fundamental method involves
measure of trigonometric parallaxes Distances
are expressed in parsecs 3.26 Ly
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Why arent all the stars in this view seen at
the same brightness ?
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Stellar Magnitudes and Luminosities
Light from any source fades as the distance
squared - referred to as geometric dilution
Luminosity, L energy emitted per unit time
units ergs/s, W. Flux, F ergs/s/cm2 is the
energy passing through area ? to the
line-of-sight per unit time. Flux measurements
depend on luminosity AND distance. To determine
a stars true luminosity, both its apparent
brightness and distance must be measured.
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Stellar brightness is defined in terms of ?
magnitudes ?
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Magnitudes

• ? An archaic but still used system to quantify
  the intensity of EM
• radiation emitted by or received from
  celestial sources
• ? Brighter objects have numerically smaller
  magnitudes
• ? Magnitudes are logarithmic quantities. A
  difference of 1
• magnitude a difference of 2.512 in
  luminosity.
• ? A difference of 5 magnitudes a factor of 100
  difference in
• luminosity
• ? Magnitudes can be negative.
• ? Sun ?26.8 Full Moon ?12.5 faintest
  star visible to
• the eye 6.

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Apparent magnitude brightness of an object as
seen
from Earth Absolute magnitude the brightness
of an object if seen
from a distance of 10 parsecs
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• Magnitudes and Fluxes
• Definition
• Difference in magnitude
• The magnitude scale based on comparison with
  standard stars.
• ? Lyra (Vega) is defined as m 0

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Magnitudes and Distance
Apparent and absolute magnitudes are related via
the distance to a celestial source
d must be expressed in parsecs
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The color of stars is a measure of the stars
surface temperature. Stellar colors can be
easily measured by taking images through
different filters. Colors of stars are most
often expressed as a blue visual color,
(B V) (B V) 1 is a red star
(B V) 0 is a blue star
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Color Index
A color (index) is defined as the difference,
expressed in magnitudes, between the observed
(or intrinsic) flux of an object between two
segments of its spectrum The Johnson color
system is the most commonly used system in the
optical near-IR spectrum Johnson bands are
UBVRIJHK
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