Download Lecture 6: Properties of Stars The Constellations The Constellations

Lecture 6: Properties of Stars
o  Overview:
The Constellations
o  Ancient astronomers grouped stars into constellations representing people, animals,
etc. These have no real significance.
Orion Constellation
o  Constellations
Betelgeuse
o  Stellar Brightness
o  Below are brightest stars that make up Ursa Major - also called ‘The Plough’ or the
‘Big Dipper’. Look north this evening. Grid lines indicate position on the celestial
sphere; Right Ascension (RA) and Declination (Dec).
o  Stellar Distances
o  Colour and Temperature
o  The Hertzsprung-Russell Diagram
NASA Solar Dynamics Observatory
Rigel
o  Chapter 6 of Introduction to Astronomy
and Cosmology
The Constellations
o 
Position on sky – the celestial sphere – is given by Right Ascension (RA) and Declination
(Dec) - celestial longitude and latitude.
Apparent Magnitudes
o  Greek astronomer Hipparchos made first known
catalogue of stars in ~130-160 BC, which was added to
by Ptolomy in ~150 AD.
o  Hipparcus grouped stars into six magnitude groups,
with 1st magnitude being brightest and 6th the faintest.
o  In 19th century, it was shown that stars of a given
magnitude are ~2.5 times brighter than those of next
fainter magnitude.
o  1st magnitude stars are ~100 times brighter than 6th
magnitude stars.
o  Observed magnitude are called apparent magnitudes.
Hipparchus of Rhodes
(180-125 BC)
Apparent Magnitudes
o  Norman Robert Pogson (1829-1891) put magnitude
system on a precise scale. Realized eyes respond to
light logarithmically.
o  If F1 is flux (Wm-2) from Star 1 and F2 is the flux from
Star 2 then
m2 – m1 = 2.5 log10( F1 / F2 )
Stellar Distances
o  Stellar distances are measured using parallax.
o  Distant stars used as reference points. Closer star appears to move relative to distant
stars during Earth’s orbit about Sun.
o  Parallax angle: p ~ 1 AU / d => d = ~ 1 AU / p
where m2 and m1 are magnitudes of Star 2 and 1.
o  The zero point of system is defined by the star Vega,
which is magnitude zero.
o  Some stars like Sirius and Canopus are brighter than
Vega, so have negative magnitudes.
Stellar Distances
o 
As angle and distance are inversely related, astronomers use a unit called parsec (pc):
Absolute Magnitudes
o  If all stars were at a fixed distance, their brightness would be a true indication of
their relative luminosity.
d=1/p
where d is the distance in parsec and p is the parallax in arcseconds.
o  Used this idea to set up absolute magnitude scale.
o  The absolute magnitude is defined as the apparent magnitude at a distance of 10 pc.
o 
A star with a parallax of 1/10th of an arcsecond will be at a distance of 10 pc.
o 
Note: 1pc = 3.26 light years
o  Using
o 
Nearest star has a parallax of 0.772 arcsec => distance in light years:
o  As flux (F) varies as 1 / d2
=> mabs – mapp = 2.5 log10( ( dabs / dapp )2 )
o  Setting dabs = 10 pc
=>
d = 1/ 0.772 pc
= 1.295 pc
= 1.295 x 3.26 light years
= 4.22 light years
mabs – mapp = 2.5 log10( Fapp / Fabs )
mabs = mapp + 2.5 log10( ( 10 / dapp )2 )
o  Example: Rigel is at 237 pc and an apparent magnitude of 0.12. What is its absolute
magnitude (at 10 pc)?
Colour and Temperature
The Sizes of Stars
o  Colour tells us about temperature.
o 
Angular sizes of a few nearby stars can be measured directly.
o  Use Wein’s Law for black bodies:
o 
Diameter = distance x angular width in radians
o 
For Betelgeuse, diameter ~1 billion km or 700 times diameter of the Sun.
T = 2.897 x 10-3 / λmax
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Blue stars:
White stars:
Yellow stars:
Orange stars:
Red stars:
λmax
~ 20,000 K
~ 10,000 K
~ 6,000 K
~ 4,500 K
~ 3,000 K
o  Temperature has a major effect on the spectral lines seen in atmosphere of a star
The Sizes of Stars
o 
Optical interferometer uses two or more mirrors separated by tens of metres to measure much
smaller stellar diameters.
o 
Two 8.2 m telescopes of Very Large Telescope at European Southern Observatory in Chile
used to measure diameter of Promixa Centauri, nearest start to Earth ~ 1/7th that of Sun.
Stellar Luminosity
o  Stars have a very wide range of intrinsic luminosity – their energy output
across the whole electromagnetic spectrum.
o  Luminosity can be calculated using Stefan-Boltzmann Law:
L = σ A T4 watts
where A is area, T is temperature and σ is Stefan-Boltzmann constant
(5.671 x 10-8 W m−2 K−4)
o  Luminosity of Sun, Lsun, is 3.86 x 1026 watts.
See more at www.eso.org/public/images/eso0134a/
Stellar Spectra
Stellar Spectra
o 
In 1666, Isaac Newton, using a prism, showed that sunlight composed of all the colours of
spectrum.
o 
In 1804, William Wollaston observed dark lines in spectrum. In 1911, Joseph Fraunhofer
mapped many of these “absorption lines” with reasonable accuracy - called Fraunhofer lines.
‘Na D lines’
In late 19th century, spectra of thousands of stars were photographed and classified into
spectral types by a Harvard astronomers – mainly women.
o 
O type stars are hottest (30,000 – 60,000 K). Contain ionised helium (He+ or He II).
o 
M type stars are the coolest (<3,500 K). Molecular lines present.
Slit position
60,000 K
λ
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Temperature
Kirchoff and Bunsen found wavelengths of absorption lines seen in Sun corresponded to
emission lines observed when the atoms of a particular element are excited e.g., salt gives
orange colour due to pair of sodium “D lines”.
3,500 K
λ
Hertzsprung-Russell Diagram
Hertzsprung-Russell Diagram with Sizes
o  In early 1900s, Hertzprung and Russell independently plotted stellar luminosity
against temperature – the H-R diagram.
o  Main Sequence contains 90% of stars.
o  Sun is on Main Sequence – G2 star.
o  Top right has bright cool stars – Red
Giants.
o  Betelgeuse in Orion is a red supergiant.
o  Rigel in Orion is blue supergiant.
o  White dwarfs are small and hot.
Sun
Lecture 6 – Practical Task
o  Identify the constellations of Lyra and Cygnus in the night sky – they are almost
overhead in the early night sky.
Vega: 5th brightest star
in sky. It is a bluewhite star having
magnitude of 0.03,
and lies 26 light years
away. It weighs ~3MS
is ~50 times brighter.
Cygnus X-1: X-ray
source ~14.8MS
Thought to be a
black hole.
M57 - Ring Nebula