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StarTalk
Sanjay Yengul
May 2016
"To know ourselves, we must know the stars."
Twinkle Twinkle …
 How many stars are there?
 How big are these stars?
Picture of night sky
 What are they made of ?
 Why do they shine?
 Are they all the same?
 How are they born
 how do they die?
PHOTO COURTESY NASA
The constellation Orion as seen from the space shuttle
Endeavour (STS-54)
Astronomical Units
 1 AU = distance between earth and the sun
 1 Light Year = 63,000 AU = 10 trillion km
 Speed of light: 300,000 km/s (186,000 miles/s)
 The Sun is about 8 light minutes away!
 Our nearest star: Alpha Centauri – 4 light years away!
What is a star?
 A big glowing ball of gas!
 Contains mainly H and He
 They have a core that is dense and super hot!
 Nuclear fusion is the source of their energy!
 They vary in size, temperature, brightness & color!
How far is that star?
For stars < 400 light years away:
 Parallax or Triangulation
For stars farther way:
 Indirect Method based on
brightness measurement
Galaxies
 A collection of stars
 Trillions of Galaxies in the Universe
 Our Galaxy is called the The Milky Way Galaxy
 It contains billions of stars
 Age: ~10 billion years, Size: 100,000 light years
Scale of Galaxies
Spectra
Spectral Classes
Spectral Class
Color
Ave.Temp. (K)
O
Blue-violet
30,000
B
A
F
G
Blue-white
White
Yellow-white
Yellow
20,000
10,000
8,000
6,000
K
Orange
4,000
M
Red-orange
3,000
Familiar
Examples
Mintaka (delta
Orionis)
Rigel, Spica
Vega, Sirius
Canopus, procyon
Sun, Capella
Arcturus,
Aldebaran
Antares, Betelgeuse
Star Classification
Classes of Stars by Luminosity
Class
Description
Familiar Examples
Ia
Bright Supergiants
Rigel, Betelgeuse
Ib
Supergiants
Polaris (the North star), Antares
II
III
Bright Giants
Giants
Mintaka (delta Orionis)
Arcturus, Capella
IV
Sub-giants
Altair, Achenrar (a Southern
Hemisphere star)
V
not classified
Main sequence
White dwarfs
Sun, Sirius
Sirius B, Procyon B
PHOTO COURTESY OF NASA
The Hertzsprung-Russell Diagram. The Sun, 12 brightest
stars of the Northern Hemisphere and white dwarf
companion stars to Sirius and Procyon are shown.
Star Classification
Classes of Stars by Luminosity
Class
Description
Familiar Examples
Ia
Bright Supergiants
Rigel, Betelgeuse
Ib
Supergiants
Polaris (the North star), Antares
II
III
Bright Giants
Giants
Mintaka (delta Orionis)
Arcturus, Capella
IV
Sub-giants
Altair, Achenrar (a Southern
Hemisphere star)
V
not classified
Main sequence
White dwarfs
Sun, Sirius
Sirius B, Procyon B
PHOTO COURTESY OF NASA
The Hertzsprung-Russell Diagram. The Sun, 12 brightest
stars of the Northern Hemisphere and white dwarf
companion stars to Sirius and Procyon are shown.
Birth of a star
 A gravity disturbance causes clumps of
gas and dust to collapse inward, heat up
 In ~ million years a small dense core
forms – the protostar!
 When T = 7 million K, fusion starts
 If the mass is big enough (0.1 solar
mass)  a new stable star
 If the mass is not big enough 
brown dwarf!
PHOTO COURTESY OF NASA
Death of a star
Stars like the Sun

Several billion years later, run out of hydrogen fuel
– the core contracts under gravity, outer layers
expand due to heat  Red Giant  Burns
Helium  Planetary Nebula  White Dwarf 
Black Dwarf
The Rotten Egg planetary nebula
More Massive Stars than the Sun:

Burn H, He  C  Fe  Explode into a
supernova  Neutron Star or a Black Hole

Meanwhile the shock from the supernova can
initiate star formation in other interstellar clouds
Hubble Space Telescope photographs
COURTESY OF NASA
The rings around Supernova 1987A
Scale of the Universe
References
 http://stars.astro.illinois.edu/sow/star_intro.html
 http://stars.astro.illinois.edu/sow/spectra.html
 http://www.iflscience.com/space/how-big-our-place-universe
 http://www.enchantedlearning.com/subjects/astronomy/stars/startypes.sht
ml
 Colors and Temperatures: https://docs.kde.org/trunk5/en/kdeedu/kstars/ai-
colorandtemp.html
 http://science.howstuffworks.com/star5.htm