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Astronomy 1020
Stellar Astronomy
Spring_2017
Day-27
Course Announcements
• Mar. 31 (Fri) – Last day to drop a class with W, F, FA
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SW chapter 14 – Due Wednesday 3/29
SW chapter 15 – Due Monday 4/3
SW chapter 16 – Due Monday 4/10
SW chapter 17 – Due Friday 4/14
SW chapter 18 – Due Wednesday 4/19
SW chapter 19 – Due Wednesday 4/26
SW chapter 20 & 21 – Due Thursday 5/4 – 10:30 am – no late
accepted
• EXAM-3: Mon. 4/3 OR Wed. 4/5
• Chapters 13, 14, 15 (+??)
Course Announcements
• Next Dark Night observing: Tues. 3/28
• 8:30pm at the observatory
• Weather dependent
• There will 2 more after this – but don’t wait if the weather is
good.
• The LAST 1st Quarter night is Monday 4/3 at 7:30pm
• On campus
• Reports are due at Class Time on Monday April 24.
• NO LATE REPORTS ACCEPTED!
Lab This Week
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Hertzsprung-Russell Diagram
NAAP lab (ClassAction Web site)
What you need to know:
Reviewing Chapter 13 will help.
Where do Stars Form?
Spirals
Where don’t Stars Form?
Ellipticals
 The interstellar medium: gas and dust
between the stars.
 Near the Sun, chemical composition is similar.
 Most is gas; 1 percent is interstellar dust.
 The gas is very tenuous: about 1 atom per
cubic centimeter.
 It emits various kinds of light, depending on its
temperature.
 Dust is in the form of
solid grains.
 “Interstellar soot” (iron,
silicon, carbon, and
more).
 Dust blocks visible light
from stars, galaxies,
etc.: interstellar
extinction.
 Size of dust particles:
large molecules up to
300 nanometers.
 Therefore, dust blocks
short wavelengths more
efficiently.
 More red light is let
through: interstellar
reddening.
 Long wavelengths
(infrared and radio)
penetrate dust.
 A star viewed through dust is fainter and
redder due to interstellar extinction and
reddening.
 The position of absorption lines are not
affected.
 Dust also emits light when it blocks a star.
 Energy from absorbed light is transferred to
the dust, heating it.
 Dust grains are typically cool (10–300 K).
 Therefore, they emit infrared radiation.
iClicker Question
 ISM & Star Formation
 Wavelengths
 Most gas and dust is concentrated in
relatively dense interstellar clouds.
 The material found between clouds is called
intercloud gas.
 Some regions can be very hot (106 K).
 X-rays emitted, but extremely tenuous.
 The Sun resides in a local bubble of milliondegree gas.
 Most intercloud gas is 8000 K.
 H II regions (about 104 K):
 Hydrogen heated and ionized by ultraviolet
light from hot, luminous stars (O and the
hottest B).
 Ionized: stripped of one or more electrons.
 At lower temperatures, hydrogen is in single,
neutral atoms.
 This gas emits radio waves with  = 21 cm.
 Light of this wavelength penetrates the dust.
 Good for mapping the Milky Way.
 Many clouds are cold enough for hydrogen to
be in the H2 molecule.
 These are called molecular clouds.
 Dense and cold, and appear dark.
 Temperatures are around 10 K, with densities
as high as 1010 molecules/cm3.
 Emit radio waves.
 Many other molecules are in the mix.
 Some clouds can have masses as large as
10 million times that of the Sun: giant
molecular clouds.
 On average, 120 light-years in size.
 Stars form in these molecular clouds.
 Molecular clouds are cold and dense.
 Some places in the cloud are denser than
average.
 Self-gravity will make these regions collapse.
 Rate of collapse is slowed by magnetic fields,
turbulence, and angular momentum (spin).
 Collapse and fragmentation lead to dense
star-forming molecular-cloud cores.
 The Sun began in one of these cores.
 Molecular cores
collapse under their
own gravity.
 Center shrinks
fastest; outer layers
later.
 This produces a
dense protostar.
 Spin of core
produces a disk of
material around the
protostar.
 Material falls onto
the growing
protostar from the
disk.
 Our Solar System
began this way.
iClicker Question
 ISM & Star Formation
 Molecular Clouds
 Star Formation Masses
 The interstellar material that becomes stars
and planets must have the key elements for
life if those planets are going to have it.
 Water and oxygen have been detected in
some star-forming regions.
Protostars
 Protostars are large, cool, and luminous.
 They will emit infrared light.
 Infrared studies of molecular regions reveal
protostars and their disks.
 The protostar
continues to accrete
more material.
 It continues to
shrink and radiate
away energy,
balancing pressure
and gravity.
 The interior
temperature and
pressure rise.
MATH TOOLS 15.1
 The low temperature of dust means that it
glows in the infrared.
 100 K dust:
 10 K dust:
 The protostar’s
energy source is
gravitational energy.
 As it shrinks,
temperature rises in
the core.
 Hydrogen fusion
begins in the core:
It becomes a main
sequence star.
 The temperature in
the core must be
hot enough for
fusion, 10 million K.
 Very low-mass stars
(< 0.08 M) never
start hydrogen
fusion.
 These are called
brown dwarfs.
CONNECTIONS 15.1
 A brown dwarf is not a star, nor a planet, but is
in between.
 Classified as L, T, or Y (cooler than M stars).
 Glow in the infrared due to internal heat from
gravitational contraction.
 Over 1,000 have been found since the mid1990s.