Transcript astrocoursespring2012lec5-1-1

V Spring 2012 Astronomy Course
Mississippi Valley Night Sky Conservation
The Night Sky Around Us
Program developed by
Mississippi Valley
Conservation Authority
Royal Astronomical Society
of Canada
Ottawa Astronomy Friends
Instructors:
Pat Browne
Stephen Collie
Rick Scholes
Course Assistant:
Amy Booth
Earth Centered Universe
software for illustrations –
courtesy David Lane
Announcements:
1. Donated set-screw to FLO
2. Transit of Venus courtesy Rick
3. Lab time/Observing after
Night Sky V - Observable Cosmic distances
As we continue, peering deeper and deeper into space,we climb the distance ladder to
find star cluster distances, galaxy distances and beyond
Astronomers speak of a
distance ladder
Each rung gives a leg-up to
the next indirect way to
determine distances
Most indirect methods use
the idea of a "standard
candle", i.e. something that
you believe you know how
luminous it is and you can
determine its distance by
measuring its brightness.
Examples: White Dwarf
Supernovae, Globular
Clusters distributions, The
Tip of the Red Giant
Branch, Surface Brightness
Fluctuations.
http://cse.ssl.berkeley.edu/bmendez/ay10/2002/notes/lec17.html
First Rung - Distance Ladder
Rung 1 Estimating Distance to
Nearby Star - Parallax
Star measured 2
different points and
times of Earth Orbit:
Nearby stars have proper motion
when measured against the more
distant background stars For
relatively nearby stars we use
Trigonometric Parallax
p
d
Point 1: Earth is on
one side of the Sun
Point 2 (+ 6 Months)
Opposite Side.
The nearby star
appears to shift its
position, relative to
more distant stars,
because we are
viewing it along two
slightly different lines
of sight.
Based on
http://www.astro.gla.ac.uk/users/martin/
ase/runaway_ase.htm
1 AU (astronomical unit) = Sun-Earth
Tan(p) = AU / d
Parallax angle and using the Astronomical Unit
Distance = 1 AU / Tan(p)
No more than a few arc secs (or 1/3600 degs)
Distance = 1 / theta ( very small angle)
For p of 1 arc sec, this distance corresponds to 206,265 AUs or
3.26 light years (63,115 aus/ly)
(This is the definition of the parsec
Definition of parallax arc-sec - parsec:
http://www.youtube.com/watch?v=6zV3JEjLoyE&feature=relmfu
Historically and Currently Speaking… Transit of Venus June 5/6, 2012
Transit of Venus expeditions:
Using Venus parallax angle we can get
distance to the Sun (1 AU)
Astronomers mounted expeditions at 2
different locations to determine parallax
angle of Venus against the Sun…
Only possible when a Transit of Venus
occurs…
Distance Ladder Rung 2
Variable Stars example Delta Cephei
http://en.wikipedia.org/wiki/Delta_Cephei
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Cepheid Variables are giant stars
that pulsate with a regular
periodicity 1-50 days
•
Mechanism: Outer atmospheres
puff outwards, making them
larger and brighter, then cool off
and fall back, making them
fainter.
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Progenitor Star – Delta Cepheii
Distance Ladder – Rung 2
Cepheid Variable Stars in Star Clusters
Cepheids are found in open
clusters, globular clusters,
and nearby galaxies. Here
we see a study of Cepheids
in globular cluster M3
(40,000 lys distance).
http://astro.unl.edu/naap/distance/cepheids.html
http://vger.pa.msu.edu/posters/M3M5Talk.pdf
Astronomical Procedure: From Apparent Magnitudes of
Variable Stars to their Distances
Plot apparent magnitude values from
observations at different times results in a
light curve for a Cepheid in the Large
Magellic Clouds – our closest extragalactic neighbour.Henrietta Leavitt did just
that.
She discovered a simple relation
between the brightness of LMC
Cepheid variables and their
periods ... Since the variables are
probably at nearly the same
distance from the Earth, their
periods are apparently associated
with their actual emission of light,
as determined by their mass,
density, and surface brightness
The actual emission or luminousity
is a direct measurement of absolute
magnitude.
http://upload.wikimedia.org/wikipedia/commons/b/ba/1777_Variables_in_the
_Magellanic_Clouds_Henrietta_Swan_Leavitt.png
Next Rung – Beyond Globular Clusters – Magallenic Clouds
Discovery of the Period-Luminosity
Relation was extremely important for
measuring distances It meant that, by
measuring the pulsation period of certain
Cepheid variable star, one could deduce its
luminosity from the Period-Luminosity
relation, and thus determine its distance from
its apparent brightness.
The longer the period, the more luminous the Cepheid
http://www.astro.gla.ac.uk/users/martin/ase/runaway_ase.htm
Note: Using the Hipparcos Space Mission data which
has done measurements (parallax angles with
miliarcsec precision), we can ‘truth’ the closer
Cepheids by this astrometric reference.
See What is astrometry – Hipparcos site
http://www.rssd.esa.int/index.php?project=HIPPARC
OS&page=astrometry
Distance From Known Distances in our Galaxy
Question: How far away is the Large Magellanic
Cloud? If we don't know that, we can't convert the
relative distances to absolute distances in parsecs. The
LMC distance needs to be established from Cepheids
within our galaxy.
http://spiff.rit.edu/classes/phys240/lectures/lmc/lmc.html
We can use the main-sequence
fitting technique to compare more distant open
clusters to nearby open clusters, and thereby
determine further distances. This eventually
leads to Cepheid distances within the clusters
contained in the LMC.
Hence we climb up the next rung of the distance
ladder.
We have different Cepheids and other variables
to choose from to confirm these indirect
measurements.
Distance Modulus
Given Apparent Magnitude and
calibrated measurements of Absolute
Magnitude
Distances to Unknown Clusters are
Calibrated
Distance Modulus:
apparent – Absolute = f(D)
m-M = m – M = f(Distance in pc).
Here the best fit
m – M = 5.5
m – M = 5 log d – 5
D = antilog ((m -M + 5)/5)
M
.
Other Variables… confirm distances to the LMC and Beyond
The distance to the LMC – practical considerations
Different methods to measure the distance to the LMC,
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RR Lyrae stars are another class of pulsating
variable stars. They are much fainter than
Cepheids, and much more common. Bright
enough that we can see them easily in the
Magellanic Clouds, and in a few other members
of the Local Group – not so beyond that
•
Cluster Main sequence fitting, as described ,
compares the color-magnitude diagram of stars
in clusters. Good News : Star clusters in the
LMC; the bad news is that the stars in them
have a somewhat different chemical composition
than stars in the Milky Way.
•
Eclipsing binary stars in the LMC. By
combining measurements of their light, as the
stars eclipse each other, with measurements of
their radial velocities, as they move in their
orbits, we can calculate the distance to such
systems. It is necessary to use theoretical
models of stellar atmospheres in the process,
however, which lends some significant
uncertainty to the resulting distance.
•
Tip of the Red Giant Branch (TRGB) Stars at
a certain point in their lives evolve in the colormagnitude diagram to a particular point, which
appears to have roughly the same luminosity for
almost all stars. Many of these stars are close
enough
Distance to LMC – Simple Calculation based in Cepheids within the Milky Way
For this example, based on Cepheids within the Milky Way
apparent magnitude: 15.57
Absolute Magnitude: -3.6
We can derive the distance
d = 10 ^(m - M + 5)
d = 10 ^(15.57 - (-3.6) + 5)/5
d = 10^ 24.17/5
d = 10 ^4.834
d = 68,230 parsecs
This means that the Cepheid in the LMC is about 68.2 kpc
(or about 222,000 light years away).
Since the Cepheids as a group are at relatively the same
distance
this is the derived distance to the LMC
As mentioned, astronomers try and observe as many
Cepheids as possible in another galaxy in order to determine a
more accurate distance. As the number of stars observed go
up
http://outreach.atnf.csiro.au/education/senior/astrophysics/variable_cepheids.html
the uncertainties involved in calculations
for individual stars
can be statistically reduced.
V Night Sky Around Us
Beyond the Milky Way, The Realm of the Galaxies
WHERE
Locating Galaxies by star-hopping
Observing individual island Universes
(poetic term)
Observing interacting galaxies
Observing clusters of galaxies (Virgo)
WHEN
Are they Visible?
Spring time ! –
We are pointing out towards the NGP
(North Galactic Pole, located in Coma
Berenices)
WHAT
Types of Galaxies
(Face on, Edge on, Elliptical, Spiral,
Barred Spiral, Irregular, Peculiar)
– depends on our viewpoint and their
intrinsic geometry
WHERE: > 2 Million Light Years beyond the
Milky Way
Famous Examples:
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M64 – Blackeye Galaxy - Spiral
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M51 – Whirlpool Galaxy - Interacting
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Virgo Cluster of Galaxies – Our Local
Group
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M65,M66 (and NGC 3628) – The Leo
Triplet, M65, face on, M66 Edge On)
M51
M51 Whirlpool
Galaxy
BlackEye
Calaxy M64
M64
M65.M66
Leo
Triplet
Fanciful Descriptions …
BlackEye Galaxy M64 is brighter than 10 billion suns (luminousity).
However this depends on our distance estimate.
Estimates of 10 to 40 Million Light Years are used for M64.
Black band is dust which obscures part of the nucleus of the
galaxy. (Turn Left at Orion,Consolmagno and Davis)
M64- long black cloud stretches across its face. Its dust lane is raw
material that someday will be part of stars and planets, and just as
long-gone dust clouds within our own galaxy are now a
part of you, dear reader, and me. (Deep Sky Objects, Levy)
http://www.asod.info/?p=1759
astronomy sketch of the day – Getting a Black Eye in Coma Berenices
M65
Leo Triplet: Messier 66 is part of a really delightful trio of galaxies,
of which M65 and NGC 3628 are the other members.
While M65 is almost edge-on in appearance, M66 is angles so that we
see more of its face, including one spiral arm that hangs more limply tha
the other, as if the galaxy had suffered some cosmic fall that injured its
shoulder – David Levy, p. 193
http://www.asod.info/?p=1699 (image asod – Dale Holt
)
NGC 3628 hides its spiral structure because we see it edge-on
The dust lane here is very prominent.
M66
Clusters of Galaxies
Past the Milky Way… to other systems with billions
of stars…
As we dart away from our home galaxy at many
times the speed of light to get to the next cluster
of galaxies in the constellations of Virgo and Coma
Berenices, we travel some 50 million light years
As we reach the galaxies of Virgo
and Coma Berenices, we realize that our
Local Group is bound to this cluster –
thousands of galaxies are sharing the same part of
space, sharing the same destiny…
(Deep Sky Objects, David Levy, p 188)
https://community.emc.com/people/ble/blog/2012/02/28/space-is-flat-what-does-it-mean
Standard Candles in other Galaxies
Just as we use Cepheid Variables to gauge distances in and around our galaxy, we use
Supernova Type 1a beacons found in other galaxies to obtain
absolute Magnitude Measurements.
Extra-galactic Standard candle - a particular type of exploding star known as a type Ia
Supernova. These objects are thought to occur in binary systems when a white dwarf star,
orbiting around a red giant companion from which it is "gobbling up" matter because of its strong
gravitational pull, is pushed over the limiting mass which such a white dwarf star is allowed to
have: the Chandrasekhar Limit, about 1.4 times the mass of the Sun. When this limiting mass is
exceeded, it causes a violent thermonuclear explosion, which releases a huge amount of energy making the type Ia supernova an extremely luminous object. Moreover, since the explosion
always happens once the Chandrasekhar Limit has been exceeded, the luminosity of all type Ia
supernovae is remarkably consistent - making them excellent standard candles.
http://en.wikipedia.org/wiki/Type_Ia_supernova
See also http://www.astro.gla.ac.uk/users/martin/ase/runaway_ase.htm
http://www.bautforum.com/showthread.php/38030-Supernova-in-M100-Spiral-Galaxy?s=5c7d93be72c3a684d130139fe3fd9513
The Ultimate Spectral Distance Ladder
Question Raised: What is cosmological redshift ?
It is the spectral shift in wavelength due to the velocity
of
the space-time fabric between the observer and the
distant object (galaxy).
It is a measurement of the recession velocity – a
velocity
that is not intrinsic to the motion of the object, but due
to the fact that the universe is expanding according to
Hubble’s Law:
Recessional Velocity = Hubble's constant times
distance
V = Ho D
In cosmological redshift, the wavelength at which
the
radiation is originally emitted is (only) lengthened as it
travels through (expanding) space. A Cosmological
redshift results from the expansion of space itself
and not from the motion of the object. So the
recessional
velocity is not the galaxies motion, but the motion of
space-time. This is a very special spectral shift indeed!
Night Sky V - Extreme Cosmic distances
Then… as the telescope looks outward the realm of the
superclusters stretches into unmapped deserts of time…As a
telescope looks backward into time (or out into space) the galaxies
appear smaller and fainter.
When a telescope probes about 5 billion light years into look-back
time, it can detect only the brightest galaxies, giant, elliptical
galaxies
– because spiral galaxies similar to the Milky Way are too dim to be
seen at that distance
First Light, The Search for the Edge of the Universe, p. 56
Richard Preston
Quasi-Stellar Objects
Quasars – Deeply Red-shifted Luminous objects
At first it wasn't understood what these objects were, since their spectra were unlike
those of any known stars. Its spectrum did not resemble that of any normal stars
with typical stellar elements. 3C 273 was the first object to be identified as what we
now know quasars to be — extremely luminous objects at cosmological distances.
“Maarten Schmidt found an object among the most distant galaxies
that burned with a terrifying light”.
First Light, The Search for the Edge of the Universe
by Richard Preston
p. 174
The clock or the computer is finite
To know it is to exhaust its potential for exciting wonder.
The night sky is more like a human being, inexhaustibly complex
and finally beyond reach.
Chet Raymo 365 Starry nights, Introduction
Night Sky Around Us – Putting it All Together
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Sun is a Star – Day and Night
– When/why can we see something
– Where can we see it depends on our place on Celestial Sphere
our point in Earths Orbit
The Solar System and our own satellite, the Moon – First Quarter Observing
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Secrets of Stars – Stellar behaviour from Stellar Spectra
History of Stars - Stellar evolution
Types of Stars Binary Stars, Variable Stars, Red Giants, White Dwarfs,
‘Planetay Nebula’, Supernovae (we are stardust)
Catalogues of Celestial Objects – Messier, NGC
Open Clusters and Stellar Nursuries, Emission Nebula, Reflection Nebula
Globular Clusters – OLD stars – Back to pre-history!
Distance Ladder – Absolute Magnitude and the Distance Modulus
Cepheid Variables in our Galaxy and Beyond…
Extreme Cosmic Distances, and yet we can see quasar 3c273 near eta
Virginis1
Tools for Lab Exercises
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Observing Exercises: http://www.millstonenews.com/the-night-sky/
Software programs: ECU, Stellarium, Sky Safari
Celestial Sphere 3D and Planisphere 2D Visual aids
Using Star Charts – Pocket Sky Atlas, Sky Atlas 2000, Deep Map 600
Observing Tools – Types of telescopes – reflectors, refractors
Types of mounts - equatorial, dobsonian, alt-azimuth
Types of aids
- clock drive, go-to
Resources
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Gros Merci to MVC for providing the laboratory space!
Lab Exercise April 13 2012
Western Sky has changed since
then…
Constellation
Celestial Object
Taurus
M1 Crab Nebula
Taurus
M45 Pleiades
Gemini
M35
Auriga
M37
Auriga
M36
Auriga
M38
Orion
M42
M43
M78
Puppis (not Monoceros)
M47
M46
Cancer
M44 Beehive
Cancer
M67
2700
6.1
30
M3 - Globular Cluster
33900
6.2
18
M51 - Whirlpool Galaxy
37000000
8.4
11x7
Ursa Major
M81
12000000
6.9
21x10
Ursa Major
M82 - peculiar galaxy
12000000
8.4
9x4
Leo
M65 - Leo Triplet
M66
Canes Venatici
The Night Sky around us … tonight
Compare May Sky to Early April…
New objects to observe, new place in our orbit…
Planisphere
Observing Plan May 11 2012 - Western Sky then Towards the Meridian South
Start with the things that are
going to set first Planisphere does not record the
planets because they change from year to
year.
Constellation
---------------Cancer
Leo
Corvus
Virgo
Object
--------M44 Beehive Cluster , Open Cluster
M67 faint Open Cluster
Planet Mars (no longer on the meridian)
Leo Triplet Galaxies M65,M66…
M104 Sombrero Galaxy (dust lane)
Virgo Galaxies M86,M84
Planet Saturn
Quasar 3c273 (near eta Virginis)
Coma Berenecies Melotte 111Great Star Cloud(OC)
M53 Glocular Cluster
M64 Black Eye galaxy
Canes Venatici M3
M51 Whirlpool Galaxies
M51
M3
Coma Star Cloud
M44
M64
M65,66 Mars
M53
M67
M84,M86
eta Virginis
Saturn
M104
Observing Plan May 11 2012 - Towards the East (ushering summer in)
Eastern Sky:
Constellation Object
------------------- -----------------Hercules
M13- Great Hercules Globular
M92 compare and contrast
Ursa Major
Lyra
M81,M82 (peculiar galaxy)
Epsilon Lyrae - Double Double
M57 – Ring Nebula
(planetary nebula)
the Cosmic Cheerio…
M92
http://www.asod.info/?p=5946
M13
Final Assignment
• From this Messier medley, you can identify
all of the objects you saw tonight
see http://messier.seds.org/
• Write your observations in your astronomy
log book
The Night Sky Around Us … Night Sky Friends …
Conserving the NightSky by sharing it …
Welcome to the universe of friends that never ends…
NightSky Friends http://tech.groups.yahoo.com/group/MoK_NSC/
Some feedback…
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What helped you most in understanding the NightSky material?
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What suggestions would you like to make?