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The Distance Ladder
LACC §18.3, 19.1, 19.5

• Within the Solar System
• Within the Galaxy
• Within the Universe

An attempt to answer the “big questions”: What is
out there? How big is the universe?

Thursday, April 22, 2010 1

Stellar Parallax
(Distance Ladder)

http://www.astro.umd.edu/educationalresources/astro/sprop/parallax.html


Stellar Parallax: Hipparcos
Hipparcos is an acronym for HIgh Precision PARallax COllecting
Satellite. Appropriately the proununciation is also very close to
Hipparchus, the name of a Greek astronomer who lived from 190 to 120
BC. By measuring the position of the Moon against the stars,
Hipparchus was able to determine the Moon's parallax and thus its
distance from the Earth. He also made the ﬁrst accurate star map which
lead to the discovery, when compared with other data from his
predecessors, that the Earth's poles rotate in the sky, a phenomenon
referred to as the precession of the equinoxes.

...distances accurate to ... 10%.
For a typical measurement accuracy of 1
milliarcsec, this will mean stars with
parallaxes ... 10 milliarcsec, i.e. stars within ...
100 pc of the Sun.

http://www.rssd.esa.int/index.php?project=HIPPARCOS&page=FAQ


The Closest Stars

http://calgary.rasc.ca/stellarmagnitudes.htm


HR Diagram

http://outreach.atnf.csiro.au/education/senior/astrophysics/stellarevolution_hrintro.html

Main Sequence Fitting
(a type of Standard Candle)
If you have a cluster of stars of
unknown distance, you can
compare how bright the stars on
its main sequence appear with
how bright the stars in a cluster of
known distance appear. In the
example above, the stars in the
Pleiades (an open cluster) are 7.5
times dimmer than the stars in the
Hyades (also an open cluster), and
so must be further away.You can
use the inverse-square law
(apparent brightness = true
luminosity / 4 pi R2) to show that
the Pleiades is sqrt(7.5) times
further away than the Hyades.

http://www.bramboroson.com/astro/apr29.html


Variable Stars

http://www.mhhe.com/physsci/astronomy/arny/instructor/graphics/
ch13/1312.jpg

Variable Stars
disregard the colors in these diagrams

http://abyss.uoregon.edu/~js/ http://astronomy.nmsu.edu/
ast122/lectures/lec15.html tharriso/ast110/class22.html


The Distance Ladder
LACC §18.3, 19.1, 19.5
• Within the Solar System: Radar Ranging
• Within The Galaxy: Stellar Parallax, Main-
Sequence Fitting (aka Spectroscopic Parallax),
Cepheid Variable
• Within the Universe: Tully-Fisher Relation,
Type Ia Supernova, Brightest Cluster Galaxy,
Hubble’s Law


LACC HW: Franknoi, Morrison, and
Wolff, Voyages Through the Universe,
3rd ed.

• Ch 18, pp. 413: 5 (choose your answers from the following:
standard candle using main sequence ﬁtting, radar ranging, standard
candle using RR Lyrae and Cepheids’ period luminosity relation, stellar
parallax).

• Ch 20: Tutorial Quizzes accessible from:
www.brookscole.com/cgi-brookscole/course_products_bc.pl?
http://

ﬁd=M20b&product_isbn_issn=9780495017899&discipline_number=19

Due at the beginning of the next class period.


The Interstellar Medium
LACC §18.3, 19.1, 19.5

• Nebulae
• Dust
• Gas



Where did all the stars go? What used
to be considered a hole in the sky is
Molecular
Cloud
now known to astronomers as a dark
molecular cloud. Here, a high
concentration of dust and molecular
gas absorb practically all the visible
light emitted from background stars.
The eerily dark surroundings help make
the interiors of molecular clouds some
of the coldest and most isolated places
in the universe. One of the most notable
of these dark absorption nebulae is a
cloud toward the constellation
Ophiuchus known as Barnard 68,
pictured above. That no stars are visible
in the center indicates that Barnard 68
is relatively nearby, with measurements
placing it about 500 light-years away
and half a light-year across. It is not http://apod.nasa.gov/apod/ap041219.html
known exactly how molecular clouds
like Barnard 68 form, but it is known
that these clouds are themselves likely Note: molecular clouds are
places for new stars to form. It is dark in color. Why?
possible to look right through the cloud
in infrared light.


HII Regions

M42 Orion Nebula and NGC1977
Note: HII regions are reddish in color. Why?
http://autostarsuite.net/photos/snoleopard/picture3566.aspx


Orion: Visible vs. Infrared

Betelgeuse

Orion
Nebula
Rigel

http://www.compadre.org/informal/features/featureSummary.cfm?FID=642


Planetary Nebula
aka The Saturn Nebula

http://www.ucl.ac.uk/star/research/stellarenvironment/cieresearch/planetarynebulae


Reflection Nebula

Note: reflection
nebula are
bluish in color.
Why?

Unspeakable beauty and unimaginable bedlam can be found together in the Trifid
Nebula. Also known as M20, this photogenic nebula is visible with good binoculars
towards the constellation of Sagittarius. The energetic processes of star formation create
not only the colors but the chaos. The red-glowing gas results from high-energy starlight
striking interstellar hydrogen gas. The dark dust filaments that lace M20 were created in
the atmospheres of cool giant stars and in the debris from supernovae explosions. Which
bright young stars light up the blue reflection nebula is still being investigated. The light
from M20 we see today left perhaps 3000 years ago, although the exact distance
remains unknown. Light takes about 50 years to cross M20.

http://apod.nasa.gov/apod/ap070813.html


Supernova Remnant

SN 1006 Supernova Remnant
A new star, likely the brightest supernova in recorded human history, lit up planet Earth's
sky in the year 1006 AD. The expanding debris cloud from the stellar explosion, found in
the southerly constellation of Lupus, still puts on a cosmic light show across the
electromagnetic spectrum. In fact, this composite view includes X-ray data in blue from
the Chandra Observatory, optical data in yellowish hues, and radio image data in red.
Now known as the SN 1006 supernova remnant, the debris cloud appears to be about 60
light-years across and is understood to represent the remains of a white dwarf star....
Because the distance to the supernova remnant is about 7,000 light-years, that explosion
actually happened 7,000 years before the light reached Earth in 1006.

http://apod.nasa.gov/apod/ap080704.html


The Local Bubble

http://www.sslmit.unibo.it/zat/images/
cartography/M-Way_2.htm


Dust Grains

A cosmic dust cloud sprawls across a rich
ﬁeld of stars in this gorgeous wide ﬁeld
Very small solid particles or "dust grains";
telescopic vista looking toward Corona
smoke-like. These absorb and redden
Australis, the Southern Crown. Probably
light passing through them.
less than 500 light-years away and
Absorption by dust creates "dark clouds"
effectively blocking light from more
seen against bright sources such as the
distant, background stars in the Milky Way,
Milky Way.
the densest part of the dust cloud is about
8 light-years long. http://www.astro.virginia.edu/
class/oconnell/astr121/
http://antwrp.gsfc.nasa.gov/
guide11.html
apod/ap040715.html


Dust Grains

The grains appear to be loose conglomerations of
smaller specks of material, which stuck together
after bumping into each other in the depths of
space.
http://spiff.rit.edu/classes/phys230/lectures/ism_dust/ism_dust.html


The Interstellar Medium
LACC §18.3, 19.1, 19.5
• Nebulae: Dark Dust Clouds (aka Molecular Clouds),
HII Regions (e.g. star-forming regions, planetary
nebulae; HII emission line), Reﬂection Nebulae
(reﬂection off dust), Supernova Remnants
• Dust: 1 grain / 1012 atoms; silicates, carbon, ices
• Gas: 90% H, 9% He, 1% heavier (by number); 1
atom / cm3 (1000 / cm3 is an excellent laboratory
vacuum), 100 Kelvin (but varies)
An attempt to answer the “big questions”: What is out
there? How big is the universe?


LACC HW: Franknoi, Morrison, and
Wolff, Voyages Through the Universe,
3rd ed.

• Ch 19, p. 435: 13, 14.

• Ch 21: Tutorial Quizzes accessible from: http://
www.brookscole.com/cgi-brookscole/course_products_bc.pl?
ﬁd=M20b&product_isbn_issn=9780495017899&discipline_number=19

Due at the beginning of the next class period.


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