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The Contents of the Milky Way
• Our Galaxy has a disk about 50
kpc (160,000 ly) in diameter and
about 600 pc (2000 ly) thick, with
a high concentration of interstellar
dust and gas in the disk
• There are about 400 billion stars
in the Milky Way
Where to find things
Dept home page (www.astro.washington.edu) Department Tools
Astr322 home page (www.astro.washington.edu/astro322/ ) Links
Catalogs: positions, brightness, type, distance, velocity, references
Astronomical Almanac - everything http://aa.usno.navy.mil/
Yale Bright Star Catalog (6th mag)- HR#, 1900, 2000 coords
HD catalog - spectra- HD#
SAO Catalog (10th mag) SAO# 1950 coords (HEASARC,etc)
GSC (14th mag) http://gsss.stsci.edu/webservices/GSC2/GSC2WebForm.aspx
GCVS >28000 http://www.sai.msu.su/gcvs/gcvs/index.htm
Astrophysical Quantities by Allen - general info (book)
Star charts:
SAO
DSS - Digitized POSS http://archive.stsci.edu/cgi-bin/dss_form/ (Dept Tools)
USNO - coords, mags http://archive.eso.org/skycat/servers/usnoa
AAVSO - link from my home page
General star info: http://simbad.u-strasbg.fr/simbad/ (Dept Tools, 322 Links)
Astronomical research:
ADS - http://adsabs.harvard.edu/abstract_service.html (Dept Tools)
Astro-ph - http://xxx.lanl.gov/archive/astro-ph (Dept Tools)
Terms to be Familiar with:
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Distances:
AU (earth-sun distance) = 93 million miles = 1.55x108km
LY (distance light travels in 1 yr) = 3x105km/s x 3x107s = 1013km
parsec (astronomer’s unit) = 206265 AU = 3.3 LY
Earth Diameter = 8000 mi = 12756 km
Earth-moon = 240,000 mi = 384,000km
Earth-sun = 93 million mi = 155 million km = 1AU
Sun-Pluto = 40 AU
Sun-Oorts Cloud = 50,000AU
Sun-nearest star = 300,000AU
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c= 3x105 km/s = 186,000 mi/s
1 lightsec = 3x105 km, 1 lightmin = 3x105 km/s x 60s = 18 million km, 1 LY=3x105 km/s x 3x107s=1013 km
Earth-moon = 1.3 lightsec
Earth-sun = 8 lightmin
Earth-Pluto = 5 lighthours
Sun-nearest star = 4 LY
Disk diameter of Milky Way = 150,000 LY
Nearest galaxies ~ 150,000 LY
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1 pc = 3.26 LY = 206265 AU
Diameter of Milky Way = 50 kpc
Nearest galaxies ~ 50 kpc
Milky Way- Andromeda ~ 700 kpc
“radius of universe” ~ 4 billion pc
2.
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SKY terms:
horizon - where the sky meets the ground
zenith - point over observer’s head
celestial sphere - NCP, SCP, CE
meridian - circle through zenith and NCP and SCP
altitude - angle above horizon
azimuth - degrees E from N point to object along horizon
ecliptic - apparent path of Sun on celestial sphere
vernal equinox  - where Sun crosses CE from S to N
declination - angle N or S of CE for object in sky
hour angle - angle W along CE from meridian to hour circle
right ascension - angle E along CE from  to hour circle of star
ST = RA + HA
3.
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TIME terms:
UT = GMT = local time at Greenwich (UT=PST+8 hr = PDT+7 hr)
JD = Julian Date (Jan1, 2013 0h UT = 2,456,293.5)
HJD - set to Sun
MJD = JD - 2,400,000.5 day (used by spacecraft)
Astronomical Coordinate Systems:
• Horizon (altitude and azimuth)
• Celestial (RA and Dec)
• Galactic (b and l)
The Celestial Sphere
CE
horizon
View from south
CE
z
NCP
90-ø ø
N
ø
90-ø
horizon
In Northern hemisphere:
altitude of NCP = latitude ø
S
The Horizon (altaz) System of Coodinates
• horizon is the reference frame
• altitude is height above horizon (0-90o)
• azimuth is angle measured E along horizon
from north point (0-360o)
Horizon system
azimuth = ?
Due to Earth rotation, stars circle celestial Poles
Stars between CP & horizon are circumpolar stars
The earth orbits the Sun (proof is parallax)
Due to Earth’s revolution around Sun
We see different constellations at different times of the year
Earth’s orbit causes a difference between solar and sidereal time
360o/365 days = ~1 deg/day
1 deg = 24h x 60min /360o = 4 min/day
stars rise 4 min earlier each day
4 x 30 days = 2 hrs/month
Tilt of Earth rotation axis to ecliptic (23.5o) causes
seasons and forms basis for Celestial coordinates
ecliptic
The Celestial (equatorial) Coordinate system
• celestial equator is reference frame
• declination () is height above or below CE
(-90 to 0 to +90)
• right ascension (RA:) is angle E along CE
from vernal equinox point (0-24 hrs)
Sun on June 21:
RA= ?
Dec = ?
Advantages of Celestial Coord:
• Coords don’t change during night
• Coords don’t change for different
locations on earth
• Objects can be found in catalogs
Advantages of Horizon Coord:
• Easier to describe objects during night
• Telescopes can handle heavy instruments
2.
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SKY terms:
horizon - where the sky meets the ground
zenith - point over observer’s head
celestial sphere - NCP, SCP, CE
meridian - circle through zenith and NCP and SCP
altitude - angle above horizon
azimuth - degrees E from N point to object along horizon
ecliptic - apparent path of Sun on celestial sphere
vernal equinox  - where Sun crosses CE from S to N
declination - angle N or S of CE for object in sky
hour angle - angle W along CE from meridian to hour circle
right ascension - angle E along CE from  to hour circle of star
ST = RA + HA
3.
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TIME terms:
UT = GMT = local time at Greenwich (UT=PST+8 hr = PDT+7 hr)
JD = Julian Date (Jan1, 2013 0h UT = 2,456,293.5)
HJD - set to Sun
MJD = JD - 2,400,000.5 day (used by spacecraft)
23.5o
dL/dt = <>
P=26,000 yrs
F1 > F2

Precession affects
coordinates
Observation
Explanation
Sky rotates about N point from
E to W
Earth spins W to E,
P = 1 day
Sun moves E among stars
(1 deg/day); stars rise 4 min
earlier each day
Earth orbits sun,
P = 1 year
Earth has seasons
(temperature, daylight changes)
Spin axis tilted 23.5 deg
to orbit
Moon moves among stars (12
deg/day), rises 50 min later/day;
has phases
Moon orbits earth,
P = 1 month
Moon and sun have eclipses
Angular sizes same,
alignment of orbits
Planets move east among stars
and have retrograde motion
Planets orbit sun with
different periods