The Contents of the Milky Way Our Galaxy has a disk about 50 kpc (160,000 ly) in diameter and about...
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![Page 1: 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.](https://reader034.fdocuments.net/reader034/viewer/2022051401/56649e685503460f94b64ee3/html5/thumbnails/1.jpg)
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
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Where to find thingsDept home page (www.astro.washington.edu) Department ToolsAstr322 home page (www.astro.washington.edu/astro322/www.astro.washington.edu/astro322/ ) Links
Catalogs: positions, brightness, type, distance, velocity, referencesAstronomical Almanac - everything http://aa.usno.navy.mil/Yale Bright Star Catalog (6th mag)- HR#, 1900, 2000 coordsHD catalog - spectra- HD#SAO Catalog (10th mag) SAO# 1950 coords (HEASARC,etc)GSC (14th mag) http://gsss.stsci.edu/webservices/GSC2/GSC2WebForm.aspxGCVS >28000 http://www.sai.msu.su/gcvs/gcvs/index.htmAstrophysical Quantities by Allen - general info (book)
Star charts:SAODSS - Digitized POSS http://archive.stsci.edu/cgi-bin/dss_form/http://archive.stsci.edu/cgi-bin/dss_form/ (Dept Tools)USNO - coords, mags http://archive.eso.org/skycat/servers/usnoahttp://archive.eso.org/skycat/servers/usnoaAAVSO - link from my home page
General star info: http://simbad.u-strasbg.fr/simbad/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-phhttp://xxx.lanl.gov/archive/astro-ph (Dept Tools)
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Terms to be Familiar with:1. 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 = 384,000kmEarth-sun = 93 million mi = 155 million km = 1AUSun-Pluto = 40 AUSun-Oorts Cloud = 50,000AUSun-nearest star = 300,000AU*****************************************************c= 3x105 km/s = 186,000 mi/s1 lightsec = 3x105 km, 1 lightmin = 3x105 km/s x 60s = 18 million km, 1 LY=3x105 km/s x 3x107s=1013 kmEarth-moon = 1.3 lightsecEarth-sun = 8 lightminEarth-Pluto = 5 lighthoursSun-nearest star = 4 LYDisk diameter of Milky Way = 150,000 LYNearest galaxies ~ 150,000 LY****************************************************1 pc = 3.26 LY = 206265 AUDiameter of Milky Way = 50 kpcNearest galaxies ~ 50 kpcMilky Way- Andromeda ~ 700 kpc“radius of universe” ~ 4 billion pc
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2. 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. 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)
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Astronomical Coordinate Systems:
• Horizon (altitude and azimuth)
• Celestial (RA and Dec)
• Galactic (b and l)
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CE horizon View from south
The Celestial Sphere
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In Northern hemisphere:
altitude of NCP = latitude ø
ø90-ø
ø 90-øhorizon
SN
z
NCP
CE
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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)
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Horizon system
azimuth = ?
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Due to Earth rotation, stars circle celestial Poles
Stars between CP & horizon are circumpolar stars
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The earth orbits the Sun (proof is parallax)
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Due to Earth’s revolution around Sun
We see different constellations at different times of the year
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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
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Tilt of Earth rotation axis to ecliptic (23.5o) causes seasons and forms basis for Celestial coordinates
ecliptic
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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)
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Sun on June 21:
RA= ?
Dec = ?
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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
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2. 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. 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)
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dL/dt = <>
P=26,000 yrs
Precession affects coordinates
F1 > F2
23.5o
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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
Observation Explanation