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 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)

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

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)

Astronomical Coordinate Systems:

• Horizon (altitude and azimuth)

• Celestial (RA and Dec)

• Galactic (b and l)

CE horizon View from south

The Celestial Sphere

In Northern hemisphere:

altitude of NCP = latitude ø

ø90-ø

ø 90-øhorizon

SN

z

NCP

CE

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. 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)

dL/dt = <>

P=26,000 yrs

Precession affects coordinates

F1 > F2

23.5o

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