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 ( Department Toolswww.astro.washington.edu Astr322 home page ( ) Linkswww.astro.washington.edu/astro322/ Catalogs: positions, brightness, type, distance, velocity, references Astronomical Almanac - everything 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) GCVS > Astrophysical Quantities by Allen - general info (book) Star charts: SAO DSS - Digitized POSS (Dept Tools) USNO - coords, mags AAVSO - link from my home page General star info: (Dept Tools, 322 Links) Astronomical research: ADS - (Dept Tools) Astro-ph - (Dept Tools)

Terms to be Familiar with: 1.Distances: AU (earth-sun distance) = 93 million miles = 1.55x10 8 km LY (distance light travels in 1 yr) = 3x10 5 km/s x 3x10 7 s = km parsec (astronomer’s unit) = AU = 3.3 LY Earth Diameter = 8000 mi = 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 ***************************************************** c= 3x10 5 km/s = 186,000 mi/s 1 lightsec = 3x10 5 km, 1 lightmin = 3x10 5 km/s x 60s = 18 million km, 1 LY=3x10 5 km/s x 3x10 7 s=10 13 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 **************************************************** 1 pc = 3.26 LY = AU Diameter of Milky Way = 50 kpc Nearest galaxies ~ 50 kpc Milky 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, h 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 ø 90-ø ø horizon SN z NCP CE

The Horizon (altaz) System of Coodinates horizon is the reference frame altitude is height above horizon (0-90 o ) azimuth is angle measured E along horizon from north point (0-360 o )

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 360 o /365 days = ~1 deg/day 1 deg = 24 h x 60 min /360 o = 4 min/day stars rise 4 min earlier each day 4 x 30 days = 2 hrs/month

Tilt of Earth rotation axis to ecliptic (23.5 o ) 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, h 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 F 1 > F 2  23.5 o

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 eclipsesAngular sizes same, alignment of orbits Planets move east among stars and have retrograde motion Planets orbit sun with different periods ObservationExplanation