Measurements

• Magnitudes– Apparent magnitude (m)– Absolute magnitude (M)Distance modulus formula

m-M = -5 +5 log (d) d in pcm-M == distance as given by magnitudes

m-M = 10 d=1000 pcm-M = 20 d=105 pcm-M = 30 d=107 pc

M related to luminosity

Typically luminosity not often quoted.

Filters

Johnson UBVRIJHKL

Mv = Abs. V magnitude

V = App. V magnitude

B = App. B magnitude

Colors

B-V, U-B, etc

Temperature

Dust

WattsL

M

L

L

MML

LLog

sun

sun

MM

Sun

sunsun

sun

26

)(04

1086.3

85.4

10

)(4.0

• Spectra– Radio– Microwave– IR– Visible– UV– X-ray– Gamma-ray

• Environment• Non-thermal –

Synchrotron• Spectral Features

– Absorption– Emission– Continuous

Velocity - Redshift

Low Velocity - Doppler shift formula

z is the Redshift (usually)

z used as a distance indicator

High Velocity – Relativistic Redshift formula

lab

labobs

lab

labobs

z

skmv

/000,300

skmz

zv /000,300

11

112

2

Scales of Distance

AU = Astronomical Unit = 150 million km

Parsec (pc) = 206,265 AU = 19 trillion miles

Lightyear = 0.307 pc =5.9 trillion miles

1 pc = 3.26 lightyears

Milky Way – 100,000 ltyr, 30,000 pc

100 kilolightyears, 30 kiloparsecs (kpc)

Largest galaxies = 10 x MW

Smallest = 100 pc

Sun – MW center = 7.5 kpc

Local Group = 3 million light years

= 1 Mpc

Galaxy Classifications• Hubble’s Tuning Fork

Sa Sb Sc Sd

SBa SBb SBc SBd

E0 E6 S0

Irregulars

Ellipticals

• E#

# = 10 (a-b)/a

• No systematic rotation axis

• Random star motion

b

a

cD galaxies

• Largest

• Brightest*

• Diffuse

• Multiple nuclei

• Few Mpc size

• Centers of clusters

Spirals

• S and SB

• Tightness of arms

• Size of bulge

• Sloppy arms

Barred Spirals

• Why?

• Not always visible

• Secondary/tertiary

• 25% - 65% of spirals(!)

S0 - Lenticular

• Spiral-less spirals

• Disk, no structure

• Bars

Irregulars

• Irr I – OB Stars– H II regions– Population I

• Irr II– Just plain weird

Sm & Im Irregulars

• Magellanic cloud like

• Sm – spiral like

• SBm – barred spiral

• Im - small

Dwarf Galaxies

• Irregulars (dIrr)

• Ellipticals (dE)

• Spheroidals (dSph)

Gerard de Vaucouleurs

• Sm, Im

• Degrees of Bar-ness

A - B

• Degrees of Ring-ness

s - r

Other measurements

• Sizes – poorly known

• Luminosity

Classification I – V

Wavelength dependent

Galaxy type, structure, content, history – all interrelated – how?