Stars

• Composed of ~98% H and He

• Fusion in the core supports the star

• Full spectrum of masses

Key Properties

• Apparent Brightness• Luminosity• Temperature / Color• Mass• Evolutionary State

Brightness

• Absolute brightness– Luminosity– Power emitted by star

into space– Only depends on star– Lsun = 4 X 1026 Watts

• Apparent brightness– How bright star appears

in the night sky– Power per unit area– Depends on star’s

brightness and distance

Inverse square law for light

24 d

LB

• Apparent brightness measured in watts per square meter

• Drops off as square of distance

Measuring Distance

• Stellar Parallax– Caused by motion of

Earth in its yearly orbit– d = 1/p

where p is in arcsecs and d is in parsecs

– 1 parsec = 3.26 lyrs

Magnitudes

• Logarithmic• Large values are dim

objects• Small values are bright

objects

2

121 log5.2

F

Fmm

Magnitudes

Absolute Magnitudes• A bright a star would

appear if it were 10 pc away

• Does not depend on distance

Apparent Magnitudes• How objects appear from

here on Earth

• Depends on distance

• We can only see objects with m≤6

pc

dMm

10log5

Color and Temperature

• Color is the difference between intensity in two filters

• B-V color is a good proxy for temperature

• Color is independent of distance

Spectral Type

• Spectral types are subdivided for intermediate temperatures

• Values run from 0-9

• Smaller numbers are hotter

• Larger numbers are cooler

• Eg. B1 is hotter than B7

Spectral Types

• Order was alphabetical depending on strength of Hydrogen line– Williamina Flemming

• Revised to follow a more natural order– Annie Cannon

Measuring Stellar Masses

Using Binary Systems

Visual Binaries

Eclipsing Binaries

Spectroscopic Binaries

HR Diagram

• Main Sequence

• Giants

• Supergiants

• White Dwarfs

HR Diagram

• Luminosity class gives size and luminosity information

Main Sequence

• Mass is the most important property for a star on the MS

• Stars spend 90% of their lives here, burning H in their cores

• MS lifetime depends on mass

Main Sequence

• More massive stars live much shorter lives– Burn fuel very

quickly to support such a large star

• Less massive stars live longer– Less fuel, but burn it

more slowly

Life After the Main Sequence

• When stars run out of H in their cores, they evolve off the MS

• Giants and Supergiants expand to extremely large sizes– Temperatures are very low– Luminosity is very high

• White dwarfs are small and hot– Have no nuclear fusion– Heated by collapse of gas

424 TrL

Star Clusters

• All stars in the cluster formed about the same distance from Earth

• All stars in the cluster formed at about the same time

• Very useful in understanding stellar formation and evolution– Can use them as clocks

• Most of what we know about stars comes from studying clusters

Open Clusters

• Only a few million years old

• Contain lots of luminous blue stars

• Contain several thousand stars

• ~30 lyrs across

Globular Clusters

• Often several billion years old– Some of the oldest

objects in the galaxy– Contains mostly

smaller stars• Around 105-106 stars

concentrated in a relatively small volume

• 50-150 lyrs across

Age of Cluster

• Main Sequence Turnoff (MSTO) – more massive stars have evolved off of the Main Sequence

• MSTO gives age of cluster– Lifetime of cluster same

as MS lifetime of stars at the MSTO

MSTO

• Young clusters still have their massive stars on the MS

• Old clusters are missing the massive blue stars on the MS