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 Apparent 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 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 All stars have very small paralax angles (<1 arcsec), which is why ancient Greeks were never able to measure it

Magnitudes Logarithmic Large values are dim objects Small values are bright objects

Magnitudes Absolute Magnitudes Apparent Magnitudes A bright a star would appear if it were 10 pc away Does not depend on distance How objects appear from here on Earth Depends on distance We can only see objects with m≤6

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 With spectral type and luminosity class, we can completely classify a star

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 A 10M(sun) has L of about 10,000L(sun) 10 times the H, burned 10,000 times as quickly, so the star only lives for 1/1,000 the length of the sun

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

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 Picture is of M36

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 Picture is of M15

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