Lecture 21: Stars. Review from last time: from observations of nearby stars, we can determine: distance to star apparent brightness  luminosity spectral.

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Presentation transcript:

Lecture 21: Stars

Review from last time: from observations of nearby stars, we can determine: distance to star apparent brightness  luminosity spectral type  temperature (for binary systems: mass) radius

Spectral Classes

H-R Diagram

Mass-Luminosity Relationship

Main Sequence Lifetime stars stay on the main sequence as long as they can convert Hydrogen to Helium in their core the hotter/more massive a star, the faster it burns up all of its Hydrogen the more massive a star, the shorter its ‘main sequence lifetime’.

Estimating the lifetime of a Star Estimate the lifetime of: the Sun (M = 2.0E30 kg; L = 3.8E26 J/s) a B star (M = 10 M sun ; L = 3000 L sun ) an M star (M = 0.1 M sun ; L = L sun )

Main Sequence Lifetimes

mass increases lifetime decreases

larger radius

Stellar Luminosity Classes ClassDescription ISupergiants IIBright giants IIIGiants IVSubgiants VMain Sequence

The Stellar Luminosity Function there are many more low-luminosity stars than high- luminosity stars there are many more low-mass stars than high- mass stars

Selection Effects Bright stars are easier to see. Most observed samples contain a bias: bright objects are over-represented compared to fainter objects. How does this affect our ‘census’ of other stellar properties?

Variable Stars Some special stars change their luminosity with a regular pattern. For example, Cepheid Variable stars have a relationship between their luminosity and the period of variation. this can be used to determine the distance to the star.

Cepheid Variable in M100

Light Curve for a Cepheid Variable Star

Why do Cepheids Pulsate? He + + light  He ++ atmosphere becomes more opaque radiation pressure pushes the atmosphere out atmosphere cools, He ++  He + pressure decreases, star contracts.

We can use Cepheids to find the distance to far-away galaxies… measure apparent brightness (flux) of Cepheid measure period of variation (t) find luminosity L using Luminosity- Period relationship f = L/(4  d 2 )  d = (4  L/f) 1/2

Star Clusters Open clusters are loose, irregular groups of young stars, found mainly in the disk of the Galaxy. Globular clusters are round, regular balls of old stars, found mainly in the halo of our Galaxy.

Star Clusters Open Cluster Globular Cluster

Star Clusters are useful because: All the stars in the cluster are at about the same distance from us All the stars in the cluster formed at about the same time (so they are about the same age)

Temperature Luminosity main sequence turnoff Pleiades

Palomar 3 What is the age of this cluster?

The Age of the Universe Stars in the oldest clusters have ages of billion years From the expansion rate of the universe, we can estimate the time since the Big Bang. Current values are around 13 billion years. Are there stars older than the Universe???