e2 - stellar radiation & stellar types

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Stars Stars are the things you see most of in the night sky. You already know all about the Sun, which is a pretty good example of an average star But what exactly is a star??? E2. 1

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Topic E2 (first part) of IB Physics Astophysics

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Page 1: E2 - Stellar Radiation & Stellar Types

Stars

Stars are the things you see most of in the night sky.

You already know all about the Sun, which is a pretty good example of an average star

But what exactly is a star???

E2.1

Page 2: E2 - Stellar Radiation & Stellar Types

Stars

Stars are formed by interstellar dust coming together through mutual gravitational attraction.

The loss of potential energy is responsible for the initial high temperature necessary for fusion.

The fusion process releases so much energy that the pressure created prevents the star from collapsing due to gravitational pressure.

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Page 3: E2 - Stellar Radiation & Stellar Types

Very high temperatur

es are needed in order to

begin the fusion

process: usually 107

K.

Nuclear fusionE2.1

Page 4: E2 - Stellar Radiation & Stellar Types

A star is a big ball of gas, with fusion going on at its center, held together by

gravity!

There are variations between stars, but by and large they’re really pretty simple things.

Massive Star

Sun-like Star

Low-mass Star

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Page 5: E2 - Stellar Radiation & Stellar Types

What is the most important thing about a star?

MASS!The mass of a normal star almost

completely determines its LUMINOSITY and TEMPERATURE!

Note: “normal” star means a star that’s fusing Hydrogen into Helium in its center (we say “hydrogen burning”).

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Page 6: E2 - Stellar Radiation & Stellar Types

The LUMINOSITY of a star is how much ENERGY it gives offper second:

The energy the Sun emits is generated by the fusion in its

core…

This light bulb has a luminosity of 60 Watts

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Page 7: E2 - Stellar Radiation & Stellar Types

What does luminosity have to do with mass?

The mass of a star determines the pressure in its core:

Pressure

Gravity pulls outer layers in,

Gas Pressure pushes them out.

The core supports the weight of the whole star!

The more mass the star has, the higher the central pressure!

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Page 8: E2 - Stellar Radiation & Stellar Types

The core pressure determines the rate of

fusion…

MASS PRESSURE &TEMPERATURE

RATE OF FUSION

…which in turn determines the star’s

luminosity!

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Page 9: E2 - Stellar Radiation & Stellar Types

Luminosity is an intrinsic property… it doesn’t depend on distance!

This light bulb has a luminosity of 60 Watts…

…no matter where it is, or where we view it from, it will always be a 60 Watt light bulb.

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Page 10: E2 - Stellar Radiation & Stellar Types

Luminosity

The Luminosity of a star is the energy that it releases per second. Sun has a luminosity of 3.90x1026 W (often written as L): it emits 3.90x1026 joules per second in all directions.

The energy that arrives at the Earth is only a very small amount when compared will the total energy released by the Sun.

E2.3

Page 11: E2 - Stellar Radiation & Stellar Types

Apparent brightness When the light from the Sun reaches the Earth

it will be spread out over a sphere of radius d. The energy received per unit time per unit area is b, where:

d

b is called the apparent brightness of the star

24 d

Lb

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Page 12: E2 - Stellar Radiation & Stellar Types

Luminosity

Exercise 13.1

The Sun is a distance d=1.5 x 1011 m from the Earth. Estimate how much energy falls on a surface of 1m2 in a year.

d

L= 3.90x1026 W

E2.3

Page 13: E2 - Stellar Radiation & Stellar Types

At a distance of d=1.5 x 1011 m, the energy is “distributed” along the surface of a sphere of radius 1.5 x 1011 m

d

The sphere’s surface area is given by:

A = 4πd2 = 4 π x (1.5 x 1011)2 =

=2.83 x 1023 m2

The energy that falls on a surface area of 1m2 on Earth per second will be equal to:

b = L/A = 3.90x1026 / 2.83 x 1023 =

= 1378.1 W/m2 or 1378.1 J/s m2 In a year there are: 365.25days x 24h/day x 60min/h x 60s/min = 3.16 x 107 s

So, the energy that falls in 1 m2 in 1 year will be:

1378.1 x 3.16 x 107 = 4.35 x 1010 joules

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Page 14: E2 - Stellar Radiation & Stellar Types

Black body radiation A black body is a perfect emitter. A good

model for a black body is a filament light bulb: the light bulb emits in a very large region of the electromagnetic spectrum.

There is a clear relationship between the temperature of an object and the wavelength for which the emission is maximum. That relationship is known as Wien’s law:

K m 2.9x10T

constantT3-

max

max

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Wien Displacement law

By analysing a star’s spectrum, we can know in what wavelength the star emits more energy.The Sun emits more energy at λ=500 nm. According to Wien’s law, the temperature at the Sun’s surface is inversely proportional to the maximum wavelength.So:

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Black body radiation and Wien Law

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Star’s Colour and Temperature

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Black body radiation Apart from temperature, a radiation spectrum can

also give information about luminosity. The area under a black body radiation curve is equal

to the total energy emitted per second per unit of area of the black body. Stefan showed that this area was proportional to the fourth power of the absolute temperature of the body.

The total power emitted by a black body is its luminosity.

According to the Stefan-Boltzmann law, a body of surface area A and absolute temperature T has a luminosity given by: 4σATL where, σ = 5.67x108 W m-2 K-4

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Why is this important?

The spectrum of stars is similar to the spectrum emitted by a black body.

We can therefore use Wien Law to find the temperature of a star from its spectrum.

If we know its temperature and its luminosity then its radius can be found from Stephan-Boltzmann law.

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Page 20: E2 - Stellar Radiation & Stellar Types

Real spectra are more complicated than this (remember emission and absorption lines?)

BlackbodySpectrum

Emission and Absorption

Lines

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Page 21: E2 - Stellar Radiation & Stellar Types

Stars can be arranged into categories based on the features in their spectra…

This is called “Spectral Classification”

1. by the “strength” (depth) of the absorption lines in their spectra

2. by their color as determined by their blackbody curve 3. by their temperature and luminosity

How do we categorize stars?A few options:

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Page 22: E2 - Stellar Radiation & Stellar Types

First attempts to classify stars used the strength of

their absorption lines…

Williamina Fleming

They also used the strength of the Harvard “computers”!

Stars were labeled “A, B, C…”in order of increasing strength of Hydrogen lines.

E2.8

Page 23: E2 - Stellar Radiation & Stellar Types

OBAFGKM(LT)!

Later, these categories were reordered according to temperature/color…

Annie Jump Cannon

Page 24: E2 - Stellar Radiation & Stellar Types

OBAFGKM - Mnemonics

Osama Bin Airlines! Flies Great, Knows Manhattan!

Only Boring Astronomers Find Gratification in Knowing Mnemonics!

O Be A Fine Girl/Guy Kiss Me

E2.8

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Eventually, the connection was made between the

observables and the theory.

Observable:• Strength of Hydrogen Absorption Lines• Blackbody Curve (Color)

Theoretical:• Using observables to determine things

we can’t measure: Temperature and Luminosity

Cecilia Payne

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The Spectral SequenceClass Spectrum Color Temperature

O ionized and neutral helium, weakened hydrogen

bluish 31,000-49,000 K

B neutral helium, stronger hydrogen

blue-white 10,000-31,000 K

A strong hydrogen, ionized metals

white 7400-10,000 K

F weaker hydrogen, ionized metals

yellowish white 6000-7400 K

G still weaker hydrogen, ionized and neutral metals

yellowish 5300-6000 K

K weak hydrogen, neutral metals

orange 3900-5300 K

M little or no hydrogen, neutral metals, molecules

reddish 2200-3900 K

L no hydrogen, metallic hydrides, alkalai metals

red-infrared 1200-2200 K

T methane bands infrared under 1200 K

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“If a picture is worth a 1000 words, a spectrum is

worth 1000 pictures.”

Spectra tell us about the physics of the star and how those physics affect the atoms in it

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The Hertzsprung-Russell diagram

You are here

This diagram shows a correlation between the luminosity of a star and its temperature.

The scale on the axes is not linear as the temperature varies from 3000 to 25000 K whereas the luminosity varies from 10-4 to 106, 10 orders of magnitude.

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H-R diagram The stars are not randomly

distributed on the diagram. There are 3 features that emerge

from the H-R diagram: Most stars fall on a strip

extending diagonally across the diagram from top left to bottom right. This is called the MAIN SEQUENCE.

Some large stars, reddish in colour occupy the top right – these are red giants (large, cool stars).

The bottom left is a region of small stars known as white dwarfs (small and hot)

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