H205 Cosmic Origins

Properties of Stars (Ch. 15)The Milky Way (Ch. 19)

EP3 Due Wednesday

APOD

Opportunities• Kirkwood Obs. Open April 1 (weather permitting)• Solar Telescope open April 4 (weather permitting)• Astronomy Club – Mondays at 7:30, Swain West 113

(Includes PIZZA!)• Remote Observing – April 9 & 10, 9:30-midnight, Swain

West 403• PBS on March 31 (Check times!):

– Investigation into a possible comet strike

• PBS on April 21?– 8 PM: 400 Years of the Telescope - narrated by Neil deGrasse

Tyson– 9 PM: - A Sidewalk Astronomer - the story of John Dobson

(now 91 years old!)

Stars Basic Properties of Stars

distance brightness diameters

The Hertzsprung-Russell Diagram

The Brightness of Stars• Apparent brightness – how bright does it look in

the sky?

Apparent magnitude - mV

• Absolute brightness – how bright is it really??

Absolute magnitude - Mv

• The apparent brightness depends on both a star’s distance and its intrinsic brightness

The Inverse Square Law tells us how a star’s apparent brightness changes with distance• Brightness decreases

as distance squared– something twice as far

away will be four times fainter

– something 10 times further away will be 100 times fainter

– something 1000 times further away will be a million times fainter

2distance 4π

LuminosityBrightnessApparent

How Far Away Are Stars?If we know a star’s apparent AND absolute brightness, we can calculate its distance

The inverse square law describes how the brightness of a source light (a star!) diminishes with distance

But how do we get the distances to stars whose brightness we DON’T know?

brightness changes as 1/distance2

Measuring the distances to stars using

Parallax

Measuring the distances of stars

Parallax: apparent change in the position of an object due to a change in the position of the observerStellar parallax uses the Earth’s orbit as the baseline

Parallax

1 AUdistance

Angle = p

(p)sin distance

AU 1

parsecs)(in distance arcsecs)in ( p

1

A parsec is the distance at which 1 AU subtends an angle of 1 arc sec

Parsec: the distance to an object with a stellar parallax of one arc second

The parallax of Alpha Centauri = 0.76 arcseconds

A parallax of ~0.001 arc secondsis the smallest we can measure

What is a Parsec???

1 parsec = 3.26 light years

A star at a distance of 1 parsec showsa parallax of 1 arc second How big is one

arc second?

The size of adime at adistance of2.3 miles!

How Big Are Stars?We can’t see the stars’diameters through a telescope.Stars are so far away that wesee them just as points of light.

If we know a star’s temperature and its luminosity, we can calculate its diameter.

How do we determine a star’stemperature?

Luminosity depends on….

TEMPERATURE -the hotter a star is,the brighter it is.

DIAMETER –the bigger a star is,the brighter it is.

Stellar RadiiWe can’t see the stars’ diameters through a telescope. Stars are so far away that wesee them just as points of light.

If we know a star’s temperature, apparent magnitude, and distance, we can calculate its radius

Temperature from

Stars range in size from about the size of the Earth to hundreds of times the Sun’s diameter

424Luminosity TR

Luminosity from parallax and apparent magnitude

Magnitudes• Astronomers use “magnitudes” to describe how

bright stars are• Small numbers are brighter, large numbers

fainter.• The brightest naked-eye stars are around

magnitude zero.• The faintest naked-eye stars are around

magnitude six• 5 magnitudes are a factor of 100 in brightness (a

6th magnitude star is 100 times fainter than a 1st magnitude star)

The Nearest and the Brightest

Goal: – to learn about types of stars– to explore the stars near the Sun and

compare them to the stars we see in the sky

Task:– plot a Hertzsprung-Russell diagram including

both the nearest stars and the brightest stars in the northern sky

The Brightest

Stars in the Sky

(no need to copy these down!)

StarDistance

(LY)Temperature

(K)Absolute

Magnitude

Sun  0.000015 5800 4.8

Sirius  9 9600 1.4

Canopus  232 7600 -2.5

Alpha Cen A  4 5800 4.4

Arcturus  37 4700 0.2

Vega  25 9900 0.6

Capella  42 5700 0.4

Rigel  773 11000 -8.1

Procyon  11 6600 2.6

Achernar  144 22000 -1.3

Betelgeuse  427 3300 -7.2

Hadar  335 25000 -4.4

Acrux  321 26000 -4.6

Altair  17 8100 2.3

Aldebaran  65 4100 -0.3

Antares  604 3300 -5.2

Spica  263 2600 -3.2

Pollux  34 4900 0.7

The Nearest Stars

StarDistance

(LY) TemperatureAbsolute

Magnitude

Prox Cen 4  2800 15.53

Alp Cen A 4  5800 4.4

Alp Cen B 4  4900 5.72

Barnard’s 6 2800 13.23

Wolf 359 7.5  2700 16.57

Lal 21185  8 3300 10.46

Sirius A  9 9900 1.45

Sirius B  9 12000 11.34

Luyten 726-8A  9 2700 15.42

UV Ceti  9 2600 15.38

Ross 154  10 3000 13.14

The HRDiagram

Hertzsprung Russell Diagram

-10

-5

0

5

10

15

20

050001000015000200002500030000Temperature (K)

Ab

so

lute

Ma

gn

itu

de

Giants andSupergian

ts

White Dwarf

MainSequence

Key Ideas – The HR Diagram

• The intrinsic brightness or luminosity of stars depends on temperature and radius• if two stars have the same radius, the hotter

one is brighter• if two stars have the same temperature, the

bigger one is brighter

• The Hertzsprung-Russell Diagram• relates the temperature and brightness of

stars

Stars come in many sizes and colors

But only certain sizes

and colors are allowed!

HR Diagram Simulator

Most stars occur in these main groups in the luminosity-

temperature diagram

Main Sequence Giants Supergiants White DwarfsB

RIG

HTN

ESS

TEMPERATURE

The Main Sequence

The sun is an

ordinary, yellow main

sequence starB

RIG

HTN

ESS

TEMPERATURE

Giants and Supergiants are cooler and very largeB

RIG

HTN

ESS

TEMPERATURE

Supergiants

Giants

White dwarfs are small and

hotter

The apparent brightness of a star in the sky depends on distance, luminosity, and temperature

Most luminous stars:

106 LSun

Least luminous stars:

10-4 LSun

(LSun is luminosity of Sun)

Most massive stars:

100 MSun

Least massive stars:

0.08 MSun

(MSun is the mass

of the Sun)

Main-Sequence Star Summary

High Mass:

High Luminosity Short-Lived Large Radius Blue

Low Mass:

Low Luminosity Long-Lived Small Radius Red

Constructing an HR Diagram

0

5

10

15

-0.5 0 0.5 1 1.5 2B-V Color

Ap

par

ent

Mag

nit

ud

e

What’s this B-V color?• Astronomers measure the brightness of stars in

different colors– Brightness measured in blue light is called “B” (for

“Blue”)– Brightness measured in yellow light is called “V” (for

“Visual)

• Astronomers quantify the “color” of a star by using the difference in brightness between the brightness in the B and V spectral regions

• The B-V color is related to the slope of the spectrum

The slope of the spectrum is different at different temperatures

Most stars fall somewhere on the main sequence of the H-R diagram

WHYWHYWHY???

Mass measurements of main-sequence stars in binary star systems show that the hot, blue stars are much more massive than the cool, red ones

High-mass stars

Low-mass stars

Main-sequence stars are fusing hydrogen into helium in their cores like the Sun

massive main-sequence stars are hot (blue) and luminous

Less massive stars are cooler (yellow or red)and fainter

The mass of a normal, hydrogen-burning star determines its luminosity and temperature!

High-mass stars

Low-mass stars

Mass & LifetimeSun’s life expectancy: 10 billion years

Life expectancy of 10 MSun star:

10 times as much fuel, uses it 104 times as fast

10 million years ~ 10 billion years x 10 / 104

Life expectancy of 0.1 MSun star:

0.1 times as much fuel, uses it 0.01 times as fast

100 billion years ~ 10 billion years x 0.1 / 0.01

Off the Main Sequence

• Stellar properties depend on both mass and age: those that have finished fusing H to He in their cores are no longer on the main sequence

• All stars become larger and redder (and cooler) after exhausting their core hydrogen: giants and supergiants

• Most stars eventually end up small and hot after fusion has ceased: white dwarfs

Star Clusters

Star Clusters

Goals:– Understand how we learn about stellar evolution

from the properties of stars in clusters – Understand how we can determine the distances

of star clusters– Understand how we can determine the ages of

star clusters

“Globular" Clusters and “Open" Clusters

Globular Clusters

•104-106 stars•old!•compact balls of stars•high star density

Open Clusters

•10-104 stars•generally young•loose•low star density

Properties of Stars in Clusters

• Formed at the same time

• Stars are the same age

• All stars have the same composition

• The stars are held in a group by their common gravity

Cluster HR Diagrams

Hotter stars are brighter in blue light than in yellow light, and have low values of B-V color, and are found on the left side of the diagram.

Cooler stars are brighter in yellow light than in blue light, have larger values of B-V color, and are found on the right side of the diagram.

hotter cooler

Distances to Star Clusters The Sun has a “B-V” color of about 0.6.

What would the apparent magnitude of the Sun be at the distance of the cluster Messier 6?

Stars in Messier 6 with B-V colors of 0.6 have similar mass and luminosity to the Sun

hotter cooler

Stars like the SUN

Ages of Star Clusters

The “bluest” stars left on the main sequence of the cluster tell us the cluster’s age.

As the cluster ages, the bluest stars run out of hydrogen for fusion and lose their “shine”

hotter cooler

The HR diagrams of clusters of different

ages look very different

Jewelbox

0

5

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-0.5 0 0.5 1 1.5 2B-V Color

Ap

pa

ren

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ag

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M 670

5

10

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-0.5 0 0.5 1 1.5 2B-V Color

Ap

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Main Sequence Turnoffs of Star Clusters

Burbidge and Sandage 1958, Astrophysical Journal

Here we see a series of HR diagrams for sequentially older star clusters that have been superimposed

Ages of Star Clusters

Cluster Turnoff Color Age

NGC 752 0.35 1.1 billion years

M 67 0.45 2.5 billion years

Hyades 0.15 800 million years

Pleiades -0.15 100 million years

M 34 -0.10 180 million years

Jewelbox -0.25 16 million years

Thinking Question: Why has a cluster with a turnoff color of B-V=1.0 never been discovered?

For WednesdayChapter 19 – Milky Way

EP3 Finished on Wednesday