A105 Stars and Galaxies

News Quiz TodayJewelbox homework due Thursday

Today’s APOD

Announcements…• Kirkwood Obs. open Weds night

8:30-10:30 PM• Rooftop Sessions, Oct. 10 & 11,

9:00 PM• Remote Obs Oct. 14 @ 10 PM

and Oct. 16 @ 7 AM

The Sun Today• Image credit: Solar

Orbiting Heliospheric Observatory/MDI

Not very interestingtoday!

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?

• Absolute brightness – how bright is it really??

• 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

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

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.

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

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)

Stars come in many sizes and colors

But only certain

sizes and colors are allowed!

HR Diagram Simulator

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

The Main Sequence

The sun is an

ordinary, yellow main

sequence star

BR

IGH

TN

ESS

TEMPERATURE

Giants and Supergiants are cooler and very largeB

RIG

HTN

ESS

TEMPERATURE

Supergiants

Giants

White dwarfs are small and

hotter

Most stars occur in these main groups in the luminosity-

temperature diagram

Main Sequence Giants Supergiants White Dwarfs

BR

IGH

TN

ESS

TEMPERATURE

Quiz: Which star is the biggest?

A B C D

BR

IGH

TN

ESS

TEMPERATURE

Quiz: Which star is the smallest?

A B C D

BR

IGH

TN

ESS

TEMPERATURE

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

Familiar Stars

1000 ly

A little farther out

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

Plot Absolute Magnitude vs. Temperature

Hertzsprung Russell Diagram - Brightest Stars

-10

-5

0

5

10

15

20

050001000015000200002500030000Temperature (K)

Ab

so

lute

Ma

gn

itu

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The Nearest Stars

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

Adding the

Nearest Stars to the HR

Diagram

Hertzsprung Russell Diagram

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-5

0

5

10

15

20

050001000015000200002500030000Temperature (K)

Ab

so

lute

Ma

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The HRDiagram

Hertzsprung Russell Diagram

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-5

0

5

10

15

20

050001000015000200002500030000Temperature (K)

Ab

so

lute

Ma

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Giants andSupergian

ts

White Dwarf

MainSequence

Key Ideas – Stellar Census

• Comparison of Main Sequence, Giant, and White Dwarf Stars

• The Family of Stars • What are the most/least common kinds

of stars?

• Why are red dwarfs so common?

The brightness of a star 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

Stellar Properties Review Luminosity: from brightness and distance

10-4 LSun - 106 LSun

Temperature: from color and spectrum

3,000 K - 50,000 K

Mass: from binary-star orbits

0.08 MSun - 100 MSun

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

Homework #7 Due THURS. Events this week

Rooftop Oct 10 & 11 Kirkwood Open Night Oct. 11 Remote Observing Oct 14 & 16