Surveying Stars

Home , Achernar, Fomalhaut, HD 119124, Lalande 21185, Mizar and Alcor, Q star

Surveying Stars

Orin Harris and Greg Anderson Department of Physics & Astronomy Northeastern Illinois University

Spring 2021

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 1 / 124 Overview

The Night Sky Luminosity & Brightness Distance & Parallax Stellar Temperatures HR Diagrams Binary Systems Stellar Masses Star Clusters Review

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 2 / 124 Our Sun is a star. Other stars are similar but can be brighter/dimmer, bluer/redder, and live shorter/longer lives.

Sun as seen from earth Sun as seen from Voyager from 43 AU (4 billion miles) Sun

The Night Sky Cassiopeia Stars in the Night Sky Night sky star neighborhood 25 brightest (apparent) stars Resolving Star trails The Night Sky Star Movement Orion with distance Star Movement Luminosity & Brightness Distance & Parallax Stellar Temperatures

HR Diagrams

Binary Systems

Stellar Masses

Star Clusters

Review c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 4 / 124

Stars in the Night Sky

• Outside of the city, on very clear nights you can see thousands of stars. • Under the best conditions the furthest star we can see with the naked eye is a few thousand light years away. • Of the 25 brightest stars, the closest is α Cen. (4 ly), the furthest is Deneb (2600 ly). • Astronomers have catalogued the position/velocity/brightness of over 1 billion stars in our galaxy. Only a handful can be resolved as more than a point of light.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 6 / 124

Stars are mostly seen as points of light

Exceptions: just a few of the largest/nearest stars. E.g. π1 Gruis, Antares, and Betelgeuse, each roughly ∼500 ly away and ∼500 R⊙

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 9 / 124 Star Trails (Atacama Desert, Chile, by ESO) Star Movement

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 11 / 124

Star Movement from GAIA over 105 years Star Movement from GAIA over 105 years Star Movement from GAIA over 105 years Star Movement from GAIA over 105 years Star Movement from GAIA over 105 years Star Movement from GAIA over 105 years Star Movement from GAIA over 105 years Sun

The Night Sky Luminosity & Brightness Luminosity Q: L Units Stellar L η Carinae Proxima Luminosity & Centauri Blackbody Rad. Relative L Q: Star L Brightness Motorcycle Q: b vs L App Brightness Bulbs Q: L vs r Q: Units for b Magnitude Sys m chart Bright Stars Q: Visible limit Stars vs. Limiting Magnitude Dark Sky Dark Sky Abs c 2012-2021G.Anderson.,O.Harris Mag (M) Universe:Past,Present&Future – slide 14 / 124 Q: Vega Luminosity

Luminosity: The total amount of power (energy per unit time) emitted by a star or other object. Units: watts. • A 100-watt light-bulb emits half the power of a 200-watt light-bulb. It is half as bright. • The Sun has a luminosity of

26 L⊙ ≈ 4 × 10 watts

26 L⊙ ≈ 4 × 10 watts Lbulb = 100watts

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 15 / 124 Q: Luminosity Units

What are the standard units for luminosity? A) Watts B) Joules C) Newtons D) kilograms E) Watts per second

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 16 / 124 Q: Luminosity Units

What are the standard units for luminosity? A) Watts B) Joules C) Newtons D) kilograms E) Watts per second

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 16 / 124 Stellar Luminosities

Solar Luminosity:

26 L⊙ ≈ 4 × 10 watts

Eddington Limit: Upper limit for stellar luminosity. Above this, radiation pressure exceeds gravity. 6 L . 10 L⊙ Least luminous stars:

−4 L ≈ 10 L⊙

Dim stars are much more common than bright stars.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 17 / 124 η Carinae

Eta Carinae (η Car): Located in the Homunculus Nebula in Ca- rina 7500 ly away. Less than 3 million years old, but will go su- pernova soon. One of the most massive stars studied in great detail. Brieﬂy became the 2nd brightest star in the sky in 1843, now faded.

M = 150M⊙ −→ 120M⊙

6 L ≈ 10 L⊙

R ≈ 24R⊙

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 18 / 124 Proxima Centauri, part of the α-Cen. triple-star system, 0.002L⊙ Blackbody Radiation

Blackbody radiation: thermal radiation emitted by opaque bodies. (perfect absorbers)

Stefan-Boltzmann law for Luminosity:

4 L = σAT

Stefan-Boltzmann constant

σ =5.67 × 10−8 W/m2 · K4 T = Temperature

A = Area

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 20 / 124 Relative Luminosity

Relative luminosity for stars with radius R and temperature T

4 Lstar σAstarTstar = 4 Lsun σAsunTsun

2 4 σ 4πRstar Tstar = 2 4 σ(4πRsun) Tsun R 2 T 4 = star star Rsun Tsun

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 21 / 124 Q: Star Luminosity

The luminosity of a star is the: A) apparent brightness of the star in our sky. B) surface temperature of the star. C) lifetime of the star. D) total amount of energy that the star will radiate over its entire lifetime. E) total amount of energy that the star radiates each second.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 22 / 124 b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b

b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b

b b b b b b b b b b b b

b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b Q: Brightness vs Luminosity

These two stars have about the same luminosity – which one appears brighter? A) Alpha Centauri B) The Sun

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 24 / 124 Q: Brightness vs Luminosity

These two stars have about the same luminosity – which one appears brighter? A) Alpha Centauri B) The Sun Alpha Centauri is a little more luminous that the Sun, but it is much farther away.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 24 / 124 Apparent Brightness

Apparent Brightness: How bright a distant object appears to an observer on Earth. As light travels away from the source, the power (L) is spread over a sphere with surface area: A =4πr2.

Apparent Brightness:

b = L/A

Surface Area of Sphere:

A =4πr2

Apparent Brightness: L b = 4πr2

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 25 / 124 Brightness vs. Distance

6 b = L 50 W 4πr2 5 L = b(4πr2) 4 r = L/4πb 3 200 W p

2 Brightness (b)

0 012345678910 Distance (r)

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 26 / 124 Q: Luminosity and distance

If the distance between us and a star is doubled, with everything else remaining the same, its luminosity

A) is decreased by a factor of four, and its apparent brightness is decreased by a factor of four.

B) is decreased by a factor of two, and its apparent brightness is decreased by a factor of two.

C) remains the same, but its apparent brightness is decreased by a factor of two.

D) remains the same, but its apparent brightness is decreased by a factor of four.

E) is decreased by a factor of four, but its apparent brightness remains the same.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 27 / 124 Q: Luminosity and distance

If the distance between us and a star is doubled, with everything else remaining the same, its luminosity

A) is decreased by a factor of four, and its apparent brightness is decreased by a factor of four.

B) is decreased by a factor of two, and its apparent brightness is decreased by a factor of two.

C) remains the same, but its apparent brightness is decreased by a factor of two.

D) remains the same, but its apparent brightness is decreased by a factor of four.

E) is decreased by a factor of four, but its apparent brightness remains the same.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 27 / 124 Q: Units for Brightness

What are the standard units for apparent brightness? A) Watts B) Joules C) Newtons D) Watts per second E) Watts per square meter

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 28 / 124 Q: Units for Brightness

What are the standard units for apparent brightness? A) Watts B) Joules C) Newtons D) Watts per second E) Watts per square meter

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 28 / 124 The Magnitude System

Hipparchus (190-120 BCE) divided stars into six magnitude classes

Apparent magnitude (m) • m = 1, 20 brightest stars • m = 2 next brightest stars • ... • m = 6 faintest stars

Raphael’s Hipparchus from “The School of Athens” Modern deﬁnition: Five steps in magnitude as a factor of 100 in brightness. Thus, one step in magnitude represents a factor of:

1001/5 ≈ 2.51

step by 012345 6 7 8 9 10 fainter by 1 2.5 6.3 16 40 100 250 630 1600 4000 10,000 c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 29 / 124 Apparent Magnitude (m)

-30 Sun (-26.7) -25 -20 -15 Full moon (-12.6) -10 -5 Venus, at brightest (-4.4) 0 Sirius (-1.44) +5 Naked eye limit (+6) +10 Binocular limit (+10) +15 Pluto (+15) +20 +25 +30 Best Telescopes (+30)

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 30 / 124 10 Brightest Stars (Apparent)

m Designation Name Distance(ly) Class

0 -26.74 - (Sun) 0.000016 G2V 1 -1.46 α CMa Sirius 8.6 A1 V 2 -0.72 α Car Canopus 310 F0Ia 3 -0.27 α CenA RigilKentaurus 4.3 G2V 4 -0.04 α Boo Arcturus 37 K1.5III 5 0.03 α Lyr Vega 25 A0 V 6 0.08 α Aur Capella 42 G8III 7 0.12 β Ori Rigel 770 B8Iab 8 0.34 α CMi Procyon 11 F5 IV-V 9 0.42 α Ori Betelgeuse 640 M2Iab 10 0.50 α Eri Achernar 140 B3 Vpe

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 31 / 124 Q: Visible limit

The faintest star visible to the naked eye has an apparent magnitude of about A) 10. B) 6. C) 1. D) 0.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 32 / 124 Stars vs. Limiting Magnitude

m ≤ No. Stars Notes +7 15,544 Not visible to naked eye +6 5,044 Zodiacal light visible +5 1,627 Faint Milky Way +4 519 Suburbs +3 177 Major city +2 49 Center of large cities -1 1 Sirius, planets

Data from the HIPPARCOS Main Catalog

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 33 / 124

Absolute Magnitude (M)

Apparent and Absolute: Apparent magnitude m: brightness of a star measured from Earth. m⊙ = −26.74 Absolute magnitude M: brightness of a star if it was placed 10 pc (32.6 ly) from Earth. M⊙ =4.83

Five steps in magnitude represents a factor of 100 in brightness which is a factor of 10 in distance (d ∝ b−1/2):

d −1/2 = 100(M−m)/5 = 10(m−M)/5 10pc

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 36 / 124 Q: Vega

The star Vega has an absolute magnitude of about 4 and an apparent magnitude of about 0. Based on the deﬁnitions of absolute and apparent magnitude, we can conclude that: A) Vega is nearer than 10 parsecs from Earth. B) Vega has a parallax angle of 1/10 arcsecond. C) Vega’s luminosity is less than that of our Sun. D) Vega’s surface temperature is cooler than the Sun.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 37 / 124 Q: Vega

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 37 / 124 Sun

The Night Sky Luminosity & Brightness Distance & Parallax Parallax Angles Angles Angles Distance & Parallax Angles Stellar Parallax Parsec Q1: Parallax Q2: Parallax Hipparcos Gaia Stellar Temperatures

HR Diagrams

Binary Systems

Stellar Masses

Star Clusters

Review

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 38 / 124 Parallax

The apparent shift in position of nearby objects in comparison to more distant objects resulting from the movement of the observer. c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 39 / 124 Parallax

S = C θ R

Circumference: C = 2π R

Arc length (fraction of C): S = 2π R

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 40 / 124 Angles, Size, and Distance S

θ R

Circumference: C = 2π R

Arc length (fraction of C): 1 S = 2π R 2

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 41 / 124 Angles, Size, and Distance

Circumference: C = 2π R

Arc length (fraction of C): 1 S = 2π R 4

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 42 / 124 Angles, Size, and Distance

Distance S S R = θ θ R In a circle 360◦ =2π radians

Distance (θ in degrees) S 360 Circumference: R = × θ 2π C = 2π R Distance (θ in arc sec) S 360 × 3600 Arc length (fraction of C): R = × θ 2π S = θ R for 0 ≤ θ ≤ 2π radians

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 43 / 124 Stellar Parallax

b b

b b b b

b b b b Jan sky Jun sky θ

1 AU × 360×3600 d = θ 2π

1 AU

June January

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 44 / 124 Parsec

Parsec: The distance at which a star has a parallax angle of one arc-second.

1 AU 360 × 3600 d = × 1 arc second 2π 206265 AU 3.2 ly 1 parsec = = ≡ arc second arc second arc second 1 1 ⇒ distance (parsecs) = ≡ parallax angle (arc seconds) p

Example: If p =0.04′′, 1 1 d = = = 25 pc p 0.04

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 45 / 124 Q1: Parallax

Sirius, α Canis Majoris, has a parallax angle of 0.38 arcseconds, while Procyon, α Canis Minoris, has a parallax angle of 0.29 arcseconds. Which star is closer to Earth? A) Sirius B) Procyon C) we can’t tell

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 46 / 124 Q1: Parallax

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 46 / 124 Q2: Parallax

The parallax angle of a star is 1/20=0.05 arcseconds. What is the distance to the star? A) 0.2 light-year. B) 0.2 parsec. C) 20 light-years. D) 20 parsecs. E) impossible to determine.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 47 / 124 Q2: Parallax

The parallax angle of a star is 1/20=0.05 arcseconds. What is the distance to the star? A) 0.2 light-year. B) 0.2 parsec. C) 20 light-years. D) 20 parsecs. E) impossible to determine. 1 1 d(pc) = = = 20pc p(arcsec) 0.05

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 47 / 124 Hipparcos Satellite

Astrometry: The precise measurement of the positions of stars and other celestial bodies. Hipparcos: HIgh Precision PARallax COllecting Satellite, homage to Hipparchus of Nicaea.

Hipparcos satellite, ESA • Operated 1989-1993. • Target precision 0.002 arcseconds • Hipparcos Catalogue: 118,200 stars.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 48 / 124 Gaia Satellite

Astrometry: The precise measurement of the positions of stars and other celestial bodies. Gaia: Successor to Hipparcos

Gaia Satellite, ESA • Operated 2013-present. • Target precision 0.00002 arcseconds (100× better than Hipparcos) • Gaia Catalogue: > 1 billion stars. • Video: Change in night sky over 5 million years

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 49 / 124 Sun

The Night Sky Luminosity & Brightness Distance & Parallax Stellar Temperatures Stellar Temps Measuring Temp Stellar Temperatures Blackbody Rad. Wien’s Law Spectral Type recall Computers Understanding Stellar Spectra Spectral Type Computers Spectral Type Solar Spectrum Arct. Spectrum B-V Color Index Color Index Q: Spectral Type

HR Diagrams

Binary Systems c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 50 / 124 Stellar Masses Stellar Temperatures

Hottest Stars

T ≈ 50, 000K (Blue) Solar Temperature

T⊙ ≈ 5800K (White) Coolest Stars T ≈ 3000K (Red)

• Most stars in the Milky Way galaxy are smaller, cooler and dimmer than the Sun. • Most main sequence stars that you can see in the night sky are larger, hotter and brighter than the Sun.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 51 / 124 Measuring Temp

• Stefan-Boltzmann Law: L = σ(4πR2)T 4 −3 • Wein’s Law: λmT =2.9 × 10 mK • Spectral Type

• Color Index: mB − mV

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 52 / 124 Blackbody Radiation

Blackbody radiation: thermal radiation emitted by opaque bodies. (perfect absorbers)

Stefan-Boltzmann law for Luminosity:

4 L = σAT

Stefan-Boltzmann constant

σ =5.67 × 10−8 W/m2 · K4 T = Temperature

A = Area

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 53 / 124 Wien’s Displacement Law

Hottest

Intensity Hotter

Hot

wavelength λ

Emission is a maximum at λm (Wilhelm Wien 1893)

−3 λmT = constant = 2.898 × 10 mK

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 54 / 124 Spectral Type

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 55 / 124 Spectral Type

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 56 / 124 Recall: Spectral Lines

• An incandescent solid or high dense gas (such as a blackbody) produces a continuous spectrum.

• A continuous spectrum source viewed through a cooler low-density gas produces an absorption-line spectrum.

Each type of atom has a unique spectrum. The spectrum of stars was initially a mystery to be unraveled.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 57 / 124 The Harvard Computers

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 58 / 124 Harvard College Observatory Collection of Astronomical Photographs. Understanding Stellar Spectra

From 1880-1920 the Harvard Computers, analyzing photographs of stars’ spectral lines, produced a ﬁrst-of-its-kind catalog of 200,000 stars, widely used by astronomers to this day. Women assistants were paid half as much as men. Selected Contributions:

Williamina Fleming: First “computer” hired at Harvard. In 1880s classified 10,000 stars, developed spectral classification system A, B, C,... according to how much 1H in spectra (the physical significance was unclear), discovered white dwarfs, numerous famous nebulae and novae. Annie Jump Cannon: In 1900s created the Harvard Classification: stellar surface temperature goes as OBAFGKM Henrietta Swan Leavitt: Developed the Cepheid period luminosity relationship, which Hubble used to establish that distant galaxies exist. Cecilia Payne: Established that stars are mostly hydrogen, discovered the first recurring Nova.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 59 / 124 Spectral Type • Original classiﬁcation A, B, C,... based on strength of 1H lines • Diﬃculties: cooler stars have weak 1H lines, but so do hotter stars because the 1H is all ionized • Later understood OBAFGKM corresponds to surface temperature

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 60 / 124 The Harvard Computers

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 61 / 124 Spectral Type

Harvard spectral classiﬁcation

Type Tsurf (K) Mass Radius(MS) %ofMS

O > 30, 000 ≥ 16M⊙ ≥ 6.6R⊙ .00003%

B 10, 000 − 30, 000 2.1 − 16M⊙ 1.8 − 6.6R⊙ 0.13%

A 7, 500 − 10, 000 1.4 − 2.1M⊙ 1.4 − 1.8R⊙ 0.6%

F 6, 000 − 7, 500 1.04 − 1.4M⊙ 1.15 − 1.4R⊙ 3%

G 5, 200 − 6, 000 0.8 − 1.04M⊙ 0.96 − 1.15R⊙ 7.6%

K 3, 700 − 5, 200 0.45 − 0.8M⊙ 0.7 − 0.96R⊙ 12.1%

M 2, 400 − 3, 700 0.08 − 0.45M⊙ ≤ 0.7R⊙ 76.45% Annie Jump Cannon (1863-1941) et al. (Mnemonic: Oh Be A Fine Guy/Girl, Kiss Me! → Only Boys Accepting Feminism Get Kissed Meaningfully.)

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 62 / 124 Spectrum of the Sun (G type)

c Nigel Sharp, NOAA c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 63 / 124 Spectrum from Arcturus (K type)

c NASA/NOAA c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 64 / 124 B-V Color Index

Observe stars through blue (B) and visual (V) ﬁlters. The color index is based on the ratio of brightness in blue light to its brightness in visible light.

B − V color index = mB − mV

A smaller color index corresponds to a hotter object. Examples: Sun B − V =0.66, Rigel B − V = −0.03 Intensity

wavelength λ

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 65 / 124 Color Index

Class B − VU − B V − R R − I Teﬀ (K) O5V -0.33 -1.19 -0.15 -0.32 42,000 B0V -0.30 -1.08 -0.13 -0.29 30,000 A0V -0.02 -0.02 0.02 -0.02 9,790 F0V 0.30 0.03 0.30 0.17 7,300 G0V 0.58 0.06 0.50 0.31 5,940 K0V 0.81 0.45 0.64 0.42 5,150 M0V 1.40 1.22 1.28 0.91 3,840

Sample calibration colors

V≡Visible, U≡Ultraviolet, B≡Blue, R≡Red, I≡Infrared Bluer ﬁlters are used for hotter stars, redder for cooler stars

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 66 / 124 Q: Spectral Type

Sirius is a star with spectral type A and Rigel is a star with spectral type B star. What can we conclude? A) Rigel has a higher surface temperature than Sirius. B) Rigel has a higher core temperature than Sirius. C) Sirius has a higher core temperature than Rigel. D) Sirius has a higher surface temperature than Rigel.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 67 / 124 Sun

The Night Sky Luminosity & Brightness Distance & Parallax Stellar Temperatures

HR Diagrams Recap HR Diagrams Q: Star L HR Diagram Example Relative L Relative L HR Diagram HR Diagram Resolving HR Diagrams HR Diagrams Yerkes Class Q: HR Diagram

Binary Systems

Stellar Masses

Star Clusters

Review c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 68 / 124 Recap

• Stellar Luminosities: – Stefan-Boltzmann Law: Luminosity L is proportional to R2T 4

−4 6 – Range: 10 L⊙

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 69 / 124 Recap

• Apparent brightness and distance: – Apparent brightness is proportional to L d2 – Stellar Parallax used to determine:

1 d(parsecs) = p(arcsec)

b b b b b b b b b b b b

b b b b

June January

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 70 / 124 Recap

• Stellar Temperatures: – 3,000K (Red) < T < 50,000K (Blue) – Spectral Classiﬁcation: OBAFGKM – B-V Color index

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 71 / 124 Q: Star Luminosity

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 72 / 124 Hertzsprung-Russell (H-R) Diagram

Temperature (K) 40,000 10,000 7,000 6,000 5,000 4,000 3,000 2,500 106 −10 O B A b F G K M b

b b

b b ( Magnitude Absolute b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b 4 b b b b b b b b b b b b b b b

) b − b b b b 5 b b b b b b 10 b b b b b b b b b b b b b b b b b b b b b ⊙ b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b bb bb b b bb b b b b b b L/L b b b b b b b bb b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b b b b b b b b b b b b b b 2 b b b b b b b b b b b b b b b b b b bb bb b b b b b b b bb b b b b b bb b b b b b b b b bb b b b b b b bb b b b bb b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b bb b b b b b bb b b b b b b b b b b b b b b b bb bb b b b bb b b bb b b b b b b b b b b b b b b b b b bb b b bb b b b b b b b b bb b b b b b b b b b b b bb b b b b b b bb b 0 b b b b b b b b b b b b b b b bb b b b b b b b b b b bb b b b b b b b b b b b b b b b b b b b b b b b b b 10 b b b b b b b b b b b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b bb b b b b b b b b b bb b b b bb b b bb b b b b b b b b b bb b b b b b b b b b b b b b b b b b b b b b b b b b bbb b b b b b b b bb b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b b b b b b b b b b b b b b b bb b b bb b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b bb bb b b b b b b bb b b b b b b b bb b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b b b bb b b b b b bbb b b b b b b b b b b b b b b b b b b b b b b b bb b b bb bbb b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b bb b b b bb b b b b b b b b b b b b b b b b b b b b b b bb b b bb b b bbbb b b bb b b b b bb b b b b b b b b b b b b b b b b b b b b b bb b b bb b b b b b b b b b b b b b b b b b b b b b b bbb b b b b b b bb b b b b bb b b b b b b b b b b b b bb b bb b b b b b b bb bbb b b b bbb b bb b bb b b b b b b b b b b b b b b b b b b b b b bb bb b b b b b b b b b b b b b b b b bb bb b bbb b b b b b b b bb b bb bb b b b b b b b b b bb bb b b b b b bb bb b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b bb b b b b b b b bb b b b b b b b b bb b b b b b b b b bb b b b b b b b b b b b b b bb b b b b b b b b bb b bb b b b b b b b b b bb b b b b b b b b b b b b b b bb b b b b bb b b b b b b b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b b b b b b b b b b b bbbb b b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b b b b b bb bb b b b b b b b b b b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b b bb b b b b b b b b b b bb b b b b b b b b b b b b b b b b b b b b b bbb b b b b b b bb b b bbb b b b b bb b b b b b b b b b b b b b b b b b b b b bb b b b b b b b b bb b b bbb b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b bb b b b b b b b bb bb b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b bb b b b b b b bb b b b b b b b b bb b b b b b 0 b b b b b bb b b b b b b b b b b bb b b b b b b b b b b b bb b b b b b b b b b b b bb b b b b b b b b b b 5 b b b b b b b b 10 b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b

b M b b b b b b b b b

Luminosity ( b b b b b b b b b b b b bb b b b b b b b b b b b b b b b b V − b b b b b b 2 b b b b b b b b b b bb b b bb b b b b bb b b b b b b b 10 b b b bb b b b b b b b b ) 10 b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b bb b b b b b b b b b b b b b b b b bb b b bb b b b b b b b b b bb b b b b bbb bb bb b b b b b b b bb b b b b b b b b bb b b b b b b b b b b bb b b b b b b b b b b b b − b b b 4 b b b b b b b 15 10 b b −0.5 0 0.5 1.0 1.5b 2.0 Color Index (B − V )

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 73 / 124 Example

Example: Animation for cluster Omega Centauri.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 74 / 124 Relative Luminosity

Stefan-Boltzmann Law: Relative luminosity for stars with radius R and temperature T

L R 2 T 4 star = star star Lsun Rsun Tsun Main Sequence:

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 75 / 124

Hertzsprung-Russell (H-R) Diagram

Temperature (K) 40,000 10,000 7,000 6,000 5,000 4,000 3,000 2,500 106 −10 O B A b F G K M b

b b

b b ( Magnitude Absolute b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b 4 b b b b b b b b b b b b b b b

b M b b b b b b b b b

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 77 / 124 Hertzsprung-Russell (H-R) Diagram

6 O B A F G K M 10 R −10 = 10 bc R = 1000 bc Deneb

R ( Magnitude Absolute ⊙ Rigel R bc ⊙ b bc Betelgeuse b Canopus 4 b bcβCen Antares −5 ) 10 b R b b ⊙ = R R ⊙Spica Bellatrixbc Polaris = 100 b b R αPav Achernar bc ⊙ L/L bc Aldebaron 2 Algolb bc bc b bc Arcturus 10 R Vega Capella bc 0 = 0 Regulus b . Fomalhautb Pollux 1R Sirius Altairb ⊙ b Procyon

αb Cen A 0 b 10 R Sun b 5 = 0 αCen B .01 R b ⊙ b M Luminosity (

−2 V 10 R Lalandeb 21185 10 = 0 b ) b .001 Sirius B 40 Eridani B R ⊙ b Procyon B Barnardsb b −4 10 b 15 40,000 20,000 10,000 7,000 5,000 Proxima Centauri2,500 b Wolf 359 Temperature (K)

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 78 / 124 Stars are mostly seen as points of light

Exceptions: just a few of the largest/nearest stars. E.g. π1 Gruis, Antares, and Betelgeuse, each roughly ∼500 ly away and ∼500 R⊙

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 79 / 124

HR Diagrams

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 81 / 124 Stellar Luminosity Classes

A complete classiﬁcation of a star requires the star’s spectral type: OBAFGKM, and a luminosity class (Yerkes Spectral Classiﬁcation):

0 Hypergiants I Supergiants II Bright Giants III Giants IV Subgiants V Main-sequence stars VI Subdwarfs VII White dwarfs

Examples: Our Sun is classiﬁed as G2 V, Proxima Centauri is M5 V.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 82 / 124 Q: HR Diagram

You observe a star and you want to plot it on an H-R diagram. You will need to measure all of the following, except the star’s: A) mass B) distance C) apparent brightness D) spectral type

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 83 / 124 Q: HR Diagram

You observe a star and you want to plot it on an H-R diagram. You will need to measure all of the following, except the star’s: A) mass B) distance C) apparent brightness D) spectral type

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 83 / 124 Sun

The Night Sky Luminosity & Brightness Distance & Parallax Stellar Temperatures HR Diagrams Binary Systems Binary Systems Binary Stars Classiﬁcation Ursa Major Mizar & Alcor Alcor & Mizar Visual Binaries Albireo Eclipsing Binary Q: Dimming Star Doppler Eﬀect Spectroscopic Measuring Masses Q: Stellar Masses

Stellar Masses

Star Clusters c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 84 / 124 Review Binary Stars

Double star: Two stars which appear close together in the sky. A binary or optical double star. Binary star: Two stars orbiting their common center of mass. Optical double star: Two stars with no physical connection which appear close together from Earth.

Of the stars nearest to the Sun, about half are known to be in multiple star systems.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 85 / 124 Classiﬁcation

Visual Binary: A pair of stars we can see distinctly. Eclipsing Binary: A pair of stars which orbit in our line of sight. Spectroscopic Binary: A binary pair in which is inferred from periodic doppler shifts in the spectrum Astrometric Binary: Measurable, periodic, deviation in the stars position. These methods of detection are not mutually exclusive.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 86 / 124

Mizar and Alcor (Ursa Major)

b Mizar-Alcor sextuple system, d = 25pc • Mizar (4 stars)

Alcor b

b • Alcor (2 stars, eclipsing binary)

Mizar

b b

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 88 / 124

Visual Binaries

Alpha, Beta and Proxima Centauri

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 90 / 124 Visual Binary: Albireo (β Cygni) Eclipsing Binary brightness

Animations: • European Southern Observatory (ESO) • European Southern Observatory (ESO) • OSU • Addison-Wesley

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 92 / 124 Q: Dimming Star

Careful measurements reveal that a star maintains a steady apparent brightness at most times, except that at precise intervals of 73 hours the star becomes dimmer for about 2 hours. The most likely explanation is that: A) the star is a member of an eclipsing binary star system B) the star is a Cepheid variable C) the star is periodically ejecting gas into space, every 73 hours D) the star is a white dwarf c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 93 / 124 Doppler Eﬀect v

∆λ ≈ v λ c

Applet

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 94 / 124 Spectroscopic Binary

approaching

To Earth −→

receding

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 95 / 124 Measuring Masses in Binary Systems

Direct mass measurements are possible only for stars in binary star systems. Measure mass using gravity & Kepler’s Law:

4π2 p2 = a3 G(M1 + M2) where p = period, a = average separation.

Since vp =2πa =2πr, need two out of three observables to measure mass: • Orbital period: p • Orbital separation: a • Orbital velocity v

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 96 / 124 Q: Stellar Masses

From which quantities below can we calculate the masses of stars in a binary system: A) orbital period and average orbital distance B) spectral types and distance from Earth C) absolute magnitudes and luminosities D) luminosities and distance from Earth

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 97 / 124 Q: Stellar Masses

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 97 / 124 Sun

The Night Sky Luminosity & Brightness Distance & Parallax Stellar Temperatures HR Diagrams Stellar Masses Binary Systems

Stellar Masses Stellar Masses η Carinae Proxima Centauri Q: Mass and Spectral Type Stellar Lifetimes Stellar Lifetimes (Sun-like star) 4 MS Stars Properties Q: Stellar Characteristics Stellar Populations

Star Clusters c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 98 / 124 Review Stellar Masses

Very massive stars are rare. Upper (Eddington) limit on a stars mass:

M . 150M⊙ Low mass stars are common. Lower limit on a stars mass: M ∼> 0.08M⊙

Objects with M . 0.08M⊙ are brown dwarfs, not enough mass to sustain proton-proton nuclear fusion.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 99 / 124 η Carinae

M = 150M⊙ −→ 120M⊙

6 L ≈ 10 L⊙

R ≈ 24R⊙

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 100 / 124 Proxima Centauri, part of the α-Cen. triple-star system, 0.002L⊙ Q: Mass and Spectral Type

How did astronomers discover the relationship between spectral type and mass for main sequence stars?

A) By measuring the masses and spectral types of main-sequence stars in binary systems

B) By using computer models of hydrogen fusion and stellar structure

C) By measuring stellar radii with very powerful telescopes

D) By comparing stars with the same spectral type but diﬀerent luminosities

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 102 / 124 Q: Mass and Spectral Type

How did astronomers discover the relationship between spectral type and mass for main sequence stars?

A) By measuring the masses and spectral types of main-sequence stars in binary systems

B) By using computer models of hydrogen fusion and stellar structure

C) By measuring stellar radii with very powerful telescopes

D) By comparing stars with the same spectral type but diﬀerent luminosities

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 102 / 124 Stellar Lifetimes

Larger stars have more hydrogen fuel, but because of their high temperatures (L ∝ T 4), they use it up much more rapidly. Typically L ∝ M 4

So the Lifetime of a star on the Main Sequence is:

10 M L⊙ τMS ≈ 10 years M⊙ L 4 3 10 M M⊙ 10 M⊙ = 10 4 years = 10 years M⊙ M M

Increasing a stars mass by a factor of two decreases its lifetime by a factor of eight.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 103 / 124 Stellar Lifetimes (Sun-like star)

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 104 / 124 Four main-sequence stars

Sirius Upsilon Orionis A1 V B0 V M =2M⊙ M = 20M⊙ Lifetime 109 years Lifetime 107 years Sun G2 V Lifetime 1010 years

Proxima Centauri M5.5 V M =0.12M⊙ Lifetime 1012 years

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 105 / 124 Stellar Properties

Luminosity, from brightness and distance: −4 6 10 L⊙ . L . 10 L⊙ Temperature, from color and spectral type: 3, 000K . T . 50, 000K Radius, from luminosity and temperature, or optical interferometry:

0.1R⊙ . RMS . 20R⊙ . R . 1000R⊙ Mass, from binary systems:

0.08M⊙ . M . 150M⊙ Lifetime, from star clusters, or mass and luminosity: 106 years . τ . 1012 years Stage of life, from HR diagrams, as we will see Main Sequence, Red Giant, White Dwarf

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 106 / 124 Q: Stellar Characteristics

All stars are born with the same basic composition, yet stars can look quite diﬀerent from one another. Which two factors primarily determine the characteristics of a star? A) Its mass and its stage of life B) Its apparent brightness and its distance C) Its age and its location in the galaxy D) Its mass and its surface temperature E) Its apparent brightness and its luminosity

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 107 / 124 Stellar Populations

Population I: “Metal-rich.” Approximately circular orbits in the disk component of the galaxy. Have a greater abundance of elements heavier than helium compared to population II stars. Z ∼ 0.02 Population II: “Metal-poor.” Have random orbits in the spheroidal component of our galaxy. Age: 10-13 billion years old. M . 0.8M⊙. Z ∼ 0.0001 Population III: “Metal-free.” Extinct population of very massive stars that formed 106 − 107 years after the Big Bang. Z ∼ 0

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 108 / 124 Sun

The Night Sky Luminosity & Brightness Distance & Parallax Stellar Temperatures HR Diagrams Star Clusters Binary Systems

Stellar Masses

Star Clusters Stellar Clusters Pleiades NGC 346 Cluster Mosaic M80 M13 HR Diagrams Clusters & HR Clusters & HR Clusters Clusters Q: Cluster Ages

Review c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 109 / 124 Stellar Clusters

• Open clusters: A group of up to several thousand stars, found in the disks of galaxies containing young stars. • Globular clusters: spherical collection of up to a million stars. Globular clusters are found primarily in galactic halos - they orbit galactic cores as a satellite, and contain very old stars.

b b b b b b b b b b b b b b

b b b b b b b b b b b b b b b b b b b b b b b b b b b Halo b b b Globular Clusters b b b

b b b b b b b b b b b b b b b b b b b b b b b b b b b b b Bulge disk b b b b b b b

b b b b b b b b b

b b b b b b b b b b b b b b b b b

b b

b b b

b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b c 2012-2021G.Anderson.,O.Harrisb Universe:Past,Present&Future – slide 110 / 124 b M45 The Pleiades (Seven Sisters)

c Craig Lent. 35 min. exposure, Canon 10D, ISO 800, 101mm refractor @f/5.4 c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 111 / 124 Open Cluster NGC 346

M80 (NGC 6093) Globular Cluster

c 2012-2021G.Anderson.,O.HarrisGreat Images in NASA Universe:Past,Present&Future – slide 114 / 124 Globular Cluster: M13 (NGC 6205) HR Diagrams

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 116 / 124

Q: Turnoﬀ point

What do we mean by the main-sequence turnoﬀ point of a star cluster, and what does it tell us?

A) It is the point in a star cluster beyond which main-sequence stars are not found, and it tells us the cluster’s distance.

B) It is the spectral type of the hottest main-sequence star in a star cluster, and it tells us the cluster’s age.

C) It is the luminosity class of the largest star in a star cluster, and it tells us the cluster’s age.

D) It is the mass of the most massive star in the star cluster, and it tells us the cluster’s size.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 119 / 124 Q: Turnoﬀ point

What do we mean by the main-sequence turnoﬀ point of a star cluster, and what does it tell us?

A) It is the point in a star cluster beyond which main-sequence stars are not found, and it tells us the cluster’s distance.

B) It is the spectral type of the hottest main-sequence star in a star cluster, and it tells us the cluster’s age.

C) It is the luminosity class of the largest star in a star cluster, and it tells us the cluster’s age.

D) It is the mass of the most massive star in the star cluster, and it tells us the cluster’s size.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 119 / 124 Q: Stellar Clusters

The choices below each describe the appearance of an H-R diagram for a diﬀerent star cluster. Which cluster is most likely to be located in the halo of our galaxy?

A) The diagram shows main-sequence stars of every spectral type except O, along with a few giants and super-giants.

B) The diagram shows main-sequence stars of spectral types G, K, and M, along with numerous giants and white dwarfs.

C) The diagram shows main-sequence stars of all the spectral types except O and B, along with a few giants and super-giants.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 120 / 124 Q: Stellar Clusters

The choices below each describe the appearance of an H-R diagram for a diﬀerent star cluster. Which cluster is most likely to be located in the halo of our galaxy?

A) The diagram shows main-sequence stars of every spectral type except O, along with a few giants and super-giants.

B) The diagram shows main-sequence stars of spectral types G, K, and M, along with numerous giants and white dwarfs.

C) The diagram shows main-sequence stars of all the spectral types except O and B, along with a few giants and super-giants.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 120 / 124 Q: Cluster Ages

The choices below each describe the appearance of an H-R diagram for a diﬀerent star cluster. Which cluster is the youngest?

A) The diagram shows main-sequence stars of every spectral type except O, along with a few giants and super-giants.

B) The diagram shows main-sequence stars of spectral types G, K, and M, along with numerous giants and white dwarfs.

C) The diagram shows main-sequence stars of all the spectral types except O and B, along with a few giants and super-giants.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 121 / 124 Q: Cluster Ages

The choices below each describe the appearance of an H-R diagram for a diﬀerent star cluster. Which cluster is the youngest?

A) The diagram shows main-sequence stars of every spectral type except O, along with a few giants and super-giants.

B) The diagram shows main-sequence stars of spectral types G, K, and M, along with numerous giants and white dwarfs.

C) The diagram shows main-sequence stars of all the spectral types except O and B, along with a few giants and super-giants.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 121 / 124 Sun

The Night Sky Luminosity & Brightness Distance & Parallax Stellar Temperatures HR Diagrams Review Binary Systems

Stellar Masses

Star Clusters

Review Review Review II

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 122 / 124 Review

• What are the units of luminosity? • What is the relationship between apparent brightness, luminosity and distance? • What is the magnitude of the faintest star visible to the naked eye? • How is a parsec deﬁned? How big is a parsec in light-years? • If you measure a parallax angle of 0.2 arc-seconds, how far away is a star in parsecs?

• Why are there no stars with M < 0.08M⊙?

• Why are there no stars with M ∼> 150M⊙? • What is a double star? a binary star? an optical double? • How common are binary stars?

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 123 / 124 Review II

• As you move along the main sequence from the lower right to the upper left on an HR diagram. How does the mass, surface temperature, luminosity and lifetime of a star change? • How do astronomers determine a star’s mass? • How is a star’s lifetime related to its mass? • Make a quick sketch of a Hertzsprung-Russel diagram. • How do stars on the main sequence obtain energy? • What is a stars spectral type? • Who were Pickering’s computers? • Our Sun is a type G2 V star. What do those letters mean? • How can you determine the age of a cluster from the main sequence turn oﬀ?

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 124 / 124