Our Galaxy the Milky Way

Our Galaxy The Milky Way

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

Spring 2021

c 2012-2021 G. Anderson., O. Harris Universe: Past, Present & Future – slide 1 / 93 Overview

The Milky Way ISM Nebula Galactic Recycling Evolution The Galactic Center Review

c 2012-2021 G. Anderson., O. Harris Universe: Past, Present & Future – slide 2 / 93 M31: The Andromeda Galaxy NGC 4565: The Needle Galaxy SA(s)b Andromeda NGC 4565

The Milky Way The Night Sky The Night Sky Etymology Moments in History Q: Galactic Fog Milky Way M13 The Milky Way Q: Disk Thickness Q: Disk Diameter Q: Galactic Halo Milky Way Orbits Galactic Merry-Go-Round Fig 14.02 Q: Bobbing Stars Galactic Mass from Orbital Velocity Q: Orbital Speed Q: Sun’s Orbit Milky Way Spiral Arms Spiral Arm – Theory c 2012-2021 G. Anderson., O. Harris Universe: Past, Present & Future slide 5 / 93 c Matipon Tangmatitham Pan-STARRS (Panoramic Survey Telescope and Rapid Response System) in Maui Etymology

• The English word galaxy is derived from the greek γαλαξιας (galaxias) meaning milky. • The English Milky Way is derived from from Latin Via Lactea from the Greek γαλαξιας κυκλoς´ aka Milky Circle.

c 2012-2021 G. Anderson., O. Harris Universe: Past, Present & Future – slide 8 / 93 Moments in History

• Galileo Galilei ﬁrst resolved the band of light known as the Milky Way into individual stars with his telescope in 1610, demonstrating the hypothesis of Anaxagoras and Democritus (5th century BCE). • Galactocentrism: 1785 William Herschel: Sun at center of ﬂattened disk. (His view through the disk was obstructed by the “fog” of interstellar medium)

• 1918 Harlow Shapley: globular clusters centered on a point 50,000 ly 27,000 ly from us. • Heber Curtis (1917) & Edwin Hubble (1920) demonstrated that spiral “nebulae” were actually distant galaxies beyond the Milky Way.

c 2012-2021 G. Anderson., O. Harris Universe: Past, Present & Future – slide 9 / 93 Q: Galactic Fog

How does the interstellar medium aﬀect our view of most of the galaxy?

A) It prevents us from seeing most of the galactic disk with visible and ultraviolet light.

B) It absorbs all wavelengths of light.

C) It produces so much visible light that it blocks our view of anything beyond it.

D) It has no eﬀect on visible-light observations, but prevents us from studying the galactic center with radio waves or X-rays.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 10 / 93 The Milky Way (SBc)b b b b b b b b b b b b b b

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 b b b ∼ 150 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 b b Bulge b b b b b b b b b b Sgr A* b b b b b b b

b b b b b b b b b b Sun b 27,000 ly from galactic center Disk b b

b b

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12 200 billion stars, 100, 000 ly diameter, 1, 000 ly thick, M & 10 M⊙ Globular Cluster: M13 (NGC 6205) Q: Disk Thickness

What is the thickness of the Milky Way’s disk? A) 100 light years B) 1,000 light years C) 10,000 light years D) 100,000 light years E) 1,000,000 light years

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 13 / 93 Q: Disk Thickness

What is the thickness of the Milky Way’s disk? A) 100 light years B) 1,000 light years C) 10,000 light years D) 100,000 light years E) 1,000,000 light years

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 13 / 93 Q: Disk Diameter

What is the diameter of the Milky Way’s disk? A) 100 light years B) 1,000 light years C) 10,000 light years D) 100,000 light years E) 1,000,000 light years

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 14 / 93 Q: Disk Diameter

What is the diameter of the Milky Way’s disk? A) 100 light years B) 1,000 light years C) 10,000 light years D) 100,000 light years E) 1,000,000 light years

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 14 / 93 Q: Galactic Halo

What kinds of objects lie in the halo of our galaxy? A) open clusters B) hot, young blue giants C) globular clusters D) gas and dust E) all of the above

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 15 / 93 Dark Matter Halo

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

Disk stars have roughly circular Halo stars have random orbits orbits around a common axis of that pass high above and below rotation. the disk. c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 17 / 93 Galactic Merry-Go-Round

Disk stars bob up and down as they orbit the galactic center. This motion gives the disk its 1,000 ly thickness. Near our Sun, this vertical oscillation takes place over several tens of millions of years,

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 18 / 93

Q: Bobbing Stars

Why do orbits of disk stars bob up and down? A) They are stuck to the interstellar medium. B) The gravity of disk stars pulls them toward the disk. C) Halo stars knock them back into the disk.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 20 / 93 Q: Bobbing Stars

Why do orbits of disk stars bob up and down? A) They are stuck to the interstellar medium. B) The gravity of disk stars pulls them toward the disk. C) Halo stars knock them back into the disk.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 20 / 93 Galactic Mass from Orbital Velocity

b b b

b b

b b b b b

b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b rb b b b b b b b b b b b b b F = ma b b b b b b b b b b b b 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 Mm v b b b b b b b b v b b b b b b b b G = m b b b r2 r b b b b b b b b b b m b b b 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 F b b b b b b b b b b b b b b b b b b b rv b b b b b b b b b b b b b b b b b b b b b b b M(r)= b b b b b b b b b b b b b b b b b b b b b b b b b b b b G b b b b b b b b b b b b b b b b b

b b b b b b

b b b

b b b b

b b

b b ∼ Using the Sun’s orbitalb distance and orbital velocity ( 230 km/s) we

b b can determine the mass inside the Sun’s orbit:

11 M ≈ 10 M⊙ (r< 27, 000 ly)

12 Mtot ≈ 10 M⊙

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 21 / 93 Q: Orbital Speed

What aﬀects the average orbital speed of a star in our galaxy? A) The mass of the galaxy inside its orbit and size of its orbit B) The mass and age of the star C) The star’s mass and the mass of the galaxy D) The mass and age of the galaxy

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 22 / 93 Q: Orbital Speed

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 22 / 93 Q: Sun’s Orbit

Approximately how long does it take the Sun to orbit the center of the Milky Way Galaxy? A) 23,000 years B) 230,000 years C) 2.3 million years D) 230 million years E) 23 billion years

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 23 / 93 Q: Sun’s Orbit

Approximately how long does it take the Sun to orbit the center of the Milky Way Galaxy? A) 23,000 years B) 230,000 years C) 2.3 million years D) 230 million years E) 23 billion years

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 23 / 93

Spiral Arm Theory

Lin-Shu density wave theory

• Link: Density Wave Theory • Link: Spiral Arm Animation

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 25 / 93 The Milky Way’s “Great Rift” Dust Lanes in the Milky Way

Interstellar clouds of gas and dust absorb visible light and obscure our view of our galaxy.

New stars are born from this interstellar medium.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 27 / 93 Andromeda NGC 4565

The Milky Way

ISM ISM Q: ISM Sample Number Densities n = N/V Local Bubbles Scorpius- Interstellar Medium Centaurus OB Association Timescale: Human Evolution H Phases H Transitions Hyperﬁne Q: 21 cm line

Nebula Galactic Recycling

Evolution The Galactic Center

Review c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 28 / 93 Interstellar Medium

Interstellar Medium (ISM): gas and dust between star systems in a galaxy (99% gas and 1% dust by mass). Gas composition:

H 89% H 60%

He 9% He 30% 2% Metals

by number by mass Number density:

n = 10−4 − 106 molecules/cm3

n =0.5 molecules/cm3

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 29 / 93 Q: Interstellar Medium

What makes up the interstellar medium? A) open clusters B) massive stars C) red dwarfs stars D) gas and dust E) all of the above

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 30 / 93 Q: Interstellar Medium

What makes up the interstellar medium? A) open clusters B) massive stars C) red dwarfs stars D) gas and dust E) all of the above

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 30 / 93 Sample Number Densities n = N/V

1024 Diamond n ≈ 2 × 1023 cm−3 Liquid Water n ≈ 3 × 1022 cm−3 1021 Air n =3 × 1019 cm−3 1018 ) 3 1015 1012 109 106 3 Molecular Clouds n ∼ 300cm−3 10 2 −3 (particles/cm Atomic Hydrogen Clouds n =1 − 10 cm −3 ISM n 1 Ave ISM in Milky Wayn ¯ =0.5cm −2 −3 3 Hot Bubbles n ∼ 10 cm 10− 10−6

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 31 / 93 Local Bubble and Clouds

Local Bubble Size: 300 ly. n =0.05 atoms/cm3 diﬀuse hot gas emits Xrays cause: 14-20 SN ∼ 20 Ma. Sco-Cen OB association

Local Interstellar Cloud Size: 30 light years n =0.3 atoms/cm3

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 32 / 93

Timescale: Human Evolution

Pliocene Pleistocene

A. boisei A. robustus A. africanus A. afarensis A. anamensis stone tools H. habilis H. erectus H. heidelbergensis 60 Fe H. neanderthalensis 104ly 135ly 106ly 91ly 96ly H. denisova b b b b b SN SN SN SN SN H. sapiens 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 Millions of years ago (Ma) c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 34 / 93 Phases of Interstellar Hydrogen

The hydrogen component ISM can be classiﬁed based on whether it is in molecular, atomic or ionized form. Roman numerals indicate the degree of ionization.

Molecular Hydrogen H2 Observed by radio and infrared emission of associated molecules like Carbon-monoxide (CO). T . 10 K. Neutral Atomic Hydrogen (HI region) non-ionized, atomic hydrogen H1. observed by 21 cm line, UV and optical absorption lines. T ∼ 102 K. Ionized Hydrogen (HII region), H+, Emission line spectra, in particular the n =3 to n = 2 hydrogen transition Hα. T & 104 K.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 35 / 93 Hydrogen Transitions

continuous energy levels 0 n = ∞ −0.85 n = 4 −1.51 n Paschen = 3 Series −3.4 n = 2 Balmer Series Energy (eV)

−13.6 n = 1 Lyman Series

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 36 / 93 Hyperﬁne Structure

Using radio telescopes, the atomic hydrogen gas in our galaxy can be mapped by observation of the 21 cm wavelength line (1420 MHz).

nuclear electron spin spin ↑ ↑ 1S ∆U ≈ 5.9 × 10−6 eV ↑ ↓

λ = 21 cm

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 37 / 93 Q: 21 cm line

What produces the 21-cm radio line that we can be used to map the Milky Way Galaxy? A) atomic hydrogen B) ionized hydrogen C) molecular hydrogen D) carbon monoxide E) helium

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 38 / 93 Q: 21 cm line

What produces the 21-cm radio line that we can be used to map the Milky Way Galaxy? A) atomic hydrogen B) ionized hydrogen C) molecular hydrogen D) carbon monoxide E) helium

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 38 / 93 Andromeda NGC 4565

The Milky Way

ISM

Nebula Nebula Types Emission Nebula Retina Nebula Lagoon Nebula Triﬃd Nebula Nebula Reﬂection Nebula M78 Nebula Eagle Nebula Horesehead Horesehead Horesehead Inside the Eagle Nebula Eagle Nebula Pillars in the Eagle Nebula Veil Nebula Spaghetti Nebula Witch’s Broom SN 1572 Crab Nebula

Galactic c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 39 / 93 Recycling Nebula Types

Nebulae are interstellar clouds of gas and/or dust. Origin: primordial hydrogen and helium gas, material ejected by stars (supernova remnants, planetary nebula, solar wind, ...), or any combination of the above. Catagorize by their interactions with visible light: Emission nebula are glowing clouds of interstellar gas.

Reﬂection nebula reﬂect starlight without emitting visible energy of their own. Absorption nebula are dark clouds which obscure our vision of more distant light sources.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 40 / 93 Emission Nebula

Emission nebula, aka ionization nebula, aka H II regions: glowing, ionized clouds of interstellar gas. Emission nebulae are ionized by UV light from nearby hot star(s). Optical and other emission line spectra result when electrons recombine with ions and cascade down to lower energy levels. Often look red due to the hydrogen Balmer n =3 to n =2 Hα line.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 41 / 93 Planetary Nebula in Lupus

IC 4406: Retina Nebula, Image Credit: NASA/HST Active stellar nursery includes the open cluster NGC 6530

M8: Lagoon Nebula, c Ignacio Diaz Bobillo Emission, Reﬂection & Absorption regions

M20: Triﬃd Nebula in Sagittarius, c Adam Block (U.Arizona) Reﬂection Nebula

Clouds of interstellar dust which reflect the light without emitting visible light of thier own. Blue hue due to 1) reflected light from hot “blue” stars 2) dust reflects blue light more strongly.

WitchHeadnebula Pleiades

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 45 / 93 Two reﬂection nebulae

Nebula NGC 2071 & NGC 2068 (M78) in Orion Absorption Nebula aka Dark Nebula: dark interstellar clouds which obscure our vision of more distant light sources.

M16: Eagle Nebula IC 434: Horsehead Nebula in Orion Horsehead Nebula in Orion’s Belt IC 434: Horsehead Nebula (IR) Inside M16: The Eagle Nebula, c T. A. Rector & B. A. Wolpa M16: Eagle Nebula, Image Credit: NASA, STScI, WikiSky/HST Inside M16: Pillars of Creation, Credit: J. Hester, P. Scowen (ASU), NGC 6960, 6979, 6992: Veil Nebula, c Martin Pugh 40,000 year old SN remnant with NS NGC 6960: Witch’s Broom Nebula, c Martin Pugh Type Ia SN Remnant in Cassiopeia, Image Scale 19′ = 55 ly, d = 13 kly 0.95 − 1.26 keV 1.63 − 2.26 keV 4.1 − 6.1 keV

SN 1572: Tycho’s SN, Image Credit, X-ray NASA/CXC/K.Eriksen et al., Optical: DSS Link to timelapse

M1: Crab Nebula in Taurus, 1054 type II SN Remnant Andromeda NGC 4565

The Milky Way

ISM

Nebula Galactic Recycling Galactic Recycling Fig 14.03 Galactic Recycling Q: Star-Gas-Star Cycle Fountains Cygnus Loop Spectrum Bubble Nebula The Orion Complex Orion Nebula Orion Spectrum Carina Nebula Q: ISM Fate MWMW Multi- wavelength Milky Way Q: Molecular Clouds

Evolution c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 59 / 93 The Galactic Galactic Recycling

• Stars return gas to interstellar space through stellar winds, planetary nebulae (low mass stars), and supernovae (high mass stars). • New elements made by stars mix into interstellar medium. • Atomic hydrogen gas forms as hot gas cools, allowing electrons to join with protons. • Molecular clouds form next, after gas cools enough to allow atoms to combine into molecules. • Gravity forms stars out of the gas in molecular clouds, completing the star-gas-star cycle.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 60 / 93

Q: Star-Gas-Star Cycle

What happens after many generations of the star-gas-star cycle?

A) The heavy element abundance increases, the amount of hydrogen gas increases.

B) The amount of gas and heavy elements stay the same: it’s a cycle.

C) The heavy element abundance increases, the amount of hydrogen gas stays the same.

D) The heavy element abundance stays the same, the amount of hydrogen gas decreases.

E) The heavy element abundance increases, the amount of hydrogen gas decreases.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 62 / 93 Galactic Fountains

SN in the the galactic disc heats the ISM driving hot gas out of the disc, which cools, condenses, and rains back into the disk. c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 63 / 93 Cygnus Loop Spectrum

Expanding supernova remnant, 5-8 kyrs old, 90 ly in size.

Emission spectrum shows New elements made by supernova now mixed into interstellar medium.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 64 / 93 10 ly diameter bubble from stellar wind of massive 45M⊙ O star.

NGC 7635: Bubble Nebula 10 ly diameter bubble from stellar wind of massive 45M⊙ O star.

NGC 7635: Bubble Nebula The Orion Complex

c Rogelio Bernal Andreo

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 66 / 93 5 2000M⊙ star forming region. Destined to become an open cluster in 10 yrs.

M42: Orion Nebula, Image Credit: NASA/HST

Star formation region 300 ly in size

NGC 3372: The Great Carina Nebula Dust Pillar

NGC 3372: The Great Carina Nebula Evaporating Blobs

NGC 3372: The Great Carina Nebula Q: Fate of the ISM

Where will most of the gas be in 1 trillion years? A) Blown out of galaxy B) Still recycling just like now C) Locked into white dwarfs and low-mass stars

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 70 / 93

Multi-wavelength Milky Way

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 72 / 93 Q: Molecular Clouds

Which of the following molecules is the most abundant in molecular clouds?

A) H2 B) CO

C) H2O

D) NH3

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 73 / 93 Q: Molecular Clouds

Which of the following molecules is the most abundant in molecular clouds?

A) H2 B) CO

C) H2O

D) NH3

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 73 / 93 Andromeda NGC 4565

The Milky Way

ISM

Nebula Galactic Recycling

Evolution Metallicity Evolution Stellar Populations Evolution Q: Halo Stars Q: Star Formation Region The Galactic Center

Review

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 74 / 93 Metallicity

Mass Fractions:

mH • X Hydrogen: X = M

mHe • Y Helium: Y = M

m“metals′′ • Z Metallicity: Z = M =1 − X − Y Sample mass fractions:

X⊙ =0.73, Y⊙ =0.25, Z⊙ =0.02

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 75 / 93 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 76 / 93 Evolution of the Milky Way

Observation: Halo Stars: 0.02%-0.2% heavy elements (O, Fe, ...), only old stars Disk Stars: 2% heavy elements, stars of all ages

Conclusion: Our galaxy probably formed from a giant gas cloud. Halo stars formed ﬁrst as gravity caused the cloud to contract. The remaining gas settled into a spin- ning disk. Disk stars formed later, kept forming, and continue to form today.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 77 / 93 Q: Halo Stars

Compared to our Sun, most stars in the halo are A) young, red, and dim and have fewer heavy elements. B) young, blue, and bright and have many more heavy elements. C) old, red, and dim and have fewer heavy elements. D) old, red, and dim and have many more heavy elements. E) old, red, and bright and have fewer heavy elements.

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 78 / 93 Q: Halo Stars

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 78 / 93 Q: Star Formation Region

Where does most star formation occur in the Milky Way today? A) in the halo B) in the bulge C) in the spiral arms D) in the Galactic center E) uniformly throughout the Galaxy

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 79 / 93 Q: Star Formation Region

Where does most star formation occur in the Milky Way today? A) in the halo B) in the bulge C) in the spiral arms D) in the Galactic center E) uniformly throughout the Galaxy

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 79 / 93 Andromeda NGC 4565

The Milky Way

ISM

Nebula Galactic Recycling

Evolution The Galactic The Galactic Center Center GC: IR GC: Xray GC: Radio GC: VLA GC: IR Orbits Sagittarius A* Q: Galactic Center Q2: Galactic Center For Further Study

Review

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 80 / 93 Center of the Milky Way (IR): NASA/JPL Image Credit: NASA/CXC/Caltech/M. Muno et al. Sgr A*

Galactic Center Radio Arc, Credit: Farhad Zadeh λ = 6cm, size = 10 ly, VLA Image courtesy of NRAO/AUI Infrared HKL-band color mosaic

Sagittarius A*

Sagittarius A-star (Sgr A*): Supermassive black hole in our galaxies center. Discovered 1974 using the NRAO

distance = 26, 000 light − years

From orbital observations

R< 45 AU ∼ aUranus

From orbits of satellite stars:

6 Mbh ≈ 4 × 10 M⊙

For a black hole this massive, the Schwartzchild radius is:

RSch ≈ 0.08AU

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 87 / 93 Q: Galactic Center

The galactic center lies in the direction of which constellation? A) Orion B) the Big Dipper C) Leo D) Sagittarius E) Taurus

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 88 / 93 Q: Galactic Center

The galactic center lies in the direction of which constellation? A) Orion B) the Big Dipper C) Leo D) Sagittarius E) Taurus

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 88 / 93 Q2: Galactic Center

What kind of object do we think lies in the center of the Milky Way Galaxy? A) A 3 to 4 million solar mass black hole B) A gigantic X-ray binary system C) A dense cluster of young, hot stars D) An enormous collection of dark matter, explaining why we detect no light at all from the galactic center

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 89 / 93 Q2: Galactic Center

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 89 / 93 For Further Study

• Video of BH at the center of the Milky Way • Journey Through the Milky Way • Supermassive BH in the Milky Way • Diving into the Lagoon Nebula • Tycho’s Nova • Fly into the birthplace of a massive star

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 90 / 93 Andromeda NGC 4565

The Milky Way

ISM

Nebula Galactic Recycling

Evolution The Galactic Review Center

Review Review I Review II

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 91 / 93 Review I

• Describe the disk, bulge, and halo of our galaxy? • Where does most star formation occur in the Milky Way? • How do the orbits of halo stars diﬀer from those in the disk? • How large is the thickness of the Milky Way? • How large is the diameter of the Milky Way? • How far is the Sun from the galactic center? • How long does it take the Sun to orbit the Milky Way? • What objects are found in the halo of our galaxy? • What are globular clusters? • How do the ages of halo stars compare to the Sun? • Which constellation is located between us and the center of the Milky Way?

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 92 / 93 Review II

• What is the interstellar medium? What is its composition? • What are the three phases of hydrogen in the interstellar medium? • List and describe three types of nebula. • Are you living in a bubble :-)? What made the bubble? • What wavelengths of light are unobstructed by the interstellar medium? • What produces the 21-cm line we use to map the Milky Way? • Describe the star-gas-star cycle in the Milky Way. • Describe population I, population II, and population III stars. • How massive is the black hole at the center of our galaxy? How do we know it is there?

c 2012-2021G.Anderson.,O.Harris Universe:Past,Present&Future – slide 93 / 93