Assignments

• Read Sec. 20 (“Our Milky Way Galaxy) and Sec. 21 (External galaxies) of DocOnotes-ISMgal.pdf

• Submit HW5.pdf by 5 p.m Fri. Apr. 19

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HII Regions

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RosettaNebula

HII Regions

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Galaxy Song - Composers: Eric Idle & John Du Prez Author: Eric Idle Singer: Eric Idle From the 'Meaning of Life' Just remember that you're standing on a planet that's evolving And revolving at nine hundred miles an hour, That's orbiting at nineteen miles a second, so it's reckoned, A sun that is the source of all our power. The sun and you and me and all the stars that we can see Are moving at a million miles a day In an outer spiral arm, at forty thousand miles an hour, Of the galaxy we call the 'Milky Way'.

Our galaxy itself contains a hundred billion stars.It's a hundred thousand light years side to side.It bulges in the middle, sixteen thousand light years thick, But out by us, it's just three thousand light years wide. We're thirty thousand light years from galactic central pointWe go 'round every two hundred million years, And our galaxy is only one of millions of billions In this amazing and expanding universe.

The universe itself keeps on expanding and expanding In all of the directions it can whizAs fast as it can go, at the speed of light, you know, Twelve million miles a minute, and that's the fastest speed there is.So remember, when you're feeling very small and insecure, How amazingly unlikely is your birth, And pray that there's intelligent life somewhere up in space, 'Cause there's bugger all down here on Earth.

Galactic disk, bulge & halo...

About how long does light take to cross the Milky Way Galaxy?

A. About a monthB. About four yearsC. About 1,000 yearsD. About 100,000 yearsE. Millions of years

About how long does light take to cross the Milky Way Galaxy?

A. About a monthB. About four yearsC. About 1,000 yearsD. About 100,000 yearsE. Millions of years

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Stars we see in the halo of our galaxy (or in globular clusters) formed even before the Milky Way collapsed

into a disk. Since we see them now, they must be

A. Young B. OldC. Low-massD. High-massE. B and C

Stars we see in the halo of our galaxy (or in globular clusters) formed even before the Milky Way collapsed

into a disk. Since we see them now, they must be

A. Young B. OldC. Low-massD. High-massE. B and C

Why do stars in the halo of the galaxy have little of the heavy elements such as carbon, nitrogen, and oxygen?

A. Those elements have been used up in halo stars.B. C, N, and O are biological elements, and there is no life out there to

make them.C. The halo stars formed before those elements were made.D. Making C, N, and O requires massive stars, and there are no massive

stars in the halo.

Why do stars in the halo of the galaxy have little of the heavy elements such as carbon, nitrogen, and oxygen?

A. Those elements have been used up in halo stars.B. C, N, and O are biological elements, and there is no life out there to

make them.C. The halo stars formed before those elements were made.D. Making C, N, and O requires massive stars, and there are no massive

stars in the halo.

Regions of the Milky Way Galaxy

• Disk• younger generation of stars• contains gas and dust, heavy elements• location of the open clusters

• Bulge• mixture of both young and old stars

• Halo• older generation of stars• contains no gas or dust, few heavy elements• location of the globular clusters

Spiral Structure• Galactic disk has “spiral arms”

– much as seen in other galaxies

• Arms are not like fins of a fan.

• Rather are moving patterns– like “wave” at baseball game

• Compression waves propagate around disk

• Note how spiral arms appear bluer than bulge or gaps between arms– due to lots of young, hot, blue starsM 51

Spiral Arms• Compression to higher density leads to concentraton of molecular clouds• source of mass for new stars• short-lived hot, massive stars outline arms and make them blue & bright• long-lived low-mass stars pass through several spiral arms in their orbits around disk

Mass of our Galaxy• Use Kepler’s 3rd Law to estimate mass: M = a3 /P2

• Sun’s distance from center: ~25,000 l.y. = 8 kpc ~= 2 x 109 AU• Sun’s orbital period: ~200 million years = 2 x 108 yr

• Mass within Sun’s orbit is ~ 2 x 1011 Msun

Galactic rotation

=> M~V2r

Velocity form of Kepler's 3rd Law

M (R)M (8 kpc)

= V (R)210 km / s

⎛⎝⎜

⎞⎠⎟

2R

8 kpc⎛⎝⎜

⎞⎠⎟

M (8 kpc) ≈ 2 ×1011Msun

RV(R)M(R) = mass inside R

Galactic Rotation

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Mass from orbital speed and radius

RV(R)M(R) = mass inside R

M (R)Mo

= V (R)Vo

⎛⎝⎜

⎞⎠⎟

2RRo

⎛⎝⎜

⎞⎠⎟

Mo ≈ 2 ×1011Msun ;Vo ≈210 km / s ; Ro ≈8 kpc

M(R) = mass inside R

Galactic rotation speed

M (R)Mo

= V (R)Vo

⎛⎝⎜

⎞⎠⎟

2RRo

⎛⎝⎜

⎞⎠⎟= R

Ro

⎛⎝⎜

⎞⎠⎟

Spectral Slit across Galaxy

Spectrum from Slit across Galaxy

Center of the Galaxyin Sagittarius

Visual

Infrared

Center of the Galaxy

Radio

X-ray

Although dark in visual light, there are bright radio, IR, and X-ray sources at the center of the Galaxy, known as Sgr A*.

The semi-major axis of SO-2 has an angular size a= 0.1 arcsec. At distance d= 8 kpc, what is the physical size s, in au?

A. 8000 auB. 80 auC. 800 auD. 0.1 auE. 1 au

The semi-major axis of SO-2 has an angular size a= 0.1 arcsec. At distance d= 8 kpc, what is the physical size s, in au?

A. 8000 auB. 80 auC. 800 au = 0.1 arcsec * 8000 pcD. 0.1 auE. 1 au

Pop QuizWhat is mass at the ?

A. 1 Msun

B. 106 Msun

C. 100 Msun

D. 107 Msun

E. 10 Msun

Suppose a GC star has an orbit with angular size 1/8 arcsec and period P=10 year. What is the implied mass of the central black hole, in Msun?(Recall dGC=8 kpc.)

A. 1 Msun

B. 106 Msun

C. 100 Msun

D. 107 Msun = (8000/8)3/102

E. 10 Msun

Suppose a GC star has an orbit with angular size 1/8 arcsec and period P=10 year. What is the implied mass of the central black hole, in Msun?(Recall dGC=8 kpc.)