The Interstellar Medium

Astronomy 315Professor Lee

CarknerLecture 19

Exercise 18 – Dark Matter Use Kepler’s Third Law to find the total mass of

the galaxy M = a3/P2

a = (20000 pc)(206000 AU/pc) = 4.12X109 AU P = 460X106 years M = (4.12X109)3/(460X106)2 = 3.3X1011 solar masses

Luminosity of galaxy F = L/4d2, L = F4d2

L = (3.88X10-12)(4)()(5.58X1023)2 = 1.52X1037 W 1.52X1037/3.8X1026 = 4X1010 Lsun

Mass-to-light ratio M/L = 3.3X1011/4X1010 = 8.25

The Stuff Between the Stars

Called the interstellar medium

Concentrated in the disks of spiral galaxies

Exists with different temperatures,

densities, size scales, and radiation environments

Composition

We can get an idea of the composition of the interstellar medium through spectroscopy

There are two components: gas and dust Gas

Dust Account for most of the opacity

Gas

Interstellar gas is abundant in hydrogen and helium

Heavier elements have condensed to form the dust

What form is the gas in?

Radiation Environment Different parts of the ISM experience

different external radiation fields Location of bright stars

Density of cloud The inner part of a dense cloud is shielded

from most external radiation

Effects of Radiation Clouds that experience little radiation are

known as HI regions

Clouds in higher radiation environment are known as HII regions

Some clouds have so little radiation that they can form some molecules Called giant molecular clouds

Dust The clouds are much more opaque than you

would expect just from the gas Dust particles are small (~1 micron or 1

millionth of a meter) Some different types of dust:

All these different types identified from complex absorption properties

Extinction Curve

What Does Dust Do? Absorption

This causes interstellar extinction

Scattering Dust scatters blue light better than red

Stars behind the cloud look redder

Reddening in the ISM

Reddening

Nebulae

What is a nebula ?

There are three basic types of nebula, each with a distinct appearance based on the way it interacts with light

Dark Nebulae

Dense clouds can completely block out the light of stars behind them

Can see stars in front of the cloud

projected on it Smallest are called Bok globules

Star Cluster and Dark Cloud

Emission Nebulae If a cloud is near bright high mass stars it may

shine as an emission nebula The UV light ionizes the gas

Like a florescent light

Emission nebulae are HII regions

Reflection Nebulae

Dust preferentially scatters blue light

Same reason sky is blue Need bright star fairly near-by to

produce effect

Observing the ISM

With optical telescopes we see the effects of the ISM in different ways

We also need to observe at other wavelengths

Multiwavelength ISM Radio

Penetrates dust and allows us to map much of the galaxy

Millimeter

Some are very complex and must be protected from UV radiation

More Multiwavelength ISM

Infrared

Dust is still very cold (<100K or -150 C)

X-ray

Formed from supernova

Types of Clouds HI clouds

Molecular Clouds

Coronal gas

Intercloud medium

Hot, low density, about 50% of total mass A lot of mass between the well-defined clouds

Structure of ISM If we look in any direction we see many different

parts of the ISM

The sun is actually in a large region of hot gas called the local bubble

The denser parts of the ISM are the sites of star formation

Next Time

Read Chapter 24.1-24.2